Fizzy chunk chunk

[billstein]

Have a strange problem.  I can play my chunk chunk and it sounds great but suddenly it fizzes out.  It may be after a few minutes or an hour.  If I change the j201s and re-bias is good again for awhile.  Anybody ever experience this?

[rullywowr]

The first thing that comes to mind are fake J201s.  Are they from a reputable supplier? 

[midwayfair]

The first thing that comes to mind are fake J201s.

I think FAKES are rare, but out of spec is pretty common from somewhere like Tayda.

My bet's on a bad power filter cap, C20 or C21. The cap charges up, works fine for a while, then craps out and shorts the power.

[billstein]

Thanks guys. Don't thinks it's the jfets because they came from Smallbear.  The electros came from Tayda so.... I'll replace both of those and see what happens.

[billstein]

I changed out C20 and C21 with some electros from Mouser and checked the J201's and they all are within range. Re-biased and it sounded great for about 5 minutes then went all fizzy again.

I checked the voltages on the J201s and Q2 and Q3 were still right where I set them. 4.75 for a 9.5 volt input.
But Q1 and Q4 had both changed.

Q1 = 7.58
Q4 = 0.284

Any other suggestions?

[midwayfair]

Ah. Sometimes JFETs actually do take a while to settle. They start high and drop. Did you try rebiasing AFTER it goes fizzy?

[billstein]

I'll try that and see what happens.

[jimilee]

This happened to me on a tube screamer. Turns out I had a broken resistor that would separate and fizz out when it warmed up. It took forever to find because it wasn't apparent when I opened it because it had settled back down.

[rullywowr]

This happened to me on a tube screamer. Turns out I had a broken resistor that would separate and fizz out when it warmed up. It took forever to find because it wasn't apparent when I opened it because it had settled back down.

That's pretty rare but certainly possible!  Thanks for sharing, Jimi.

[jimilee]

This happened to me on a tube screamer. Turns out I had a broken resistor that would separate and fizz out when it warmed up. It took forever to find because it wasn't apparent when I opened it because it had settled back down.

That's pretty rare but certainly possible!  Thanks for sharing, Jimi.

You know me, I'm nothing if not helpful. Broken a few resistors, carbons, happens when I reflow.

[Clayford]

I've done it too on the MLCC 10n and 100n axial caps I have. Just as Jimi said, reflowing.

[billstein]

Thanks guys for all the help. I thought I had this fixed but it has fizzed out again a few times. It seems to only happen when I put a pedal in front of it that juices it a bit. It happened with a compressor and just now with a Harmonic Energizer. I can go back in and rebias it and it will be fine until I put something in front of it again.

Any thoughts?

[midwayfair]

Thanks guys for all the help. I thought I had this fixed but it has fizzed out again a few times. It seems to only happen when I put a pedal in front of it that juices it a bit. It happened with a compressor and just now with a Harmonic Energizer. I can go back in and rebias it and it will be fine until I put something in front of it again.

Any thoughts?

Remove it from the enclosure. See if it still happens on a test rig.