Q about "wrong" voltages applied to electros

[Mark Hammer]

I'm repairing a buddy's 5-knob Philosopher's Tone compressor.  The pots are mounted directly to the main board and various jacks and stompswitch are connected to the main board via Molex-like headers.  He had tried to do something himself, and gotten the power jack and output jack connectors confused.  He applied power to the and apparently let the magic blue smoke out.

My working assumption is that the +18V supply fried the 2u2/50V DC-blocking cap just ahead of the Volume pot (if not the pot itself), and that simply replacing the cap ought to restore the pedal to working order.  Since double-side boards can be a bit of a pain to desolder from, I thought I'd ask here first.  I'll simply add that all chips read good on the V+ pins.

So, would applying a substantial positive voltage to the negative side of an electro damage it, or should I be looking elsewhere for the problem?

[erio tedesco]

Mark, take a look at the "General Technical Information (mainly pg 5) in : https://www.tdk-electronics.tdk.com/download/530704/6ad5ed9e1ff4f727c328cb92da2adf2b/pdf-generaltechnicalinformation.pdf

[Mark Hammer]

Thanks for that.  My reading of it suggests that pummeling the cathode with a reverse voltage and no current-limiting between cap and supply would likely foster some damage.    I replaced the cap but still no output.  Pulled all the dual op-amps, tried them out in another pedal, and they all worked fine.  That still leaves the 741, which I'll test out later this morning.

[PRR]

IIRC (you don't give a schematic) the '741 is not essential to passing signal.

Do you need a plan for a signal tracer?

[Mark Hammer]

Naw. I'm OK there, thanks, and set up for signal tracing.  I just have to hunker down and do more than the superficial troubleshooting.

[antonis]

Mark, I know you'll not get it wrong  , so I feel free to say it..

Is there any chance for Vref divider cap to be shorted..??

[Mark Hammer]

Couldn't tell, you but after I put on my new illuminated magnifier headset to take a look, I suddenly noticed that R10 was fried to a crisp.  Didn't initially see it, as it was hidden in the shadows between the three 2-pin Molex connectors at the left and 9-pin connector to the right, just above where it says TP3.  Now to replace it and see if that does the trick.

Thanks for prompting the second look. 

[Mark Hammer]

Whoops.  Under the goop from the exploded resistor, once I removed it and cleaned up the area, I saw it was R18, which is the 10 ohm resistor taking the 18V+ feed.

[antonis]

Worked as reverse polarity protection very slow blow fuse..