Studer A820MCh Blacksides -
Chan 6 muting output when in record (with several other channels) and monitoring input (in Sync playback mode).
Signal appears on the meter bridge (meter signal tapped before output amp) when this occurs, and signal is recorded properly. No failure ever in Repro, Sync or Input mode when not recording. Problem follows the Line Amplifier board. The problem is intermittent.

I shotgunned IC 3, and 7 so far, no joy. The way the board is configured via the DIP switch options rules out IC 4.
Now trying to make it fail in another machine loop recording under similar conditions. I suspect IC 1 (74C374). Want it to fail in front of me with my meter in hand, and the board on the extender. This problem has happened before, and I've never been confident about the source. The weird part, for me is the specific conditions for failure.There are three controls for muting the output driver amp (that I can see).
1. Powering-on mute, to avoid transient spikes on the output when the machine is turned on or off (IC 7, 201).
2. Buss controlled from the microprocessor for various modes (IC 1, 3, 6, 9).
3. Output Amp overheat condition (R252, IC 7, 3, 6, 9).Couldn't make it fail in the other machine, now working it in the original machine after replacing IC 1. No failure yet.
Now trying with the original IC 1, and still no failure.

Finished rebuilding a Schoeps 221 psu with a slowish start filament. Neumann SM-2 noise.

Intersound IVP-
No output except for insert sends.

Busted wire from master volume pot to board (and to -10db output).
Jumpered insert points while I was in there.

Checking and adjusting Schoeps 221, and Neuman SM2 filament voltages.
Paranoid. Bought one of these to poke around with. I was building a similar thing from scratch like a dummy.
Crazy what's out there and for what price.
"Tube Amplifier Filament LM317 Adjustable Regulator Soft Start Power Supply Board"https://www.ebay.com/itm/153101615076

That's insane, so useful and so cheap.

Posted solution to a freelance tech headache at a local studio dealing with a Studer A827
More testing with the Studer, and checking Schoeps 221 and Neuman SM-2 PSU voltages.

Neumann SM-2 NSMa power supply (with active linear regulator for the filament voltage).
Changed R7 to lower the filament voltage to roughly 3.95v

Neumann SM-2 (A modded to C) sn193-
Noise on low capsule output after being on for a while, or when the mic is gently bumped. I'll update the original post about this.

Neotek Elite-Removed the armrest for reupholstering.Neumann SM-2 soft start filament regulator boards.

Elite armrest back, and better than new.

Updated all the Neumann SM-2 supplies with the new soft start filament regulator.