Thursday, November 21, 2019

Collecting data on the failure to autoload tapes or dump into the vacuum columns


This was quite interesting to watch. I attempted this about five times, all unsuccessful. The first was the most successful try.

The tape did indeed thread itself around the perimeter and out to the take-up reel hub but it stopped threading too early. It should have fed more tape and wound it a few times around the hub before switching on vacuum and attempting to dump into the columns.

The remaining attempts never got that far. Once the supply reel refused to turn at all. The other times it fed in part way then gave up and rewound onto the supply reel a few seconds.


I began with the tape manually threaded through the machine and around the take-up reel, pushing load with my acrylic cover held in place. In fact, the tape did dump down and form the two loops midway into their vacuum columns, but the machine then recognized a load check and dropped vacuum. I need to see what other failures can lead to the load check symptom, as my problem may be elsewhere.


First, I was concerned about the one case where the supply reel didn't move at all. It has worked reliably every other time but it might hint at some unresolved problem.

The time it threaded successfully we either ran out of time or didn't set sufficient time to wind the tape around the hub a few times. I had to investigate this.

Using my scope I watched the test point that shows me timer intervals. The 5.5s timer was measured at 5.0s, a bit short but somehow this can't be the whole story. Perhaps the turning speed of the reels is too low, or some key signal is not being processed. For example, when the tape begins to thread into the path, it has only 3.5 seconds on a timer.

During that time, the BOT sensor that looks for the bright start of tape reflector should go dark as the tape passes that point. This resets the timer to 5.5s which ought to mean that we have at least that amount plus whatever time was used to darken the BOT photocell.

The problem is that I don't really know what is normal. Is the reel turning too slowly? Is the time interval a bit too short and causing the problem? Is there another defect on the control board or with the capstan that I haven't detected yet?

Friday, November 15, 2019

Drilling down into failure of drive A 'dump' of tape into the vacuum columns


Since the visual evidence showed that the failure is occuring in the upper vacuum chamber, fed by the supply reel, I can remove the signal leads from the lower chamber and instead use those leads to monitor the pressure/vacuum differential switch signals that trigger changes in the loading state machine.

I believe that the switch recording vacuum in the chambers will activate (go low) at the start of the dump sequence. This is where the supply reel will move clockwise to allow the tape across the mouth of the upper vacuum chamber to be sucked downward.

The loop of tape in the chamber should pass across four photocells near the top of the chamber, the reel should stop at that point and the tape loop should NOT pass across the other four photocells at the bottom of the column.


I was not happy with the reliability of the data I was collecting. Depending on where I push on the acrylic cover, I get different failure results. In one case the tape edge folded over instead of sliding down the column in a loop. I think the plate flexes enough that if I push within a column it narrows below the width of tape.

The logic analyzer did record signals but I never saw the vacuum signal go low, in spite of the fact that something triggered the capture. Without seeing the signal go down, I can't trust what I am seeing. Either there is some change I need to make to the logic analyzer software in the Analog Discovery or I have to move over to my full size dedicated analyzer.

Thursday, November 14, 2019

Gleaning hints for what is transpiring behind the metal cover of the Telex 8020 tape path


Both of my drives are failing when attempting an autoload. When I hand thread the tape through the path and onto the take-up reel, it fails to dump the tape into the vacuum columns properly.

In both cases, I am not sure exactly where problems are arising because the tape path and vacuum columns are covered by a metal plate. The Telex repair people had a clear plastic plate they could substitute for the metal one, thus observing any defects in loading.


I designed a clear plastic plate and sent it out for fabrication yesterday, but won't have it back for about a week. In the interim, I am looking for clues I can gather to help narrow down the area where the load process is going wrong.

If I can access signals that tell me the location of the tape, particularly in the two vacuum columns, it might tell me what kind of failure is occuring. The tape might not enter the column at all, or it might bottom out without the servo loop properly controlling the reels.

I looked at the schematics and was pleased to see that all the signals I want to view are conveniently present on J1 near the bottom of the logic card backplane. Each vacuum column has eight LED-photocell combinations, four at the top of the column and four at the bottom. These signal the current position of the tape column.

The servo mechanism for each tape reel, while in dump or run mode, should turn the reel motor at increasingly high rates as the tape loop moves through the four photocell locations - turning in one direction, to feed in more tape when the loop is at the top of the column, and in the other direction to pull some out if the loop is approaching the bottom.

I set up my Digilent Analog Discovery as a logic analyzer, capturing the state of the sixteen signals over time. Most jacks hooked to the logic cage have a set of wire-wrap pins sticking out for convenient access to the signals on various socket pins. Not so with J1, unfortunately.

I then had to seek an alternate way of capturing the signal. If a signal arrives on one of the sockets on the back of the logic cage, I assumed it must be connected to at least one of the backplane pins behind the PCBs. The only other routing would be socket to socket, bypassing the logic cage.

I found the signals on card slot 4, pins 23, 25, 27, 29, 31, 33, 35, 37. 39. 41, 43, 45, 47, 49, 51 and 53. Nicely sequential and easy to wire up. With the Analog Discovery cable wired up, I brought out the device and hooked it to my laptop, where I configured it as a 16 channel logic analyzer. With that done, I powered up the tape drive and then plugged in the cable to the device.


The autoloader depends on air pressure at a number of points to guide the tape around the perimeter of the tape patch and out toward the take-up reel. If any of those are not producing flow, or have inadequate flow, the tape will droop into the upper vacuum column which is what I presume is occuring.

