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.

Start testing logic and power amplifiers in drive A

INITIAL SHORT TEST

I did a quick check of the power supply connectors to see if any of the loads are shorted, using the VOM to measure resistance. All the results were reasonable except for the 12VAC line which appeared to be an open circuit. There may be a relay that connects this line only at certain times; investigation is needed.

BORROWING POWER SUPPLY AND VACUUM/BLOWER FROM CABINET B

Since I am still waiting for the fuse holder for the cabinet A power supply, I swapped the power cables over from the A drive to the B cabinet power supply. Too, I moved the vacuum and pressure hoses over to the A drive, giving me a complete unit to test.

The hose lines and power cables are all a bit short, so that I could barely get them connected across cabinets. My first attempt looped the hoses over the mid level frame bar, which wouldn't permit the two drives to be pushed together.

The power cables required that the cabinets be abutting, thus I had to redo the hose connections. I completed all the connections and slid the drive cabinets together. Everything fit together with no slack but not excessively taut.

FIRST POWER UP

Power up was done with no circuit breakers tripping or fuses blowing. The operator control panel had the Power, File Protect and Load Point lights illuminated. Load Point was weakly lit, unlike the others. When I pushed the Load/Rewind button, vacuum power came on, the automatic cartridge opener activated and the drive was trying to thread tape into the vacuum columns. After a short interval, Load Check illuminated which is expected.

When I push the Unload button, the power window on the front door lowers. Hitting Reset or Load/Rewind will raise it again. This is as expected.

Power window in front door lowered

CLEANING TRANSPORT PATH IN PREPARATION FOR A TAPE TEST

I opened up the vacuum chamber on the drive and cleaned it thoroughly using Isopropyl Alcohol and KimWipes. A lot of dust had coated the walls, faces and other parts including the tape head. It had to be removed before I would attempt to load tape.

Vacuum columns, head and tape transport path

LOADING THE FIRST TAPE

I couldn't figure out how to get the tape cartridge inserted with the tape seal in place, so I removed the seal and put the tape on the drive. Hitting Load/Rewind starts the vacuum and begins slowly rotating the tape reel counter-clockwise. This continued for some time until the time-out when the control logic declared a Load Check condition.

The control logic for the drive sequences through state machine steps to load the tape. The first step is to power on the vacuum and blowers as well as the +45/-45V supply. This also activates the automatic cartridge opener which should open the wrap-around seal and lift it to give access to the tape.

The second step rotates the tape counterclockwise until the end of the tape closes a vacuum switch that indicates the end of tape was located. This should advance the machine to the next steps where it will turn the tape reel clockwise and feed the tape down through the vacuum columns, tape heads and eventually onto the take-up reel.

However, the drive never advances. This will require some diagnosis to determine if the flaw is in the vacuum sensor or the control logic board. Very fortunately, I found a binder in the garage with the maintenance manual and other helpful documents. Just in time, too!

RECAP OF PROBLEMS DISCOVERED

1. Load Point weakly lit at power up
2. Fails to load tape

More work on the operator control panel of drive B

REPLACING BROKEN DENSITY BUTTON

I installed a mini pushbutton switch on stiff leads into the PCB, after removing the broken switch. Simple and easy fix. This hides behind the operator control panel in normal operation thus I didn't need to find the same or cosmetically similar switch.

Replaced Density pushbutton switch on the operator control panel PCB

VERIFYING THE PUSHBUTTON LOGIC WORKS

First step was to beep out the connections between J1 that connects the drive motherboard and J2 which connects to the pushbuttons and lights on the panel. It was important to verify that I knew what was connected directly and what was driven by the flip flop that recorded the Online status of the drive.

I then did some beeping out of wiring on the backplane to identify places to read switch state and to command light activation. I wanted pins well enough separated for easy access with the probes and other wires.

