[Radio Wrangler]

Have you kept the original power transistors?

This is very important because it is not easy to find replacements for them and it is most probable as Nick has said that they were OK. There are two problems. Firstly though these transistors are still being made, the modern ones have been 'improved' a lot and are often unstable in the quad circuit. Secondly, these particular types have been very extensively faked and unless you really know where they came from you will most probably get something which has no resemblance to what it purports to be.

Repairing a Quad 303 with a hope of a good outcome requires a serious amount of knowledge. I agree with Paul that you need someone with the right experience, and he'll want the original transistors.

Have a search through this forum and you'll see plenty of material on Quad 303 power transistors.

The 303 design was very good in its time, but it relied on some characteristics of the output transistors to keep it stable, and it's now hard to find transistors slow enough to work safely in this circuit. Back in the 1980s I used to use this as an example when I gave lectures on design oversights!

David