Mixers - Gain pot type help

ZenOvertone

I've got a mixer with a few faulty channels, definitely due to gain pots but despite a marking of 10k, the type is unknown.  I'm sure they won't be linear types but should they be reverse log C type as they control op-amp gain stages?  

maltingsaudio

What mixer is it? Manufacturers do things in different ways and repair on cheaper and older low budget ones aren’t often cost effective. 

robinbowes

ZenOvertone said:

I've got a mixer with a few faulty channels, definitely due to gain pots but despite a marking of 10k, the type is unknown.  I'm sure they won't be linear types but should they be reverse log C type as they control op-amp gain stages?  

Depends entirely on the circuit.

What mixer?

R.

ZenOvertone

It's a Monacor/IMG stage line PMX600, discrete component fortunately.  Gain pots are marked 10k with a "C" in a different place on the casing.  Other pots are clearly labelled B10k as expected for linear taper so it's a bit of a mystery!! 

robinbowes

Maybe pull one out and measure the curve?

R.

Danny1969

They are often anti-log but sometimes linear. Using the wrong law will result in unpredictable gain changes at different points of  the pot's travel. If the circuits 2 half's of a dual TLO72 or similar with pin 2 and 3 feeding the non inverting inputs then the pot generally needs to be anti-log  ... sometimes marked with an X

Maybe you can clean get them to get them serviceable again with some decent contact cleaner  ... also bear in mind sometimes it looks like the pots at fault because the lower output voltages can't breach what's effectively a semi conductor caused by a bad joint. Turning the pot up can increase the voltage and the channel pops back into life leading to the assumption the pots faulty but sometimes it's a bad joint further on. I used to come across this all the time fixing Soundcraft mixers

ZenOvertone

Thanks all, one of the pots is broken off inside, beyond repair.  I'm going to take one of the others out to try and compare with a good linear pot and a reverse log pot to see what's closest.