DrBoogie Build. One trimmer acting weird.

[steveyraff]

Build a few of these now with no issues.

On this one, everything works fine except the fourth trimmer, just to the left of 'Q4'. I tried replacing both the Trimmer and the Q4 J201.

I also checked all cap and resistor values and everything is correct.

Basically, there is no sound at all unless that trimmer is EXACTLY where it is shown in this picture. Half a mil to either side and it goes dead. When its in exactly this postion, all the other trimmers work through a wide sweep and I get the distortion effect, but it seems a little weaker than previous builds. Any other time I build this, I had a wide range to work with on all trimmers.

Where should I look next?

[GibsonGM]

All I can suggest is to look ALL AROUND that JFET.  Each part that connects to it.  I'd get some voltages at the D, G and S after you turn the pot one way to off, the other way to off...see what it looks like.   

Odds are very good that you accidentally crossed some things not meant to be connected ("solder bridge"), it happens to everyone sometimes.  Like you said - you've done 'em before and that trim should have a range, right?  If it is the right value trim, connected right, you should have the same result.  UNLESS there is something 'downriver' shorting something else.    Check source >1k8 resistor to ground...

Don't know what schematic you're working from; Vero often doesn't come with one, which makes things more difficult.  The Dr. B that I'm familiar with has the wiper jumped to the hot lug, and is merely passing a current from "+" to the drain of the FET as a variable resistance....therefore, should be easy to determine if it is doing that correctly or is faulty...

[Elijah-Baley]

I never built this, but I use veroboard.
Did you try to check some unwanted contact between the stripes around that trimmer? I used to pass a knife between the stripes.
You could even check all the parts in contact with the trimmer, in case of some soldering is bad. Use the multimeter.
Or, maybe there's some mistake before it.

EDIT: Someone answers faster than me.

[steveyraff]

I never built this, but I use veroboard.
Did you try to check some unwanted contact between the stripes around that trimmer? I used to pass a knife between the stripes.
You could even check all the parts in contact with the trimmer, in case of some soldering is bad. Use the multimeter.
Or, maybe there's some mistake before it.

EDIT: Someone answers faster than me.

Thanks guys. Yep, I've went over it all with a jewellers loupe eyepiece and I've ran a blade between all tracks.

I built two of these recently - something just came into my mind. On one of them I recall accidentally powering it up with a J201 the wrong way around. It may have damaged it but I can't for the life of me recall which it was. I already replaced Q4. I could try replacing Q5 - would it be affected by Q4's trimmer? I can see there is a physical connection on the vero layout. Its just that I'll have to pull one off another circuit as I've none spare - worried the heat will also damage the replacement :/

[steveyraff]

Meh - changed that one too and it sounds the same. I mean - it doesn't sound bad, I just remember my last builds sounding more high gain.

[steveyraff]

Only other thing I can see is that I used an 33nf instead of where that 30nf is supposed to be. I only that 33 or 27nf.

[steveyraff]

Well...that didn't make a difference either.

[deadastronaut]

Swap the trimmer?  Ive had a few glitchy ones...

[steveyraff]

Swap the trimmer?  Ive had a few glitchy ones...

Yep - first thing I did. No difference. So now thats all trimmers and J201's swapped. I've knifed between all tracks, I've double checked all cap and resistor values, and wiring...

I'm totally stumped with this one. I've literally sat looking at it for hours. No idea. I think I have to admit defeat on this one.

[GibsonGM]

Have you actually measured the trimmer's resistance value?  I know the label say it is right.... Sorry to ask the obvious, don't be insulted!  Cuz if it were 1MEG intstead of 100k, say - you would get exactly what you describe.      Either that, or another resistor in series with it somewhere?  Shouldn't be tho.

That's all I got, odd issue!