Mackie mic pre noisy diodes

[g1]

Working on an old CR1604, some noisy channels in the XLR10 expander. All semi-conductors measured good. Swapped IC's with good channels, no change, swapped transistors, no change. Swapped protection diodes and problem solved.
Just wondering if anyone has any tricks for locating such problems. Couldn't see the noise with the scope until the output of the IC. I tried freeze spray but everything is so close together I couldn't be sure I wasn't freezing everything.
Is this a common fault? What would be a likely cause or was it probably a bad batch of diodes? The bad channels were all close together, 12-16. Was thinking maybe phantom power related but wondering why the diodes would get noisy rather than pop.

[Enzo]

Use a business card or a 3x5 card or whatever as a shield when freeze spraying. You can usually isolate small parts for frezing.

This is a very conventional circuit, parts are common, nothing special, a zillion mixers with the same stuff in them. No reason to think the Mackieness of it matters.

Sometimes you'll just never know. And it could even be something weird like the glass bodies of those diodes atracted some film from the air that made their surface noisy. A squirt of tuner cleaner might have cured that. SOmetimes it isn't the part, but the solder. Especially with the RoHS lead-free solder thing. You replace the part, you replaced the solder. Just sucking off and resoldering the old parts might have worked.

And you probably didn't take the time to determine if it was all the diodes or only the ones to the +15 or to the -15. That could have been a clue.

But the bottom line is you cured it, and it isn;t like we should expect mixers all over the world to start getting noisy diodes tomorrow. And next time yo9u encounte3r it, you will be aware of the possibility and will just lift the diodes out of circuit to see.

[g1]

Thanks Enzo. I did verify that it was not a solder issue.
As for the Mackieness, I just wanted to make sure it wasn't something like a common fault or bad batch of diodes (in which case I would replace them in the good channels too).

[jrfrond]

Even though those diodes probably were adjacent to each other on the same reel of diodes when the baord was automatically stuffed, I am not buying the "bad batch" theory.

See as the channels were adjacent, I am going to agree with Enzo here and point out that it sounds more like localized contamination. Were all four clamping diodes noisy? If anything, I've seen those diodes cook when mic pre transistors shorted, but as for noisy, never. Opamps, transistors and leaky phantom blocking caps are usually in that category. But hey, ya never know!

[bkahuna]

I've seen the diodes go noisy in these mic pres, but only when the mic pre got cooked. I've fixed a lot of Mackie mic pres over the years, and usually just "shotgun" them by replacing the 4 transistors and the IC. Those components can be marginally bad in ways that are impossible to measure (usually ends up as noisy), and it's just not worth my time trying to replace only the "bad" components, only to have to tear it apart again to replace something that measures good, but isn't! There have been a number of occasions when the pre would still be noisy, after repair, and it turned out to be the diodes, even though they measure OK, but I've never seen them go noisy without a trashed pre to go with it. It's still rare enough that I don't typically replace the diodes as part of the shotgunning process.

[Steve Conner]

If anything, I've seen those diodes cook when mic pre transistors shorted

Probably the other way around, pilot error stuffed a high voltage into the mic input, cooking both the diodes and the transistors.

Diodes with glass cases are light-sensitive, this could cause hum and noise in a mic pre application when light falls on them.