I know there is enough flow to separate the end of the tape on the reel and to blow it up onto the PEOT sensor that detects the end of the tape covering a vacuum port. There also must be air flow inside the chamber so that as the end of the tape enters the covered area it is blown upwards towards the top edge.

Once the tape is pushed along the top edge to the top-left corner, more air pressure should drive it downward through the tape head. Finally, at the bottom left there is pressure to force the tape rightward towards the exit slot where it continues to the take-up reel.

The process of loading spins the supply reel until the end of tape is sensed at PEOT, then it reverses and threads the tape which should be forced by air pressure across the top, left and bottom edges of the tape path. After the tape exits the bottom slot and reaches the take-up reel hub, vacuum holes in the hub grab the end of the tape and a sensor switch detects the Hub Vacuum condition.

The tape winds a bit more around the take-up reel before a timer expires and begins the dump sequence. The supply reel feeds tape in and the take-up reel unwinds some tape. The solenoid based valve should have switched to provide full vacuum to the two vacuum columns, so that when tape is released by the two reels it is sucked down into the upper vacuum column and sucked up into the lower column.

A servo loop between the detected tape position, using photosensors, and the reel motor will feed out or pull back some take to each reel to maintain the tape loop in the middle zone of each vacuum column. When the tape is in position and a timer expires, the drive begins to seek forward looking for the BOT reflective spot that signals the beginning of tape. It stops the motors and illuminates the Load Point lamp. This completes an autoload sequence.

With tape being manually threaded through the path and wound on the take-up reel, the drive is smart enough to detect the Hub Vacuum condition and switch immediately into the dump state. It should switch on vacuum to the chambers and cause each reel motor to feed out some tape until the loops are in the middles of the two chambers.

While the drive currently won't complete the self-threading, it used to. Back then, it would fail in the dump state. This may be the same failure that occurs when I pre-thread the tape, or a different one.


The picture I took and used to trace out the outline of the metal cover was obviously photographed at an angle and suffered parallax errors. The resulting cover is close but is particularly out of alignment near the top.
Acrylic cover over tape path and vacuum columns
Fortunately, it will fit over the tape path and vacuum columns permitting me to do some testing. The misaligned holes near the top may cause autothreading to malfunction, thus I can't totally diagnose that part of the problem with the current cover. However, if I manually thread the tape through the path and onto the take-up reel, I can watch the dump process where it tries to lower the tape into the vacuum columns.


I could see the dump process well with the acrylic cover in place. The take-up reel released tape forming a loop right in the middle of the lower vacuum column, exactly where it should stop. The supply reel released tape which fell all the way to the bottom of the upper vacuum column and caused the tape check.


Based on the testing I did with the clear cover, the failure is with the servo loop that lowers the tape into the upper vacuum column. Specifically, it is not stopping the supply reel motion when the tape hits the middle of the column nor reversing direction as it should when the tape loop passes by the lower four photocells.

The failure could be:

  • LEDs and/or photocell board failure causing logic board to miss the tape loop
  • Logic board failure so that it doesn't properly respond to the tape loop in slowing or reversing the supply reel
  • Servo loop failure so that commands to slow, stop and reverse the supply reel motor are not properly handled
I will use the logic analyzer signals to see what the logic board is being told about the upper tape loop. That will help me choose between the possible locations of the fault listed above.

Monday, November 11, 2019

Starting repairing Control Logic Board for drive B


I subjected the board to a careful examination under the stereo microscope, looking closely at every component and wire for signs of damage. I found a broken capacitor that sets the time constant for a one-shot DM96L02 thus disabling this timing pulse.

I also saw one electrolytic capacitor that looked a bit dodgy to the eye and applied a resistance test to it. Fortunately it passed the sniff test so it doesn't need replacement.


The schematic gave me the value for the capacitor and fortunately I didn't have one at hand, so off to Anchor Electronics and, nine cents later, I had what I needed. I desoldered the old one and installed the new one, putting the board back into apparent full working order.


It malfunctioned in a slightly different way, but still rotated the supply reel for a few seconds at power up. It also failed to load properly since it couldn't reverse the direction of the reel for the first step of the load sequence.

So much for the unrealistic hope that everything wrong with the board would be visible as damage and easily corrected by replacement of a part. Real debugging ahead, tracing signals around the board until I find the defects.

It would be easiest to debug if I had a card extender, but I don't. I considered designing one and sending it to fab, as long as I can find the female connector into which the card is inserted.No luck finding a 35 x 2 row .156" connector at Digikey, but I widened my search. I did find a 36x2 connector of the proper spacing on eBay and given its low price, bought it.

When the connector arrives next week I will decide whether to build an extension board using this socket. There are other ways, a bit more cumbersome, where I can put micrograbbers on specific component pins/leads and slide the card all the way into the card cage.

Building clear cover for vacuum columns to assist in debugging


The path that the tape takes past the head and into the two vacuum columns is hidden behind an aluminum cover plate, blocking a chance to observe what is working and what is failing during the autoloading. The maintenance manual identifies a service tool that was available - a clear plate to install.
Cover plate hiding tape path and vacuum columns
Since I don't have that plate and there is little chance I would ever find it, the only way I can peek inside is to manufacture my own. The plate has many holes to clear screws and tape heads, as well as holes that apparently help control the airflow to route the tape through the path. It will be a complex piece to design and build, but ultimately worthwhile.

