Strange readings on Vintage Rat!!!

[gutsofgold]

Need help debugging, no output when effect is engaged. Audio probed my way to the OPAMP and got no output there. Tried a different chip and still same issue.

Pin   V
1     8.6
2     4.33
3     3.88
4     0.00
5     0.00
6     4.33
7     8.7
8     8.6

Pins 1 and 8 are tied together via a 30pF ceramic cap. I tried a brand new cap in that spot as well and it did nothing.
According to GGG I should be getting 8.4v on PIN 1 and 4.6v on PIN 8. What's going on here! PIN 8 clearly has an issue, Any ideas?

[aron]

If you listen to pin 2 and then pin 6, you get no sound?

[gutsofgold]

Exactly. Unaffected signal heard at PIN 2, nothing at PIN 6.

[aron]

What kind of op amp are you using? All the voltages look ok except for pin 8. Do you have a socket there? If so, measure the voltages of the pins without the IC in there to see if you have a problem on pin 8.

[Projectile]

Exactly. Unaffected signal heard at PIN 2, nothing at PIN 6.

Wow, that's really strange. I can't even make any sense of that. PIN6 is the opamp's OUTPUT and PIN2 is the opamp's INVERTING INPUT. The signal from pin6 should be feeding pin2 through the feedback loop, so there is should be no way you could be hearing a signal on pin2 if there is no signal at pin6. Are you sure you have that correct? I would definitely double check that. I just can't see how that would be possible unless you have a short to pin2 somewhere before the opamp. Check all of you ground connections and make sure your not reading any voltage on them. It's possible that pin 2 could be tied to the signal path through a ground connection, and if that ground connection is broken it could bleed signal to pin2. I'm just taking shots in the dark here though. I'm pretty amateur at this stuff.

I have no idea what's going on with pin 8, but since that pin should have nothing other than a cap across it, I would assume that the problem is developing elsewhere. The  pin2 weirdness is where I'd start looking. However, I would also suggest pulling the opamp and checking to make sure you don't have anything shorting to pin8 on the pcb or something. 

Let us know how it goes. I'm really curious about this one.

[gutsofgold]

Well it seems someone stole a few parts from this thing. I already put the obvious ones back in, like the 47 ohm resistor that the 9v source connects to.  It's an actual Vintage Rat reissue so ProCo built it on one of those "Multi Rat" boards that they can build various versions of the Rat on and I can't seem to tell which empty spaces on the board should be there and which are actually needing a part.

[Projectile]

Maybe this will help. Here is a folder containing all of the rat schematics and info I've collected from the internet:

http://www.speedyshare.com/799874731.html

There are only minor differences between the different versions, mostly just a few cap of resistor changes or swapped clipping diodes for LEDs. The biggest difference is that some of the new ones have a jfet input buffer, but it's just a buffer. The ggg schematic also has some weird second bias network for the output section, which I don't really understand. I don't think it was ever part of any of the commercial pedals though.

The 47ohm resistor connected to +9v doesn't really matter. You could jumper it and the circuit will still work fine. It just sort of acts like a fuse as extra reverse polarity protection measure. I'd really focus on how you could be getting signal at PIN2 without signal at PIN6. Signal travels from PIN6 to PIN2, so there is no way that should be happening if the circuit was working properly. That is going to be the easiest place to start diagnosing the problem IMO.