Help finding potentiometer replacement

Beavs

Beavs

Active Member
I have a Denon PMA-850 that has a bad balance potentiometer. It's a dual gang 100 kOhm potentiometer. I had sketched up the attached for my own reference in trying to find a replacement (so kind of sloppy but made sense to me). This is a 6mm smooth shaft for a set screw. Also has a center detent which is nice, but not required on the new one.

I thought I had found a replacement Alps pot for this (though a bit larger case dimensions, it ended up snugly fitting), but it was pointed out to me that I ordered an Audio Taper pot rather than a Linear Taper. Searching to find a Linear Taper pot I can't find anything to work from Mouser, Digikey, PartsExpress, Newark....any ideas on where I could find a suitable replacement?

General Info:
100 kOhm
Dual Gang
6mm Smooth Shaft (slotted with smooth circumference is fine, no knurled, flatted, serrated if possible)
Approx. 25mm Shaft Length (this is inclusive of thread length between abutting face and end of shaft)
5mm pin spacing for the 3 pins on each gang
10mm pin spacing between the two gangs

Pics of original pot and dimensions attached.

Link to the Alps pot I ordered for reference because it fit just right, but to get the two channels even it was at like 3:15-3:30 instead of 12:00....also think wattage on this one may have been lower than I should've used.
https://www.mouser.com/ProductDetail/688-RK27112A00CC
20211116_181831.jpg 20211116_181814.jpg 20211115_090000.jpg
 
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avionic said:
Colfax...:biggrin:
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Ha! I generally steer clear of the Fax but would gladly make the journey if I could find the pot.

That Alps pot that I linked is actually about the space limit for width and depth. Depth could increase but would have to get creative with a couple caps.

I already had the Alps pot in, problem is that balance between L/R occurs at about 3:30 and left channel has more gain than right at full turn each way. Looks like taper is 15A on the Alps. If it was the same taper as the original the Alps would work just fine...I've looked and cant seem to find this Alps pot with linear taper (as I understand it anyway) anywhere.
 
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Also, as best as I can explain this in writing, all 3 pins of each gang on the Alps pot are "active" whereas only 2 pins of each gang are active on the original pot. By "active" I mean resistance changes...old pot no change measuring to 1 of the 3 pins on each gang.
 
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Beavs said:
Ha! I generally steer clear of the Fax but would gladly make the journey if I could find the pot.

That Alps pot that I linked is actually about the space limit for width and depth. Depth could increase but would have to get creative with a couple caps.

I already had the Alps pot in, problem is that balance between L/R occurs at about 3:30 and left channel has more gain than right at full turn each way. Looks like taper is 15A on the Alps. If it was the same taper as the original the Alps would work just fine...I've looked and cant seem to find this Alps pot with linear taper (as I understand it anyway) anywhere.
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Have you looked for an equivalent pot by Bourns? Might be a possibility.
 
DEAFASABAT said:
Have you looked for an equivalent pot by Bourns? Might be a possibility.
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I looked for any 100K pot, not specific to brand, on Mouser, Digikey, Parts Express, Newark, Ebay, and I'm sure there were a few other sites. I think I also tried Bourns website but was mostly coming up with small trimmers as I recall.
 
Have the same issue here trying to find the replacement potentiometer for the bass control of my Technics SU-V85A. Discovered the threaded sleeve broken off the main body (see pic) a couple of weeks ago when I was exploring around. It is marked “M100KohmC 98B” but don’t know what it means. Very interesting to see your findings. I did see a lot of various parts on AliExpres_ but not sure about fitment and quality.


InkedIMG_7455 (002)_LI.jpg
 
EC1 said:
Have the same issue here trying to find the replacement potentiometer for the bass control of my Technics SU-V85A. Discovered the threaded sleeve broken off the main body (see pic) a couple of weeks ago when I was exploring around. It is marked “M100KohmC 98B” but don’t know what it means. Very interesting to see your findings. I did see a lot of various parts on AliExpres_ but not sure about fitment and quality.


