PSU -ve rail voltage drops under load

[Alex R]

Here is the power supply for the switching and button filament lamps for a very large (and very nice) old studio mix/record desk. It's a DDA, about 40 yrs old I think.

The -22v rail runs the filament lamps in the buttons - maybe a hundred of them. But once you switch more than a handful on, the voltage drops away. You can draw very little current from that -ve rail. I have tested both the 22v -ve and the 22v +ve rails on the bench by running them into a low-impedance load. The +ve rail stays up fine, but the -ve voltage drops away the same as it does when running the desk.

I have tested all the semiconductors and they are fine, the diodes all give a beep, though I have not tested the zener voltage on the 12v zeners.

Any ideas where else I might look for such a fault?

Thanks in advance!

[Rick Erickson]

What does the voltage do before the regulator? If it drops significantly then it is probably a filter cap problem. With a PSU this old I would replace all the electrolytics. If that doesn't fix it then there is likely a problem with the series pass transistors or their controlling components - zeners, associated small transistors, resistors etc.

RE

[Alex R]

Thanks Rick I'll check all that out.

[Alex R]

OK I cured it - thanks again Rick! Under load the correct voltage was there on the transformer side of the 2n3055s, but dropped away the other side. They were biased right, but not conducting. They both gave healthy beeps on the diode tester both on and off the board, but I replaced them both anyway and back came the voltage.

First time I've seen power transistors that give the right beeps but don't conduct. To do with the fact that in this application they conduct DC maybe?

[Enzo]

Oh, they always conduct DC.

[Gtr_tech]

Originally Posted by Alex R

OK I cured it - thanks again Rick! Under load the correct voltage was there on the transformer side of the 2n3055s, but dropped away the other side. They were biased right, but not conducting. They both gave healthy beeps on the diode tester both on and off the board, but I replaced them both anyway and back came the voltage.

First time I've seen power transistors that give the right beeps but don't conduct. To do with the fact that in this application they conduct DC maybe?

They were leaky, and your diode test will say they're fine. You need a dedicated transistor checker that shows leakage current.

[Rick Erickson]

[I don't know "A Dorian" from "a Delorean"]
Simple - "A Dorian" is just a G Major scale played over the key of A. {A, B, C, D, E, F#, G, A}

[Enzo]

Would that be a gray scale?

[Rick Erickson]

Actually, it's one of the most common "Blues" scales. I'll be all over it tonight...

[Amp Kat]

Originally Posted by Rick Erickson

Actually, it's one of the most common "Blues" scales. I'll be all over it tonight...

That was clever Rick, A dorian scale is ....

I like the hypodorian other wise known as the Mixolydian. Enzo wrote a song called Muting Fet Blues and I wrote the music and recorded it but never finished it all the way. It was only about five years ago but anyway it was in C Dorian so I guess I should finish it ey?

[jrfrond]

Originally Posted by Alex R

OK I cured it - thanks again Rick! Under load the correct voltage was there on the transformer side of the 2n3055s, but dropped away the other side. They were biased right, but not conducting. They both gave healthy beeps on the diode tester both on and off the board, but I replaced them both anyway and back came the voltage.

First time I've seen power transistors that give the right beeps but don't conduct. To do with the fact that in this application they conduct DC maybe?

Static tests often don't reveal anything with borderline components, when in fact, they fail under a dynamic load. This is where experience comes in and your brain says "sub it out". We can check transistors with DVM's, transistor checkers and curve tracers, and not find a thing and still have failure.

Substitution is a legit form of troubleshooting.

[Enzo]

SO you didn't like my Dorian Gray reference? Perhaps if I painted a portrait...

[martin manning]

Originally Posted by Enzo

SO you didn't like my Dorian Gray reference? Perhaps if I painted a portrait...

Wilde thing...

[Gtr_tech]

Originally Posted by Enzo

Would that be a gray scale?

Vickers scale....its harder for me to explain it then it is for me to just play it...

[Rick Erickson]

[SO you didn't like my Dorian Gray reference?]

Sorry - I'm a litle slow sometimes. The police refer to it as "impaired"...