Help! MXL 991 is dead after mods

After installing the Gus 3-cap mod on two MXL 991s, one of them isn’t working. I think it may be the something on or around the gate that’s the culprit. I’ve tried several different capacitors. I’ve tried resoldering the other side also.

I don’t really know how to use a digital multimeter other than for voltage. With the mic body and capsule off, the working mic reads around 30v across C3 and also C4 (which we’re also changed) and it hums pretty loud because it’s not grounded to the body.

On the non-working mic it’s like 3 or 4v across C3 or C4. When I touch a post on the gate, a 1G resistor, the lead from the FET to the gate post, or the gate cap with one of the DMM leads, it starts to hum like it should.  Otherwise, it makes no noise.

Can anyone help me figure out how to fix it?

Photos would help. Especially since almost none of us know the components "by name" (as marked on that particular circuit board) - the roles they play are arguably more important.

Front

Back

Are you 100000% sure you managed to avoid lifting / breaking any traces? The photos are a bit on the blurry side...

That being said, what (DC) voltage can you measure across the electrolytic capacitor, closest to the XLR? Looks like that should be the first filter cap for the power.

Part of the pad did lift on C3, but it didn’t come completely off. I scraped away a little bit on trace leading up to it where the pad was intact, and bent the lead down to touch the trace.

I don’t get a reading across any of the electrolytic caps. I just tried resoldering C3 again, but still nothing has changed.

Use multimeter as diode checker funstion /continuity tester. First check aren't diodes around BJT shorted, then 6.2V, 6.8Vzener diode. Or just check all the diodes. This is what i would start here. If not then check both BJT at the output aren't shorted. Same way. If you will get "beep" between two legs of any, then you have reason. Traces also in these mics like to brake. It can be barely visible.

ln76d said:

Use multimeter as diode checker funstion /continuity tester. First check aren't diodes around BJT shorted, then 6.2V, 6.8Vzener diode. Or just check all the diodes. This is what i would start here. If not then check both BJT at the output aren't shorted. Same way. If you will get "beep" between two legs of any, then you have reason. Traces also in these mics like to brake. It can be barely visible.

Click to expand...

I checked all of the diodes, and they all show readings.

So if I'm not getting readings across the electrolytic caps, then I probably have a broken trace somewhere?

So I ran new capacitors and bypassed their traces after the pad for C3 come off earlier. Now, when I tap on the mic body I can hear the tapping, but nothing is coming through the capsule.  It’s like I’m getting closer, but all of the readings I get are still the same.