Locating Bad Capacitors

[Jason299]

I just finished cleaning a Realistic QTA-790 (a.k.a. Kenwood KR-8140), and while I've eliminated virtually all the POTS generated static, I still have a nominal buzz/hum regardless of input, speakers, cables, etc. Sound is otherwise very good, which leads me to believe I may have one or more bad capacitors.

I've tested and replaced speaker caps before, but not in a receiver. A parts list is attached and I have a complete service manual that's too large to attach, but it can be found here: https://www.hifiengine.com/manual_library/realistic/qta-790.shtml

Any suggestions on how to test or identify bad, or likely bad caps for replacement would be appreciated.

 

[HPMguy]

The only visual way to tell if a cap is bad is when it swells up and/or spews out the electrolyte from overheating, or if the shrink wrap cover has shrunk down past the ends (means it's running very hot). Not reliable. To actually test requires removing the cap and a decent quality capacitor tester, not one of the inexpensive computer ones. Those are really handy (if it's bad on the cheap one, it's bad), but given the age and fagility of old board and traces, I'd not pull them just to test, I'd replace with new.

Diagnostics for hum are pretty simple for the most part -- 60Hz is a rectifier problem, 120 Hz is a main filter problem.

Given that the equipment in question is a): 52 years old and b): made with capacitors of far lesser quality than current manufacture, I'd personally plan to replace every single one.

Hum can also be caused by poor grounds, and that receiver will be full of wires, corrosion and heat cycles are hell on solder connections.

 

[ConradH]

#1 Fixing it requires putting it on a test bench and checking through the circuitry with a signal generator and scope. Some detective work will locate a bad cap, if that's the problem at all. It usually isn't, unless it is.

#2 The other path is some degree of restoration. Given its age, you'd replace every electrolytic cap and possibly some other things. Then, it's back to #1, because doing that work isn't a guarantee of anything. You have to confirm that the unit is now working to specification. Or not. And then do more detective work to figure it out.

 

[Jason299]

#1 Fixing it requires putting it on a test bench and checking through the circuitry with a signal generator and scope. Some detective work will locate a bad cap, if that's the problem at all. It usually isn't, unless it is.

#2 The other path is some degree of restoration. Given its age, you'd replace every electrolytic cap and possibly some other things. Then, it's back to #1, because doing that work isn't a guarantee of anything. You have to confirm that the unit is now working to specification. Or not. And then do more detective work to figure it out.

Thanks, this unit has Mylar, ceramic, electrolytic, and one oil filled capacitors. Is it “primarily” the electrolytic caps that fail? I’m guessing the oil filled needs to be replaced as well.

 

[ConradH]

It's usual to only replace the aluminum electrolytics. If tantalums are present they usually get replaced as well, but fortunately they aren't that common in consumer gear. I'd leave film and ceramic alone. Small PIO (paper in oil) caps I'm no expert on. I've seen a lot that don't measure too well, but I don't know how good they were when new. I suspect they cover a wide range of quality and some will last near to forever. Others not so much. Searching the 'net will get you many opinions, mostly invoving guitars. Choose an opinion you like.