Digging into load failure on Telex 8020 drive A

STATUS OF AUTOLOAD ON DRIVE 1

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.

SUSPICION ABOUT REEL SPEED

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.

INVESTIGATING OTHER REEL MOTOR

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.

SUPPLY MOTOR REAR VENT

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.

MOVE OVER TO DRIVE B FOR A WHILE

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.

Repair work on logic circuits for drive B

BROKEN COMPONENT FOUND ON CONTROL LOGIC BOARD AND REPAIRED

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

BUILD CONFIGURATION DIP SOCKET FOR CAPSTAN PRE-AMP

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

Switching back to power supply in cabinet A for drive 1

FINISHED WITH REPAIR OF POWER SUPPLY

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

OVERVIEW OF POWER WINDOW IN DOOR

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

DIAGNOSING THE PROBLEM

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.

REPAIRING THE FAILURE

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.

Ongoing diagnosis of load function for Telex 8020 drive A

APOLOGIES - HAD TO SWITCH TO MODERATED COMMENTS

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.

MALFUNCTION DURING DUMP STAGE

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.

DETERMINING CAUSE OF THE PROBLEM

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.

Continuing diagnosis and repair of tape load on drive A of my Telex 8020 system

VERIFYING VACUUM AT TAKE-UP REEL HUB

I pulled the hose off the differential pressure switch for the take-up reel hub and held my finger over the end. When the tape threads around the reel I should feel the vacuum suck in on my fingertip slightly. This test determines whether the fault is in the switch or in the vacuum piping.

Vacuum hose on rear of take-up reel motor

Vacuum hold-down holes in take-up reel hub

Tape above on supply reel and take-up reel below

When I powered up and pushed the Load button, the tape loaded and wound around the bottom hub. I could feel the vacuum on the hose end that I had removed from the differential switch. This suggests that the defect is in the switch itself, or the cabling, since it should activate with the vacuum.

DIVING INTO THE SWITCH TO DIAGNOSE THE PROBLEM

The switch closes to connect a signal wire to ground. Other switches on the same holder are properly connecting to ground, thus my problem has to be one of these:

• Defective switch, won't activate at the vacuum level
• Blocked switch due to dirt in the passage (variant of the above condition)
• Wire lacks continuity from the switch back to J03 pin 11 where it enters the control logic
• Vacuum level too low, sufficient for PEOT switch but not for Hub Vacuum switch
• Switch activates but vacuum switched off as diverter solenoid changes to run mode

Four vacuum/pressure differential switches - hub vacuum switch on bottom

I put my scope on the switch line and set it to trigger once if the switch activates. After pushing Load and watching the tape try to load, I determined that the vacuum hub switch never closed. It is definite that the load sequence is stalling in Thread waiting for the Hub Vacuum Switch to close as the trigger to move onto Dump state.

In Dump, the diverter solenoid releases to Run condition, putting the vacuum in the tape columns. The feed reel will rotate clockwise to feed in tape while the take-up reel turns counter-clockwise so it too feeds tape into the columns. Once the loops of tape are in the proper spot in the columns, the tape would begin moving slowly forward until the reflective spot comes under the phototransistor. This is the Beginning of Tape (BOT) marker and then load sequence completes.

I am not getting into Dump because of the failing switch. I decided to test the switch with a stronger human powered vacuum. I yanked the vacuum hose from the reel hub end, waited during a load until the tape went around the take-up reel and then sucked in to try to activate the switch.

In spite of my inhaling strongly the switch never registered so the load operation again failed. I noticed that there is a fairly high flow when I suck on the hose which stops if I block the back end of the switch.

Remaining possibilities as I see it:

• Differential switch is broken inside
• Connection from switch to backplane connector J3 is broken
• Something in circuit within the logic cage is holding the signal to +5 in spite of the switch grounding

Next step was to disconnect the plug from the differential switches and pull the board. With that done I could perform a binary search. From the pin for the switch, I should see a path to ground when I inhale on the hose; that means the switch is good and the cabling or logic cage is bad.

Differential switch board with three set for vacuum, one set for pressure

Micro Pneumatic Logic, In. MPL503 part

The switch is clearly bad. I have more resistance inhaling on the hose to other switches, plus they do activate but the suspect switch does not. I removed the board and will take the switch apart to see if it has "user serviceable components inside". If not, I can buy replacements on eBay.

