api 312 problem

[thinktank]

I've got some 312 mic pre's that I made. I've got some distortion
on two of them. Basically, when I record in with a mic, the sound
distorts. The other's that I make work great. They are all transformer
balanced input and output. Lundahl 1538xl's on the input and the 1503
on the output. Any methods of using my meter to troubleshoot or other
suggestions of where to look for this type of problem? Thanks.

Nathan

 

[Rob Flinn]

Have you tried swapping the op amps from the good ones into the bad ones ?

 

[Flatpicker]

That's what I would suggest, too. If fact, swap everything you can to narrow it down. The first rule of troubleshooting is to isolate the problem.