28-01-2022, 06:54 PM
(This post was last modified: 28-01-2022, 07:04 PM by Mike Watterson.)
I test electrolytics with a current limited PSU. At battery voltage, rated voltage for cathode caps up to 30V or at 30V for HT types. Disconnect the non-chassis end. Note FM detector caps are +ve to 0V/Chassis and Germanium and some other old transistor sets use +ve battery to 0V
I've hardly ever had to replace HT electrolytics, except 1950s 120V parts on the 90V HT of Vidor battery valve sets (but not others) and the two colour plastic case ones on old transistor radios. The FM discrimator (often 4uF) next most common and then sometimes audio output valve cathode capacitor.
If the current doesn't drop to a fraction of a milliamp the leakage is too high. I might give them overnight.
If the voltage on volt meter drops to zero straight after disconnection then it's dried out.
If those pass, then I'll short it and test on a capacitor meter that shows ESR. The caps range on DVM isn't much use. Ordinary bridges can be misleading due to leakage or high ESR (drying out).
There are a few low noise audio pre-amp ICs and some low voltage headphone ICs, some dolby ICs and various IC stereo decoders. There are no other parts specifically for audio, except maybe some inductors and non-polarised Electrolytic caps in cross-over networks. I often replace non-polarised Electrolytic caps with metallised plastic foil types as they tend to dry out.
Obviously some amps or radios have audio transformers and can be burnt out (O/C) due to leaky anode to grid caps or a dead grid bias battery on really old sets.
I've hardly ever had to replace HT electrolytics, except 1950s 120V parts on the 90V HT of Vidor battery valve sets (but not others) and the two colour plastic case ones on old transistor radios. The FM discrimator (often 4uF) next most common and then sometimes audio output valve cathode capacitor.
If the current doesn't drop to a fraction of a milliamp the leakage is too high. I might give them overnight.
If the voltage on volt meter drops to zero straight after disconnection then it's dried out.
If those pass, then I'll short it and test on a capacitor meter that shows ESR. The caps range on DVM isn't much use. Ordinary bridges can be misleading due to leakage or high ESR (drying out).
There are a few low noise audio pre-amp ICs and some low voltage headphone ICs, some dolby ICs and various IC stereo decoders. There are no other parts specifically for audio, except maybe some inductors and non-polarised Electrolytic caps in cross-over networks. I often replace non-polarised Electrolytic caps with metallised plastic foil types as they tend to dry out.
Obviously some amps or radios have audio transformers and can be burnt out (O/C) due to leaky anode to grid caps or a dead grid bias battery on really old sets.
