Newbie requesting help with a dead ProCo Vintage Rat

[Dave H]

Normal pedal architecture has the ground path completed when a plug is inserted into the input jack. You do have a plug there for testing?

Yes, good point!

[scrumper]

It’s most likely because pin 4 is open circuit from ground/battery negative. With the battery disconnected check for continuity between battery clip -ve and pin 4. If it isn’t a very low resistance find out where the open circuit is and correct it then do as Juan said and check the DC voltages on the op amp pins wrt battery -ve. They should read 4.5V on pins 2,3 and 6. 9V on pin 7 and 0V on pin 4.

This was instructive - there is an open circuit there. Didn't get far figuring out exactly where last night - maybe one of the capacitors just up from pin 4 is fried but not sure yet.

This is great though - I have something to go on now. Thank

The Dude: I did have a plug in the input jack, but worth pointing out!

[The Dude]

Maybe this will help- schematic with notes/changes for the various Rat pedals.

https://music-electronics-forum.com/...6&d=1387061571

[scrumper]

Thanks The Dude - I found that very helpful indeed. That covers my circuit.

Managed to get distorted but quite crap sounding audio out of it after connecting pin 4 direct to a known-good ground point. Investigating now where the open is between 4 and the point its connected to in the actual circuit, which is the DPDT switch. I have a feeling the switch is bad so the chip might not be the only thing that isn't grounded.

I'm tempted at this point to make a clone of an LM308 rat and use my existing pots, jacks, and housing. Whatever it is that's broken will be fiddly.

Thanks all for the help.