My favorite stereo is a DVD recorder that... doesn't work with DVDs anymore,
won't even open the tray.

But, the stereo parts are wonderful, so it does all my TV sound.
A few days ago, it developed a buzz, which didn't affect any of the
digital (like blue-ray) sound inputs, only the analog ones.
Sounded like 120 Hz, so I checked the regulators for that
bunch of op amps, and sure enough, a capacitor had bulged...
an hour to disassemble, five minutes to replace the thing, and now it works
fine again (except for the DVR parts)...

But, the capacitors I replaced were the OUTPUT filters on 7815/7915
regulators (hey, they're identical, and one failed; better do both
while I'm here, or it's another couple of hours to get to them ever again).

How often do regulator output capacitors fail, and how likely is it to generate an
audible buzz when they do? I'd have thought the regulator
would reject 120 Hz and its harmonics pretty well even without those
filters. The removed part that failed tests as open, not shorted.

On Fri, 7 May 2021 15:18:01 -0700 (PDT), whit3rd
wrote:

My favorite stereo is a DVD recorder that... doesn't work with DVDs anymore,
won't even open the tray.

But, the stereo parts are wonderful, so it does all my TV sound.
A few days ago, it developed a buzz, which didn't affect any of the
digital (like blue-ray) sound inputs, only the analog ones.
Sounded like 120 Hz, so I checked the regulators for that
bunch of op amps, and sure enough, a capacitor had bulged...
an hour to disassemble, five minutes to replace the thing, and now it works
fine again (except for the DVR parts)...

But, the capacitors I replaced were the OUTPUT filters on 7815/7915
regulators (hey, they're identical, and one failed; better do both
while I'm here, or it's another couple of hours to get to them ever again).

How often do regulator output capacitors fail, and how likely is it to generate an
audible buzz when they do? I'd have thought the regulator
would reject 120 Hz and its harmonics pretty well even without those
filters. The removed part that failed tests as open, not shorted.


I'd check other caps in the chain, before the regulator. Most common
age-related failure is high z with no physical signs - just high
ripple voltage, where there shouldn't be any.

If the input cap was high z, ripple would break through as the voltage
dips below minimum reg values.

RL

On Saturday, May 8, 2021 at 7:30:57 AM UTC-7, legg wrote:
On Fri, 7 May 2021 15:18:01 -0700 (PDT), whit3rd
wrote:
My favorite stereo... developed a buzz, which didn't affect any of the
digital (like blue-ray) sound inputs, only the analog ones.
Sounded like 120 Hz, so I checked the regulators for that
bunch of op amps, and sure enough, a capacitor had bulged...
an hour to disassemble, five minutes to replace the thing, and now it works
fine again (except for the DVR parts)...

But, the capacitors I replaced were the OUTPUT filters on 7815/7915
regulators.

I'd check other caps in the chain, before the regulator. Most common
age-related failure is high z with no physical signs - just high
ripple voltage, where there shouldn't be any.

The preregulated supply is, I now see, well-enough documented (a printed wiring image) that
it's possible to look at ripple under bias; it takes a lot less than hours to get to the TOP surface
of that board, so next time I get curious, I can check that out (but those input capacitors
aren't ones that I can grab from my stock; instead of hours, it'd take a few days to order
replacements). I did check the diodes, and of course transformers rarely fail.

The replaced items included one tested-faulty capacitor, and it sounds OK now.

https://scrapsfromtheloft.com/2017/1...is-bull****/On Fri, 7 May 2021 15:18:01 -0700 (PDT), whit3rd wrote:

My favorite stereo is a DVD recorder that... doesn't work with DVDs anymore,
won't even open the tray.

But, the stereo parts are wonderful, so it does all my TV sound.
A few days ago, it developed a buzz, which didn't affect any of the
digital (like blue-ray) sound inputs, only the analog ones.
Sounded like 120 Hz, so I checked the regulators for that
bunch of op amps, and sure enough, a capacitor had bulged...
an hour to disassemble, five minutes to replace the thing, and now it works
fine again (except for the DVR parts)...

But, the capacitors I replaced were the OUTPUT filters on 7815/7915
regulators (hey, they're identical, and one failed; better do both
while I'm here, or it's another couple of hours to get to them ever again).

How often do regulator output capacitors fail, and how likely is it to generate an
audible buzz when they do? I'd have thought the regulator
would reject 120 Hz and its harmonics pretty well even without those
filters. The removed part that failed tests as open, not shorted.

Suprised it wasn't 100 Hz, twice the mains frequency.