Cambridge Audio A500 Woes

[Chrisr3521]

Hi,

Problems, problems with this amp, firstly the Sap outputs were blown, which have been replaced along with the usual resistors, trimmers and associated caps, also took the opportunity to recap the amp whilst it was apart with decent caps (mks4 for input, nichicons pw and Panasonic fc for the rest).

So switched on with dim bulb tester, adjusted bias, ok, checked d.c offset and its ~500mv on one channel and ~300mv on the other..... problems, so off I went checking transistors on my peak component tester, everyone checked ok, put back together and smoke this time, r263 burning up whilst on the bulb tester, and pin 'C' on all but one of the saps showing rail voltage, again........ problems, pulled one of the Saps but checked ok on the Peak meter, is this a reliable test? Anyone have an idea of possible faults?

Thanks

 

[Ian Finch]

R263? That's part of the RC compensation in the feedback network of the R channel and would only burn if there were was considerable HF oscillation. I'd expect unusually high temp in the output devices too if that was the fault.

If you have no other way of checking oscillation, a cheap AM radio placed near the amp should emit noise when you power up and confirm something bad is coming from the amp.

 

[JonSnell Electronic]

If R263 has burnt up, take a look at the current path. Straight into the base of the inverting transistor and would only do that if the capacitor was short circuit, (C221 12pF). Re check your changes, replace the MPSA type transistors and check the diodes, then replace all output components.