SOLVED Nobleman Overdrive Oscillation

[Iceman53]

I am troubleshooting a Nobleman OD build for a friend

Symptoms:
The pedal seems to work OK until the gain pot is increased. Once the pot's value is increased past about 32k, the circuit starts to oscillate. Backing the pot off stops the oscillation. Through an amp, it sounds like a motorboat. This occurs with no input (guitar), and with the input grounded. The oscillation signal starts out as sine wave then increases to a .6 v p-p square wave (with the corners smoothed off) riding on top of a 4.46 vdc level, at an 85 ms period.

What I tried:
I check component values in the feedback circuit, and a few near that circuit. I cannot find any wrong values. I am not sure what all of the components in the feedback loop are supposed to be doing. I'm guessing there has to be oscillation protection built in somewhere. I tried a different op amp. Same symptoms.

Any ideas or suggestions are appreciated

 

[Chuck D. Bones]

That side looks OK. C22 is 22nF instead of 27nF. Some flux residue. Are those sockets for R100? How are the In and Out jacks grounded?

Can we see the other side?

 

[Iceman53]

That side looks OK. C22 is 22nF instead of 27nF. Some flux residue. Are those sockets for R100? How are the In and Out jacks grounded?

Can we see the other side?

I think you mean C12, not C22. He has another cap in parallel attempting to get the required value. The spring sockets are for R100. He tries different values to get the LED intensity he wants. I have a small test box that I built to use to troubleshoot his non-working boards. I have in and out (signal) ground connected to DC ground inside of the box. The only connections he has coming from the board are power (+ & -), and Signal In and Out. I make connections to the PS, Guitar in and Amp Out inside my box. Like I mentioned in a different post, his soldering skills aren't that good, as you can see from the rear of the board. Virtually impossible to follow the traces.

 

[music6000]

Chuck, Is Isopryl what you use to Clean your Boards?

 

[Iceman53]

Chuck, Is Isopryl what you use to Clean your Boards?

I tried Isopropyl on a different board that he built. It made the board 'sticky'.

 

[Chuck D. Bones]

yep, IPA and lots of it!

3 recommendations:

Run separate ground wires for IN return and OUT return. Using the power ground as signal ground is a recipe for noise & oscillation. Not saying it is the cause, but it might be.
Fix all of the solder joints. I see what looks like quite a few cold ones.
Clean the board. Since you don't want to flush dirty IPA thru the pots, I'd recommend removing them first. There is a ton of flux on the board, so it's gonna take 5 tons of IPA to clean it off.

Your buddy owes you a few beers for this. ?????

You're right, I meant C12. 22nF is probably close enough anyway. The SPECTRUM control is interesting. Bass & Treble cut at one end, Bass & Treble boost at the other. Those Germans think they're so clever. It's actually a pretty interesting design, I might build one myself.

If you end up punting, here's one for $50.

 

[Iceman53]

yep, IPA and lots of it!

3 recommendations:

Run separate ground wires for IN return and OUT return. Using the power ground as signal ground is a recipe for noise & oscillation. Not saying it is the cause, but it might be.
Fix all of the solder joints. I see what looks like quite a few cold ones.
Clean the board. Since you don't want to flush dirty IPA thru the pots, I'd recommend removing them first. There is a ton of flux on the board, so it's gonna take 5 tons of IPA to clean it off.

Your buddy owes you a few beers for this. ?????

You're right, I meant C12. 22nF is probably close enough anyway. The SPECTRUM control is interesting. Bass & Treble cut at one end, Bass & Treble boost at the other. Those Germans think they're so clever. It's actually a pretty interesting design, I might build one myself.

If you end up punting, here's one for $50.

I will reconfigure the grounds, re-solder the cold joints, and flush the board as you suggest, but probably not till tomorrow. Busy this afternoon. Priorities... Thanks for all of the help

 

[Iceman53]

I will reconfigure the grounds, re-solder the cold joints, and flush the board as you suggest, but probably not till tomorrow. Busy this afternoon. Priorities... Thanks for all of the help

Update: I did as you recommended (Chuck) - Ran separate grounds for signal and power from the board to my test box. Gave the board an IPA bath and scrub with a brush. Touched up all of the solder joints on the rear of the board. Let the board soak in IPA for a while then scrubbed it again until I could see all of the runs. I scraped of what could have been 2 possible solder shorts (I didn't meter them) with an Xacto knife. Hooked it all up and...it worked! I'm going to pass along the IPA bath/scrub & cold solder joint suggestions to Greg. I'm sure it will save both of us some time down the road. Looks like I owe you a few ??for the excellent help. And I was getting ready to punt...

 

[Chuck D. Bones]

Yay!

I was reading more about this pedal and running some simulations. Seems like D3 & D4 only come into play during the initial note transient because the signal is pretty well limited by D1 and D2. Another pedal board supplier makes a version of this with a Bass control in series with R7, similar to the Timmy Bass control.

 

[mikeperez83]

I was excited to see this heading as a friend is having the same issue with his build, but without knowing where either 2 maybe solder joints were. Oh well.