Scratchy Trimpots

[antojado]

Just wondering if this is common or not. I have a ROG Eighteen that I built a few months ago. It's been sounding great (especially into a TS808 clone!), but it's recently developed a scratchiness to the sound. I decided to check the bias and when I adjust the trimpots (all of them) they make that scratchy noise. I've read some other posts where it sounds like others have had some trouble with trimpots being noisy. Is there a particular brand that's better or are they all like that? I bought these from Jameco Electronics and they're the cheap no-name brand cermet. Maybe the ones smallbear sells are better? Thanks.

[petemoore]

  yea...trimpots.
  Had troubles with those.
  mostly they scratch and introduce AC as a result when theyre being adjusted. and provide a stable R value otherwise.
  options...
  use a smaller value trimpot, and 'bump it up' so the small range is within the target R rating...using fixed resistors, series.
  use a regular pot to find the resistance you want fixed there, make a fixed resistor of that value [two resistors = R value you want] to place in the R position.
  ..set the pot to sound or voltage desired, measure that R value [don't turn knob], match value with fixed resistor[s, install fixed value...or find the nearest value and put that there..?
  my 18 and supreaux had the exact thing your'e describing, no 'definitive on whether trimpots were the issue, strongly suspected though.
  K...turn the pot all the way up, use pot to determine parallel resistance value [probably a 'bigger value], bias, use a chosen [from parallel pot reading, start with...500k pot...1meg? should cover the range...which of course depends on the value used in the trimpot position...and assuming it can be set to a high enough resistance] choose fixed resistor value [same as the parallel pot] in parallel with the trimpotwafer value [the wiper will be out of the picture]. This way you can just tag [shape leads for quick/ez solder job] that resistor right to the trimpot lugs, from the top of the board.
  I just trim, bend and crimp the first R lead to the trimpot leg, then the other side can hold itself when shaped, 'pushed to directly' as cooling occurs to the other lug so metal to metal joint occurs there.

[antojado]

Thanks for reply. I think I'll see what value they're at just install a fixed resistor. It doesn't seem like there's much need for the variability except when first setting it up.

[petemoore]

  Yupp, I'd try a couple Jfets just for 'inconsistancy testing', using a regular pot [maybe 50k -100k]...chosen for it's ability to tune to bias [somethimes too big a pot value can make fine adjustments really touchy]...sometimes I use a smaller pot or clip a resistor across the big one so I can fine tune.
  Then, once the JFet is chosen and biased, the pot value can be replaced with a fixed value.