Power Rail Short

[drummer4gc]

Hi all,

I've got an older EHX Graphic Fuzz in for repair - schematic found here: https://www.scribd.com/document/329409984/aagraphicfuzz-pdf#

The V+ rail is getting pulled down to like 2.5v (12v expected), so there must be something shorted down the line. I'm wondering how you all go about hunting down a guilty component responsible for pulling down a power rail in a big circuit like this.

I've checked that C42 in the power supply isn't the culprit, and the 3094 is the only socketed IC so I removed that, but that's not it either. No obvious signs of burnt up stuff, except evidence that the 120R resistors in the power circuit have not been enjoying the extra stress related to this issue.

Any nice trick for hunting this down? Would love to not have to pull every IC and transistor on the board if possible...

Thanks!
Matt

[Rob Strand]

What input supply are you using.   To me it looks like it has a12V reg for one rail and a 13V zener for another rail.  That means you need more than 25V DC going in maybe 30V or so.
Actually, when I zoomed i I can see it says 40V in.  It could be a 30V unregulated supply which produces 40V.

[theehman]

Is this an early reissue or an original?

[drummer4gc]

It's an original, I think. Uses a 40 vdc power supply.

I actually fixed this issue - plugged it in and felt around the opamps real quick for signs of warming. Felt one start to heat so I replaced it and voila! Good +\-12 where it should be.

The resistors and voltage regulator are working but are browning from heat due to the issue, so I'm planning to replace them anyway - otherwise I would've been a little more cautious around plugging 40v into a known short. Any other good tricks for tracking down shorts more safely?

And since Ron chimed in and I believe has good knowledge of these units, maybe you could help me track down another issue? The fuzz circuit isn't working - no signal comes through when the switch is turned to on. Switch is good, I get fuzz signal out of U2B, but I don't understand enough about OTAs to really know how to check if the CA3094 is acting properly. I get that it's basically acting as a sort of compressor for the fuzz circuit, but I don't know how to verify it's proper functioning. The schematic is also drawn in a non-linear way that is throwing me, so I can understand that the FETs are acting as switched when voltage is applied to turn the effect on and off, but I'm having a hard time putting all the pieces together to really get how it all works, which makes it a tough debug.

Without fuzz, it works, but with a decent volume drop compared to bypass when volume is set at unity - I suppose that could be incorrect as well, and I'm hoping to track that down once I wrap my head around the circuit.

Thank you!

[drummer4gc]

Got things working - the CA3094 was bad, and fuzz is back now that it's been replaced.

Wanted to ask once more though - can anyone help me know what the available gain should look like with this circuit? Fuzz off, I don't get unity until the volume slider is near 100% (with EQ sliders flat). I've done some audio tracing and don't observe any unexpected signal drop, but I'm wondering if I should be expecting gain somewhere where I'm not getting it.

Thank you!