Back in Aug 2022 I built a TS808 for a good friend. Got the PCB from PedalPCB...
https://docs.pedalpcb.com/project/LGSM.pdf (https://docs.pedalpcb.com/project/LGSM.pdf)
Not too long ago he told me that the pedal stopped working, no output. When I finally got it back from him there was something rattling around inside the enclosure. Turns out that one of the electrolytic caps in the power section (C11 47u 25V) had blown up and the rattling was the cap's case which had blown off leaving a little mess in its wake.
I cleaned things up a bit and replaced the cap with a 47u 35V. But still no output. With an audio probe I found strong output from Q1 but almost no output from pin1 of the 4558. Replaced the 4558 and now the pedal works.
Question is... WTF happened? What blew the cap and fried the opamp? The pedal worked great for about two years before this happened. Is it just going to happen again? Nothing else looks amiss to me. I asked my friend if he might have accidentally plugged a wrong power supply into it, but I haven't heard back from him yet.
I recorded two sets of voltages for each component. The first is after replacing the cap, the second is after replacing the opamp.
PSU 9.68v 200mA
IC1 JRC4558
pin1 8.91 4.73
pin2 7.82 4.78
pin3 7.76 4.73
pin4 0 0
pin5 8.73 4.74
pin6 8.94 4.75
pin7 8.98 4.75
pin8 9.60 9.52
Q1 2N3904
E 4.50 3.11
B 4.96 3.63
C 9.60 9.52
Q2
E 4.40 3.03
B 4.86 3.56
C 9.60 9.52
Vref (R16/R17) 6.51 4.74
Anyone have any thoughts or ideas about what might have happened? I would like to tell him that it won't happen again. But I don't think I can say that right now. Thx!
Sounds like over voltage. Just enough to fry the cap and opamp but not fry the transistors.
Quote from: moosapotamus on Yesterday at 12:18:20 PMPSU 9.68v
Vref (R16/R17) 6.51
No bueno for 10k/10k unloaded divider..
Right, that 6.51v Vref was when the fried opamp was still installed. I am still puzzled by how a fried opamp woould cause that. But 4.74v after I replaced it is mucho bueno, no?
Quote from: GGBB on Yesterday at 01:30:45 PMSounds like over voltage. Just enough to fry the cap and opamp but not fry the transistors.
My guess, too. I'm anxiously waiting for my friend to explain himself. :icon_cool:
It must have been a decent amount of voltage, no?
I mean, a 25V cap will survive 25V for a fair period, so it was probably more than that. A typical op-amp can handle 30V (+/-15V) indefinitely, and the maximum supply is more like 36V, so it was probably more than *that* too. This is not a case of "I tried it on 18V and...". The 1N5187 protection diode should deal with reverse voltage, so it's not (shouldn't be, at least) a case of a lower voltage but backwards.
Feeding 40V or so into a pedal is probably never going to end well. The question is "How?!?" :icon_eek:
40 Volts won't kill a '4558 real soon. And I can't picture a loose 40V supply on stage.
2SC1815 is a 50V part, and in this circuit it only feels ~~half the supply, cool to 100V.
Lightning hit would be 5 or 6 fried amps pedals appliances.
If it was only the cap then the cap could have been installed backwards, or, the cap polarity was mislabeled.
To fry both the cap and the opamp you have to think overvoltage.
A lightning strike would do it.
1N5817 did not damaged?