Testing rig for voltage regulator card being set up, believe I found and fixed the problem

CONCEPT FOR TESTING THE 6V AMPLIFIER CARD OUTSIDE THE POWER SUPPLY

Based on the schematics of the supply and the regulator card, I believe I can connect the input power supply across the K,L,M pads for negative and the positive end hooked to pad R which is the +Sense connection. Then a variable supply is set up across the +sense and -sense pads (R and Q) so that I can see how the regulator responds as the voltage is below, on and above target. 

The output of the card at B,C would be drive current that flows into the base electrodes of the bank of power transistors, returning out of the emitter through a .10 ohm resistor to the K,L,M pad side of the input power supply. I will wire up one spare 108 transistor and resistor for this purpose. For the load side I will connect a 15 ohm resistor which should draw about 600ma at the target output voltage.

I played around with circuit simulations until I felt comfortable with how this will work before I applied power and possibly damaged the card. I am ready to head over to the shop, breadboard this out and check the card.

WIRING PRACTICALITIES

I either need to find a spare SMS socket or do something more involved to connect to the various pads B, C, K, L, M, R and Q. Wiring in resistors and connections to the bench supply is easy. I did indeed have a spare SMS socket, so I was ready to go. I decided to check the Overvoltage Protection card first, just to see at what voltage it would trigger.

OV CARD HAS WEIRD HYSTERESIS BUT TRIGGERS ABOVE 6.5V

Setting my bench supply to current limit at 500ma, I began applying increasing voltages to the crowbar card circuit until it fired, latched up and hit the current limit of the supply. That seemed to be somewhere in the range of 6.5 to 6.6V, well enough above the top of the valid range for the +6V supply that it shouldn't spuriously trigger.

When it fires, it locks down regardless of reductions in the input supply. However, when I switch off the supply entirely it should immediately reset, since it is a SCR thyristor which will unlatch when current goes to zero.

However, it didn't unlatch right away. I found that for many tens of seconds after it triggered and I disconnected the power, when I reapplied even a very low voltage it would still be conducting. There are no capacitors on the card, so this is some weird internal capacitance or other phenomenon of either of the two SCRs. Not an issue, just an odd behavior.

OBSERVATION SUGGESTED MAIN DRIVER TRANSISTOR WAS BAD

Driving the regulator card with my test setup, I would see that its ability to hold the output voltage would stop at about 13.5 volts on the input, where it began to oscillate up and down with excursions well into the overvoltage trip range. 

As I checked the output, it looked to me like the driver transistor was bad - which makes sense because if it can't drive the bases of the main power transistors, they won't be able to regulate their current and establish the voltage level desired.

The other two 108 transistors which failed all exhibit a characteristic behavior on the VOM diode test setting. While a good 108 will have a voltage drop in only one direction between emitter and base, the bad transistors show voltages in both directions. I look for the open circuit to determine that a 108 transistor is still good.

REPLACED 108 TRANSISTOR, RESULTS MUCH BETTER

After I unsoldered the bad one and installed a replacement, the behavior in the test rig was much better. I moved the card over to the actual power supply, hooked up my bench supply and electronic load, then tested again. 

At 4A load, the supply held rock steady even as I advanced the input voltage from 9V all the way up to the 16.5V that comes out of the unloaded DC Power Supply in the 1130. No oscillations, no excursions, just clean behavior with nary a hiccup as I varied the load.

I am glad it was the 108, for which I have spare units, rather than the two 026, the 086 or the 123 transistors that are on the regulator card (or the two 033 transistors on the overvoltage card). It could also have been failed zener diodes, one in the regulator and one in the overvoltage, for which I don't have a written spec for the voltage level making replacement challenging. 

READYING FOR A FULL LOAD TEST IN THE MACHINE

While this looks like I have turned the corner and fixed the power supply, I can't be sure until I see how it behaves with a full load on it. I have the resistor complex that will let me set up loads of 6, 12, 18 and then 24A but the supply is going to have to be fed by something that can deliver more than 24A of input voltage. 

As I mentioned earlier, the way I intend to perform this test is to put the power supply into the 1130, but leave the load side disconnected - the logic circuits won't touch the supply until I feel it is truly solid. Instead I will wire in my resistor load complex and test it that way. If it survives the full 24A of load with steady output voltage and no overvoltage incidents, I will wire it back to the logic circuits and move forward. 

DC Power Supply for raw DC voltages