Wow, thanks for all of the responses!
I took all of your suggestions and poked around on it again, and I think I finally found the offending component
.
Firstly, my 400F is a later build with the 0950A serial number prefix. I do have the latest manual downloaded from Keysight, but as Ted mentioned, it's a lousy scan and is still missing some correct info. I had noticed that CR12 and CR13 are in the correct positions and correctly labeled on my board, but are swapped relative to the board layout in the manual. The A2 circuit board in mine is p/n 00400-66514 REV A. The A2 board in the manual is 00400-66504. I also noticed the change in value for R18 from 100 ohms to 806 ohms in the latest rev, as well as the addition of CR24 across the base and emitter of Q6 (can someone explain the reason for that to me?).
I verified the transistor pinout as suggested and they match the original components that were removed. The original Motorola transistors had EBC clearly printed on them. The replacements that I obtained from Digikey did not have the pinout marked, but the data sheet indicated that they were the same, as well as a quick check using the diode function on my Fluke 8050A.
Ok, down to the nitty-gritty. When I first turned it on today, I noticed that the negative rail voltage started at ~-19V and was drifting down while bouncing around a bit. I started poking around with the voltmeter and noticed that the voltage feeding the base of Q7 was moving ever so slightly, so I measured across C24 (4.7 mfd tantalum) and it was moving quite a bit. I suspected that C24 might be breaking down under power so I lifted one side and my -26V rail came up from -12V to -25.3V
. I didn't have a 4.7mfd tantalum handy, so I stuck a 4.7 electrolytic in there temporarily and the negative rail has maintained -25V since.
The only thing I'm not sure about is how critical it is that my positive and negative rail voltages match exactly.
I'm reading +26.27 volts on the positive rail and -25.65 volts on the negative rail. Is that any cause for concern?