Tape path around left perimeter and two vacuum columns

I took the plate off drive B so that I could carefully measure it out for entry into a CAD program of some sort. I suspect that the number of curves and drilled holes take this beyond the capabilities of TAP Plastics, thus it likely will require laser-cutting.

Cover plate to be replicated in clear acryllic
I will use to do the cutting. I first have to build the design using Inkscape, an open source and free alternative to Adobe Illustrator. My door fits in a 23 7/8" by 11 1/2" rectangle but the closest common material size offered by Ponoko is P3 which is 31.1" x 15.1".

The shape is complex, even before all the holes and cutouts get added inside the remaining outline. I began by cutting down the outer shape, which took some time and ingenuity in the case of the arcs of unknown radius. After I traced the arc and continued it around a sheet of paper, I could determine the radius and distance from the perimeter to the far edge of the cover.

I then decided I would get better results if I were to take a picture of the cover, import it into Inkscape as a layer and then trace everything on the next higher layer. I had sized the picture so the image dimensions matched quite well, ensuring that my final design will correct. That gave me a good set of circles, paths and other shapes that fit well.
Laser cut file
I then uploaded it to and selected clear acrylic of the appropriate size as the material. This gave me a set of costs, including premiums for faster production and for speedier shipping. They started at just over $21 not counting shipping, which was a minimum of Priority Mail. I upgraded to 3 day manufacturing which resulted in a total of $40. It should arrive late on the 18th of November.

Friday, November 8, 2019

Attempting manual load of tape on drive in Cabinet A of Telex 8020


The drive logic is set up for manual loading (or when a drive loses power with tape wound on the take-up reel) where it will dump the tape into the vacuum columns and then begin seeking the BOT marker.


Threading the tape through the transport is annoying since the drive is really designed for autoloading, but I was able to get it through and wound around the take-up reel. In fact, as I wound the tape I saw the Load Point light flick on, which helps validate that the Beginning of Tape sensor and logic works properly.

However, when I hit the Load button, the top reel allowed its tape to buckle out of the entry slot in the tape transport and not load down into the upper vacuum column. Sigh. I will need to do more tape path and loading debugging. Sure wish I had the clear plastic alternate cover for the transport chamber.


My recent tests with drive B were done with two PCBs I had swapped in from drive A - the control logic PCB and the reel pre-amplifier board. Since I was going to move back to drive A for the manual loading test I described above, I wanted to move those boards back where they belonged.

When I put the original two boards into drive B, the place they sat when I acquired the drives, at power-up the supply reel began rotating slowly clockwise. This occurred without having pushed the Load button at all.

Through process of elimination I determined that the Control Logic Board is the one that is malfunctioning. None of the LEDs light up on the board. I have the board pulled out and will have to do a very careful examination as the first step in diagnosing this problem.

Updating channel specification for 370 features


I found a new manual that defines the 370 channel specifications. It is a replacement for the earlier document but has a similar name. It is GA22-6974-x IBM System/360 and System/370 I/O Interface Channel to Control Unit Original Equipment Manufacturers' Information.


Additional Signals

One of the enhancements was to provide a two byte wide data path through the addition of a third cable which hosts a second set of Bus In and Bus Out signals. To differentiate use of this from the usual two cable single byte interface, IBM added some control tags in the Bus cable. These are Mark 0 Out, Mark 0 In, Mark 1 Out, Mark 1 In, Mark Out Parity and Mark In Parity. They are ignored by most control units but Mark 0 In is used for command retry.

IBM added Data Out and Data In tags to support a high speed feature. These are ignored by control units that don't use the facility. I will probably need to implement these just in case future uses of this channel will attach to very high speed devices.

IBM also added a Disconnect In tag that signals some I/O errors. It was a previously reserved signal on the cable thus it will only be generated by control units designed to work with 370 channels. Only some control units have this feature.

Additional Protocol Sequences

Disconnect In is used to assert that a control unit has an internal malfunction such as a processor error that won't allow it to use normal channel protocols to respond, thus the channel should abandon efforts to communicate with this control unit.

Mark 0 In is used to request Command Retry, where the control unit asks the channel to send a command another time because something stopped it from working properly the first time. The logic behind this is that the control unit expects that it may be able to successfully execute the command at a later time. This is either immediately or when the control unit signals it is no longer busy.

High Speed Transfer alternates the use of Service Out and Data Out tags (and correspondingly Service In and Data In) which allows operation at higher rates than the interlock specs dictated using only Service Out/In.

Beginning design of an FPGA based 370 channel, needed to drive my Telex tape drives


Once I have the tape drives able to load tapes and move them forward and backward, I will run out of tests I can accomplish solely with the tape subsystem. To be able to send commands to the drive over the channel, as well as cause it to write and read back data from the tapes, it needs a 370 Input-Output Channel such as would be on an IBM mainframe of the era.

This means I can't really verify whether the read/write circuits work properly without a channel. Simple movement doesn't prove it can handle commands such as forward space past next tape mark. Also, the status and sense bytes that need to be returned can't be viewed without the channel to request them.


I chose to implement a 370 channel in an FPGA because this will be useful for a number of future projects in addition to testing my tape drives now.  This will require me to control 16 outputs and receive 15 inputs. In addition I will output a periodic Clock out and steady low Meter Out, terminating and ignoring Meter IN.

