Home |
Search |
Today's Posts |
|
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. |
Reply |
|
LinkBack | Thread Tools | Display Modes |
#1
Posted to sci.electronics.repair
|
|||
|
|||
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... |
#2
Posted to sci.electronics.repair
|
|||
|
|||
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 |
#3
Posted to sci.electronics.repair
|
|||
|
|||
dual-dual potentiometer failure?
|
#4
Posted to sci.electronics.repair
|
|||
|
|||
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 |
#5
Posted to sci.electronics.repair
|
|||
|
|||
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? |
#6
Posted to sci.electronics.repair
|
|||
|
|||
dual-dual potentiometer failure?
|
#7
Posted to sci.electronics.repair
|
|||
|
|||
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 |
#9
Posted to sci.electronics.repair
|
|||
|
|||
dual-dual potentiometer failure?
On 03/14/2019 10:36 PM, bitrex wrote:
On 03/14/2019 03:52 PM, wrote: 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 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. 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. |
#10
Posted to sci.electronics.repair
|
|||
|
|||
dual-dual potentiometer failure?
On 03/14/2019 03:52 PM, wrote:
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 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. |
#11
Posted to sci.electronics.repair
|
|||
|
|||
dual-dual potentiometer failure?
On Friday, 15 March 2019 02:45:19 UTC, bitrex wrote:
On 03/14/2019 03:52 PM, wrote: 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 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. transplant time. NT |
#12
Posted to sci.electronics.repair
|
|||
|
|||
dual-dual potentiometer failure?
In article , bitrex 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. |
#13
Posted to sci.electronics.repair
|
|||
|
|||
dual-dual potentiometer failure?
On 03/15/2019 04:32 AM, wrote:
On Friday, 15 March 2019 02:45:19 UTC, bitrex wrote: On 03/14/2019 03:52 PM, wrote: 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 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. transplant time. NT 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. |
#14
Posted to sci.electronics.repair
|
|||
|
|||
dual-dual potentiometer failure?
On 03/15/2019 02:03 PM, bitrex wrote:
On 03/15/2019 04:32 AM, wrote: On Friday, 15 March 2019 02:45:19 UTC, bitrexÂ* wrote: On 03/14/2019 03:52 PM, wrote: 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 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. transplant time. NT 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. Ah, right this one. https://en.wikipedia.org/wiki/Finagle%27s_law |
#15
Posted to sci.electronics.repair
|
|||
|
|||
dual-dual potentiometer failure?
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. |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Remote controlled audio potentiometer (OT?) | Electronics Repair | |||
Technics SE-9060 DC on input potentiometer | Electronics Repair | |||
Safe to use WD40 as switch or potentiometer cleaner? | Electronics Repair | |||
Dual Potentiometer sources? | Electronics Repair | |||
Potentiometer and Capacitor Questions - Can anyone help me ? | Electronics |