Sunday, October 20, 2019

Power supply testing results for Telex 8020 tape drive system - almost ready


The schematics show the wiring in the power supply across four terminal blocks - TB1, TB2, TB3 and TB4. Some of the wiring does match, for example TB2 which is behind the PCB cage next to the triacs is hooked up the same way, but others seemed wide of the mark.

For example, I checked some points on the terminal blocks that should be connected to the neutral and to the hot lines, but they aren't hooked there. The wiring on TB4 doesn't seem to be connected to the 45V power supply transformer at all, but it should be. That is where the connections are swapped to set the input voltage from 110 up to 230 volts.

I resorted to a time consuming process of disconnecting various wires and beeping out continuity in order to build up a map of where each of the terminal block lugs actually connected. This took abouit an hour to complete. It was sped up by the realization that most of the wiring did indeed match the schematics.

The big error was on a schematic showing now the large transformer was wired up - the supply for +45, -45V - which was erroneously drawn as hooked to TB1. Once I realized that this was false and that TB1 was wired for an entirely different purpose, the output of the hi voltage supply (ground, +45 and -45), I could move on to sort out the actual transformer wiring.

No terminal block is involved in wiring the input of the high voltage transformer. Instead, six wires are plugged onto lugs on the transformer body, those wires running through harnesses to other locations. These were hooked to the cooling fan, the neutral power bus, the output of the triac which switches on power to this supply, and formed a hidden jumper function.

Two wires are to the cooling fan - terminals 2 and 5 - which is a 115V unit. Those terminals are part of an autotransformer that drops the input voltage (220 in my case) down to 115. Terminals 4 and 8 had separate wires but they are shorted together somewhere remotely, thus they form the jumper that is needed to configure these windings for 208, 220 or 230V.

Terminal 2 is connected to the neutral leg - actually just one of the legs of the 3 phase when this is connected to the 3 phase distribution panel, but as I am wiring it to household single phase 220, it is just one hot that I chose to call neutral because it is not switched.

The switched power from the triac was wired to terminal 10, which is the proper winding for 208V operation. I moved it over to terminal 9, thus converting the transformer to 220V input voltage.


I performed some quick tests of the electrolytic capacitors that I could isolate from the circuits, both for capacitance and for ESR values. I was trying to rule out obvious problems before I energized the circuit. The two capacitors appeared to be in excellent condition.

Next I considered hooking up a variac which will allow me to vary the voltage from extremely low levels up to the 220V target input. By beginning softly, I would limit the current inrush to capacitors, giving the surface time to 'heal' if there are any tiny shorts due to long periods of disuse. An alternative method would be to put limiting resistors in line with the capacitors and gradually lower the resistance, but a variac would be the quickest way to deal with the bringup.

However, the variac I own is a 110V unit, not 220V. In order to use this variac I would have to temporarily rewire each supply from 220 to 115V configuration then use the variac. Once satisfied that the capacitors are not going to die, I would rewire and run them at full voltage. Not worth the effort given how clean everything looks, so I will just watch carefully as I test each unit.

Each of the separate supplies - 45V, PS1, and PS2 - were powered up and the output voltages checked under partial load. I used some power resistors to load down the different supplies while I checked the resulting voltage levels and ripple with a scope.

TESTING PS2 - +6.4 AND unregulated 8V SUPPLY

I set up a plug for the 220V (actually 240 as I measure it) of my home supply, which normally has attached the fast charger for the Sonata Plug In Hybrid. I had a spare plug which I wired to the connector from the tape drive. That connector was inserted into the rear of the power supply, my plug was inserted in the extension cord to the home 240 supply.

I had isolated the main circuit breaker allowing only the PS2 supply to be energized. When I switched it on, I measured the unloaded output levels as 6.55 on J8 and 11.2V on J7, which is close enough particularly for the unregulated output.

Last step was to install resistors to put the supply under load, within its specs of course, and measure the output voltages again. With a small load (100 ohm resistor) I only had 65ma draw and under a half watt of consumption. I was still over voltage by about 2.3% but I don't think that is critical at this point so I moved on to test the next supply.