These are designed for SLT logic levels (+3 for high, 0 for low) with specific thresholds, currents and protections. This will be addressed by use of converter PCBs that build drivers with an LVCMOS (3.3) input and receivers that produce an LVCMOS output, but are SLT at 92 ohm impedance on the cable end.

This FPGA channel must be fast enough to handle the 781,250 bytes per second coming from a tape drive, thus a data rate of more than 6.25MB/s. I have to define a connection and protocol from the fpga to some computer where I can generate the software channel programs, source data for writing and receive data and sense information.

In my initial implementation my link to the computer does not need to support 6.25MB/s as each channel program and its data can be preloaded at slower rates, then results fetched at slower rates, as long as the actual transfer over the bus and tag occur at the full rate.


I will use several references in building the specifications for my FPGA channel. The goal is to have definitions of state machines and logic equations for all output signals based on the states. When that is ready, coding it in VHDL will instantiate the machines.

IBM publishes a document that locks down the specifications over the Bus and Tag cables. It is A22-6843-x, IBM System/360 I/O Interface Channel to Control Unit Original Equipment Manufacturers' Information. This will be the first source. I need to understand updates that may have occurred by the time of the 370 version of the channels, which may be a different manual title or otherwise documented by IBM.

Telex schematics show me how they implemented the channel interface for the tape drive controller, which I can cross reference to be sure I understand the IBM documents. Some of this is implemented in firmware in the control unit ROM, thus may require quite a bit of effort to extract for understanding. One has to understand the architecture of the controller engine then must examine each instruction to see what it does.

We (will) have access to the full schematics of the IBM 360/50 mainframe. We already have all the microcode. The channel microcode and schematics show how IBM implemented the channels in the 360 model 50 which is another cross reference to validate what I learn from the documents.

The material from the 360/50 will not include any enhancements made for 370 channels and upon which the tape drive may rely. Hopefully I can back-fit the enhancements to my understanding of 360 channels.

The 360/370 channels come in three flavors, although one of them is not appropriate for use with high speed devices such as the tape drives. These are Byte Multiplexors, Selectors and Block Multiplexors. IBM spells their multiplexer channels with an 'o' - Multiplexor.

Selector channels are for high speed devices and will be busy during the entire channel program. Thus, if the tape has a 10,000 byte record and transfers at 781,250 bytes per second, it will tie up the channel for 12.8 ms until the transfer is done. Worse is when the channel program is searching for a specific record header on disk where up to an entire rotation of time can be required; the channel stays busy for this. Selection is for the transfer of an entire block at a time.

Multiplexor channels are for low speed devices. They select for the transfer of a byte of data then disconnect. This allows multiple slow devices to be overlapping their data transfer, as they multiplex their bytes over the channel.

The Block Multiplexor channel allows for concurrent command chained operations for each device, thus more than one device may be searching for a disk sector but they remain disconnected until the condition is satisfied. Data transfer overlap depends on the speed of the devices, the speed of the channel and the overhead of the control unit requesting the reconnection.

Thursday, November 7, 2019

Restoration of power supply and bring-up of integrated 3803 equivalent control unit for Telex 8020 tape system


These tape drives are intended for use on an IBM mainframe type system, which uses input-output channels connected via a pair of cables called bus and tag. The IBM equivalent tape system was the 3420. A set of 3420 drives had a separate cabinet, the 3803 control unit, into which the bus and tag cables were connected.

The 3803 then had cables hooked to the 3420 tape drives which implemented a simple protocol, with lines to command specific mechanical actions and wires to transport 9 bits of data in and out. All the error checking, formatting and decoding was done in the 3803 control unit.

Telex designed the 8020 tape drives as a compatible alternative to the IBM 3420 system. To offer space savings as an additional inducement to customers, on top of a substantially lower price, Telex designed their control unit to fit inside the first tape drive of a string.

The Telex tape drives implement their own simple protocol, over ribbon cables, to command mechanical actions and read/write 9 bits of data. The control unit speaks this Telex protocol on the ribbon cables and uses the IBM channel protocol on the bus and tag connectors. It performs all the error checking, formatting and decoding too.


I began to move the big, heavy box out of my drive cabinet A to begin the restoration effort. It is more than a foot high, the width of the cabinet and spans from front to back. Coming out of the box are ribbon cables that run to the bus and tag connectors as well as to the various tape drives that would be controlled by this system.

Telex directly soldered them to the bus and tag connectors, thus removal of these would have to come from inside the control unit enclosure. I opened the top to get the lay of the land but the cable routing is quite dense.

I discovered that the entire card tray can slide out and then pivot up to give access to the backplane pins, thus there is no need to remove the box from the drive to do my debugging and restoration. I replaced the anchoring hardware and moved on to the first step of restoration - good power.


I can get access to the power supply from inside the front door of the tape drive, hopefully enough to do any repairs it might need. I first removed the front cover plate, but behind it was a solid heatsink wall so that went back in place. Next I removed the top plate and looked down at the innards including three large electrolytic capacitors.

Control Unit power supply electrolytics
I unscrewed one lead on each of them in order to test the capacitor's condition. I also pulled the connector from the power supply to the backplane in advance of applying first power assuming the capacitors check out okay.

