Cambridge Audio A300 amplifier (hum and buzz problem)

[cwlau5450]

I got a CA A300 amplifier from my friend before. It has hum and buzz problem before I received it.
The symptom is that the speakers has hum sound consistency during power on. The speakers has buzz sound after power off. Just like the sound is discharging from some components and very annonying. And I quite worry this may harm my speaker
I have recorded the problem and post on here:


YouTube


For the amplifier board, I have replace:
- All E.cap (except few small value ceramic capacitors)
- All transistors
- All regulators/Op-amp
- Adjust the current by tunning the variable resistor
- Most diodes, (except some 1n4148, and one 3V3 zenor diodes which I have verified by Fluke multimeter is ok)


I have also replaced the followings to verify this issue but no avail.
- Transformer
- Apply contact cleaner on volume, balance, tre, and bass.
- Check all interconnection by multi-meter


I appreciate that if anyone here got experience on fixing this issue or provide any valuable hints for me to fix the amplifier.

Thanks!

cw

 

[Mario Pankov]

Buzz is usually a ground loop. Check your grounds. Hum is usually bad PS section, check the caps there. If there are bleeder resistors in the PS section see where they are - they may cause buzz in conjunction with the ESR of the caps. Do you have a scope?

 

[cwlau5450]

Hi Mario,

Thanks for your advices. I have attached the PS circuit. As you can see the circuit is pretty simple. In fact, i have replace all the components. The large caps are Panasonic FC series capacitor. It might not the best capacitors and it brand new. And yes, i have a very cheap scope, low frequency, X-channel functioning(damaged X-channel) to observe some output waveform. I can see some abnormal waveform from circuit and still don't know where the root problem is.


Thanks!

CW