On 3/14/19 1:02 AM, bitrex wrote:
I have a Behringer rackmount EQ on my desk again, exhibiting
intermittent distorted or no output, no meter movement, etc:

Example image:

https://imgur.com/a/SCVFbGq

Problem affects both channels, I've traced it down to what seems to be
open/damaged dual-gang potentiometers, the big ones on the far right in
the pic for "warmth" (i.e tube distortion/coloration) control.

Here's the relevant portion of the schematic, the dual gang Alps pots in
question are labeled VR27A/B and VR28A/B:

https://imgur.com/a/eFmo1Zq

I have my own functional unit to compare against; when I test those pots
on mine in-circuit with the control board unplugged and removed from the
rest of the circuit, with the pots at center position, measuring from
the ground lugs to the other pins I get various resistances in the
kilohm range, 4k, 8k, etc.

When I do the same on the faulty unit I measure open circuit on
everything. on the pots on _both_ channels.

An open-circuit pot failure on both channels seems unlikely but can't
really see what else it could be, the TL074 op amps IC1 and IC2 tested
good, all power supply and tube bias and plate voltages look OK with no
signal, etc.

Anything else to test to be sure before I yank them out and replace?
De-soldering and replacing this kind of PCB-mount pot from plated-thru
holes is super-annoying...


Wanted to follow up thanks everyone for the help it was a indeed a
dual-dual potentiometer failure, cracked, possibly from the unit being
dropped. PLUS a damaged or cracked trace on the TL072 V+ bus causing
lack of power. $5 in pots and a couple jumpers and it's back up and
working fine. oddly the exterior looks undamaged so who knows how
exactly it occurred.