Pioneer SX850 volume control pot - Page 1
Forum section: electronics / repair

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 16, 2017, 08:36:49 pm
content: I'm working on this classic receiver for a friend. It was re-capped about 7 years ago and someone at that time also replaced the volume control pot. Now this pot has issues and needs to be replaced.The original potentiometer was a 50k B taper dual gang pot -- with center taps, so 4 terminals on each gang. These are pretty much unobtainable now. I did find one actual correct pot, used, for around 30 dollars US, assuming it hasn't already been sold. The replacement that was installed is an ordinary dual gang 6-terminal pot, and the technician just left the former center tap connections going back to the Loudness contour switch, unconnected. So the Loudness switch doesn't work. Also the tech needed to shorten the shaft so he cut it in two, removed a section and then reglued the knurl back on so it would hold the original knob. This worked, sort of, but over the years the glue softened and failed. Needless to say this kind of "fix" is unacceptable and even if I just put in another 6-pin dual ganger it will have the right shaft length at least.So here's the main question: I read on another forum, not related to SX850, that in audio applications one may use a regular 3-terminal pot in place of the unobtanium 4 terminal center-tapped pot, by connecting the part of the circuit that would have gone to the center tap, to the wiper through a 2.2k resistor.Will this work acceptably well, or at all even? If it does I'll be able to save my friend some money and time. I'd like to get some opinions before I open the thing up again and solder in a couple of 2.2ks, since I'm waiting to buy a pot anyhow.(Blows my mind to think that this thing cost about 550 dollars in 1976 money... that's like 4 grand today. It sounds great, too.)

author: Toasty
author_rank: Regular Contributor
timestamp: September 16, 2017, 09:51:40 pm
content: Can't offer any advice, but there is a brand new one on eBay for $44:http://www.ebay.com/itm/232490671971PN: ACV-164-0Looking at it, I'd say the tap is at 20k.Hope you get this fixed. Love the old stuff!T

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 16, 2017, 10:17:37 pm
content: Wow, thanks for finding that, somehow I missed it in my Ebay searches.I'll have to see if my friend wants to spend that much, but really, I think it is worth it to get this unit happy again.I'd still like some opinions/facts on the question of using the 3-term pots instead of the 4 terminal ones by using the 2.2k (or other value) resistor to the wiper.ETA: friend just texted me that he is ordering the new pot! Let's hope he gets the order placed in time. It's the right thing to do to put this beautiful oldster back on her feet.

author: SeanB
author_rank: Super Contributor
timestamp: September 16, 2017, 10:30:37 pm
content: The Rc circuit simply places a gentle slope low pass filter in the volume path, that will be less effective at high volume, just like the original pot but with a slightly less pronounced effect. Will do the job of boosting the lower frequencies at low volume though.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 16, 2017, 10:34:23 pm
content: Thanks, I will give it a try while waiting for the NOS part to arrive.