Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old March 14th 19, 06:02 AM posted to sci.electronics.repair
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Default dual-dual potentiometer failure?

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...


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Old March 14th 19, 12:14 PM posted to sci.electronics.repair
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Default dual-dual potentiometer failure?

A common spike could take out both, certainly. Before going ahead and replacing the pots - can you determine a source for such a spike, or how it could have happened?

Could also be a manufacturing defect common to both sections.

Peter Wieck
Melrose Park, PA
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Old March 14th 19, 12:36 PM posted to sci.electronics.repair
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Default dual-dual potentiometer failure?

On 3/14/19 6:14 AM, wrote:
Could also be a manufacturing defect common to both sections.


Behringer is known as the "Pep Boys"of studio equipment.

--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com
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Old March 14th 19, 01:26 PM posted to sci.electronics.repair
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Default dual-dual potentiometer failure?


Behringer is known as the "Pep Boys"of studio equipment.

I would be more inclined to use "Harbor Freight" for comparison. Pep Boys does stock a great deal of OEM equipment, as well as the cheap stuff. Harbor Freight does not.

Peter Wieck
Melrose Park, PA
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Old March 14th 19, 01:26 PM posted to sci.electronics.repair
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Default dual-dual potentiometer failure?

On 14/03/2019 05:02, 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...


What happens if you solder 5K, say, across the o/c parts of all pots?



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Old March 14th 19, 08:52 PM posted to sci.electronics.repair
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Default dual-dual potentiometer failure?

On Thursday, March 14, 2019 at 12:20:27 PM UTC-4, bitrex wrote:
On 03/14/2019 08:26 AM, wrote:

Behringer is known as the "Pep Boys"of studio equipment.



A mechanical impact to the knob may be a common factor that damaged both channels of the pot.

m
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