Cambridge audio a500 blown resistors

[Finlay]

I purchased this amp on ebay and after a few days of use I smelt burning plastic and the left channel died. After opening it up I saw that 2 resistors had blown R216 and R214 (I also replaced the surrounding resistors incase damaged). I powered it up and it worked fine it was stable untill I connected it to a my speakers (old b and W 686s), at which point both the same resistors burnt up in a second. Would replacing the sap15s help as I see that on the board that they are the tings connected to these resistors? And advice would be great thanks

 

[tomchr]

A schematic would be helpful.

 

[PRR]

A schematic would be helpful.

Easily found:
Cambridge Audio A500 Stereo Integrated Amplifier Manual | HiFi Engine

I don't see any way to cook these resistors unless the power transistor is blown?

 

[tomchr]

Yeah. If you get enough base current to kill R216, the SAP15P is very likely toast. I'd probably swap Q208 as well.

Tom

 

[Finlay]

What is the power transistor? And thanks this is very helpful

 

[tomchr]

What is the power transistor?

U202 (SAP15P).

Tom

 

[Finlay]

Thanks Tom,
Is it worth upgrading the power transistors really, I've read some stuff about getting recent one that don't burn out as easy, or is it just a better idea to improve the cooling of the chips?

 

[tomchr]

The reason I suggested that you swap out the SAP15P is that I bet that device has died and is now killing those two resistors. I bet Q208 has died as well.

I would not arbitrarily upgrade or change anything until you have the amp working.

Tom

 

[Audio Service]

Check also RV201 the bias trimmer, when the outputs fail more often than not the trimmer will also fail with no visual indication.
If you could attach an aerial photo of the Amplifier circuit board that would be great.

 

[nigelwright7557]

I would suspect the output transistor those resistors are connected to.
Simply replacing components doesnt always fix the underlying problem.
As said previously a fault in the bias circuit will blow the outputs.