I used my capacitance meter first and confirmed they were all at about 72,000 uf, appropriate for filtering the three supplies. Next I hooked up my ESR meter and confirmed that the equivalent series resistance of these is under 0.06 ohms. At the current of 1A that would only drop .06V when the circuit was fully pulling from the capacitor, much less for handling ripple. These are all good.
Capacitance meter used to verify electorolytics

Meter to measure equivalent series resistance (ESR) of capacitors
The power supply produces three voltages - +15V, -15V and +5V - thus the three filter capacitors we see. The control unit is built with TTL logic and op amps, thus determining the voltages required. Other components such as discrete transistors are designed to work within these voltage levels.

Another early check I applied was to test the jack from the controller logic for dead shorts across the three power supply rails, since I have already found two small filter capacitors on logic boards with dead shorts while restoring other portions of the system. All power rails exhibited the behavior of charging up the filter capacitors, quickly getting to acceptable resistance levels. As an arbitrary example, a 20 ohm total load for the 5V rail would demand 1.25 A from the supply.

Next up I wired up a plug for the controller. It uses an ordinary household style plug given its much lower consumption, so I needed to wire up a socket for that sized plug that would in turn have wires poked into the 240V socket. A bit sketchy but okay for testing purposes.
240V socket for controller (see sketchy wires pushed in bottom socket)
I plugged it in, pushed the on switch and watched for signs of life (or magic smoke escaping). Time to test the three voltages while plug J3 is unhooked from the logic drawer. I saw +5.0, +15.0 and -15.0, exactly what should be generated.

The next and probably final step at this time is to hook up J3 to everything else and turn it back on.
Since I don't have a powered-on tape drive nor legitimate channel cables hooked up, this won't be happy but I can at least watch the status LEDs for boot-up of the processor and indications that the two ends are unconnected.

The signs of successful startup of the controller are LEDs producing a repeating moving dot display on the processor board and two LEDs that indicate the channel adapter initialized properly.  That is exactly what I saw, suggesting that all is good. The processor runs self-diagnostics during power-up and didn't find anything wrong.
LEDs lighting to show successful startup of controller

Tuesday, November 5, 2019

Debugging of Telex 8020 cabinet B drive


After hooking up all the wiring and hoses, I powered up the drive with a tape on the supply reel and pushed the Load/Rewind button. The vacuum powered up but no reel movement occured. I watched the diverter valve solenoid to see it pull in to select load mode versus its default run mode.

No solenoid movement. I put the meter across it and no voltage was delivered to the coil. I then began probing the connections to the driver board. It is provided with +12V and -12V plus a signal control line. The line is pulled up by the driver board and grounded by an open collector inverter on the control logic board.

Testing the lines quickly showed me that the +12V rail is not active. I know that the power supply will shut that off if there is a short circuit, such as I had with the bad capacitor on the dump card when restoring the power supply.

I decided to yank cards out of the machine until I figured out which one is shorting the +12V bus, without harm to the rest of the machine. I switched off the +45V and -45V so they don't provide power to the motors, pulled the power amplifier and pre-amp cards, then monitored +12V.

The voltage was zero again, so I yanked all the cards except for the control logic to ensure I had valid +12V as a starting point. I installed the read, write and AGC boards next had the voltage disappeared. A bit of a binary search led me to the write card which had a dead short across the +12V rail.

It was another of those black polarized capacitors, a 15uf 20V molded unit, similar to the one that failed shorted on the dump card. At this point I distrust all of these, but only the one is bad at this time. If I were to keep the drives in service, I would go through and swap out all of these for new tantalum axial capacitors. As it is, I am ordered a batch of 10 as replacements since these are so troublesome.

Shorted 15uf filter capacitor on the Write board
Now the diverter valve works, the supply reel turns to try to find the end of the tape on the PEOT vacuum sensor, but I seem to be missing the blowing air that forced the tape end up in the air. This may be because of the broken plastic part where the forklift crunched the tape drive.

Indeed, it is because of the damage. Further, the allen bolt that holds one end of the plastic down is bent, thus unable to get the part to seat firmly enough for the rubber gasket to keep the blown air channeled to the proper place.

Damaged plastic part with air hole visible

Channel for air to blow the end of the tape off the reel

I should make a rubber or plastic right angle diverter that sits in the air outlet hole and blows it towards the edge of the tape to make it lift up. It may be possible to buy a new allen bolt and flatten the area to make a proper seal but I am doubtful, so auto cartridge load is ruled out for drive B.

Damaged surface where o-ring and plastic part need to seal

I realized that the control logic will see tape in the path and a vacuum on the take-up reel hub, understand that tape is already wound on the drive, and simply attempt to dump the tape into the vacuum columns. I hand threaded the tape and hit Load/Rewind, but the tape in one of the columns bottomed out causing a Load Check. One of the servos or one of the LED sensors is malfunctioning - it appears to be the upper column.

Digging into load failure on Telex 8020 drive A


When a threading attempt failed with the latest problem, it left the end of the tape down in the upper vacuum column instead of threaded through to the tape path down past the heads.

I trimmed the bent end of the tape and now it fails getting down near the bottom of the tape path, perhaps 6" shy of reaching the take-up reel. It times out but with a different situation inside the hidden area of the drive.


I observed that the speed with which the take-up reel rotates is much faster than the very slow rotation of the supply reel. Since that reel lets out the tape to thread through the path, if it is too slow then there is insufficient time for the tape to make it all the way to the take-up reel.

The maintenance manual has a procedure to set the rotational speed of the reels during the autoload. Unfortunately it involves a special Switch board that replaces the Control Logic board; I don't have this board nor do I have the clear plastic vacuum chamber cover I mentioned earlier, nor the card extender which is also used for the procedures. . Still, I know the rotation speeds based on the desired settings. I can observe the speeds during the stages of the autoload and grossly adjust the speed for any stage where it appears wrong.

