ProCo Rat Debug

[sarpedon]

I'm working on building a ProCo Rat, I finished the build but I don't get any output from it.

I followed this schematic http://www.diystompboxes.com/pedals/PCRAT1.GIF and the only change I made was using 500k pot with a 220k resistor in parallel to replace the 150k pot.

I checked the output from Q1 and Pin 2 of the IC and I'm getting the proper output there, but nothing from pin 6 of the IC.

The voltages on the IC are
1 8.15
2 4.5
3 3.9
4 0
5 0
6 0.5
7 8.9
8 0.3

Here's the perfboard layout I made and used and the schematic for that with the parts labeled. Sorry for all the wires that are cluttering everything up, I'm not sure of a better way to do them on the layout image so they aren't very much in the way.



Thanks for any help with this.

[Projectile]

That schematic and layout looks very wrong. Where did you get that from? Every Rat I've ever seen uses the LM308 in a non-inverting configuration. The IC appears to be wired backwards in this schematic as an inverting amplifier, and I don't even think it's going to work at all like that. That's probably the issue. There also appears to be numerous other mistakes compared to the schematics I've seen. The transistor input buffer is unnecessary, and it wasn't on the original Rat. The 1K on the output transistor's source lug seems too low. Every schematic I've seen has that value as 10K.  Check everything against this schematic:

http://home.centurytel.net/flanneldrawers/ProcoRatOriginalschem.gif

I've used that schematic before, so it's verified and works as it should. You don't have to change the two 33K resistors that make the bias voltage. Those values should be fine, but I'd modify everything else to match that schematic.

Good luck, and next time use a verified schematic and layout.

EDIT: I just realized that you made your own layout. That's great! I don't have time to check it, but since the schematic you used looks wrong, the layout is probably going to have to be changed.

[sarpedon]

That schematic and layout looks very wrong. Where did you get that from? Every Rat I've ever seen uses the LM308 in a non-inverting configuration. The IC appears to be wired backwards in this schematic as an inverting amplifier, and I don't even think it's going to work at all like that. That's probably the issue. There also appears to be numerous other mistakes compared to the schematics I've seen. The transistor input buffer is unnecessary, and it wasn't on the original Rat. The 1K on the output transistor's source lug seems too low. Every schematic I've seen has that value as 10K.  Check everything against this schematic:

http://home.centurytel.net/flanneldrawers/ProcoRatOriginalschem.gif

I've used that schematic before, so it's verified and works as it should. You don't have to change the two 33K resistors that make the bias voltage. Those values should be fine, but I'd modify everything else to match that schematic.

Good luck, and next time use a verified schematic and layout.

EDIT: I just realized that you made your own layout. That's great! I don't have time to check it, but since the schematic you used looks wrong, the layout is probably going to have to be changed.

I'll check it out, thanks for the help.  The layout is right for the schem as far as I know...but that's not really much help if it's based on a bad schem.

[sarpedon]

Thanks for the help Projectile!

The main problem seems to be the swapped pin 2 and 3 on the IC.  I was able to reroute those on my perf board, but I'll have to do a whole rebuild for any of the other differences.  The sound isn't perfect, but it's actually functioning now at least.

[Projectile]

Try this:

-Swap R6 and R13 for 10K resistors.
-Remove R12

That should get it pretty much working properly.

Then if you want it to match the original Rat more closely, you could remove the input buffer like this:

-Remove R5, R6, Q1 and C3
-Jumper the output of R4 to PIN3 of the opamp (keep c10 and r7 connected also)

As far as I can see, that should get you 99% of the way to the original Rat schematic. The only major difference is that you are using a 150K pot for gain, which will mess with the gain range a bit, but it should still work okay. Note, that I haven't checked your layout to make sure it is correct. I'm just basing my modifications on the incorrect schematic.

Where did you find that schematic anyway?

[sarpedon]

Try this:

-Swap R6 and R13 for 10K resistors.
-Remove R12

That should get it pretty much working properly.

Then if you want it to match the original Rat more closely, you could remove the input buffer like this:

-Remove R5, R6, Q1 and C3
-Jumper the output of R4 to PIN3 of the opamp (keep c10 and r7 connected also)

As far as I can see, that should get you 99% of the way to the original Rat schematic. The only major difference is that you are using a 150K pot for gain, which will mess with the gain range a bit, but it should still work okay. Note, that I haven't checked your layout to make sure it is correct. I'm just basing my modifications on the incorrect schematic.

Those are pretty manageable changes, thanks.

Where did you find that schematic anyway?

Off of the schematics link at the top of the forum here, under Pro Co http://www.diystompboxes.com/pedals/schematics.html.  One is marked OK, but the link didn't work.  This one claims to be drawn by R.G. Keen with some corrections by Jack Orman, and from my time reading around here sounded like 2 reliable sources to go by.

[Projectile]

Wow, knowing that the schematic was looked over by both RG and Jack Orman I would have figured it was correct too. It's hard to believe that those guys would publish a bad schematic. Weird... but it does in fact work now with the changes?  That really surprises me. I've never, ever seen an opamp wired that way though, so I can only assume it's an error.