Rat oscillation

mdc

mdc

Well-known member
Hi folks - not a pedal pcb problem per se, but it did arise during an a/b test and mostly you folks are very smart and have good suggestions!

I recently picked up a 1980s proco rat. I've had a bunch of them over the years and love them, and additionally I recently built a Muroidea and I wanted to see how close the clone actually was to the original. The good news is that the muroidea sounds extremely close to the vintage unit. They're not identical, but I don't think there's any more difference between the two than there would be between any two production rats.

However, the vintage unit oscillates wildly when the distortion knob is past 1 or 2:00—you can sort of tune it out with the filter, but it's pretty severe. I did a bit of reading around and this seems to mostly be a layout or wiring issue when people are building clones. Just curious if anyone here has encountered similar oscillations in a proco-made unit and if there's a common cause for it. Bad LM308N? A particular cap that tends to go?

It happens whether the pedal is being powered by a PSU or a 9V battery, and just to see if it was a problem with how/where the wires were laying, i took the back off and pushed the wiring harness around to no real effect.
 
Sort by date Sort by votes
Do you have hot pickups? Could be the 308, I’ve had oscillation problems with Rats of which I’ve built about 20...the circuit is really built around exploiting the instability of the 308. I’ve solved it by building a small buffer and inserting it pre-input. Oscillations only ever occurred with hot humbuckers.
 
Upvote 0 Downvote
Vintage jazzmaster single coils, so not even a little bit.

I should add that this isn't an issue on the muroidea with the same guitar/signal chain/cables/power supply/etc.
 
Upvote 0 Downvote
Hardwired, unfortunately—but not an impossible task to desolder it. I actually have an old motorola LM308N that's exactly the same as the one in it—I could test that in the muroidea first to make sure it's good and then see if I can get the old one out without destroying the board.
 
Upvote 0 Downvote
I don’t think so, but it’s a good enough fix that I use it when it happens. There’s a tiny vero board JFET buffer on tagboardeffects that will fit just about anywhere...I’d build that and stick it in there haha
 
  • Like
Reactions: mdc
Upvote 0 Downvote
Fair point, I mean if the worst thing to happen with it is that I need to keep a tuner in the chain then I think the world is probably better off. Feature not a bug as they say...
 
Upvote 0 Downvote
Have you tried replacing the 30pF cap connected to pins 1 and 8 of the op amp? It's the external compensation cap to help with oscillations. Also check the 2u2 and 4u7 caps in the op amp feed back loop, I've had the 2u2 go bad and cause oscillations when turning up the distortion pot past the 3 o'clock position. Since the rat circuit runs on the hairy edge, if you upset the LM308 in any way, it gets upset and doesn't play nice!
 
  • Like
Reactions: mdc
Upvote 0 Downvote
Beef up the 30pf. Crocodile clip to each leg and test different values since caps in parallel sum. Something like a 470pf would clamp down a lot more and depending on the chip might do the trick. No substitute for testing.
 
Upvote 0 Downvote
You must log in or register to reply here.
Back
Top