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?
The first thing that comes to mind are fake J201s. Are they from a reputable supplier?
Quote from: rullywowr on April 07, 2014, 06:27:20 PM
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.
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.
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?
Ah. Sometimes JFETs actually do take a while to settle. They start high and drop. Did you try rebiasing AFTER it goes fizzy?
I'll try that and see what happens.
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.
Quote from: jimilee on April 08, 2014, 03:52:15 AM
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.
Quote from: rullywowr on April 08, 2014, 04:29:28 AM
Quote from: jimilee on April 08, 2014, 03:52:15 AM
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.
I've done it too on the MLCC 10n and 100n axial caps I have. Just as Jimi said, reflowing.
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?
Quote from: billstein on April 15, 2014, 07:14:39 PM
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.