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. We established the fault was the primary winding on the mains transformer having gone open circuit.

Cambridge Audio A500 Amplifier

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!

The circuit diagram shows the main amplifier DC voltage as +/-45v so working backwards the transformer needed a +/- 32v secondary winding*. Each power rail has a 4A fuse so that’s a clue to the transformer power requirements.

Cambridge Audio A500 amplifier PCB

* The DC voltage after full wave rectification and smoothing is roughly the AC voltage x 1.414 so working backwards from the 45v (required voltage) / 1.414 = 31.82vac (transformer secondary voltage).

The only transformer that I could find that fitted into the space with a suitable output was a 160VA one with two 30v secondary windings. In the photo, the new one is on the left, the original on the right.

Cambridge Audio A500 mains transformer

Before buying a replacement transformer I checked out the amplifier power transistors were OK as they’re unusual 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 obtained and fitted that brought the amplifier back to life.

YouTube video of the repair below

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