Serious problems with my A12

[jlearman]

Hey guys/tim,

I've built 5 other perfectly functioning sca preamps, but something must have gone very wrong with this last a12 build. It barely passes audio & makes all sorts of weird extra-terrestial noises when I turn the gain & trim knobs. I've uploaded 2 short mp3 files of me turning the switches & talking into 2 different mics. Can anyone point me in the right direction? I have no idea where to even begin with trouble shooting this thing.

watch your ears when listening to this....keep your volume down.
http://users.rcn.com/joe.learman/audio/a12_sm57.mp3
http://users.rcn.com/joe.learman/audio/a12_319.mp3

many thanks!

[tpryan]

Are the DC voltages correct?
Are you able to adjust the output offset voltage to 0V?
Does the op-amp or voltage regulators get unusually hot?
Does it make noise at all gain settings, or only certain ones?
What happens if you swap op-amps from one of your working A12s?

[jlearman]

I switched the opamps, and the "bad" opamp in the good a12 worked fine. When I moved the opamp from the good a12 to the bad one, the behavior was very similar, although not as extreem as the mp3's in the previous post.

I checked the DC voltages, and the negative side was fine at -18v, but the positive side is sitting at approximately 8.2 & adjusting r25 does nothing.

The DC offset voltage fluctuates between 1.9 mV and 4.0 mV. I've turned R19 like crazy and it doesn't seem to be making any difference.

As far as I can tell, nothing is getting abnormally hot.

And finally, yes it behaves poorly at all gain settings, not just particular ones.

Thanks for your help Tim,
Joe

[tpryan]

Remove the op-amp and measure the voltages again. If you can't adjust R25 to get the proper B+ voltage, you have a serious problem. Check to make absolutely sure that C10, C12, and C4 are installed the right way round.

[jlearman]

Those voltage were actually checked with the opamp removed. But I checked again just to make sure, and they're the same {-18V +8.2V}. Turning R25 still does nothing. I verified that c4, c12, and c10 are installed correctly. I double checked with my working a12 & with the pictures of the board in the assembly instructions.

Any other ideas? I really appreciate the help.

Thanks,
joe

[tpryan]

Measure the voltages at both ends of D2 and D4 and tell us what you find. Are you sure that R25 is marked "102"? Are you sure you have the correct resistors installed at R26 and R27. Is D5 installed the right way round?

[jlearman]

Working a12:
D2: 29.84, 29.10 (stripe side)
D4: 18.02, 29.11 (stripe side)


Bad a12
D2: 29.84, 29.21 (stripe side)
D4: 3.21, 29.21 (stripe side)

R25 and R29 both say y 103 on the top, and r 19 says y 103. R27 reads 1k2 and r26 is 118 on both preamps. And yes, D2, D4, and D5 are all installed the right way 'round.

Could u2 be fried?

[tpryan]

Well, R25 and R29 should be marked 102, but that's not causing your symptom. Has the voltage at U1, pin 3 changed? It should be the same as at the anode (the "unstriped" end) of D4, and above you were measuring about 8V. What happens if you remove C4? If that doesn't make a difference, try a new LM317. Try C4 first, it's much more likely to have shorted than for the LM317 to have failed.

[jlearman]

sorry....r25 and R29 are marked 102. I'm not sure why things have changed, but I am now getting 8V at the anode of D4. I guess I'll order a new c4 and LM317 from mouser and see if that fixes the problem.

Thanks Tim,
Joe

[jlearman]

Hi Tim,

Sorry this has taken so long, but just got around to installing a new u2 and C4, but am having the exact same problem. Getting 29.25 volts at D2 and the striped side of D4, but only ~8 V at the pin 3 and the non-striped side of D4.

D4 is definitely installed the correct way around, and R25 is the right part (has Y 102 on the top)

Any ideas?

Many thanks,
Joe

[tpryan]

Did you try powering it up with C4 out of the circuit? You're still measuring voltages with the op-amp removed, right?

[jlearman]

No didn't power it up without C4, and yes, I removed the opamp for all these tests

[jlearman]

Hey Tim, just wanted to bump this....any other ideas? If not, maybe this is to the point where I should send it to you and have a pro take a look at it.

Thanks again,
Joe

[tpryan]

Send it in, I'm curious to see it. Email sales@seventhcircleaudio.com for return details.