The supply reel speeds during the stages are:
  • While looking for the end of the tape to start threading, counterclockwise at 2/3 rps
  • While threading into the path, clockwise at 1/2 rps
  • When lowering tape into the upper vacuum column, counterclockwise at 1.3 rps
The take-up reel speeds during the stages are less varied:
  • While threading into the path, clockwise at 2.1 rps
  • when lowering tape into the lower vacuum column, counterclockwise at 2.8 rps
Each reel has its own power amplifier PCB in the card cage - slots 1 and 2 are assigned to reel amplifiers. The pre-amplifier for both reels is a single PCB in slot 4. The Control Logic board in slot 5 gives commands to the pre-amp board, where it generates the drive voltages that are amplified by the boards in slots 1 and 2.

I may have problems in any of the cards in slots 1, 2, 4 and 5 as they are all involved in loading operations. I began to study the theory of operations and the schematics to figure out which board(s) could result in overly slow supply reel motion. Further, I set up some observation points to narrow down the fault, if any, to specific circuitry.

I took video of the drive attempting an autoload, allowing me to go back and assess speed after I break the sequence into the first couple of stages. This gave me the ability to validate the speed against the targets above.

I can see that the supply reel is moving far too slowly both when locating the tape end and when trying to thread it through the path. This is the cause of the failure to autoload, but we need to dig further to see which board at fault.

I swapped the two reel power amplifier boards. If the take-up reel slows down and the supply reel goes faster, I know it is one of those. If the problem remains associated with the supply reel, the potential locations of the fault are:
  • Reel pre-amplifier board section controlling the supply reel
  • Supply reel motor itself or wiring to its field coil
  • Incorrect adjustment of both speeds for the supply reel
  • Error in control logic board signals to the reel pre-amplifier
The swap of reel amplifier boards made no difference to the behavior of the drive. I then swapped the reel preamplifier boards in drives A and B to see if this changed things. If anything it was a bit slower than before, so I have to rule out adjustments on the reel preamp board in addition to the previous exclusion of the two reel amplifier boards. This means it must be a fault in the motor or wiring itself as everything else has been swapped.

Digging further into the schematics and theory of the reel motors showed me two areas to check next. I have to verify that the field coil ramps to the proper value on the supply reel motor, something I can detect using my voltmeter on the sense resistor. The other potential source of the slow drive is the autoload analog switch and its reference voltage, as these supply the voltage that is used by the preamplifier and amplifier to set motor speed.

I can do comparative testing between the take-up reel voltages, since these seem approximately correct, and the supply reel voltage. I don't know the voltage to speed function; could be linear with voltage or some other curve that makes it harder to determine the desired supply motor value.

Similarly I can do comparative testing of the field current as a way of checking that the supply motor itself seems healthy. That is, compare the voltage on the take-up reel sense resistor with the voltage at the supply reel resistor.

There are practical considerations that may complicate getting the meter on the sense resistors or the command voltage to the reel pre-amplifier board. I had to study the schematics and then the boards and backplanes to figure out how to access what I need.

The analog switch and reference voltages are produced on the reel preamplifier board, which I had swapped. This tends to rule out the problem being on that board, but I found test points to measure what I wanted to see. The test points are deep back on the PCB, suitable for access when using the extender card I don't have. Instead I need to attach mini-grabbers with long leads to carry the measured signal out to where I can reach them.

First to test were the reference voltages used for speed control - +4.8V and -4.8V. These were good of course since the take-up reel wouldn't work properly if they were bad. Next up were the driving levels for the two reels, picked off from two other test points.

The test points showed me the output of the analog switch levels. The take-up reel signal was at 0.7v when it was rotating during threading. The supply reel signal was at 0.8v when seeking the end of tape and then about 0.7v when threading. Based on these I would expect both motors to turn at the same rate, but they are not.

I then moved to the signal that exits the reel pre-amplifier and drives the reel amplifier, which I could pick off from the backplane easily. I hooked up to the take-up reel drive first, measuring
2.1V on drive 1 and .7 volts on drive 2 lines. I then hooked to the supply reel drive signal and saw 2.7V on drive 1 and .7 volts on drive 2, blipping to over 1V for a brief period during threading.

This suggests that the supply reel should be rotating much faster than it is, based on the drive current from the reel pre-amplifier board. Next up I starting measuring the current going to the two motors, making use of the sense resistor built into the power amplifier boards. This develops a voltage across it based on the current going into the motor.

The goal was to compared the voltage on the sense resistors of the supply and take-up power boards. That will tell me what the electronics are driving through the main coil of the motor. The results were odd, with the faster rotating take-up reel motor producing 21 millivolts and the slow supply reel displaying over 600 mv.

Since the sense resistor is 0.1 ohms, we can calculate the current flowing by I = E/R as 210ma on the take-up reel and 6A on the supply reel. These motors are designed to peak at about 22A of current. The higher current in the supply reel seems like it is trying hard to get itself spinning further but having no luck.

The preamplifier board sums the target speed and compares to the speed it has projected based on the current, acting as an artificial tachometer. The output of the summing point is the drive voltage to the motor.

There are two test points, TP5 and TP11, which let me see the voltage presented the power amplifier boards for the two reels. These will range from -10V to +10V depending on direction and speed. They are set at about + or - 0.8 volt when the reels are turning.


