A bad ceramic capacitor. I'd never come across one until I was working on an AKer's Realistic SAF-40B. The capacitor across the 470k resistor going to the plate of the phase splitter 12AX7 measured 500k. That would explain the high voltages and crappy sound in one channel. Replaced that cap in both channels and the amp is singing sweetly now.
Well that's a first...
- Thread starter RossW
- Start date
Register to hide this ad
Nice find. Hard to do, isn't it?
Ceramic caps and resistors are usually pretty trustworthy but anything can fail. They just aren't the "usual" suspects.
I chased my tail for a LONG time until I found some bad solid aluminum caps. BOTH channels too. They are supposed to be good "forever" too.
Ceramic caps and resistors are usually pretty trustworthy but anything can fail. They just aren't the "usual" suspects.
I chased my tail for a LONG time until I found some bad solid aluminum caps. BOTH channels too. They are supposed to be good "forever" too.
I had a NOS Sprague ceramic disc cause horrible hum in a Fisher 70A. Another one from the same package worked fine. Sometimes they're just bad.
I have given up on items like this more thanonce. I would put the amp back on the shelf and come back to it a year later. At that point, if I couldn't figure it out, I would order all new caps(ceramics, silver mica's, etc) and replace them all. It has solved my problem more than once. I can't say I really trouble shot it succesfully, but I can say I repaired it successfully.
You just have to be methodical.
Test/replace the high-failure parts first (paper/wax and/or electrolytic caps, etc).
If that doesn't fix it, start looking at other parts, in the order of likelihood that they will be bad.
As is attributed to Arthur Conan Doyle's Sherlock Holmes: "when you have eliminated the impossible, whatever remains, however improbable, must be the truth". Once you KNOW that every usual suspect is innocent (by testing- even of new replacement parts!), it only can be something else...
I have, on at least one isolated occasion, had a bad ceramic cap. IIRC, it was a coupling cap in a HH Scott phono stage. Leaking B+ into the following stage. It got replaced by a nice mica 500v cap, as did its corresponding mate in the other channel...
Also, remember- at least some of the 'death caps' that need replacement- some of them are ceramic caps. Those do fail- especially those old enough to be not X/Y rated (to fail open)... those can be dangerous when damaged...
Regards,
Gordon.
Test/replace the high-failure parts first (paper/wax and/or electrolytic caps, etc).
If that doesn't fix it, start looking at other parts, in the order of likelihood that they will be bad.
As is attributed to Arthur Conan Doyle's Sherlock Holmes: "when you have eliminated the impossible, whatever remains, however improbable, must be the truth". Once you KNOW that every usual suspect is innocent (by testing- even of new replacement parts!), it only can be something else...
I have, on at least one isolated occasion, had a bad ceramic cap. IIRC, it was a coupling cap in a HH Scott phono stage. Leaking B+ into the following stage. It got replaced by a nice mica 500v cap, as did its corresponding mate in the other channel...
Also, remember- at least some of the 'death caps' that need replacement- some of them are ceramic caps. Those do fail- especially those old enough to be not X/Y rated (to fail open)... those can be dangerous when damaged...
Regards,
Gordon.
I had a couple flaky mica caps in the oscillator section of a Philco TV. It wouldn't hold vertical or horizontal to save it's life. I poked and prodded and messed with everything that I could find, and finally just shotgunned the 3 caps. the problem went away. I don't know which was actually at fault, but by that point I didn't care anymore. I could have done them one at a time but I didn't feel like hauling the chassis in and out of the cabinet potentially 3 more times.