redpackman
Active Member
Still problems.
Last edited:
Have checked caps C77 and C78, in fact C77 was totally missing. I put in a 470uf 6.3v cap in the C77 position. Would its absence be enough to have caused this? It's in the schematic, but it wasn't there in my machine when I got it.
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So why does the collector of Q15 have -29 v instead of -20 v?
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Would Q15 even be in play for the thrystor problem blowing???
I agree, and the DBT does nothing to protect the thyristor from the initial current dump from the power supply filter cap. Once triggered it stays on until power is removed, DBT or no DBT.The way I would troubleshoot this is to run it with the triac removed and use a scope to see what triggers the triac, then trace the source of that signal. Running it on a DBT prevents the fuse from blowing so the triac has no protection and when it crowbars the supply, it conducts current for longer than it normally would and is destroyed.
OK, put in a new thrystor. Using a dim bulb tester with 60 watt bulb. Have checked caps C77 and C78, in fact C77 was totally missing. I put in a 470uf 6.3v cap in the C77 position. Would its absence be enough to have caused this? It's in the schematic, but it wasn't there in my machine when I got it.
Turned the amp on... bulb glowed slightly and died down to nothing. OK that's good.
Testing voltages gingerly. Switching machine on and off for ten seconds at a time to test. No outrageous voltages around thrystor. Have not checked all voltages but noted that the collector of Q15 is supposed to have -20v if I'm reading the schematic right. it's got -29v ! I've pulled and checked R100, R101 and R123. All are supposed to be 2 watt resistors 62 ohms each. They all check out (out of circuit) at around 62 ohms. So why does the collector of Q15 have -29 v instead of -20 v?
Nothing's blown yet, but I'm not leaving it on to cook like the last one did. I think I remember checking the voltage before R 100, R101 and R123 and I believe it's -40v as it should be. Would Q15 even be in play for the thrystor problem blowing???
Oh, also checked the one fuse in the machine. It's supposed to be 3A. It was 3A. Also checked R110 and R111 (both 2 watt .5 ohms). Both checked out, out of circuit.