Strange Problem

[JustMike]

I just built 4 A12's and 2 N72's and everything works. EXCEPT when I rack it. One of the A12's doesn't work when I rack it and plug my snakes in. But If I take the chassis out of the rack and test with THE SAME MIC AND CABLE it works.
I tested all of the pre's this way and levels are comparable.

I also tried known good snake channels/mic cables/mics etc.

Could there be a bad component or a misinstalled component that could cause this kind of behaviour? Something in the chassis ground circuitry perhaps?

[tpryan]

If you have a mechanical symptom, there's a mechanical solution. Something is loose, or perhaps broken, but you'll have to find it. My first guess would be bad solder joints at the XLR connectors, but it could easily be elsewhere. Does the offending module misbehave in other positions in the rack?

[JustMike]

I'm going to try that next, but I don't see the logic. The only thing that would be different would be the power connector and I've already confirmed that.
I'll keep you posted.

[JustMike]

OK, I said I'd keep you posted. So I'll swallow my pride and tell you I had 2 bad cables. Jeez...what are the odds?
Module sounds fine.
A little OT, but why do the trim pots on the A12 and the N72 act differently? With the trim pot all the way down, the N72 passes no audio. But on the A12, it still does. Did I build something wrong?

[tpryan]

The N72 has two gain stages, the A12 has one. The trim on the N72 sits between the gain stages and can be configured to act as a full fader or a trim control. Configuring it as a full fader allows you to add distortion by running the amp at very high gains while still allowing control of the output level. This mode of operation is not possible with the A12. If you'd rather have the N72 behave like the A12, replace the 0 ohm jumper at R20 with a 10K resistor.