Saturday, October 26, 2019

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


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

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.


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.


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.

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