View attachment 2409368
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Is your bass control still working properly or is the pot entirely shot? If it's still working properly, you could very carefully get some JB Weld on there to hold the threaded sleeve to the body, no? Let the nuts/sleeves of the other pots do the bracing work to the faceplate and just handtighten the nut on this sleeve.
 
@Beavs,

Yes, it is still working. As you mentioned, the other pots are doing the bracing work to hold this board to the faceplate. JB weld? Never thought of that, might try that. Thanks.

Another thing makes me wonder, I see a lot of “grease” inside that broken pot, how does the Deoxit D5 + F5 as I learned will clean and replace the original lubricant in there if you really need to clean it? Seems impossible to replace that much of grease with F5, maybe I am wrong.
 
EC1 said:
@Beavs,

Yes, it is still working. As you mentioned, the other pots are doing the bracing work to hold this board to the faceplate. JB weld? Never thought of that, might try that. Thanks.

Another thing makes me wonder, I see a lot of “grease” inside that broken pot, how does the Deoxit D5 + F5 as I learned will clean and replace the original lubricant in there if you really need to clean it? Seems impossible to replace that much of grease with F5, maybe I am wrong.
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Grease would be for lubricating the shaft inside the sleeve...typically no good way to re-grease that but you seem to have the opportunity to do so if it needs it.

The D5 and F5 can flush that lubricating grease out if you overdo it and then the pot would get a loose or gritty feel when turning it. I wouldn't put any grease on the shaft because the sleeve would probably push most of the grease into the resistive element inside the pot...if you see any grease in the body of the pot, probably try to remove it so it doesn't mess up the resistance of the pot. You could put some grease inside the removed sleeve and then allow the shaft to push the excess grease out the front of the sleeve. Will need a clean surface though on sleeve and pot body for the JB weld.

Also, F5 deoxidizes but also leaves a thin film of conductive lubricant for the resistive element to slide along, D5 mostly just flushes and deoxidizes. I'm no expert on this, so take it with a grain of salt. The pots with stepped individual contacts I use D5 on. I believe your pot is a smooth continuous contact element and those I use F5 on...lack of lubrication can cause one the contacts to scrape off the continuous resistive element. Look at the Alps volume pot I took pictures of in this thread...metal contacts slide along the continuous resistive element and F5 helps to lubricate....again, no expert, just my understanding:

https://audiokarma.org/forums/index.php?threads/denon-pma-850-troubleshoot.978577/#post-15207392
 
Also, VERY small amount of JB Weld...get that inside the pot and better start looking for a replacement pot!
 
@Beavs, thanks for writing this for me. And sorry for hijacking your post.

I kind of understand it now, still new to this hobby. The grease I saw is to lubricate the turning shaft and nothing to do with the actual conductive and resistance electricity function (wiper maybe it is called?). So the D5 and F5 are the ones to clean and lubricate them to restore to their original state.

I saw this post about how member leesonic replaced his pot and hope will help a bit. Will also interest in what you find and resources so I can find mine

https://audiokarma.org/forums/index.php?threads/technics-su-v85a-amplifier-restoration.963896/
 
EC1 said:
@Beavs, thanks for writing this for me. And sorry for hijacking your post.
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No problem. Finding the correct replacement parts is probably the hardest part of this. There are some folks on here that seem to know some of these parts off the top of their head....just have to dangle the right bait I guess to get them to bite on your thread.

If you unsolder that pot and get good pics of it so folks know how many pins/arrangement, approx dimensions, and any marking, you may get some folks to chime in that have an idea of a replacement. I'll even take a look since the search for the pot I need has fell pretty flat, but I already made the mistake of ordering the wrong Alps pot for my application so proceed cautiously with whatever I find!
 
