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Pioneer sx-2000 mode switch

The value written on the pot is 500k, same for the volume pot but both read much higher at their max resistance. When reading from end to end I get 700k on the front and 1200k on the back side.
 
is it a dual section pot or single? Dual section are often log / antilog pots. If its single section with the middle grounded, its probably just a linear pot, nothing special.

I think Japanese "B" taper is log, where US "B" taper is linear. Nothing like being extra confusing.

anyway depending on the particulars, if you can't find a log/antilog it may have to be an MN taper. Those are pretty close to zero from one end to center and 500k from center to the other end where a log/antilog will be approximately 50k from center to one end and about 450k from center to the other end with the pot set middle of travel.
 
So the parts list says "Balance 500kilohm dual tandem" as far as how it's grounded here's a pic of the ground connections before I removed it PXL_20221122_180141318.jpg PXL_20221122_180131291.jpg
 
I was looking at this one since the specs are right and it looks to be the same size shaft. The price isn't bad either and they also have the 15k resistors I need to finish the job
 
Is it odd for a pot to drift like that? Ive rebuilt a few of these old tube amps and have always been able to get them clean and working properly without actually replacing the pots. Maybe I've just been lucky so far haha
 
not terribly unusual, carbon pots are pretty terrible and having the sections drift relative to one another is a headache. Fun when it happens in the tone control circuits, you get different response channel to channel. Or in the volume pot when it moves the balance around the room whenever you change it.

so dual pot. With the pot in the middle, measure from center to the left terminal on both sections and then center to right. Is the same side much larger than the other, or is the high side on the right on one section and on the left of the other? If its the same on both sides its a dual log pot. If its opposite, its log/antilog. If its about the same resistance center to either end, its dual linear.

I honestly have no idea what "dual tandem" is supposed to mean. I'd just assume its log/antilog since thats what a lot of balance controls are.


let me see whats in that SX-990 parts hulk when I get home. Doubt its the right thing but maybe we'll get lucky here and I have something that will be a drop-in for you.
 
That makes sense, and thanks for the testing info. I'll check it when I get back from lunch. And thatd be awesome if you happned to have a drop in on hand, if not I'll track something down I'm sure
 
Alright so after checking it's opposite sides so that makes it log/anti log. The pot I posted in the link I was going to replace it with is linear taper. Will that work? It has the right shaft and stuff so I really hope so!
 
ok finally got to this, Per the SX-990 service manual, the balance is a dual 500k pot, Pioneer p/n C85-048-0. I'll measure it next I'm out in the shop, hopefully its more on value. If its usable, PM me an address and I'll stick it in the mail for you.

twin linear might work, but the resistance will be off and it will probably have less volume than the original. Bigger problem is it may give some extra HF roll-off in the process. An MN taper more closely matches a log/antilog pot. Its finding something that fits the hole and knob thats usually the bigger chore.
 
Sweet! Sounds like that just might work, lmk what you come up with when you test it. And thanks for the info if yours isn't going to work I'll look for a different one
 
On the second pic, front section, across the outside terminals from black to white I get 555k. Black to purple 550k, purple to white 5 ohms.

Rear section, orange to black 520k. Orange to blue 180 ohm, blue to black 519k.

That's approximately center, so it looks like it might be an MN taper. I think it would be good enough though. The resistance from signal to wiper is low in the middle so it shouldn't cause any imbalance problems. Can probably tweak the knob just a bit and get it very close. Its within 10% of tolerance overall too which isnt terrible.
 

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Alrighty, I had time today to finish up this pioneer. Thanks to Gadget hooking me up with a replacement balance pot this thing sounds great now. Unfortunately I broke the tuner string when testing components in the volume/balance pot area but I was able to get it restrung today. I'm happy to finally be closing the case on this one and ready to get down to some serious listening and see how this thing stacks up against some of my other vintage tubes gear. As a nice bonus, since rebuilt the entire phono section thinking my problem was there originally, it's now dead quite and perfectly balanced! Thanks for everyone's help and hopefully I don't run into this much trouble with any other projects anytime soon haha
 
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