Carlsbro Showman parametric/1 oscillating amp , 1990

Removed the triac speaker prot cct just in case a reaction with that, same result. Checked ESR of all cap types. Upto +/-40V rails , below oscillation, all DC agree near enough on both channels all TO3,TO220 and TO92, zeners. Go up to about +/-45V and the 6V, 5W bulbs start glowing that I added into the supply rails , alternately swapping dim and bright pairs very odd. Next I'll disconnect 1 of each complementary pairing of MJ15024 and see what happens. Any other suggestions? Next stage is sketching out the cct

Are you posting from a cellphone? Do you have an internet service provider charging you by the byte?

On 6/16/2010 1:13 PM AZ Nomad spake thus:

Nah, he just uses this public forum to post badly-formatted, cryptic notes to himself for future reference, apparently.

Still oscillatated with one pair and also with the other pair. Checking over the board again there was no continuity between line level i/p ground and the pa ground. An innocuous 1/4W resistor the same body colour as all the others but overlooked at 2.2 ohm bands was o/c linking the grounds. No discolouration except I managed to originally see the third band as yellow , not gold. No overheating on the adjascent pcb , so presumably a fusible resistor. In hindsight the slow "hunting" effect of the osc giving alternate loading to main + and - rails should have suggested a ground or zero problem.

The "good channel" also had an o/c 2.2R but presumably because full o/c no oscillation there. The failed one in the oscillating channel had 3.7K across the ends so presumably not fusible resistors. Must have been a very high current surge to knock out normal MO resistors without any sign of overheating.