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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…

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