QX8000 volume pot?

slayer44

Well-Known Member
Has anyone used a replacement for this pot?...scratchy as hell and bought a replacement from ebay with the same issues. Seems to be a bad design. I deoxed it about 10 times and spun it a 1000, still scratchy. Was wondering if anyone found a new version of this four-ganged pot. I hate to think about cobbing something together. This pot doesnt come apart easily, but if that is my only recourse, then so be it. Just wanted a second opinion. Thanks.
 
Did you scope the circuit(s) under power before the volume pot? There's a hell of a lot of switches before the volume pot, that could be marginal or flat out dirty. Make sure every switch and pot on it is clean (in some cases you may have to dis-assemble and physically clean, not just DeOxit). Same with after the volume pot.

The only probable trouble maker transistors I see in it would be the 2sc871/870's. They aren't as bad as the 2sc458 or the 2sc725/726, so I'll leave it up to you on replacements. KSA992's are replacement.

Most likely the signal caps in it are just plain wore out, unless you've recapped it recently. Dried or drying out caps can cause this noise also, and also can cause systemic circuit failure. If it's all original, I'd recommend doing a recap of the signal chain from input to output, including the POWER SUPPLY (Do this board 1st as a failing P.S. won't supply the correct voltages.).
 
Hi Larry!...I did not scope the circuit, however I know the volume pot is bad because it has dead spots and turning it incrementally I can hear the contacts scraping through the speakers. I busted my old one open to see what could go wrong and the graphite(?) pads on the wafers just started disintegrating. This is a 1971 unit so I think the volume switch is beyond help. If I find a good spot in the volume position and work the rest of the switches, I get good sound, so my component upgrades shouldn't be that bad, but I can't go any further until I replace that pot. I could easily find a new four-gang 500k volume pot, but this one has a fourth leg at the bottom of the graphite which I have to find the reason for. The schematic shows it going to the switch unit AWS-004. I am still looking for a new switch, though.
 
QSilver...thanks Man...that might be an option, I am just concerned about the fourth leg of the pot going to the above mentioned board...see the attached pic (borrowed from Ebay, not mine). it has a leg attached to the center of the graphite track in addition to the three usually there. I could buy another one from ebay but might run into the same problem. I am already out 30 bucks for a bad one I bought from there. I appreciate it.
 

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That's for the LOUDNESS switch. One other option would be to contact Mark Oppat @ www.oldradioparts.net. He might be able to rebuild yours depending on who built it,etc. Is there any part information on the side of the shell of the original? He'll need that.
 
Oh I see...pushing the loudness button taps into a louder position on the vol control. Crafty. I went ahead and bought another switch from ebay which was cheaper this time and I contacted the seller first. He told me it was his personal amp and had no issues with the volume pot, but his pre died, so he just parted it out. I'll take one more stab at this and if it doesnt work, I will look into your suggestion. Thanks Larry. Stay tuned.
 
I took a look that the schematic and yeah... if push comes to shove, you could just build up a resistor to take the place of that extra feed from the volume pot?
 
I might be able to but I couldnt tap into the pot with it. I havent really studied the schematic yet because I am holding off until my replacement gets here. If that doesnt work, I will explore options for bypassing.
 
Did you follow up your cleaning with Caig Fader (Faderlube). If you did not then you are scraping the inside parts to their doom. Deoxit, especially the D-5 will remove the essential lubrication from the device. You have to replace that loss. Damage can be done very quickly so leave your pot be until you get some Fader or other equivalent product inside of it. I won't explain how I found this out. :confused:
 
Oh yeah I did. I use de-oxit to clean and work it, then blow out residue with compressed air, then faderlube. Rinse & repeat, Rinse & repeat...The pot is just disintegrating, nothing I can do. And I am not sure it's the graphite track that's the problem. Inside the diameter of the track there are two metal fingers that contact a metal ring attached to the pot shaft. This contact is surrounded by a white plastic disk that I don't think allows de-oxit to penetrate and clean the fingers. I will wait for my new pot then go from there. If that doesn't work, I will put a standard pot in there and work something out.
 
Sometimes it is the solvent in the D-5 that does the damage to the pads and traces. Fir that reason, I have always used the D-100 and F-100. I hope the new pot will clear this up for you.
 
Sometimes it is the solvent in the D-5 that does the damage to the pads and traces. Fir that reason, I have always used the D-100 and F-100. I hope the new pot will clear this up for you.

Ah hah!! D-100 is good IF it can GET to the corrosion,
so without the cleaner in D-5,
you need to use some other sort of cleaner to achieve what I have been known to comment as: "clean corrosion"...

At the very least, use D-5 in switches and avoid it in the pots?

IIRC you were having some problems with long term deoxiting performance on switches?
 
IIRC you were having some problems with long term deoxiting performance on switches?
That is true when the job was done by others. I keep a needle bottle of 50% Deoxit and 50% low aqueous IPA. It is very effective at flushing gunk from all of the above. I have never had it fail; including the most recent issues that I was having. I am, however, going to have to wait to deal with the dreaded filter switches.
 
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