Blown (or just scorched) mica caps?

[Gazucha]

 Hi all,

 Was planning on pulling these mica caps to test but read that it was not a good idea in devices with precision tuning. This is a HP 3312A Function Generator which has a waveform fault on the main output channel. i.e no waveform output!
 
 Some caps are clearly darkened. Surely not a good sign, but can that be read as bad?

Photos included

 Cheers chaps.

[mmagin]

I would wonder if the color of the (epoxy?) dip was more variable than we would expect.  This doesn't look like the sort of circuitry that's going to damage 300V rated capacitors.  I see a 50v rated cap (non-silver mica) in there, and I don't see power semiconductors that I would associate with heavy RF currents through those capacitors.

Also, my impression from the people who repair old (consumer-grade) tube radios is that film and electrolytic capacitors are often bad, but silver mica very rarely fail.

(I haven't a lot of experience fixing up old stuff -- Would certainly be interested to hear I'm wrong from anyone with firsthand experience to the contrary.)

[Alex]

Discolouration is a sign of overheating, although it is surprising to see this in a signal section of a circuit. It is unlikely to be due to aging as other caps are fine.

I would not worry about calibration when you don't even have an output. Remove the caps one by one, test properly and resolder in original position. Then clean the flux residue. If the failure is due to the caps I would expect a major failure of the caps like a short.

If you do have to replace the caps, chances are that something else is wrong; mica caps have high reliability. You should also check the power supply levels according to the service manual. Temporarily insert ceramics and check operation. When you replace the caps the circuit calibration will be off, but noone can avoid that.

In any case calibration has probably expired anyway, so repair freely as normal but with care.

Alex

[Gazucha]

 Thanks chaps,

 will pull them and see.

  Only just got this so haven't had the time to get 'into' it yet and now it's weekend, but I like the idea of of testing with ceramics. 

 

[Conrad Hoffman]

I see that a lot on low voltage low power circuits and those caps are very likely fine. I also have a lot of NOS s-m caps with the same discoloration! No idea what causes it, but it seems to be common.

[Gazucha]

Actually, I pulled a few and they were all fine, so I just re-soldered several suspect looking joints elsewhere and the unit came back to life. 

Just need to do some fine tuning now.