The sequence of testing planned is:

1. verify that the 'start state' line on J4 pin 3 was high when I powered up
2. verify that the Reset button connected ground and J4 pin 4 when pushed
3. verify that the Unload button connected ground and J4 pin 5 when pushed
4. verify that the Load/Rewind button connected ground and J4 pin 7 when pushed
5. verify that pushing the Start button changed J4 pin 3 to low
6. verify that pushing Unload doesn't connect ground to J4 pin 5
7. verify that pushing Load/Rewind doesn't connect ground to J4 pin 7
8. verify that SW1 toggled to offline and back to online causes J4 pin 3 to go high again
9. verify that the Power lamp is always illuminated
10. verify that grounding J4 pin 17 turns on the Select light
11. verify that grounding J4 pin 11 turns on the Load Point light
12. verify that grounding J4 pin 9 turns on the Load Check light
13. verify that grounding J4 pin 15 turns on the Ready light
14. verify that grounding J4 pin 10 turns on the End of Tape light
15. verify that grounding J4 pin 12 turns on the File Protect light
16. verify that pushing the Density switch on the PCB will connect ground and J4 pin 1

Status of first set of tests:

1. Failed - no signal seen on pin 3
2. Success
3. Failed - no connectivity
4. Failed - no connectivity
5. Failed - no signal on pin 3
6. Failed - no connectivity
7. Failed - no connectivity
8. Failed - no connectivity
9. Success
10. Success
11. Success
12. Success
13. Failed - no light
14. Success
15. Success
16. Success

Moving a VOM directly to the flip flop I did see that the Start push-button turned it HIGH and SW1 toggled to offline toggled it LOW, so indirectly I confirmed tests 5 and 8. However, something is still wrong as the signal isn't reaching J4 on the motherboard via the cable from J1 on the operator control panel PCB.

Starting testing of logic and amplifiers in drive B

SCHEMATICS OUT OF SYNC WITH THIS DRIVE

The book of schematics I own are a match for cabinet A, both the drive and the integrated control unit, but I found noticeable differences in the cabinet B boards. If I have to debug or repair any of the boards that differ from the diagrams, it is going to force me to compare and partially reverse engineer.

One example was the operator control board, which had some additional ICs and a hand executed engineering change that involved cutting board traces and a jumper wire. That one was a simple board and not many changes but still took 30 minutes of tracing before I was sure I had found all the changes.

SHORT TESTING OF LOGIC CAGE BEFORE APPLYING POWER

After inserting all cards in the logic cage, I used the VOM to test for any short circuits before attempting to energize the power supply. With that successful I could move forward to fire up the system.

VERIFICATION OF DRIVE SPEED CONFIGURATION

Boards 7, 9, 10, and 11 have configuration differences depending on whether the drive is to operate at 75 inch per second or the full speed at 125 ips. I verified that boards 9, 10 and 11 were properly set. However, board 7 has a problem.

On board 7 the specific part values for the configuration are all encapsulated in a DIP 16 package that plugs into the U27 spot on the PCB. This provides six resistors and two capacitors to the rest of the circuitry on the board, which is the pre-amplifier that controls the capstan motor. The package is missing, thus my board can't work as it sits.

I have the proper values and wiring required, thus I can build the parts onto a DIP socket and install this onto the board to complete its configuration. I won't be able to have tape move forward or back, nor read nor write, until this is completed.

INITIAL POWER ON AND OBSERVATIONS

With the boards installed and everything wired up , I applied main power to the drive power supply. Many of the PCBs in the logic cage have small red LEDs which illuminated and the fans for the +45 and -45V supply came on. No fuses blew or breakers tripped.

The operator control panel had the Power and File Protect lights illuminated. No button presses changed anything. I believe that I will need to place a tape on the drive before the logic will attempt to load - firing up the vacuum/blower motor and rotating the reels trying to thread the tape end into the vacuum column and through to the take-up reel.

CLEANING TAPE PATH

I chose to clean the tape path at this point, using lab low-lint wipes and 99% isopropyl alcohol. There was a lot of dust picked up but finally I am ready to try to fit on a tape reel and test out the loading sequence.

Converting the Telex drives to different speeds

DOWNGRADE MODELS AND PRICING STRATEGIES FOR MAINFRAME PERIPHERALS

Mainframe makers had long pursued a strategy to build a peripheral device with multiple speeds of operation and optional features, allowing them to win business with more price sensitive customers while protecting the prices and margins for those who valued the highest performance and added features.

The device was designed for its top performance and features, then methods are developed to slow it down. This can be by changing the size of a pulley on a mechanism, or adding delay logic. An advantage of having one design with slowed models is that a field upgrade can be offered, allowing a customer to pay the market price difference and receive the benefits of the faster model.

A sizeable price difference may be implemented by a pulley or jumper, with the cost of parts changed way out of proportion to the price changed. It is most visible in processors with slowed speeds, where the CE key, used to change metering of hours used between the customer and the service meter, also increased the machine to its full performance.