FIXING THE VACUUM SWITCH

With a bad switch, I have four options:

1. First is to grab a good switch from my drive B; that eliminates the chance I can get the second drive operational. 
2. Second is to swap the role of the Cartridge Detect and the Hub Vacuum switches, by moving both wiring and hoses; this eliminates the ability to use autoloader cartridges and forces me to manually remove the tape seal band. Assuming the other switch is good, this will allow me to continue further on the load cycle. 
3. Third is to buy a suitable vacuum operated switch, hook it up to the hose and wire it to the signal line bypassing the bad differential switch; this adds non-original parts to the drive but allows for both autoloading cartridges and a potential working second drive. 
4. Fourth is to find a replacement of the exact same switch component and install it. 

I removed the switch from the board, which was easier than I initially assumed. I began by desoldering the pins that fit onto the PCB, also removing the screws for the plastic holder on the top side of the board. After removal, I realized that the soldered pins where lugs that slide into the switch body, and the plastic holder remains in place.

The side of the switch had a small screw, which I tried adjusting. I discovered that when rotated fully clockwise, the switch was closed. I then backed it out about a third of a turn from when the switch opened and tested with vacuum. It worked perfectly! Somehow this was misadjusted at the Telex factory when assembled onto the PCB.

I don't have to buy any replacement switches or disable any functions of either drive. Good news indeed. I had to reinstall the board onto the tape drive and insert the four hoses correctly. This took a bit of time because of limited access, but eventually it was done.

TESTING LOAD AFTER REPAIR OF THE VACUUM SWITCH

Pushing Load resulted in the tape starting to turn until it hit the PEOT sensor but not threading into the tape path, immediately signaling a Load Check condition. Time to check my hose connections to the differential pressure board because it seems that one of the switches is operating or failing to operate when it should.

Monitoring the switch that I repaired, I watched the voltmeter on the signal line as I pushed Load. As soon as vacuum came up, but before the tape had even began to advance into the tape path, the switch closed. My setting to was too sensitive now. I need to pop the switch out and adjust it for less sensitivity, iterating until I have it switch on only when the tape wraps round the hub of the take-up reel.

Wow, that switch setting is sensitive. There is a narrow zone where it works. Too sensitive and it triggers when the vacuum comes on. A bit less sensitive and it triggers when the PEOT sensor sees the end of tape early in the load cycle. A magic zone that is about 10 or 15 degrees of rotation of the screw where it works properly, but less sensitive than this it will never see the tape on the hub.

Once I had the Hub Vacuum Switch working properly, the load sequence moved to the 'dump' state where it lowers tape into the vacuum columns in preparation for moving forward to the reflective BOT marker. The diverter value switches off, routing vacuum to the tape columns.

The take-up reel winds counterclockwise allowing the tape to be sucked into the lower column. The supply reel winds clockwise feeding in more tape to the upper column. LED sensors in the tape columns will detect the tape level and stop rotating the reels when the tape is in the proper position.

The load goes wrong in a new way. Now, I hear a weird vacuum and scraping sound, the end of the tape on the takeup reel is completely pulled off the hub and we go into a Load Check condition. My debugging moves on to figuring out what is happening inside the covered tape path with the vacuum columns.

My suspicion at the start is that the load process should wind the tape further on the take-up reel before it begins the dump state. If too little is on the hub, it will slip off during loading of the vacuum column. Time to dig through the maintenance manual and begin scoping signals and timing events.

VACUUM AND AIR PRESSURE PARTIAL SPECIFICATIONS

In the maintenance manual, they indicate that the air blower must produce over 50 inches of pressure and the vacuum pump needs to run at around 30 inches. Test procedures at specific points in the drive, with a tape loaded, so 49+ for blower and 26 inches minimum for the vacuum.

I don't know the CFM requirement for the pumps in order to buy substitutes to power the second tape drive. I can probably estimate from air-holes and the pressure to sort out the probably flow rates. The cross section area of all the holes combined is probably less than .20 square inch.

ROUGH, INEXACT AND SKETCHY CALCULATION OF CFM REQUIREMENTS

That is .0014 square foot thus to get one CFM we need air to move around 733 feet in a minute or 12 feet per second or almost 90 miles per hour. If it flowed at anywhere near this rate we would hear some serious whistling and tooting. From this I expect that any pump or blower with just a few CFM will be adequate.

