EDC 522 repair question

[Testtech]

Working on an EDC 522 Voltage Calibrator.
 Upon receipt it had a blown -15V regulator; replaced that. No other signs of Magic Smoke. All PS voltages are correct.
Powers up in OVERLOAD, and sure enough it is.
The Output is pinned at the -150V rail regardless of the front panel settings, the 0 output relay is closed due to the OL condition, causing the 50ohm 2W current limit resistor between the power amplifier and the output attenuator circuitry to get real hot RIGHT NOW!
So far I have tested all the semis in the output stage using a curve tracer, and all are fine. I replaced the op amp. I have measured all the resistors for value, and the caps for leakage, again all apparently OK.
I have run continuity of the feedback loop, this too seems normal.
The DAC is indeed telling the output stage the correct level, but the output stage has a mind of it's own.
Still the same problem, what am I missing???

Any and all comments welcome!

[dacman]

Did you test the parts in-circuit or out?

Referenced to SG, what is the voltage value at INP and FB?

What is the value at pin 6 of Z303?  (This one is normally not stable, but should be trying to correct.)

What is the voltage drop across the higher power resistors, for example R317?

I would first suspect an MJE350 or a supporting part, such as R317.

I may try disabling the 150 Vdc rails while troubleshooting.

[Testtech]

Thanks for your response!

To expand on this;
I am also testing with the 150V supply disconnected, and using a +/- 30 V lab supply, with that connected the measurements are:

522 set on the 10V range

522 output -28V
INP=-1.3
FB= -28
V across R317= 0.6
Z303-6 = +14
Z303-2=-1.9
Z303-3=-1.3

The transistors were tested out of circuit using a Tek 576. The diodes were tested in circuit, as were the resistors. In addition, I substituted devices for Q308 and Q306 without success.

[dacman]

The voltage at FB should be half the output voltage on the 10V range.  Find out why R334 & R335 are not dividing the output in half.  Seems like there is an open or short somewhere.  There should be 16681 Ohm between SG and FB (R334).  Check for solder splatter.

[TiN]

Also check knobs on the FP. I had issue on MV106 when one of wires got open and output was stuck with overload as well.

[Testtech]

I think I misstated the FB voltage, I measured in the wrong place..
The voltage at the junction of R334, R335, and R318 is -3.1.
Can it be half the output voltage, when INP is -1.2, plus the diode clamp drop of D301/D302?

[dacman]

EDC placed FB, INP, and SG test points in the middle back (on at least some units).

The voltage across R318 should be close to zero.  It's an isolator between FB and Z303.  There should be no significant current flow from FB into R318.

D301 & D302 will keep Z303 pins 2 & 3 within 0.7 Vdc of each other.  Z303 is trying to compensate.  I would guess that there is 1.3 Vdc across R302 (in the DAC output, if the dials are set to 0.00000).

The voltage across R302 should be close to zero.  It's the isolator between the DAC and Z303, and should have insignificant voltage drop.

If the MSD is set to 10, then both inputs to Z303 (pins 2 & 3) should be at 5V, pin 3 input comes from the DAC, and pin 2 comes from FB.

There are two +/- 15 Vdc power supplies in the unit, one for the DAC and the other for Z303.  There should not be any current flow from FB to R318 to D301/D302 and to the DAC.

[dacman]

Could you give us the voltage values around the transistors in the output stage, such as Q301 and Q308?

[Testtech]

Thanks for the help. I have not been able to revisit this repair. I will update when I get back on it.