Tuesday, October 22, 2019

Begin restore of power supply in Telex 8020 drive A


Visually it is different from the other unit, having no space on the front for plug-in boards, using fuses instead of circuit breakers for some power levels, and employing a bat handle toggle switch along with a circuit breaker for main power control.

Front of new power supply
It does have the same sockets as the other enclosure - J4, J5, J6, J7 and J8 - and undoubtedly does the same things just with a different cabinet arrangement. AND - what a difference internally. This is packed with almost no spare room, with all the PCBs designed and built by Datapro/Telex. The first one I did had two power supply units they sourced externally and simply integrated into the enclosure.

View inside from the top

I can see that the regulated power supply is a printed circuit board just behind the front cover. The AC switch control logic is a small PCB on the bottom, but with no card cage. Similarly, the dump function is implemented internally without a plug in card. The triacs are mounted on a mid level block, while power transistors are spread across blocks on the right and on the front PCB.

Low voltage supplies board, behind front panel
The two 300W resistors are mounted vertically along the right edge and the huge transformer is stuffed in behind them. With the compactness of this arrangement, I will take more time to isolate sections and test them, with key parts of the wiring buried beneath all the other parts.

Top cover
Fortunately, I believe I have the schematic pages that fit these units. The major downside of this enclosure is that there is no labeling on the various terminal blocks, unlike with the first unit. I have to study how they seem to be wired and match what I see to schematics until I can definitively label them TB1, etc.


This enclosure is mounted down at the very bottom of the cabinet, in front of the cage where the IBM bus and tag cables are connected. This is unlike the supply in the B cabinet which sits on a shelf about 18-20" above the bottom level.

I unscrewed the socket head bolts and then fought to slide it out the side of the cabinet and then to haul it up to the table top level. I believe each of these weights about 125 pounds and is unwieldy to boot.

Nothing appears damaged or out of place. I had to disassemble layers to figure out what parts are where and how the wiring runs, in order to isolate each section for independent testing. It is much more tightly integrated - no separate PS1 and PS2 components, instead one board that produces +12, -12, +5, +6.4, 12VAC and +8 unregulated.


I fired this up and found that all low voltages are working properly. This part is good to go. Proper voltages delivered for +12, -12, +5, +6.4, +8 unregulated (about 11.5) and 12VAC.


This is controlled by grounding pin 13 of J4, just like the other power supply, but there is also a bat handle switch here that blocks the power to the high voltage supply transformer. When I switch on the toggle switch, but don't ground pin 13, no 45V power is available.

When I use the jumper to pin 13, but leave the toggle switch, there is still no 45V. This is the expected behavior. When I flip on the switch the main circuit breaker instantly pops! Something is shorting the high voltage supply, probably a bad filter capacitor or other failed component. This is going to take some work to debug.

The circuit is pretty straightforward. The power flows through the transformer, through a full wave rectifier bridge and then into two huge filter capacitors. Large fuses sit between the filter capacitors and the rest of the circuitry - mainly the shunt regulation since nothing is plugged into the power supply.

My next test was removal of the fuses to isolate the transformer, rectifier and capacitors. I also removed the capacitors from the circuit by taking the wire off one lug. Triggering power with pin 13 and then flipping the switch, I will either see 45V unfiltered or the breaker will still pop.

With the fuses out, it still popped. I then removed the capacitors and it still popped the CB. That leaves just four rectifiers and the transformer. Removing the transformer leads from the diodes, the circuit breaker was happy, no tripping.

I removed all the wiring from the rectifier terminal block (also has the two transistors that shunt current to the 300W resistors), then quickly ascertained that I had a dead short across at least one of the 1N1186A diodes. This is a 40A, 200V stud mount rectifier. I began unwiring and unmounting them for the final check.

A full wave rectifier is built from four diodes, usually drawn in a diamond shape. At two opposite apexes, the AC input from the transformer is connected. At the other two opposite apexes, the + and - rectified current appears. The two bad diodes were both connected to one of the AC inputs, but failed differently. One of them failed open circuit, the other in a short.

I ordered the replacement diodes and expect them to arrive later this week, at which time I will install them and continue with the power supply checkout. There may have been a sizeable short, well more than 40A, to cause one of them to short out. Interestingly, the lines from the rectifier and capacitor are fused with 30A cartridges, leading me to suspect the filter capacitor as the short that took out the diodes.

Using my ohm-meter and then an ESR meter, I saw reasonable readings from both of the large filter capacitors. The ohmmeter charged up the capacitor slowly and then I could read the charge in millivolts, when the capacitor is good. My ESR meter measured less than .01 which is fine for the usage here where the power dissipated across that resistance would be about 3 or 4 watts at full draw of the supply and the voltage drop about a quarter of a volt.

The big decision I had was whether to wait for the original equipment rectifier diodes to arrive at the end of the week or whether to convert this to use of a modern rectifier bridge. I have an HBPC5010 component on hand that supports higher current and much higher peak inverse voltage, but in a small package. However, I chose authenticity since it will only be a few days and there is plenty of other work to do on this drive.


Given the breaker trip I get when I jumper the first triac, I know this part of the circuit is working. It is delivering 240V to the toggle switch and through that to the large transformer for the high voltage supply.
Triacs and related components
The vacuum/blower motor jack J6 appears to have 240 across it, just like on the other supply, regardless of whether the control line is grounded or not. At this point I have to wonder if the issue I had is due to a misunderstanding. Either the control line needs to be pulled up to 5V to turn off the outlet, or perhaps something else is going on that I am missing.

AC switch control board, below triacs
Nothing in the schematic differentiates the vacuum/blower circuit from the hi voltage supply circuit, they use the same design and thus should be switched on by grounding the appropriate pin. I decided to plug in the blower motor and see what happens. I also should look carefully through the power supply board to be sure it isn't pulling the line down on its own.

I find it highly suspicious that both cabinets have the same apparent failure, leaving the vacuum/blower motor powered continuously. I reviewed the circuitry that switches the HV and motor power, assuring myself that they are absolutely symmetrical in design and should operate the same.

The AC Control Switch board generates +5V to power the operator control panel but it uses it internally through opto-isolated triacs. The LED in those triacs on the board is driven by the +5V supply and routed out as control signals. When the control signal is pulled down to ground, the LED lights and triggers the triac.

The board also generates 12VAC separately for each of the two triacs. That 12V is switched by the opto-isolated triac to provide an AC drive signal off the board and out to the main triacs that actually switch the AC power. The main triac gate is fed the 12V AC only if the control signal was pulled to ground, the LED lit and fired the on-board triac, and then that triac feeds 12V.

I finally decided that my meter might be sensing 240V due to the capacitor network across the triac but if it isn't on, there wouldn't be any power to drive the motor. I hooked up the blower to my repaired power supply in cabinet B, provided 240 to the power supply and turned on the circuit breaker. No motor running. I think grounded J4 pin 12, the control pin, and the motor turned right on. In fact, this was working correctly on the cabinet B supply and probably works well on the cabinet A one too.

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