When to recap a Fisher 400?

satterfi

Member

• Mar 26, 2007

• #1

The preliminary survey of my Fisher 400 with my new ESR meter indicates that most caps are in pretty good shape. There aren’t any really bad ones, but a few seem to be worth notice.

All coupling caps have the same reading of about 3 ohms. A new 715P is near zero. So are they bad? How did mhardy know that?

C75A is about 2 ohms, the other sections a bit lower. This multi-section cap runs about 125 F after a few hours. That seems high to me, but I don’t have the experience to know.

All other caps measure very low ESR.

What else do I need to check before using this reciever daily?

Jon

 

mhardy6647

Lunatic Member

• Mar 26, 2007

• #2

They're bad because they are pushing 40 years old... I "knew" it on BOTH of the ones I've serviced because the plates of at least one output tube were glowing red. Not a good thing. Experience dictates that the caps that Fisher used do not last this long.

If the big cans are running cold, they're OK. If they are getting warm, they're probably not.

I'd also replace the bias supply double cap "out of hand".

 

Tom Bavis

Audiophool

Subscriber

• Mar 27, 2007

• #3

ESR is only one potential problem... leakage is another. Leaky electrolytics get warm... REALLY leaky ones can explode from overheating. Leaky coupling caps will only destroy output tubes... a voltmeter on each grid terminal will tell you. ANY positive voltage (on output tubes, less negative) points to a leaky cap.

 

mhardy6647

Lunatic Member

• Mar 27, 2007

• #4

Thanks, Tom, after I posted I realized I should say "check the grids" and/or "check for DC"... but I forgot.

 

satterfi

Member

• Mar 27, 2007

• #5

I appreciate the advice. It’s likely that I will replace all electrolytic and film caps even if they measure as good as new. It’s not very expensive. My Mouser project order to replace every non-ceramic cap is less than $80.

But before I do that, I want to document the current operating parameters. That way I can say “I replaced all caps because 10% were bad”.

If you want a copy of my documentation, send me an email and when I’m done, I’ll email you my data.

mhardy, I’ll build you a working ESR meter, just cover my costs and I’ll send you one.