I cross wired the motors (drive B reel motor hooked to drive A electronics) to see how fast the reel turned during load. It looked somewhat faster but nothing like the take-up reel rate. Not sure this was enough to explain the load problem.


I looked closely at the supply motor rear and found a bizarre repair was done on it, with scotch tape wound around an outlet that is otherwise closed with a screw placed in the vent opening. I compared it to the motor on drive B which appears intact and correct.

Outlet on drive B motor (example of a good fitting)

Sketchy scotch tape repair on the suspect supply motor
This rear section of the supply reel feeds pneumatic pressure to the hub lock, to keep the tape itself from coming off the hub. Unless I feel air escaping, this should be fine. I have no noticed any issues with the tape seeming loose on the reel but I will test it. For now, this is closed as a non-issue.


I decided to switch over to restoration of drive B for a while, moving boards from the logic cage if necessary until it began to attempt a load. I moved the vacuum and blower hoses over to the other drive and shifted the main power input too.

Monday, November 4, 2019

Repair work on logic circuits for drive B


Upon inspection of the control logic board I found that an engineering change was made to this board, adding one jumper wire and one 470 ohm resistor. The resistor was broken in half. I didn't have that value on hand but once I picked one up I could repair the board.

Broken resistor on control logic board (05)
Resistor replaced on Control Logic Board

I had to build a replacement for the configuration "chip" that plugs into the socket for U27 on the capstan pre-amp board. It houses five resistors and two capacitors of specific ratings for the 125 inch per second capstan speed:
  • 3.6K
  • 39.2K
  • 2.94K
  • 4.12K
  • 6.8K
  • .01 uf
  • .027 uf
These are placed across the narrow width to opposing pins. In other words, pins 2 and 15 are opposite each other and connect to a 6.8K resistor. Pins 8 and 9 are opposite each other and connect to a 39.2K resistor. Only pins 1 and 16 are unused.

I collected these components at Anchor Electronics and soldered them onto a IC socket that will plug into the socket below it on the PCB. Everything was going well, verifying the values with my capacitor and ohm meters, when I realized that I had bought an .022 capacitor, not the intended .027 uf.

I had to drive out to the store again just to buy the ten cent part I had miswritten on the sheet, having transcribed my sloppy handwriting incorrectly. After the wasted 45 minutes I completed the DIP socket with the proper configuration components and had it plugged into the PCB.

With this installed, the board is complete and configured for the high speed model 6 behavior. Between the broken resistor for the control logic board fix and these parts, including a second try at the .027 capacitor, my bill was just over $4.

Configuration block for 125 ips on Capstan Pre-amp PCB

Sunday, November 3, 2019

Switching back to power supply in cabinet A for drive 1


Having received the fuse holder to replace a damaged part, I could now use the power supply that came in the base of cabinet A to power the first drive. It was installed and the wiring harnesses plugged into the J4, J5, J6, J8 and J9 sockets on the supply.

Hooking up the main power cord was more challenging until I found that the bus and tag connectors on the rear were on a hinged panel. Lifting that panel gave me access to plug in the power cord to the rear of the supply.

Using the VOM I verified the +5V, +5V for operator control panel, 12VAC, +12V, +6.4V, +8V and -12V levels. Once these were good, I could plug J4 back in and test a power-up to see the +45V and -45V supplies come on.

Restoring operation of Power Window on Telex 8020 drive A


The front door of the tape drive has a glass window that slides down under power to allow the operator to insert or remove a tape from the supply reel, then slides up when the drive loads and uses the tape. It should be triggered by a push of the Unload button, to open, and of the Load/Rewind or Reset button, to close. At power up, the glass is lowered.

The mechanism has switches at the bottom and top of the travel range of the glass, to signal to the logic board that it has fully opened or fully closed. It also has a momentary contact switch on a plate at the top of the glass, which detects if a foreign object, e.g. operators hand, is in contact with the window while it is trying to move.

The PCB in the logic cage, slot 06, looks at the Reset, Unload/Rewind and Load/Rewind buttons, the current state of the window, as well as whether the tape is loaded. It commands closing or opening the door by pulling the appropriate signal line to ground. That line goes to the logic card in the door.

The card in the door looks at the switches at the ends of glass travel, the safety switch on the top of the glass, and the commands coming from the card in the logic cage. It drive the motor one way or the other to accomplish the desired position.

In addition, the door holds the motor and related components that interact with the local logic card. Among these are a full wave bridge rectifier for the logic board.

Power window motor and logic board, plus switches

The wiring is spread across four diagrams in the schematics - an overall tape drive level, the power door level, a control card inside the door, and a control PCB in the main logic cage. I had to take time to map out the interactions - situations such as pin X of one terminal or jack hooked to pin Y on another schematic, which in turn is wired to Z on the door.

Once I had the wiring clear in my head, I could work out a testing strategy that took voltage readings from various signal lines and verified proper operation of the switches. Initially, I was presented with an unasserted command to close the window and an apparently asserted (grounded) command to open the window. Since it is physically sitting in the open position, that is consistent.

The two logic outputs that should indicate whether the window is in the opened or closed state are both at ground. The two command signals didn't change state regardless of keypresses of Reset and Load/Rewind buttons.

I decided to first verify the presence of the input voltages (+45, -45, +12, -12, and +8). If these aren't present the logic and motor aren't going to work.All present and accounted for, after the testing.

