Smoking Trimpot.

[mcasey1]

Ive recently assembled a professor tweed, the vero layout here in the gallery.  I was turning the second trimpot in order to raise the voltage on the drain of Q2, and the trimpot started to emit a stinky smoke.  Has anyone else experienced this?  What would be the cause, too much voltage?  These are radioshack trimpots, decent, certainly not turned out, as it was the first turn on it. 

[Paul Perry (Frostwave)]

There is only one way to burn out a trimpot (and I have done it more than once).

Here's how: connect one end pin to ground, and the wiper pin to the power (or vice versa). Now move the wiper till the resistance is so low that enough current can flow to burn the trimpot track.

How does one connect it like that? Usually a solder short

[oldrocker]

With Radio Shack trim pots don't use a metel screwdriver to adjust the trims.  If you stick the screwdriver too deep and it doesn't take much it will short something out and cause the trim to smoke.  Use a plastic trim adjustment screwdriver if you have one.
Remember what you were told.  NEVER SMOKE POTs. 
By the way if you already made a Tube Screamer try coming out of the TS-808 and into the Professor Tweed and out to the amp.  This is one of my favorite setups.

[mcasey1]

I checked for bad joints, solder bridges, the like.  Put in a new one, and it keeps smoking them.  Where should I look for an error?  From the +9V rail to the wiper?  I am afraid to check voltages, should I?  I just dont want to connect power to the circuit again.

[markm]

Remember what you were told.  NEVER SMOKE POTs. 

That's right oldrocker, remember "Reefer Madness"? 

[TELEFUNKON]

Smoking Is Hazardous to your Pots!

[aloupos]

I'll put $50 that you have a short between + and - in the pads where the trimpots 3 legs are connecting. 

At this point, unplug the board / disconnect the battery.  You will continue to burn trimpots until the short is resolved.  Check all of the connection points around the trimpot.  look for two solder pads that are accidentally bridged.   

Good luck

A

[markm]

Fifty 
Is that ALL? 
I'm not a Vero guy but, maybe double check and see if your component placement is correct or trace-cuts perhaps?
I agree with the above, until you resolve the issue, your trimpots are doomed 

[mcasey1]

Im sure its my error, but Mark, have you had success with your vero Prof Tweed layout?

[mcasey1]

Im going to post some debug readings soon, as the trimpot situation seems to have stabilised.  Is it normal for the pot's taper to abruptly jump from 2.2 to 8V?

[mcasey1]

Nevermind, now they keep going up in smoke again, and I cant see where it is that is making them do that.  I dont see any shorts.  MarkM is this layout verified?

Another thing is that the LED is on regardless of whether the imput is plugged or not.  Wiring from sitch to jacks is correct.  Wonder if the LED is always seeing the voltage because of an error on the board.

Could I debug here or what?

[zjokka]

Im going to post some debug readings soon, as the trimpot situation seems to have stabilised.  Is it normal for the pot's taper to abruptly jump from 2.2 to 8V?

my experience with these jfet tube amp simulation circuits is that in your case the trimmer is too big in value, maybe try a smaller one which might allow for more precise adjustment if you don't need the 100k.

[mcasey1]

Thanks for the tip, zjokka.  Im reallylooking for guidance on how to go about determining the cause of the trimpots getting fried.

[aloupos]

your led is always on .. this is a definate indication of a short.   Honestly, using a PCB is IMHO the easiest way to assemble these projects.  Take a break from it, clear your head, and go back and check any possible pad thats close to another pad but not connected.  Use your meter to see if there is a short there.  You may also want to take a digital photo of the backside of the board, maybe others will spot the short. 

Because of the lead always being on, the odds go down from 2:1 to 1:1, but I'll raise my bet to $100.00  

Anthony