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:

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:

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...

Reply to
bitrex
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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

Reply to
peterwieck33

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

--
"I am a river to my people." 
Jeff-1.0 
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Reply to
Fox's Mercantile

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

Reply to
peterwieck33

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

Reply to
N_Cook

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

Reply to
bitrex

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

Reply to
makolber

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.

Reply to
bitrex

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.

Reply to
bitrex

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.

Reply to
bitrex

transplant time.

NT

Reply to
tabbypurr

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.
Reply to
Dave Platt

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.

Reply to
bitrex

Ah, right this one.

Reply to
bitrex

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.

Reply to
bitrex

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