This Cambridge Audio amplifier required repairing as one of the speaker outputs wasn’t working.
These old amps use unusual darlington transistors called SAP15 in the final amplifier power stage. They come in both NPN and PNP variants.
I repair hifi and electronics near Stroud in Gloucestershire. Please contact me for a repair quote.
There’s nothing unusual in darlington transistors but these have additional resistors and diodes in the same package. They went obsolete years ago!
The SAP15 could be replicated with a couple of normal power transistors but the PNP one that had failed has five series connected diodes in the package.
The additional diodes would be tricky to replicate as they’d need to be attached to the heatsink to prevent thermal runaway.
To save the faff I took a chance and got one from eBay. The one I bought had tell tale signs it had been pulled out of an old amplifier which suggested it’s likely to be a genuine part. That’s good as it’s not a Chinese clone.
The transistor checked out OK with a multimeter.
When a power transistor fails in an amplifier it can take out a number of other components with it so I removed and tested all the small driver transistors on the faulty channel.
There were also a few burnt out resistors and some copper track on the PCB that needed repairing. This amplifier is somewhat lacking in the auto switch off fault protection circuitry so a fault can cause damage.
After replacing the SAP15 transistor I used a variac to bring the mains supply up slowly to establish it all worked properly.
Finally the output bias was checked and adjusted.
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