Back in Aug 2022 I built a TS808 for a good friend. Got the PCB from PedalPCB...
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 April 21, 2025, 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 April 21, 2025, 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. 

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?!?"

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?

Back around 2000, there was a rash of capacitors that self-destructed because they were made from a stolen formula that had a mistake in it that caused the early demise of a lot of laptops.  Maybe the capacitor was from one of these production runs.

Quote from: amptramp on April 22, 2025, 06:58:41 AMBack around 2000, there was a rash of capacitors that self-destructed because they were made from a stolen formula that had a mistake in it that caused the early demise of a lot of laptops.  Maybe the capacitor was from one of these production runs.

That's possible, but it doesn't explain the dead op-amp.

Lightning would definitely be enough, but like Paul said, you might expect a trail of carnage, or at least enough other stuff fried to know.

Or, you know, you heard rain and thunder?! There *are* clues with Lightning!

The cap was a newer Nichicon with a gold colored case. It was not installed backwards. 

I'm assuming the 5817 is OK. Not really sure how to evaluate that.

I considered that a short somewhere might have done it. But again the pedal was working fine for over two years. Seems like it should have happened a lot sooner if it was a short.

None of his other pedals were damaged. But I like the lightening strike idea. Seems like whatever did it must have been something relatively rare or out of the ordinary. My buddy hasn't gotten back to me with any ideas about what might have happened. I need to hit him up again.

Would an AC instead of DC power supply do it?

Quote from: JustinFun on April 22, 2025, 04:32:41 PMWould an AC instead of DC power supply do it?

Not with a healthy 1N5817 ..
(just an unregulated semi-rectified supply..)

I may never find out exactly what happened. But based on the damage, could some additional circuitry be devised that would protect against the same thing happening again?

Quote from: moosapotamus on April 22, 2025, 04:51:48 PMI may never find out exactly what happened. But based on the damage, could some additional circuitry be devised that would protect against the same thing happening again?

Without knowing the cause it's tricky.  You can build higher and thicker walls, which give the appearance of protection, but if the problem is someone dropping bombs from above it does nothing.

It's probably going overboard to protect against a direct lightning strike.  I've seen communications equipment where is looks like someone has got in the box with a welder.   To protect against that would need something bigger than the main circuit.

A resistor in series with the input supply is simple and could fend of some abuse.  You could also use zener or TVS after the resistor (say 15V) for more protection.   You could end up with the resistor frying. No major parts dead but at the end of the day the device stops working.

Quote from: ElectricDruid on April 21, 2025, 07:11:12 PMIt 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?!?"

I had somebody plug in a 48V AC plug pack to a 6SN7GTB pedal I built (which I had supplied a 18v plug pack for, had stamped it into the aluminium enclosure AND written on it with permanent marker)- just before cooking the 1N5819 it managed to nuke the 35v electros on board as well as the 7806 regulator.

Every time you make something idiot proof, they seemingly design a better idiot.