Let's talk internal trimpots!

bassteban · #1 06-19-2010, 10:16 AM

Specifically(to begin with- derails & tangents are welcome & invited

), the DOD FX75B's trimpots- does anyone here know what they do? There are three- one of which seemed to be a gain-type thing as cranking it made the effected signal sound unpleasantly overdriven, one produced some ugly feedback & one I couldn't tell what it did.
I also have tweaked the trimpots on my Soundtank Echo Machine to make it self-oscillate nicely- it's now convinced it's analog.

i_got_a_mohawk · #2 06-19-2010, 10:27 AM

Had a quick look at the schematic, which only shows 4 variable resistors (well, to my count) which would relate to the pots. So, dunno what the trim pots do, they don't seem to be on the schematic!

cheapbasslovin · #3 06-19-2010, 10:34 AM

I looked at that schematic, tried tracing the circuit, and that schematic is wrong.

I messed with this once. The one that produces distortion I believe is a bias for the Bucket Brigade chip. One is for resonance and the other I can't remember, but I think may affect delay time. The one that creates the hellfire feedback works in conjunction with the regen knob as well as the third unidentified trimpot. I externaled the resonance trimmer and made a clip of me tweaking all the parameters on it.

http://soundclick.com/share?songid=8846703

bassteban · #4 06-19-2010, 10:35 AM

This is by far my most genuinely productive thread ever. Thank you both.

Edit: DANG, cheapbasslovin, the latter half of that clip were absolutely nauseating(I mean that in the nicest way imagineable, mind you- it seems that may have been the intent

).
I'd love to see some pics of the modded box.

cheapbasslovin · #5 06-19-2010, 10:48 AM

I was going for, "hey look it's a flanger, and also... this... thing."

i_got_a_mohawk · #6 06-19-2010, 10:54 AM

Quote:

Originally Posted by cheapbasslovin

I looked at that schematic, tried tracing the circuit, and that schematic is wrong.

I messed with this once. The one that produces distortion I believe is a bias for the Bucket Brigade chip. One is for resonance and the other I can't remember, but I think may affect delay time. The one that creates the hellfire feedback works in conjunction with the regen knob as well as the third unidentified trimpot. I externaled the resonance trimmer and made a clip of me tweaking all the parameters on it.

http://soundclick.com/share?songid=8846703

That'll explain that then!

bigchiefbc · #7 06-19-2010, 01:50 PM

Someone already said something along these lines, but what the heck. Since the FX75B is an analog flanger, it is has a bucket-brigade delay chip in there. In a lot of analog delay pedals I've seen, each bucket brigade chip has a bias trimpot which has to be individually tweaked in each pedal, as no two bbd chips are made exactly the same. Then there is usually a noice cancel trimmer for the clocking chip.

bongomania · #8 06-19-2010, 02:42 PM

I'm sure this is just "preaching to the choir" here, but my one bit of advice about internal trim pots is to always mark the original position! I usually use a razor blade to scratch an exact mark in; but grease-pencil, paint-pen, toenail polish, or liquid paper corrector ("White-Out") all work for this purpose.

Swift713 · #9 06-19-2010, 05:20 PM

Now that's the best sounding flanger I've heard.

iamdenialNJ · #10 06-20-2010, 01:33 PM

I never understood internal trim pots...why not just make more external knobs?!

Step · #11 06-20-2010, 02:06 PM

Quote:

Originally Posted by iamdenialNJ

I never understood internal trim pots...why not just make more external knobs?!

because most internal trimpots are set and forget type of stuff (input gain, bias). Stuff most people (the sane kind

) don't want to mess with.

also, I imagine putting a knob 95% of people aren't going to use on top isn't that economical

cheapbasslovin · #12 06-20-2010, 02:27 PM

Quote:

Originally Posted by iamdenialNJ

I never understood internal trim pots...why not just make more external knobs?!

Having trimmers makes sense with components that have inconsistencies from the factory that are unacceptable. Still, they are fun to play with.

BarkerBass · #13 06-20-2010, 03:02 PM

hello lovely tb electronics boffin types and effects tinkerers!

Consider this a noob question since I am not an effects tinkerer but please humour me:

Can a trim pot be replaced with a standard pot of the same value? Or vice versa could a standard pot (say a 250K volume pot) be replaced with a 250K trim pot?

I ask because I'm trying to reduce the space taken up by a sunken pot on a preamp in a bass of mine, if it could be replaced with a trim pot a lot of space would be saved.

Cheers

cheapbasslovin · #14 06-20-2010, 03:13 PM

Quote:

Originally Posted by BarkerBass

hello lovely tb electronics boffin types and effects tinkerers!

Consider this a noob question since I am not an effects tinkerer but please humour me:

Can a trim pot be replaced with a standard pot of the same value? Or vice versa could a standard pot (say a 250K volume pot) be replaced with a 250K trim pot?

I ask because I'm trying to reduce the space taken up by a sunken pot on a preamp in a bass of mine, if it could be replaced with a trim pot a lot of space would be saved.

Cheers

Yes, that would work perfectly, if you didn't want to adjust without taking it apart.

BarkerBass · #15 06-20-2010, 03:40 PM

Amazing, Thank you, that was just what I wanted to hear! Cheers