Next I pulled the PCB and beeped out the two range switches to be sure they are in the proper position and indicating the correct status. Each switch is a DPDT momentary type, which switches both poles to its normally open (N/O) position when the glass hits the lever, otherwise the poles are at the normally closed (N/C) positions at intermediate glass positions.

Aha! One of the two switches had a bad contact for the N/C position. It was for the upper range (window closed) position, however no movement can occur if the N/C contact isn't made while the window is below that point. It was extremely bad so I removed the switch to try to deoxidize the contact.


I used my trusty Deoxit spray, although it took some doing since the switch was all-but-sealed, with no entry points for the spray. I found that there were very tiny gaps where the electrodes entered the plastic body, over which I could put the applicator straw and then force the fluid inside. After a couple of rounds of this, my switch was restored to like-new condition.
DPDT switch with bad N/C contact (top right)
My backup plan was to swap connections, since one pole only has a N/C connection and the other pole only has a N/O connection. As long as one side had a good N/C and the other a good N/O connectivity, rewirind would do the trick. Fortunately, this wasn't necessary.

The switch was reinstalled, the PCB put back in place and I powered up for the test. Voila! The door now closes on its own when the Reset or Load/Rewind button is pushed and opens on its own with Unload button activation.

Friday, November 1, 2019

Ongoing diagnosis of load function for Telex 8020 drive A


Sleazebag spammers promoting services and goods have been relentlessly posting fake comments on my blog using profile names such as "American Football". I have had to moderate comments now, so that I can keep them from benefiting in any way.

There has always been a low level of such spammery, with generic comments that include links to other sites in the body of the text. When it was infrequent I didn't mind them sitting there for a day or two, but now that I can get 4-6 comments a day cropping up, it became intolerable.

To legitimate viewers of the blog, it means that your comments will be delayed as much as a day until I can spot the legitimate comment and publish it. I wish I didn't have to moderate, but it is the only defense I have. Reporting dozens of spam posts has done nothing to stop the relentless waves of unwanted comments.


We are zooming in on the interval from when the Hub Vacuum sensor detects that the tape has wrapped around the take-up reel hub, until the 'dump' stage causes the tape to pull off the take-up hub. More specifically it is the timing of the transition to 'dump' that is the likely problem.

It seems that the correct behavior after detecting the hub vacuum would be to continue threading for a few more seconds, to achieve a few more turns on the take-up reel. Once sufficient tape is on that reel, dump state can begin. Its first action is to switch the vacuum and blower into run mode, where the vacuum is in the columns inside the tape path and the air pressure flows into the air bearings.

In dump state, the supply reel rotates clockwise to feed tape into the tape path and the take-up reel rotates counterclockwise to also feed back into the tape path. The vacuum in the columns will pull the tape down or up into the column as the reels rotate. When the tape in each column is in its proper position, the rotation of that reel stops. When both are in the proper position, the drive transitions to moving tape forward seeking the BOT reflective spot.

The symptom we see is that the tape has pulled entirely off the take-up reel and thus falls into the lower vacuum column, losing vacuum and causing a Load Check. This can occur for a number of reasons:
  • Not enough tape on the take-up reel so that dump can feed tape to the proper position in the lower vacuum column without the end of the tape falling off the reel.
  • Sensors in the lower vacuum column are not detecting the tape position correctly leading to continual feeding of that column
  • Servo loop for the take-up reel not working correctly, thus continuing to feed tape even though the tape reached the proper position in the column.

A properly equipped Telex customer engineer would have a very useful tool to help diagnose this - a plastic tape path cover. After swinging the metal cover open, the clear plastic plate fits atop the path permitting the CE to watch the tape movement. I don't have this so I have to work blind, instead recording sensor signals and watching external actions.

From detection of the Hub Vacuum condition, the logic should set a 5.5 second timer during which the tape continues to thread around the take-up reel, before it switches to dump state. In dump state, the diverter solenoid is dropped and vacuum switches to the tape columns. The rotation of the reels change to feed tape into the vacuum columns. Do I see the 5.5 second pause and is the take-up reel still rotating?

I set up my iPhone to record the loading process, with the VOM in view to see when the Hub Vacuum is detected. I recorded it in slow motion mode so that I could better observe timing, rotations of reels and other behavior. The VOM will slightly lag the signal change, but a missing 5.5s delay will be really visible.

As the old saying goes, a watched pot never boils. In this case, there are two possible ways that could be true. It could have successfully loaded (Not today). It could fail to thread down to the take-up reel even after dozens of tries. Sadly, that is what happened.

By the end of the day, I ran somewhere between 70 and 100 load attempts and exactly once the tape wound around the take-up reel. I know it wasn't 8.5 seconds of turning after the tape touched the hub vacuum ports, so my attention will turn to the timers on the control board.

This board uses a single 555 timer chip but has four transistor circuits to switch in different resistor values. These determine the different time delays used in the control logic. I thought I needed to use micrograbbers to route signals out from the control board where I can hook them to my scope. I want to see the various time durations of the 555 and relate that to the intended intervals. Fortunately, there is a test point (TP4) on the edge of the board for exactly this purpose.

Selectable resistance for 555 timer delays
When I figure out whether the duration is odd for all or just the 8.5s wrapping interval, it helps set the strategy for which components to check next. There may be multiple transistors conducting, thus changing the resistance in the circuit to the parallel equivalent, or one not conducting, or components that drifted off value.