Oh boy, what have I done? 9090DB volume pot rebuild

tnsilver

Stereo Puppy
I've lost a right channel. Again... This time all the cleaning and deoxit in the world could't help. I could see my 1KHz signal from the switch board entering the volume pot, but on the tone control PCB, only the left channel was live. Darn it! So I've watched @Kale do it, and I got a little jealous, and I also figured, what the heck, if I don't get this right I'm gonna need a replacement volume pot anyway, right? So I started taking it apart like this:

zGUtJ9I.jpg


and pretty soon, wafer after wafer it was reduced to this:

bkzpugg.jpg


and finally to this:

3hTYcvp.jpg


I used about half a can of D5 and a toothbrush, plus some autosol metal polish to get it all clean and shiny:

AkahbBS.jpg


and then it was time to put it back together (I used JBWeld to lock the shaft back in place on the last wafer):

kAfgikY.jpg


I don't know why that back panel reads 250KΩ for the VR's on this thing, all of them are well above 360KΩ, but whatever... so, I closed it up:

G0KyYPK.jpg


and voila - disco! :banana:
 
Nice work !!! Takes a brave man to go that deep!!

I have done the function switch and dolby switch completely apart on a QRX9001, the good thing is, you know its going to work when you get it back together.
Sometimes you just cant remove all the oxidation and debris that can be worked loose by the deoxit.
 
Takes a brave man to go that deep!!
Oh no, not me. It totally freaked me out. That's why I took dozens of photos - just in case. Let's see some of those because there's never enough when you really need them:

4th wafer - volume control: Under the locking washer, a rubber spacer, then a flat nylon disk plus a wiper plate on the inside. The edges of the shaft are spread inside the rectangular opening in the center of the retainer washer. I squeezed them tight with long pliers. Unfortunately, it's irreversible and I had to jbweld it:

wMIdq2z.jpg


zGUtJ9I.jpg


XQRDHlJ.jpg


3rd wafer - volume control: Nylon disk with full turn restriction and wiper on the other side

YKNqr9F.jpg


2nd wafer - balance control: 2 springy plates, one steel, one brass, locked in position inside the tin cup. This gives (the otherwise smooth) balance control the little click feel in the center, by making the brass semi-circle spring into a shallow dip in the nylon disk

SgoDFJn.jpg


...and behind all that, the nylon disk with the dip and the wiper inside. Here's where the balance shaft begins (or rather ends) by spreading it's lips and clinging to the nylon disk for retention

QIObjrJ.jpg


1st wafer - balance control:

uLsKbec.jpg


ffNvzGB.jpg


...and that's pretty much all of it:

3hTYcvp.jpg
 
Last edited:
Braver soul than I sir, what did you use for that thin shaft grease that seems to congeal on those old multi section potentiometers?
Makes me really want to purchase a small ultrasonic cleaning machine
-Lee
 
I would definitely lose sleep over tackling that pot if it was me. Great pictures and I really appreciate the effort you went to.
 
Well done sir. There is always a feeling of accomplishment when you get something like that back together in working order. And now it's good for a another few decades :)
 
Well done, a braver man than me. Did you compare zero volume before and after? It would take that level of disassembly to cure that problem.

- Pete
 
This is neat, but it doesn't sound like it would fix the common issue with these, where the volume control doesn't completely cut off the volume when it's turned down all the way. I guess if you found the right donor pot you could replace the wafers...
 
This is simply outstanding work. I've not had one of these dual function pots torn down yet to that extent, but seeing this thread at least tells me it can be done successfully. I'd had doubts about this particular pot due to the design of it, but it has now been proven "doable". Thank you for taking the time to document this.
 
Back
Top Bottom