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August 25, 2014

Granularity of Repair: fixing the volume control potentiometer in a Pioneer VSX-3800 receiver

Filed under: blog,electronics,environment,repair,Uncategorized — admin @ 5:14 pm

For several months, the primary audio system in Gimli’s media room (well, the couch is Gimli’s)  has had the annoying problem of an intermittent left channel due to a problematic volume control element, one part in a triple-ganged, motorized potentiometer in an otherwise great Pioneer VSX-3800 receiver . I’d tried the usual fixes such as a couple of “control cleaner” sprays, a few times, with only temporary success at best. So, based on previous similar experiences, this electronics doctor decided that it was time to operate on the patient. This time, I had no replacement part on hand, and it was looking impossible to get one shipped to me quickly. Here’s the procedure:
Pioneer VSX-3800 the problem pot

The receiver cover is easily removed after taking out a few obvious self-tapping screws. The motorized volume control in the VSX-3800 sits on its own small circuit board on the front-right of the unit.

 

Two machine screws hold the pot onto the front panel of the receiver, so I took those out. After that, there was enough working space around the volume circuit board to do this job. Next step was that I de-soldered all the pins (13 leads for the potentiometers and motor and volume LED, plus 2 lugs that just hold the motor mount in place), using a soldering iron and a “SoldaPullt” spring-loaded desoldering vacuum. That freed the volume control potentiometer assembly such that I could remove it from the circuit board. That was the easy part!

Next, I needed to take apart the volume control to get at the problem left channel potentiometer section. First, I was able to isolate which potentiometer was the problem one by tracing the circuit board and seeing which output was for the left channel. I used a small standard jewellers’ screwdriver and a couple of small pairs of needlenose pliers to lift up and bend upright the metal lugs at that potentiometer, which made it somewhat movable but not enough to get at the innards of the pot. Next, I opened up the motor gear area by bending up the lugs on its case. There still was not enough play in the system to get at the problem pot. Finally, by prying very gently with a thin blade, I was able to pop off the metal retaining disc on the pot system axle beside the final plastic gear in the motorized system. That freed up the whole pot assembly at last.

Now to fix the actual problem. Close inspection of the potentiometer resistor surface showed some form of dirt deposit on it (maybe from some spilled wine from some party in the receiver’s long history. I only got this receiver as a free castaway on craigslist so I don’t know the real history of the unit).  Seems the cleaning solutions were not able to remove that. So, I very gently sanded the resistive surface with 600 grit sandpaper (careful if you try this yourself, since you can destroy the thin resistor surface if you are too aggressive) to clean it. I also gently bent the metal wiper contacts so that they would apply more force to the surface.Then, I reassembled the unit piece by piece, referring back to my pictures, and bent the lugs for each section back into position. I put the spring loaded gear on the pot axle back on, and pressed the spring clip into place on it -that was difficult but eventually it went on. Finally, I put the motor housing back together with the pot and bent the lugs there back into place.I did some resistance checks with a DVM -all good. I soldered the assembly back onto the circuit board and put the support screws in, and then closed up the case.

As a first test, I played Neil Young’s great Live at Massey Hall album (1971) [no disco allowed on the premises] and adjusted the volume control from quiet to LOUD. All good -no more left channel issues – a successful repair with no new parts required. This was an instance of a fairly “granular” repair, in that an individual component was operated on and fixed, rather than replacing a whole assembly. Granularity of repair is one of many repair topics that you’ll be seeing in that upcoming book that I keep telling everyone I’m writing. Coming soon…

 

8 Comments »

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  1. I’m working on the exact same repair right now, but I’m having a difficult time getting the pot out. You mention the two screws that affix the volume control to the front panel of the receiver, but how do you get to those screws? It seems like you need to pull the volume control knob off from the front, but I don’t know how to get it off. Being that it’s plastic, I don’t want to try and pry it out and wreck it if that’s not how it comes off. Any hints?

    Comment by Jeremiah Johnson — September 19, 2014 @ 7:06 pm

  2. Never mind. I got it off.

    Comment by Jeremiah Johnson — September 19, 2014 @ 8:03 pm

  3. I just finished doing the same repair. Cleaned everything up and reassembled. Now the receiver plays at really loud levels no matter where I turn the volume knob. Any thoughts as to what I did wrong?

    Comment by Jay M. — October 13, 2014 @ 3:40 pm

  4. Jay, I’d guess that somehow you ended up with the contact for the potentiometer wiper shorted to the high, static output side of the potentiometer, or some other similar thing. Check the soldering there -maybe something on the circuit board got shorted during the repair. Or maybe the potentiometer wiper got moved high on the resistor and then decoupled from the shaft, so that nothing happens when you rotate the volume dial. I’d look into both those possibilities.

    Also, measure the total resistance of the pot out of circuit if you can, both channels, both the fixed resistance, and also the variable. The fixed I think should be 100K ohm, and the variable of course should vary as you rotate the pot.

    Hope that’s any help.
    Best regards,
    Dave

    Comment by admin — October 16, 2014 @ 7:31 am

  5. I have a Pioneer reciever with the same volume setup. Only thing wrong with it is the motor that turns the pots. I had the left channel drop out as well but it was in the AB speaker selector switches. I spent a couple hours last night taking them apart the gold slides that make contact with the rail were smashed by a tiny piece of plastic. I would like to know if anybody has tried to replace just the motor on the volume co trol and what is a good replacment without buying the entire assembly.

    Comment by Justin — December 16, 2014 @ 4:23 pm

  6. Might be worth a closer look at the failed motor in case minor surgery will fix it. It’s probably a brushless stepper motor type, so likely the issue is not contacts. Maybe it is mechanically jammed/stuck internally or the external parts are stuck. A resistance check might still be worthwhile before surgery, to check for open circuit e.g. in windings.

    Comment by admin — December 18, 2014 @ 12:47 pm

  7. I bought the Pioneer VSX-451 at the thrift store recently for $25. When I got it home I noticed the right speaker was noisy occasionally and sometimes cut out. Damn! I thought I had traced it to the volume control that looks just like the model you are working on so I sprayed tuner cleaner in the holes of the 2 pots, hell the motor too, and the volume knob LED light pot. No Go.

    So I plugged the speakers into the B-channel and nothing unless you wiggled the switch button.
    A-button same thing but B was really bad.

    Disassembly time. I took her all apart. Unscrewed the power supply transformer. Set it aside. Unscrewed the framework that holds the A/B switches. Disabled brackets, 2 of them. Removed the A switch cover held in place with a small tab. Corroded tiny silver contacts 12 of them in the A switch, 6 in the B plus 6 gold plated sliders, 4 in the A, 2 in the B. Cleaned them all up with 400 grit sand paper and scraping with X-acto knife plus spraying with carb spray. Upon reassemble I noticed the small tab on the A cover that locked the cover on was broken. 25 years makes nylon bridle. So I carefully epoxied the cover on being careful not to get goop in the moving slide part. B switch went back together without a hitch. Good to go.

    Comment by fieldguy7 — January 10, 2017 @ 5:17 pm

  8. Thanks for the great post.

    Comment by Jason Quest — January 16, 2017 @ 10:23 am

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