Many thanks for the great advice. I'm off to the shops tomorrow to buy a 5w bulb. Both the diodes I had the arcing problem with were 1n4148.

Thanks once again.
Geoff.

Hi All,

Many thanks for the great advice. Althouh I do use the old light bulb techneque, I guess I've been unlucky a few times or not used a small enough wattage bulb. Do you think I should go down to a 25w bulb for these DC coupled amps?
Also on the subject of silicon devices checking O/K out of circuit and then failing when fitted, I've had two occasions recently when the audio sounds like electric arcing going on and it has been a diode breaking down under load. At least replacing the affending diode has solved the problem.

Regards
Geoff

Hi everyone,

I've worked on a number of valve and solid sate amplifiers over the years but never had much success with fault finding on DC coupled power output stages. For example, on occasions I've found the power output transistors blow, replaced them and checked all other transistor/diode junctions and all looks O/K. Switched on and the expensive power transistors have blow again.
I was just wondering if anyone can suggest a clever (and safe) way of fault finding on these DC coupled circuits to avoid having to keep replace expensive parts.
Regards
Geoff