Fixing an Audio Amplifier Used Capacitors

My old Technics audio amplifier (nowdays you’ll use these only if i) you’re an audio aficcionado or ii) you got one by inheritance) started to produce noisy outputs and the trend was to become worse. After some thinking and searching I decided to have a look inside it. Hardly I could do anything on this complex board circuits.

Fortunately there was a capacitor slightly bumped and instead of just replacing it I decided to remove and get some numbers from it, for instance, do a rough characterization to answer “Does it still have 4700uF as specified?”.

Looking at the picture it seems the capacitor is not working as expected. Using a logaritmic plot we can extract the RC parameter and know that the resistance was 128 Ohms, we get an estimate of the capacitance value.

So, it’s clearly damaged since this cap specification is 4700uF and its giving around 1000uF which is way off. I decided to do the same test using a new capacitor brandless. Results follow:

The curves are much more similar and the capacitance value is 36% off the expected value (2200uF) so this capacitor looks OK.

To conclude, an interesting thing on this power amplifier, the PA modules when unmounted it’s possible to see boundwires (as we would see in integrated circuits)!