U1 pins 3 & 4 = + 28 V and 5 & 4 = - 28 V

[leo]

I just finished a new A12 and I am getting +/- 28V between 3 & 4 and 5 & 4 on U1. The trim pots do nothing. I checked this A12 against one that I build about a month ago that is working and all of the diodes and polarized caps are oriented correctly. I triple checked that I had the right trim pots installed and they are correct. My other A12 works fine and there is no visible difference between the two.

I am assuming that I also have bad trim pots like the person from the other post who was only having this problem at 3 & 4 of U1. Is there any chance that there was a bad batch of trim pots that were sent out?

Is there any way for me to easily check to see if the trim pots are bad? If they are bad, is there any chance that when I powered up damage was done to C4 and C5 or U2 and U3?

Any help would be appreciated.

[tpryan]

First, make absolutely sure that you haven't inadvertently swapped U2 and U3. Double check the orientation of D4 and D6.

Is there any way for me to easily check to see if the trim pots are bad?

With the pots turned fully clockwise, you should measure close to 1K ohms across the outermost pins. Turned fully counter-clockwise, you should see 0 ohms at the same pins.

is there any chance that when I powered up damage was done to C4 and C5 or U2 and U3?

Not from an open or shorted trim pot. Let us know what you find.

[leo]

I am probably an idiot. I know I checked U2 and U3 when I was installing them to make sure I put them in right but I bet I screwed that up. That was the only thing I did not check last night when I was racking my brain for the problem. I will check them when I get home and make sure that is the problem.

The next question is, if I did put U2 and U3 in wrong, in your estimation would I have damaged anything that would need to be replaced?

Thanks for the help Tim. I was hoping I would be careful enough not to make a dumb mistake. I guess not.

[leo]

Okay that was it, I swapped U2 and U3. I cannot believe I did that. Thanks Tim for the trouble shooting help. I am going to order a new U2 and U3 but I was curious if there was a chance I should consider replacing any other parts I might have damaged.

Any help would be greatly appreciated.