[Radio Wrangler]

In common with most solid-state audio power amplifiers, these designs consist of several stages all DC coupled together. Each stage is only correctly biased if the whole lot is working correctly.

This means that if one part fails, there is a high risk of a chain-of-dominoes sequence taking out a number of active devices. This may sound bad enough, but it has a nasty consequence in that the chain of dominoes can easily form a ring. There is now no way to fix it if the method is to replace one part at a time and then apply power to see if that one fixed it. Each part you fit is immediately killed by its neighbours when power is applied and the repairer thinks 'that new part can't be bad, I've just fitted it'.

Transistor power amps have acquired the reputation of being little devils to fix and have driven some people to refuse to touch anything other than valves ;-)

The way to fix them is to take the long way round and not be tempted by quick fix shortcuts. With power off the amp you need to find ALL the bad parts and replace them ALL before applying power. In a commercial environment where time costs money, the most efficient fix might be to pull all the semiconductors, measure all the resistors and fit a known good full set of semiconductors - then maybe check the removed set to see if any of the higher value parts can be recycled.

There are some quick fixes which work, but you only find out about them by learning from seeing which parts pop most often.

I've designed a few audio power amps from scratch. Not copied a mix of circuits from here and there or done some mods, I mean starting with the maths and trying novel architecture. When firing up a prototype you lack the comfort-blanket of knowing it once worked! Techniques involve lighting up one stage at a time with temporary feedback arranged to control the bias before the output stages are put on, and using current-limited bench power supplies. This would be tediously slow for doing a single repair, but on a prototype it allows checking the performance of each stage against the design intent.

Douglas Self's website is quite illuminating regarding solid state amps, it might go deeper than you want, but it does explain what's going on.

David