Why does nobody talk about capacitor ESR drift in old amps

I was going through a 1980s stereo receiver last night and decided to actually measure the ESR on the power supply caps instead of just replacing them blindly. Found out three of them had ESR values over 10 ohms even though they looked fine and tested okay on a basic capacitance meter. That detail came from a forum post by a guy named Mike in Austin who specializes in vintage gear. Has anyone else found that the physical condition of old caps can be totally misleading?

2 comments

2 Comments

mitchell.dakota18d ago

Sucks when you find that, right? Especially after all the time spent just assuming caps are bad based on looks. @mia_anderson those readings were on the big main filter caps, the 10,000uF ones that everyone just swaps out without testing. Physical size definitely plays a role though because the larger cans tend to have more internal layers that can dry out weirdly compared to the smaller coupling caps. I had a similar experience with a Pioneer SA-8500 where the caps looked perfect on the outside but the ESR was through the roof on three of them. Makes you wonder how many "recapped" amps out there still have bad caps inside that nobody bothered to measure.

mia_anderson18d ago

wait, were those 10 ohm ESR readings on the smaller filter caps or the main big ones? wondering if the physical size matters for that kind of drift.