dual-dual potentiometer failure?

bitrex

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Mar 14, 2019, 1:02:11 AM3/14/19

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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...

peterw...@gmail.com

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Mar 14, 2019, 7:14:18 AM3/14/19

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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

Fox's Mercantile

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Mar 14, 2019, 7:37:04 AM3/14/19

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On 3/14/19 6:14 AM, pf...@aol.com 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

peterw...@gmail.com

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Mar 14, 2019, 8:26:18 AM3/14/19

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> 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.

N_Cook

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Mar 14, 2019, 8:26:34 AM3/14/19

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

bitrex

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Mar 14, 2019, 12:20:27 PM3/14/19

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Eh, I think they get ragged on unfairly sometimes. It's not an otherwise
cheaply made box, the enclosure is solid heavy steel enclosure, massive
toroidal power transformer, linear power supply with
conservatively-rated parts, the pots are Alps, the soldering-work looks
great (compared to some truly awful crap I've seen from China.) It's
essentially a solid-state EQ with a over-drivable tube exciter stage
prior to it going into the four parametric bands I think it sounds quite
nice strapped across the output of a mixer with some bass and treble
boost and the "honky" mid-range cut a bit before going into a PC's ADC
for recording lil jam sessions.

They're all pushing 20 years old tho stuff just breaks sometimes.

The most reliable piece of studio gear I have probably is my Behringer
MX602 line mixer, running most every day for 19 years and all it's
needed so far is a replacement headphone volume pot

mako...@yahoo.com

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Mar 14, 2019, 3:52:10 PM3/14/19

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On Thursday, March 14, 2019 at 12:20:27 PM UTC-4, bitrex wrote:
> On 03/14/2019 08:26 AM, pf...@aol.com 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

bitrex

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Mar 14, 2019, 10:36:36 PM3/14/19

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I think we might have our suspect, here. I took a closer look at the
pots and I overlooked it initially. It's subtle but the metal housings
of both have some slight bending/warping that the other, more recessed
pots don't. Looks very much like impact damage to me.

I'll swap them out and see what happens.

bitrex

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Mar 14, 2019, 10:38:13 PM3/14/19

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Like it was in a rack that tipped over forward onto the floor or
something, and those two big knobs that jut out took the brunt of it.

bitrex

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Mar 14, 2019, 10:45:19 PM3/14/19

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On 03/14/2019 03:52 PM, mako...@yahoo.com wrote:

Mmmhmm. I've pulled 'em and opened them up and the phenolic contact
boards inside are both cracked right in half.

That'll do it.

tabb...@gmail.com

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Mar 15, 2019, 4:33:01 AM3/15/19

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transplant time.

NT

bitrex

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Mar 15, 2019, 2:03:44 PM3/15/19

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Replacements of the same type are $2.50 at mouser but _sigh_ there must
be some corollary to Murphy's Law that while 2 day shipping is pretty
cheap you'll always discover you need something on a Thursday.

bitrex

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Mar 15, 2019, 2:06:58 PM3/15/19

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Ah, right this one.

<https://en.wikipedia.org/wiki/Finagle%27s_law>

Dave Platt

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Mar 15, 2019, 2:08:06 PM3/15/19

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In article <ScEiE.33408$h85....@fx37.iad>, bitrex <us...@example.net> wrote:

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

>I think we might have our suspect, here. I took a closer look at the
>pots and I overlooked it initially. It's subtle but the metal housings
>of both have some slight bending/warping that the other, more recessed
>pots don't. Looks very much like impact damage to me.
>
>I'll swap them out and see what happens.

I picked up a nice subwoofer system with integrated amp at a local
electronics flea-market last year, for about 15% of the original
price. The seller said "It works fine, but the volume adjustment knob
is missing." It wasn't just the knob - the whole potentiometer shaft
was snapped off, inside the mounting flange. Tried it when I got it
home, and the amp powered up but wouldn't play any music.

When I opened up the amp I found the dual pot bent backwards and
smashed up - both halves were open-circuit. Apparently somebody had
struck the pot knob quite hard, somehow, and completely wrecked it.

I removed it and tacked a couple of resistors into place to simulate a
mid-scale setting on each channel, and the amp came back to life and
the subwoofer worked. A $6 purchase from eBay, for a package of three
dual-section 20k linear pots of the right size, and another session of
soldering, and the subwoofer was as-good-as-new. Works beautifully,
alongside my heavily-refurbished (from-the-same-flea-market) Minimus
77 speakers and refurbished (also-from-the-same-flea-market) Proton
receiver and (made-almost-entirely-from-recycled-and-eBay-parts)
Raspberry Pi media streaming player.

bitrex

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Mar 31, 2019, 5:07:31 PM3/31/19

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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.