SOLVED Now my Mofeta Build is Squealing/Oscillating!!

[debrad]

After setting aside my Boogie Monster which continues to squeal at high volume settings, I put together a Mofeta build...and now it's squealing on me too!

For the record, I've managed to tame the squeal in the "normal" channel by setting the bias on Q2 at 4.5v; however, with the "bright" channel maxed, the squealing/oscillation starts anywhere between Noon and 2:00 on the "master" regardless of whether I have Q2 biased to 4.25v, 4.5v, 5.5v, 6.0v or 6.5v...the only difference being the amount of "dirt" I'm able to get (i.e. the bright channel has more dirt earlier in the pot sweep with lower bias settings).

I've scoured this forum and the internet in general but haven't really found any comments from others having oscillation issues with their Mofeta builds so I am curious to know if I might have done something specifically wrong here or whether I just can't build pre-amp pedals in general (I hope not!!).

My only deviation from the original is the use of SMD 2N5457 JFETs on adaptor boards rather than through hole or soldering the SMDs directly to the board:

 

[Robert]

Try moving the input/output wires as far away from the PCB as possible.

Same question as before, does the volume control on your guitar affect the oscillation?

 

[debrad]

Thanks @Robert

Just moved the input and output as far away from the board as I can. Pedal is still squealing with BRIGHT fully CW and the MASTER around 1:00.

I can set both controls full CW without oscillation with the guitar's volume at ZERO but the squeal comes on once I turn the guitar's level between 1 and 2...pickup selection does NOT seem to matter.

Next step will be shielded wiring for input and output but that didn't help me with the Boogie Monster so I'm wide open to other suggestions.

Any chance my charge pump could be an issue? This Mofeta has a TC1044SCPA but I do have other options (TC7662BCPA, ICL7660CPA, LT1054CNB, and Max1044CPA)...

 

[Robert]

I doubt it's the charge pump.

Try removing the input jack from the enclosure (keep it connected) to get it as far away from the Master pot as possible. The Master pot is directly under the Input jack and those jacks are sitting awfully close to the pots.

 

[JTEX]

This is caused by unwanted capacitive coupling between the output and the input. True Bypass switching tends to be the culprit, since it places input and output wires and switch contacts in close proximity.

Try one or both of the following:
- while it squeals, stick a very thin, sheet of metal (aluminum foil?) between the output and input pins on the 3PDT switch, trying not to make a short. Make sure the metal sheet is grounded (touch it to some ground point). Reposition it to see if/how it affects the squealing.


- shield your input jack with copper or aluminum tape, connected to ground.

 

[debrad]

OK...something dawned on me while test driving the input jack suggestion from @Robert

My current basement rig has the guitar running to a 2 loop switcher then out to my amp and I have been testing the Mofeta and the Boogie Monster in the first loop. With that arrangement, the input and output jacks of the loop are close together AND the cables that run from those jacks to the pedal jacks are just as close, if not closer.

Just now, I pulled the Mofeta from the loop...ran my guitar straight to the pedal and then ran a cable from the output straight to the amp. Guess what?

You guessed it, NO OSCILLATION!

I put the Mofeta input jack back inside the pedal...NO OSCILLATION!

I replaced the Mofeta with the Boogie Monster...NO OSCILLATION!

So, like many previous oscillation problems, it looks like the issue was the proximity of the input and output wires; however, in this case it was the cables OUTSIDE the pedal instead of the wires INSIDE.

Now I'm going to go back to the trim pots for these two pre-amp pedals and see if I can dial them back to the recommended values without inducing any squeals or oscillations.

Thanks everyone!!!