I've been renovating a Quad 405 for a couple of days. Recapped it, installed a OPA604's, changed the D1 and D2 diodes and lowered sensitivity. Installed the boards and did some initial test, everything looked fine. DC offset sat at -0.8 mV and the other board at 1.6 mV. I let it sit idling for an hour yesterday evening. Plugged it in today and let it idle for a while, again DC offset looked fine on both channels. Moved the unit to test with a pair of speakers, but before that I thought I'd measure DC offset a final time. Plugged the multimeter to the left channel, set it at mV and turned the amplifier on. The DMM immediately went OL, and I heard a pop from the amplifier. Turned it off, removed the hood and smoke was coming out from it. Was sure I had managed to short circuit it with the DMM, but I had not it turns out. R22, D5, Tr4, Tr5 and Tr3 have all gone up in smoke. R35 looks cooked too, Any ideas what would cause this? I've checked my soldering more than once, no solder bridges, no cold solder joints. Everything is installed with the right polarity. I feel really bummed out about this to be honest. The board number is 12368 ISS 9. EDIT: I see now when inspecting the board that R35 has a cold solder joint, would that cause these components to go up in smoke?
I can't just match those transistor numbers to either the Quad manual I'm looking at or the Quad 'Evolution' series of updates.
My best guess is the triac has fired because of an offset and that has taken the output stage and drivers out. The triac may be shorted. I would remove that for fault finding.
Was the OP604 a genuine part? These as far as I know are hard to get in recent years.
A dry on R35 (0.18 ohm ?) I think would trigger the triac becaue the output looks as though it go hard against the negative rail.
Be aware that the Quad is very fussy on output and driver transistors and modern parts with high ft can cause stability issues.
My best guess is the triac has fired because of an offset and that has taken the output stage and drivers out. The triac may be shorted. I would remove that for fault finding.
Was the OP604 a genuine part? These as far as I know are hard to get in recent years.
A dry on R35 (0.18 ohm ?) I think would trigger the triac becaue the output looks as though it go hard against the negative rail.
Be aware that the Quad is very fussy on output and driver transistors and modern parts with high ft can cause stability issues.
Sorry about your mishap...
I will try some speculation... :
TR5 is part of the overload protection, and is driven by R35; but a cold solder joint on R35 would just disable the current sensing, and not cause TR5 to blow.
TR3+TR4 is the "class A" amplifier.
The relation between these two, would put TR3/TR4 "on the cause side" and TR5 as the "consequence".
Could it be that the fast OPA604 Op Amp went into oscillation and caused the class-A amp to overdrive the output, which activated the protection TR5 ?
(and eventually the class A amp blew...)
Have you added supply decoupling caps near the op amp? (pins 4 and 7, I think)
When you test the output DC, do you have the inputs connected to ground?
Maybe some noise or "undefined" disconnected input triggered the oscillation?
I will try some speculation... :
TR5 is part of the overload protection, and is driven by R35; but a cold solder joint on R35 would just disable the current sensing, and not cause TR5 to blow.
TR3+TR4 is the "class A" amplifier.
The relation between these two, would put TR3/TR4 "on the cause side" and TR5 as the "consequence".
Could it be that the fast OPA604 Op Amp went into oscillation and caused the class-A amp to overdrive the output, which activated the protection TR5 ?
(and eventually the class A amp blew...)
Have you added supply decoupling caps near the op amp? (pins 4 and 7, I think)
When you test the output DC, do you have the inputs connected to ground?
Maybe some noise or "undefined" disconnected input triggered the oscillation?