Question about trim pot leads


2005-12-24 4:34 am
I have a question regarding trim pot leads. I have the following schematic:

amp board schematic

There are two trim pots (for BIAS and DC offset).

This is the type of trim pot I am going to use:

bourns trim pot

Wiper is easy to figure out but how do I know where the 1 and 3 legs go on the board?

I can't reference the old trimmers as the board was totally shot and I am trying to rebuild it.


Ian Finch

Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
I assume you only want to know whether CCW will be say, minimum bias and CW maximum, as this a logical direction of adjustment.
Unfortunately, when you only have an electrical schematic, there is nothing indicating which way the trimpot should face physically
to get the logical direction of adjustment.

You could need a pic or assembly diagram for your original device to know how it was designed and that could well have been reverse -
annoyingly, some amplifiers are backwards and that can be due to access problems after the design stage.

All you really have to know about the the old trimpots is which way they faced because the connection sequence will be the same
and there will then be no difference in the direction they operate, whether enclosed types or not. I doubt that you can't determine that,
whether the old parts were shot or not.

For VR502, the bias trimpot, this design should have the transistor turned on least with the transistor base and wiper (2) connected
to the CW (3) end. That allows bias to be be maximum in that CW position too. I think that's clear enough to proceed.

It doesn't make any odds which way the offset adjustment (VR501) is wired. You start off with the wiper middled and adjust for minimum
offset which could be in either direction, regardless of which way you wire the pot.