Peavey 400R 15V regulation issue

[tdlunsfo]

Looking for some advice on this one. I can't find any bad parts and I only get +/- 9V on the 15v rails for the preamp and this is with the preamp disconnected. When the preamp is connected the voltage falls to 1v. Any thoughts? I tried replacing the 15v zener and no change. I'm down to the regulator transistors but they pass the diode checks. Caps pass the ESR check.

Here is the link to the schematic:
https://www.audioservicemanuals.com/...dule-schematic

[g1]

Are you getting a solid +/- 40V at regulator collectors?

[tdlunsfo]

Kinda, -49v and +47v

[g1]

How about at the 431 base where you changed the zener?

[tdlunsfo]

-9.8v

[g1]

I missed the 'kinda' with regard to the HV rails, check them for excess AC volts (ripple).

[tdlunsfo]

42mV AC ripple on the collectors of 430/431

[Helmholtz]

430/431 are just simple emitter followers, where emitter voltage follows base voltage within 0.7V.
So what are base voltages and base-to-emitter voltages?

[g1]

Is the 4K7 in the 431 circuit good? The 100uF filter at the -15V rail?

[tdlunsfo]

431 B = -9.16v , B-E = 0.66v
430 B = 9.28 , B-E = 0.529v

[tdlunsfo]

Yes on the resistor and caps. Measures 4.74k and the caps are 220u in this circuit and measure ~240u and zero ESR in circuit.

[tdlunsfo]

So something strange happened about 15 mins ago. I all of a sudden started getting +/-14.3v but as soon as I plugged in the preamp it collapsed to 1v and now I am back at 9v with the preamp disconnected and unable to repeat the 14.3v. Weird issue..

[g1]

It almost sounds like some kind of intermittent. Maybe it would be easier to just leave the preamp connected.
With the +/- LV only at 1V, recheck voltages at the 430 and 431.

edit: nevermind

[tdlunsfo]

Got it. Bad 431.

[Helmholtz]

431 B = -9.16v , B-E = 0.66v
430 B = 9.28 , B-E = 0.529v

These measurements do not indicate a malfunction of 431.

You could have (had) a bad solder joint.