A12 PCB rev1.3 and Neg Regulator?

[drunton]

Has anyone out there built an A12 on the newer Rev1.3 PCB? I just got my 2nd pair of PCB's a couple weeks ago and am having some problems with the negative supply side.

I am doing some experiments, so here is what I have:

A. on one PCB, only the regulators and supply resistors, supply caps and the offset circuitry are populated (no tantalums C4/C5) and no feedback resistors.

B. on the other PCB, the whole thing is populated except for the tantalums (C4/C5). The feedback resistors are different values to try and use a 990 with different transformers.

Both boards behave the same, so I must have done something wrong on both....

I want to go to +/-24, and the +24 is fine, but the - only goes to -22, it starts at -13V (not 14-ish). Also, I have noticed that the 47uF (c15) causes it to drop to -10 and only go to -18 - sure sign of a leaky cap, but it was installed correctly and BOTH PCB's act the same way ???. And when, I check the caps they seem ok on the meter, so right now C15 is pulled.

I have replaced the regulators, checked the resistor values, double and triple checked diodes. The Vin is proper for both regulators +/-26V

Have to say I am a bit baffled, like I said, both are working the same way. My other pair of A12's are fine and I compared polarity of diodes and caps to make sure I did everything right.

Any one got a pointer...

[tpryan]

For the LM337, you need to have a cap across the output and a current draw of at least 2.5 mA or else all bets are off. Install the tantalum caps and see what happens.

[drunton]

I was hoping you would say something like that. I'll check it out when I get the caps later this weekend.

I assume then that these regulators work similar to a zener and need some current draw to "bias" them. Ran into this on a high voltage pass transistor regulator on a tube circuit.

Thanks as always for your wisdom, Tim!

[drunton]

Yep, that worked just fine. Added the tantalums and everything fell right into place.

Thanks Tim! And, by the way, love your comments in the latest TapeOp about building kits. I've built quite a few things, yet still learn. It's amazing what you run across when you really do it yourself.