With the IBM 3420 tape drives, IBM offered models with three different speeds - 75, 125 and 200 inches per second. The Telex 8020, a competitive drive that was program and plug compatible with the 3420, offered models at 75 and 125 ips.

In looking through the schematics for the tape drive, I found that the same PCB design would support both speeds, differing only in the value of some components on the board. If my drive was a lower performance model, I could easily modify the machine to give myself a field upgrade.

Of course, if my drive was already set up to run at 125 ips, there is no need for changes. I pulled a board (the capstan drive pre-amplifier) from slot 7 and looked for the parts that vary based on speed. I discovered that Telex had been a bit clever, making it harder for a customer to upgrade the drive themselves.

Board 7 schematic where parts values determine speed of the drive

The differences in speed for this board are set by the values of six resistors and two capacitors, as shown in this chart on the bottom of the schematic page. I looked over the board to find parts with numbers like R302 and C302 but quickly realized that they were all installed inside a pseudo integrated circuit U27 that had a DIP 16 mounting but was a big encapsulated block with an obscure part number.

Table of parts values to set capstan tape speed

Pseudo-IC containing the discrete parts

Using the pin numbers on the schematic, I checked the values of the embedded components and found that my drive A was the full 125 ips version. If it had not been, I would need to remove the DIP 16 block and wire up discrete components of the proper value on a DIP socket to plug into the board.

I pulled board 07 from cabinet B to check which speed it implemented. To my surprise, the pseudo-IC has been removed from the board. I can see that the board has a flush level socket into which the IC 'plugs' instead of being soldered. The good news is that I know the values and can build a DIP 16 socket with the parts.

Flush socket with removed pseudo-IC on drive B board 07

This cabinet had been destroyed by a forklift through the window of the front door and subsequent fall that was hard enough to bend the frame and break some welds. Typically in those cases the insurance carrier pays the owner or vendor and the machine is supposed to be scrapped.

The missing chip may be because the normal installation procedure at the customer site would have had the CE plug in the appropriate chip to set the speeds, but it is shipped sans IC. However, this board appears beat up and used, indicating that the chip was removed deliberately. Telex may instruct their CEs to remove these modules because they can be used to upgrade other tape drives in the field - before release the unit for scrapping.

Nothing else appears to be removed, although I need to look more carefully at all my boards just in case this was stripped for some parts before it was turned over to the scrapper. I definitely need to restore the drive in cabinet A first, as it is the most likely to have had a full complement of working parts.

Start repair of operator control panel & finish repair of dump card from cabinet B of Telex 8020 tape system

CONDITION OF CONTROL PANEL

The panel has a number of wires dangling loose and I wasn't sure that everything was even still there. This drive took such major trauma I assume that when it was hit with the forklift tine to cause the front damage, it knocked the unit over backwards strongly enough to twist the frame and jar things loose.

Damaged operator control panel

RESTORATION OF WIRING AND REPAIR OF PARTS

My first step was to take the schematics and write down the wiring points for the pushbuttons that are disconnected - which plug pin they are hooked to and whether the wire is on the normally open, normally closed or pole side of the button. I also have disconnected wires that I think go to the indicator lamps.

I did continuity testing and identified all the wires and switch positions to which they belong. It was a quick task to solder on all the wires and verify their operation. The two lamp connections are more problematic. These lamp holders have a top tab that represents one side of the bulb and two side tabs for the other contact of the lamp. On the first two lamp holders, the top tabs have broken off clean at the level of the case.

I can see a touch of metal where the tab broke off on each housing; I should be able to insert a wire and solder it to the remaining metal or at least make a good friction fitting. I made a first stab but didn't get a good solder connection onto the tab that was broken off flush. With a bit more work, however, contact was restored.

I was not getting reasonable results either looking for the switches to give me continuity when pressed nor for the lamps to light (except for the Power On which is hardwired to light). I decided to plug in the main power supply to the unpopulated card cage and test with the plug on the rear that is connected by ribbon cable to the operator panel.

After some study of the card it became obvious that my schematics do not match the card. There are two additional ICs on the board, an open collector hex inverter and a dual JK flip flop. Only two of the six inverters are wired and only one of the flip-flops. Clock and D input were wired to ground, so this operated solely but using the notSet and notClear inputs. For some reason, the notQ output is sent through one of the inverters however the output of that inverter is not connected to anything.

