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Quad 303 power amp problems

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New Member
We have a Quad 303 power amp. its been in the loft a few years (and possibly misused by interfering tenants). at present it won't work.

problem is severe mains hum and very little sound. the unit consists of a heaty metal case containg a very large transformer a rectifier 4 1000uF 100V condensers, 4 beafy power transistors, 2 preamp circuits and one unidentified circuit (see picture) my guess is its a power regulator for the preamps. This circuit has some burning on it.

any ideas of the possible problem ? we assumed the smoothing condensers were shot but they look ok.



The capacitors might loo all right but have you actually tested them?

It still sounds like a case of capacitor plague to me.


New Member
thats what I thought but the circuit in the pic has burning from one of the resistors so i was wondering if it was a preamp power issue that was ruining the sound even before it got to the high power stage. I suppose my dad is loath to let me change them as the modern ones will not be anything like the old ones and we were concerned about the burnt board

Nigel Goodwin

Super Moderator
Most Helpful Member
First off, there's no preamps in a 303 - it's just a power amp.

The three boards are two power amplifier boards (with the output transistors mounted off board on heatsinks), and a regulated power supply board, again with the power transistor mounted off board on a heatsink.

If you google for Quad 303 there is lot's of information on the net, including full circuit diagrams and upgrade modifications.
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