I am not sure if your potentiometer is ALPS or NOBLE.. If it is a balance potentiometer it might be a special kind that has 50% of the carbon tracks metalized so they have constant near zero ohms resistance in that 50% of the track. The two gangs are arranged so that the metalized portions are opposite (i mean the front gang has metalized the first 50% while the rear gang has metalized the second half of the travel). I believe that the "1Z" code of the part number means that. Upon rotating the shaft from the center detent, the resistance of one channel drops while that for the other channel remains constant at nearly zero ohms. I happen to have a 250K pot like this which looks similar to yours and I verified that mine has the behavior that i described upon rotating the shaft.

Beavs said:
Anybody have a good source for pots?
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Have you considered this brand?. They are made in Taiwan, reliable, and reasonably well constructed. This model will require an extension of the shaft to fit in your application.
Alpha also makes the balance potentiometers with the center detents and with the metalized tracks like I described. This will be the alpha "MN" type.
I have one here to show how it looks like:20211204_191518.jpg
This particular part I bought it in Bangkok, Thailand, but Mouser and other providers carry this brand in the US, and I have seen other companies carrying them. Pin spacing is 5mm.
20211204_191542.jpg
20211204_191608.jpg

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You can get parts from Thailand, in fact TAYDA is based in Bangkok. They also have the MN potentiometers. Jus make sure it is alpha brand. There is an American company that goes by the same name. You can give them a call and see if they have a suitable part for you.
 
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JoseHH said:
I am not sure if your potentiometer is ALPS or NOBLE.. If it is a balance potentiometer it might be a special kind that has 50% of the carbon tracks metalized so they have constant near zero ohms resistance in that 50% of the track. The two gangs are arranged so that the metalized portions are opposite (i mean the front gang has metalized the first 50% while the rear gang has metalized the second half of the travel). I believe that the "1Z" code of the part number means that. Upon rotating the shaft from the center detent, the resistance of one channel drops while that for the other channel remains constant at nearly zero ohms. I happen to have a 250K pot like this which looks similar to yours and I verified that mine has the behavior that i described upon rotating the shaft.


Have you considered this brand?. They are made in Taiwan, reliable, and reasonably well constructed. This model will require an extension of the shaft to fit in your application.
Alpha also makes the balance potentiometers with the center detents and with the metalized tracks like I described. This will be the alpha "MN" type.
I have one here to show how it looks like:View attachment 2412444
This particular part I bought it in Bangkok, Thailand, but Mouser and other providers carry this brand in the US, and I have seen other companies carrying them. Pin spacing is 5mm.
View attachment 2412445
View attachment 2412446

EDIT:
You can get parts from Thailand, in fact TAYDA is based in Bangkok. They also have the MN potentiometers. Jus make sure it is alpha brand. There is an American company that goes by the same name. You can give them a call and see if they have a suitable part for you.
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Hey Jose, this looks like the best lead I've got so far on this. The 50% of the track, opposite sides for front and rear (or R/L channel technically), are just as you describe where resistance at 12:00 is at about 1.5 Ohms and only drops to about 0.7 Ohms when fully rotated to the 5:00 or 7:00 positions in opposite directions, whereas the other channel will increase up to the 100 kOhm value.

Smooth shaft will be preferred since the knob has a set screw and not serrated...but serrated could maybe work if I can get the knob aligned properly.

One question, you mentioned the model you showed would require a shaft extension. If I can't find one with a long enough shaft, have you extended the shaft on one of these before and if so, could you give me an idea of what you did? I'm thinking maybe a set screw sleeve type of coupler with a 6mm smooth shaft piece...finding those may be easy or a huge pain, I've yet to look.
 
I have never try to extend the shaft of the potentiometers. What I would do is to use a lathe to make a part from aluminium. Alternatively, if you find a bushing (could be a plastic bushing or tube you may be able to glue a piece of potentiometer shaft.
But Alpha makes many types pf potentiometers and they also make some with a round shaft. You need to search in TAYDA. Mouser also carries this brand but I do not know if they will have the MN type. There are some on ebay.
What happened to the original part is it damaged?.
 
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