03-03-2019, 07:40 PM
Yes indeed; see post #13.
However, that doesn't change the point that 30 seconds with a dual current-limited bench power supply would prove that the rest of the amp was OK and that it was worth pursuing the repair. If that test showed that there was an additional fault elsewhere, then I'd personally turn it away - those output MOSFETs are long-obsolete. I've done more than my fair share of these amps, and I'm in no rush to go back!
Obviously, some people come from a different background to me, and might not have had the luxury of even relatively basic test equipment when they were working in the repair trade, but given the price of gear today - both new and s/h - there's really no excuse to not have the tools to make the job easier (and more profitable). Unfortunately, there are still too many people who rely on little more than an AVO 8 (because "it was all anyone needed in my day"). When I worked in the trade I argued daily with my colleague about using the oscilloscope - which he would only countenance after all other avenues had been exhausted (and hours wasted).
With any luck the new bridge will arrive tomorrow - but if not, perhaps a temporary bridge can be installed for testing purposes...
For what it's worth, we didn't see this switch problem all that often because we rarely used the power switches - in most installations, operational power was switched on and off by the room breaker - and in that case, the usual problem with the switches was that they couldn't be turned off. The same switches were used in countless other items, and were an absolute menace. We suspected that the small amount of heat from the neon/resistor was enough to soften the plastic over the years, and as the springs inside were quite strong, deformation of the case seems quite likely. Non-illuminated variants were practically 100% reliable by comparison.
Anyway, here's a picture showing what can happen when these things go wrong!
However, that doesn't change the point that 30 seconds with a dual current-limited bench power supply would prove that the rest of the amp was OK and that it was worth pursuing the repair. If that test showed that there was an additional fault elsewhere, then I'd personally turn it away - those output MOSFETs are long-obsolete. I've done more than my fair share of these amps, and I'm in no rush to go back!
Obviously, some people come from a different background to me, and might not have had the luxury of even relatively basic test equipment when they were working in the repair trade, but given the price of gear today - both new and s/h - there's really no excuse to not have the tools to make the job easier (and more profitable). Unfortunately, there are still too many people who rely on little more than an AVO 8 (because "it was all anyone needed in my day"). When I worked in the trade I argued daily with my colleague about using the oscilloscope - which he would only countenance after all other avenues had been exhausted (and hours wasted).
With any luck the new bridge will arrive tomorrow - but if not, perhaps a temporary bridge can be installed for testing purposes...
For what it's worth, we didn't see this switch problem all that often because we rarely used the power switches - in most installations, operational power was switched on and off by the room breaker - and in that case, the usual problem with the switches was that they couldn't be turned off. The same switches were used in countless other items, and were an absolute menace. We suspected that the small amount of heat from the neon/resistor was enough to soften the plastic over the years, and as the springs inside were quite strong, deformation of the case seems quite likely. Non-illuminated variants were practically 100% reliable by comparison.
Anyway, here's a picture showing what can happen when these things go wrong!
