Intermittent A12 and a Quiet A12

[room4studios]

Hello,

I've just finished building 8 modules, 6 A12s and 2 J99s. They all passed the voltage tests etc, so i put them all in the rack and discovered the following.

Preamp1 is intermittent, sometimes it works fine and sometimes it doesn't work at all. I noticed a few things that might shed some light: I was monitoring some music through a phantom powered mic when it cut out, when i switched the phantom power off it immediately kicked back in, but obviously the level died away, and when i switched the phantom power back on it was working. When i was doing the voltage set up on this one, i remember winding it up towards either + or - 24v (it has has SC10), the voltage dropped to close to 0v and remained there. I came back to it and it was working fine again, so i continued winding it up.

Preamp4 is quiet. It produces a signal around about 7 db quieter than all the rest. The trim control only increases the level by about 1 db as opposed to 5 in all the rest.

Preamp 3 (a12) and Preamp 8 (which is a J99) don't null effectively when the trim level is on maximum but they do when the trim in on minimum. All the rest null effectively at both settings.

Please let me know where to start in trying to troubleshoot this lot.

Cheers

Phil

[tpryan]

The intermittent preamp almost certainly has some bad solder joints somewhere. If B+/B- is intermittent, try re-flowing all of the components associated with the LM317/LM337 regulators.

The quiet A12 and troublesome J99 may have each others trim controls. Verify that the A12 trim control is 10K ohms.

[room4studios]

I will reflow the intermittent on asap, however i checked the trim controls and the a12's are all 10k and the J99's are 1k.

[room4studios]

Hello, I've double checked all the components on the quiet A12 and everything is present and in the right place and orientation. Do you have any other input as to what the problem might be?

With regards the two modules not nulling effectively at maximum trim do you have any other input? I have double checked these too.

Cheers

Phil

[tpryan]

Send an e-mail to rma@seventhcircleaudio.com for return instructions, or take them to a local repair technician.

[room4studios]

Just incase this benefits anyone in the future here is what the problems were.

Intermittent A12 had a poorly soldered contact at one of the LM317 or LM337 (can't remember which)

On the Quiet one i had neglecting to solder poles of SW1, the gain switch.

The units that didn't null was done to a poor test. The mic took a while to reach full capacity after turning on the phantom power and the first piece audio recorded was different to the second out of phase bit through no fault of the preamps.

Many thanks for your help and patience Tim

Cheers

Phil