Diagnosing failure in tape load operation in Telex 8020 drive cabinet A

AUTOLOADING CARTRIDGE AND TAPE CARTRIDGE OPENER

IBM introduced a feature on its tape drives, during the 360/370 era, where the operator only had to push the closed tape cartridge onto the drive and hit the Load button. The drive would open the seal belt around the outside of the tape and thread the tape automatically. My drives have that feature as well, but I am unable to test it because I don't have the right kind of tapes.

The normal seal is a band that clamps shut around the perimeter of the tape to keep out dust, but is not designed to work with the automatic openers. A different band, called either the wrap-around or autoloader cartridge, is instead placed on the perimeter of the reel of tape. With this band on the tape, it can be opened by the drive.

All the tapes I own have the standard seal on them. I have dim memories of having to take new tapes out of the box, back in the mainframe days, take off their normal band and put on the autoloader types before putting the new tapes into the tape library. I am seeking such a band but so far can't find any. Without that, I won't be able to test out that part of the loading function of my drives, instead having to remove the seal and stick the tape reel on sans band.

REVIEWING LOAD SEQUENCE

At the point that the load fails, it has powered up the motor for the vacuum pump and air blower, begun rotating the feed reel slowly counter clockwise, and energized the diverter solenoid. It then waits for the PEOT sensor to indicate that the tape is present. After a watchdog timer goes off without any PEOT signal, the logic records a tape load check condition and stops.

There are a number of possible errors that could cause this:

• Diverter valve not operating to switch vacuum
• Insufficient vacuum or air leak
• Inadequate blower volume or air leak
• PEOT sensor opening clogged
• Component error on board responsible for sensing PEOT and advancing load state machine

DEBUGGING THE SIGNALS

My first action was to watch the diverter valve to see if it operated. My VOM showed the operating voltage applied to the coil and I felt air coming out of the loading shoe. Additionally, I could feel vacuum at the PEOT sense port. The shaft in the solenoid moved fully and promptly.

This narrows the problems down a bit. The diverter is working and I have some vacuum and air pressure. Time to move the meter to the PEOT switch and see if that is tripped. I can see this on J3 on the motherboard, pin 15.

I then realized that the vacuum front plate wasn't closed properly. It was snapped shut at the top but there is a second snap at the bottom. Without that closed firmly, the vacuum wasn't able to pull the tape where it had to go. With the door shut properly, the drive would almost complete loading.

Notice that the vacuum front plate is not on my list of possible causes. This is always a possibility, thus debugging requires flexibility and adaptability if you want to succeed.

The tape was threaded through and onto the takeup reel, but then it just stopped. This may be caused by the previously noted problem with the Load Point light dimly lighting. If the drive believes it found a load point, that would stop the seek operation.

I also noticed that it won't unload from this point. Pushing unload does nothing, while it should rewind the tape and then unload it completely.

I watched the BOT and EOT lines (beginning of tape or load point; end of tape) and they were properly low while tape was in the column. They are not detecting a reflective tape spot. These should trigger a clear logic 0 or 1 level on the motherboard, driving the indicator with a logic 0 to light the bulb.

I watched the output that drives the control panel light and it stayed at logic high, meaning the bulb should not illuminate. I don't know why it is partially lit but it is not confusing the load logic.

The load process seems to stop at the point where it should enter the 'dump' state where the two reels move to lower tape into the columns now that the vacuum has switched back to run state from threading state. I found a pin where I can watch to see if the state machine advances to 'dump'.

It does not. Vacuum drops after the tape winds on the take-up reel but before it tries to lower the tape into the vacuum columns. The state machine never advanced to this point. I will now check a vacuum switch that should indicate that the tape has wound onto the take-up reel. If that doesn't occur within the timer limit, the drive would give symptoms like I am seeing.

After hooking the VOM up to the switch at J03 pin 14 it is definitive. The switch is not closing. This is hooked with a hose to the take-up reel hub which has three holes that are hooked to the vacuum line. When the tape winds around the hub, it blocks the holes, producing the vacuum that should trip the switch on.

My action plan tomorrow is to verify that vacuum is applied to the hub, then verify that we see a vacuum when tape covers the hub. If the vacuum is felt then, the problem is in the switch itself.