Cambridge Audio A500 – Not Switching On

Cambridge Audio A500 Amplifier

An old Cambridge Audio A500 amplifier came in for repair that wouldn’t switch on. The fault was tracked down to the primary winding on the mains transformer having gone open circuit.

Cambridge Audio A500 Amplifier

Contact me at if you need the transformer replacing in your Cambridge Audio A500.

Amplifiers often have circuit diagrams on the Internet but I’m yet to find one that states any useful information about the mains transformer and this was no exception. Being a very old amplifier Cambridge couldn’t give me the information so some working out was required to find a suitable replacement.

The circuit diagram shows the main amplifier DC voltage as +/-45v. That’s NOT the voltage from transformers secondary winding, which will be lower. To obtain the required secondary voltage there’s a simple equation you apply to the DC voltage to get the AC voltage before it’s rectified and smoothed.

Cambridge Audio A500 amplifier PCB

With this amplifier you need the biggest correct voltage transformer that will fit into the space for it which is quite narrow. The photo below shows the old and new transformer.

Cambridge Audio A500 mains transformer

Before buying a replacement transformer the amplifier power transistors were checked as they’re unusual (SAP15N and SAP15P) and obsolete.

They’re darlington devices with additional resistors and diodes in a five pin package. They tested OK on a multimeter so a new transformer was fitted that brought the amplifier back to life.

YouTube video of the repair is below.

If you have a Cambridge Audio amplifier or other electronics equipment you need repaired please contact me for a quotation.

Contact me at Please include model numbers and any photos of obviously broken parts.