Powering up A12 - not so good

[jarbar]

Hi there.

Tested A12's J7. Pins 1/2 and 2/3 briefly read somewhere between 1200ohms and 1800ohms, then meter goes back to 1. (normal? or is it my cheapie DMM?) For pins 1/2, positive probe must be on 1. For pins 2/3, positive probe must be on 2. Otherwise, I get no reading.
(This is just info that may help the real problem: )

PS powers up with all its LEDs glowing. However, when I plug in an A12, things get weird. With the first A12, the large external LED was dim and slowing pulsing off and on. When I tested U1 pins 3/4, I got a negative reading of what I believe was less than 1V. Pins 4/5 gave nothing.

The second A12 seemed to prevent the PS from lighting its LEDs at all. When I wiggled the J7 connector on the A12, the LEDs came on, but I got a spark from somewhere on the A12.
I quickly powered down. I can't see anything that looks burnt.

Now I'm nervous to even connect the things for fear of blowing something. I've tripled checked diodes D2-D7, caps C4,C5,C10,C13, and nothing looks to be shorted out. But I don't want connect to do some DMM testing because of that spark.

Thanks for your time and I really truly appreciate the help. I'm looking very forward to getting these things up and running.

jar

[jarbar]

UPDATE:

The A12 that spark seems to be fine. I'm just about to put the SC25 in. I think the spark was a product of me dicking around with the harness.

However, any feedback regarding the pulsing big LED on the power supply when the other A12 is powered up would be wonderful.

Thank you.

[jarbar]

Well I got the second one powered up and the PS big LED isn't pulsing anymore. (have no idea what happened last time)

However, on U1 pins 3/4, I get 27.8V. Pins 4/5 is saying -14.4V and I got that one to -18V no problem.

I see in another thread that this was due to a bad potentiometer.

Hopefully that will get this thing going.

[jarbar]

All right. I got the working A12 tested. It seems to sound fine, but there's a fairly loud pop when I click the gain up or down. Is this normal?

Also, when I plugged a mic in (condenser), I mistakenly had the phantom on and there was a spark at the XLR input. Should this happen? I know it's a good idea to have the phantom off, but I've never seen something spark before.

Thanks for any feedback on my continuing troubleshooting saga.

[jarbar]

I just tried my working A12 again.

This time the phantom was OFF, and no mic plugged in, but when I connected the mic cable to the XLR input, I saw and heard a spark that seemed to come from somewhere in the circuit (ie, not at the XLR jake). I seemed to be from the pre, not the PwrS.

[tpryan]

If you can't adjust the voltages at U1, pins 3 and 5 to +/-18V, you need to stop and figure out what's wrong. Don't plug anything in, don't apply a signal, it's just a waste of time.

Any sparking is a sign that something is very wrong. Stop and figure it out before connecting a microphone. You should be able to measure +48V at pins 2 and 3 of CONN1 when phantom is switched on, and 0V when it's off. Make absolutely sure that you've installed the correct resistors at R33 and R34. Let us know what you find.

[jarbar]

Thanks for getting back to me. Much appreciated.

The problem with the unadjustable 27V at U1 (pins 3/4) was a hard to see solder bridge on the top side of the board at U2.

The spark as it turns out, was a result of the underside of the board touching the metal divider that sits between the Pwr Supply and the first pre amp. I hadn't fully screwed the pre amp into place as I was only testing it and when I plugged the XLR in, something touched and sparked. I trimmed any longer leads on the underbelly and now that it's firmly in place, it doesn't move much and seems to be working great.

Thanks so much. I look forward to trying these out this weekend.