I also found signs that this board was reworked to change its function. Two traces were deliberately cut and a jumper wire was soldered elsewhere on the back. It is clear that I would have to reverse engineer the card to figure out the schematic before I can do more debugging.

After I beeped and traced out the circuits, I realized that the one of the cut traces was done to wire the flipflop and inverter into the circuit, breaking the straight connection of Start pushbutton to output. I would have expected to have the flipflop on the main control logic board down in the card cage, but I suspect that the rework may give more flexibility in resetting the drive.

The other cut was for pin 2 - the gate to turn on the +45 and -45 power supply - which was originally directly connected to ground on this PCB. The result would have been immediate powerup of the power supply, but I believe that this rework ensured that the drive could intelligently power up and down rather than always driving the 45V supply.

I checked for continuity of the +5V supply to the VCC pins of the TTL chips, but the circuit wasn't connected! I found that the choke coil was poorly soldered to the board. That was quickly repaired. I then tested the various light driver signals, grounding each gate signal to verify the lamp illuminated.

Testing the switches will be a bit more complicated since some of the signals are now driven by the flipflop that is set by the Start button and reset by the RESET button or when the switch S1 on the PCB is turned to the offline position. That means I can't directly check the Load/Rewind or the Unload buttons, these are only active when the Start flipflop is energized. I can and did verify the basic behavior where the button wiring enters the PCB but can't check the outputs of the PCB right now.

The pushbutton on the back for Density (S2) is partly broken, but I can can wire in a new pushbutton switch to replace it. I am assuming that the drive will normally select density automatically upon reading and picks the highest density by default to write; the button must be to override this behavior.

MOUNTING THE PANEL

The reason for this damage is the failure and loss of the top bar across the frame, caused by the traumatic bending damage it sustained. That bar mounted hinges that were attached to the bottom of the operator panel. It tilts up for service and rotates down to position the buttons and lights facing forward.

I need to create a replacement mounting for the panel of some sort. The more significant decision facing me is whether to repair the frame first. If the drive had the vacuum pump/blower assembly, I could conceptually restore it to full operation, however that doesn't exist. If I gamble that I can find pumps and blowers that match the flow requirements of the drive, restoration is possible.

To repair the frame would entail removing all the covers and all the interior parts. It is not all that arduous a task. I probably would need some kind of clamps and a hydraulic piston to attempt to straighten this. Alternatively I could take the stripped down frame to a automobile body shop where they do this kind of repair all the time. They could also weld on a replacement top bar.

The downside is expense. Frankly I don't have a use for these drives so the restoration is simply a hobby effort, after which I will find a home for the working drives. I have about $150 sunk into them so far including the replacement parts I bought. I am not sure I want to step up to the expense of the substitute vacuum/blower plus the frame repair work.

DUMP CARD DIAGNOSIS AND REPAIR

The plug-in board for the cabinet B power supply, the "Dump circuit", drives the shunting of +45 and -45 power supplies through big 300W resistors. Based on a suggestion by a blog reader and after some study of the schematics, it appears that the dump circuit is used to sink energy from the reel motors when they are slowing down from a high speed rewind, a form of dynamic braking.

With my dump card installed, the +12V power supply output went to zero. It returns when the card is pulled out. The card itself is not very complicated. It uses two op-amps to compare the 45V levels against the 12V levels. Since the +12 and -12 supplies are regulated using zener diodes, they are used as a reference standard.

Appropriate resistor dividers take the +12V and the +45V  down to a common voltage when both are on spec - thus the inputs to the op amp are balanced and it does not drive any output voltage. If the +45 level increases above nominal, the difference is amplified by the op-amp, with its output further amplified by a transistor on the card. There is a second circuit comparing -45 and -12, a mirror image of the first circuit, thus the card has a +dump and -dump output plus inputs of +12, -12, +45 and -45.

Dump card with the shorted capacitor clipped out ot the circuit

I found that the +12V input contact was shorted to ground, which certainly matches the symptoms observed. Looking at the schematic, the most obvious source of the problem would be a short in the electrolytic capacitor sitting between +12 and ground. I clipped off one lead and indeed, that capacitor is a dead short. The contact itself is no longer shorted with that capacitor removed from the circuit.

Our culprit

I found a suitable replacement for the 15uf, 20V part, soldered it onto the board, and tested my cabinet B supply with the dump card installed. +12V is working exactly as intended. Whether dump is working properly really can't be tested until I get the whole drive working and can put it into high speed rewind then monitor the 45V rails.