Help! Ross Comp doing some weird stuff with trimpot

[shake__appral]

Was hoping to get some help with a Ross Comp I’m building. The “Level/Sustain” pot doesn’t seem to be cooperating with the trimpot (2k). Got it to pass signal but only as I decrease the “Level” pot will the signal start to come in (and is also thumpy). When the pot is fully cranked it cuts sound completely.

Here’s a link to the schematic (using the LM13700 build with no “switch mod”).

https://guitarpcb.com/wp-content/uploads/2018/07/BD_MoRC.pdf

 

[Mcknib]

The trimpot should be set in the middle position if you get thumping you should turn it in small increments afaik it adjusts the OTA input bias currents to balance them

Yours looks like it's set to one extreme or the other your trimmer screw slot should be parallel to the 2 inline lugs turn it 90°

Did you check your level pot is OK It's wired as a variable resistor so as you turn it resistance should change across the two outer lugs 1 and 3

With lugs 1 and 2 joined resistance should decrease clockwise

Your main pic is out of focus so it's difficult to check all components are correct

Edit check resistance across the trimpot too set it to the middle position and check resistance between the middle and two outer lugs both should be1K ish and resistance across the two outer lugs 2K ish that'll let you know it's doing what it should do

 

[shake__appral]

The trimpot should be set in the middle position if you get thumping you should turn it in small increments afaik it adjusts the OTA input bias currents to balance them

Yours looks like it's set to one extreme or the other your trimmer screw slot should be parallel to the 2 inline lugs turn it 90°

Did you check your level pot is OK It's wired as a variable resistor so as you turn it resistance should change across the two outer lugs 1 and 3

With lugs 1 and 2 joined resistance should decrease clockwise

Your main pic is out of focus so it's difficult to check all components are correct

Edit check resistance across the trimpot too set it to the middle position and check resistance between the middle and two outer lugs both should be1K ish and resistance across the two outer lugs 2K ish that'll let you know it's doing what it should do

Ok I got the trim pot squared away, got the pedal working… but the pots are tapering backwards (??). So when I turn them counterclockwise the volume and sustain go up, opposite of a normal pedal. Never had this happen on a board with fixed pots, so I’m assuming it has something to do with the trim pot. God I hate those little fucks. Any ideas?

 

[BuddytheReow]

The trimmer won't affect how the pots behave from a clockwise/CCW perspective. You could just take the pots out and mount them on the other side of the board. Or just deal with it, like the original tone knob on a Rat.

 

[jimilee]

Check over the pedal again, you have a short or a wrong component.

 

[Mcknib]

Ok I got the trim pot squared away, got the pedal working… but the pots are tapering backwards (??). So when I turn them counterclockwise the volume and sustain go up, opposite of a normal pedal. Never had this happen on a board with fixed pots, so I’m assuming it has something to do with the trim pot. God I hate those little fucks. Any ideas?

It's a guitarpcb board the pots go on the underside but the other way around so lugs 1 and 3 are the wrong way around on yours

 

[Nostradoomus]

It's a guitarpcb board the pots go on the underside but the other way around so lugs 1 and 3 are the wrong way around on yours

This. They don't mention it in the build docs which is kind of a big no no. But I'll shut up because it's THE BEST.

 

[Mcknib]

There's usually a small 1 denoting pot lug 1 on the pcb shown in the build doc

I'm obviously used to it so if one pads not square for lug 1 I'd always look for lug numbering yours is the V4 Morc pcb