I removed PS2 from the circuit breaker and reattached the rest of the supply circuitry. Both plug in boards were moved from the card cage. The Dump card will short the 45V supply if the 12V sensor supply levels drop, not something I want to have happen. The AC Control card will switch on the triacs that energize the 45V supply and the vacuum/blower motor, which I am not yet ready to do.

My first test was to leave PS1 disconnected and turn on the circuit breaker, verifying that the fan doesn't run and all is otherwise good. Once that was verified, I could insert PS1 in the circuit and get ready to test its levels.

This supply delivers four different levels but all is accessible from the front panel plug J4. I was quite satisfied with this supply. The +12, -12 and +5 circuits were only a trace over the exact value, something I attribute to the operation of the sense lines. The 12VAC was solid as well.

Supplies with sense circuitry allow attachment of a second thin wire to the load, paralleling the heavy distribution wire that will carry all the load current. There is some voltage drop on the distribution line, varying as the square of the current draw, which makes the voltages different between the output and the sense pin at the supply.

The circuitry then boosts the voltage of the output slightly, compensating for the line loss, attempting to get the sense wire at the target voltage by delivering enough more on the output pin to cover the wire loss. Since I didn't have the sense wire added, I believe that the minor voltage boost was being applied, so that my 5V was reading at 5.03V.


I bypassed the triac to directly power the high voltage supply. The fan came right on and I measured +52 and -52V at the outputs. Since this isn't a regulated supply, those didn't seem bad. The "Dump" PCB drives a shunt of the excess voltage across huge 300W resistors, a very wasteful power supply design. Since I don't have the dump card in place, the voltages will be high.

Two 300W resistors running left to right near the top of the pitcure
I rewired the transformer to its highest input setting (230V) in an attempt to minimize the raw voltage of the power supply, since this will lower the wasted power consumption across the resistors. The result is just a hair over 45V, just what I want to see. Ah, the days before switching power supplies, when power waste was the norm.


Once I had the power supplies operating, it was time to verify that the AC supply control circuit and triacs were working. This plug in card has two control inputs. Grounding the input will fire the associated triac and provide 220V AC to the circuit it is switching. One switches the vacuum/blower AC motor on, the other feeds 220V to the 45V power supply..

What I found was that the high voltage supply was switched off, but the vacuum/blower motor had full power applied. I need to investigate this, as the blower should also be off. The card is supposed to deliver a  low capacity +5v supply, which is also used internally with the opto-isolated drivers for the triacs. I don't measure any supply, which is a problem.

With no supply, neither triac should be firing but I definitely have supply going to the vacuum/blower circuit. The internal 5V supply runs through a photo-diode and a resistor to the control signal line. These are not wired to anything but need to sink current to ground in order to fire the photodiode and turn on the triac.

Turns out I forgot to plug in two wires that on the side of the card, inside the card cage, to deliver power. With that installed, the +5V was working great and when I grounded the control line for the triac switching on the 45 volt supply, it fired right up.

The problem remains that my other triac is always firing, thus the vacuum/blower motor will be running continuously but should only run when the control line is grounded. Even when I remove the AC control plug in board, the triac is firing. It appears there something wrong on the back of the card cage.

Based on my diagnosis, but without actually testing the failing part, the only part that could be bad is the triac itself. I bought a new one and should have it on hand in about a week, after which I will finish the repair of the power supply. The leads have to be desoldered and the new one soldered in place.


Finally it was time to completely reassemble the power supply and all its parts, then feed power to the main input. Each and every power level was again verified to be present and correct. Lastly,  I cycled the 45V which depended on external control signals, by grounding the related pins on the connectors of the power supply. I had the 'dump' card (actually the voltage regulator card) installed and checked that the +45 and -45 seemed goo.

Suddenly while completing the final tests of the voltages, I found that both +12 and -12 had dropped out. All others are working fine, but this adds a second fault to be addressed once I have the triac in hand.

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