Accidentally put 18V into a Mofeta - what did I fry?

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Aidan M

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I have an MXR iso-brick mini power supply mounted under my test pedalboard, usually outputting 9V DC to the pedals up top. However, there is a recessed dip switch on it that changes one of the outputs to 18V. It looks well designed and unlikely to be accidentally 'knocked', but of course... sh*t happens!

So, my PedalPCB Mofeta (Sunn Model T) was one of the pedals connected to the 18V output at the time. It had been working beautifully (I especially love it on bass), but now it no longer passes any sound when engaged, although the LED still lights up. I assume I've fried something, but nothing appears visibly 'blackened' inside. I know there is a charge pump in the circuit (TC1044SPA) that normally increases the 9V up to something higher... could that be the damaged part? Is there a simple way to check before ordering a replacement chip?

Also note that I had added a 100nF capacitor across pins 8 and 3 (ground) of the charge pump, per the suggestion of the original designer of this circuit (the Model feT) - John from Electronic Audio Experiments.
 
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Putting +18VDC would at first, fry the TC1044S. Next, C101, C102, C103, C24, D101, D102 and all transistor / FET components would have to be checked. The charge pump may have generated 31VDC for a few hundred milliseconds, which could wipe out 25V caps, The transistors and FETS might have handled it, but an over spike during failure could take them out - so check for shorts between contacts.

I took out a Morer Flanger during a recording session a month ago by accidentally using an 18V supply. I know how it feels :(
 
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Thanks both. I checked each diode in the power section (D100, D101, D102), and they all seem to be ok (~0.2V voltage drop). I'm not sure what to check on the capacitors with a DMM, but I can see that those in the power section (C101, C102, C103, C24) are only rated for 25V.

However, when powered on, I measured slightly under +9V on pin 8 and -9V on pin 5 of the charge pump (TC1044S), and ~+17V at the output of D102. I interpret this to mean that the issue is probably not in the power section, right?

I'm still fairly new to all this (~10 builds in), so I’m just going off Google suggestions for testing transistors with DMM… Fortunately the JFETs (2N5457s) in Q1 - Q4 were socketed and all have a ~0.68V voltage drop across the gate-source junction, and the BJTs (2N5088s in Q5 & Q6) have similar across the base-emitter. Is there some other measurement I ought to make to confirm whether the transistors are functional?

Also, is there some general strategy for how I should proceed in hunting down the culprit from here, or do I really just need to test each component in turn until I find it? And how do I test capacitors? Thanks!
 
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Is your op-amp IC socketed? That's really easy to try. You should be able to measure the resistance across the caps in-circuit; if any of them measure very low resistance they might be cooked.

Do you have an audio probe[link] of any kind? If the previous advice doesn't help narrow it down, you can step through the schematic along the whole path that the audio travels with a probe, listening for the point where the audio goes away/drops out. e.g. if it is a transistor that is blown or a cap that's bad in a non-short-circuited way, that might help you find it.
 
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Aidan M said:
I measured slightly under +9V on pin 8 and -9V on pin 5 of the charge pump (TC1044S), and ~+17V at the output of D102. I interpret this to mean that the issue is probably not in the power section, right?
Click to expand...

Aidan M said:
do I really just need to test each component in turn until I find it? And how do I test capacitors? Thanks!
Click to expand...

Reading back on your original post, I wasn't clear on exactly what it is or is not doing. Is it lights and no sound when the effect is engaged?

18v into that charge pump circuit would have created a burst of 32+ volts (it may have surged even higher). A TL072 is rated at 40V but can be quickly checked with a continuity beep test. Measure each pin to each pin and you are okay if there are no beeps (near zero ohms).

2N5457 or substitutes are 25V max and could be blown. 2N5088 are rated at 30V and some substitutes are 25v. Again, continuity beep test between all the pins should tell you if it is bad (beep) or good.

Metal Film caps (boxes, not cans) are typically good for 50v+ and should be good. 1/4w resistors on the 'source' side of a bad FET should be checked (R4 is an example). Otherwise, the 1/4w resistors elsewhere are unlikely to have gone bad.

+17V at D102 suggests you did not blow up the charge pump or the power section components. When a cap goes over voltage, it goes short and often internally burns to an open, which makes an ohms or continuity test ineffective. You have to do a capacitance test and the measurement will have no display for any range of capacitance tested. If C103 was a 25v value, I would replace it
 
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Thanks @andrewsrea ! Yep, the LED comes on but there is no sound when engaged.

I tested IC1 (TL072 or whatever it actually is - I can't make out the faded print on it and it was from a kit provided by Musikding), and there are no shorted pairs of pins. I also checked the resistors on the FET sources and they were fine except for R10 on Q4, which is supposed to be 3K3 Ohms but measured at 2K... not sure whether that's damage-related or just poor tolerance, but I replaced it.

I'll try replacing C103 next as you suggested, once I get my next Mouser order in.

@z2amiller - thanks for the audio probe suggestion - I may give it a try also.
 
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