M90 Volume pot.

[Tinman]

I just wanted to throw my 2 cents in on the M90 volume pot subject.
Here's one of the threads that discussed it:
https://boomboxery.com/forum/threads/for-those-that-have-m90-and-maybe-on-there-wishlist.18208/page
When I took my current M90 apart, I found a tiny metallic piece that was sitting in the bottom of it.
After it sitting on my work bench for a week or so, I inextricably threw it away.
As I realized much later, it was one of the wipers from the volume pot.
The other wiper was gone before I took it apart so I needed a replacement anyway.
I started reading some threads here and found the EWCT5AF20B54 pot as a possible replacement that has the elusive seventh tap.
I ordered one from Greece (It doesn't look like they're available any more).
I installed it and found the balance would only work when centered and it sounded fine.
I didn't know if the balance was working previously since the volume pot wasn't.
After verifying the balance pot was good, I took the new pot apart and compared it to another M90 volume pot.
I found the two inner most wiper arms were in a different position to the originals but didn't know enough about pots to know if this would cause the balance to not work correctly.
After reassembling everything, I was going to live without the balance working.
Then, testing the radio, I noticed the treble wasn't working correctly so I took it back apart and tore into the treble pot finding missing plastic retainer heads on both wipers.
I took the bass pot apart finding the same two heads missing.
I know JB weld or some form of epoxy has become the preferred way of fixing them but I'm partial to good old Krazy Glue.
It's loose so it creeps under and into small crevices so that's what I applied to both sets of wiper arms and let them set overnight to make sure everything was dry.
I ended up taking the new EWCT5AF20B54 shaft/wipers and metal cover from the volume pot apart and crimped it to the bass's board.
I crimped both original Krazy glued pot shaft/wipers onto the treble and volume boards.
I reassembled the amp boards back onto the chassis and powered it up.
Volume works, treble works, bass works and holy $hit, the balance works.
I was and am still ecstatic that after the ridiculous amount of time and effort I put into this box (this was one of many, many problems with this box).
Having all controls working, including the loudness feels awesome.
I'd suggest, if you can find a similar pot, crimp that shaft/wipers on the bass or treble boards and crimp an original pot shaft/wipers on the volume board.

Here is the new EWCT5AF20B54 wipers on the left compared to an original wiper assembly.
My assumption is that the original wipers are basically sawing the plastic heads off over years if being twisted back and forth.
Both treble and bass wipers were missing the top heads only, eventually the bottoms would've broke off.


Here is the original volume board on the left and the new EWCT5AF20B54 board on the right.
You can see my little black jumper for the seventh tap is sticking down.
The original board is bad.
I'm assuming the wipers fell off and cut it up while being tumbled around before falling out.




Thanks to caution and chuckulaa who's previous work and posts led me in the right direction to get mine up and working.

 

[caution]

Nice work! This would be a death sentence for most M90 owners, it's no small task repairing these.
I guess if the volume pot ever died for good, it could be replaced with a normal one and run the loudness tap to a loudness switch, just have to turn it off if you go past 40%

 

[Tinman]

Thanks.

Something that might save somebody some time, while I was dismantling the other pots, I took one of the rec volume pots apart thinking maybe they'd be of some use.
Unfortunately, those two look like the balance pot so they might be able to repair that one but they look to be useless as far as the bass, treble and volume pots are concerned.

 

[Eric]

Hi Guys,
Need some advice with my M90 volume pot. I noticed my balance control was messed up -no sound unless placed in the middle. Switching from stereo to mono also didn't produce stereo imaging even though the light was on. Tried line in and I'm getting sound out of both channels with only one side plugged in. Sure enough the volume pot upper wipers had broken lose. Luckily they were not damaged. I was able to apply a small amount JB plastic Weld and they are holding nicely. But it did not fix my balance issue? Took the Pot back apart and pin 2 (left loudness tap) does not seem to connect to any traces (I used tinman's image below). I assume it should connect to the trace my arrow is pointing to? Once the signal hits the green area (tip of my arrow), it does not show back up on any traces. All the other connections ohm out fine. Are the bass or treble control boards an option. All the other pins seem to be working and have correct resistance readings. I'm not really sure if this is my problem or if there is another problem. Balance pot tested fine out of circuit, the bass and treble controls work. Loudness seems to work fine too, but there is definitely something going on and the L and R signals are getting combined at some point. What's your thoughts?

 

[docs]

Im not certain as to this specific pot issue but I have successfully restored connectivity of traces before by using 3 or 4 layers of conductive paint built up over several hours. If it is confirmed that the trace is disconnected you can restore it.

 

[caution]

This is a multi-layered pot board so it's not easy to see, but pin 2 is the midpoint tap for the left channel, so once it disappears where your red arrow is, it likely runs under the outer ring, isolated by a layer of soldermask, up to the center of the outer carbon ring, where that little green dot is way up there.

Pin 4, the right channel midpoint tap, is a little easier to see. Its trace runs north, where it disappears under the left side of the inner conductive ring, and reappears above it, where it then ties into the midpoint of the right channel (inner carbon ring).

 

[Eric]

It does indeed run up to the green dot near the top and I do have connectivity with my ohm meter. Thanks caution for that info. Guess I will trace the line in circuit and see where the signal is getting combined. I suppose the base or treble wipers could be broke and shorting some traces out, but it sure seemed like they were working fine? Its not the easiest to follow with these multitap pots. Seems like others have had this same symptom with the balance going screwy and the volume pot was the issue. Thanks, docs for the conductive paint tip. I'm sure I'll need it some time.
Let me know if you guys have any thoughts. Stereo mono switch tested fine (in circuit), I can't recall if that affects line in and makes it mono or not.

 

[caution]

Every pot wiper in this box is vulnerable to wipers detaching. They're under constant pressure in order to contact the carbon track firmly, and eventually the plastic posts that were melted down to create "rivets" to hold them in place on the shaft plate begin to weaken and fail.

If it helps visualize it better I drew the layers in the order which they're applied. First the main wiring (green) is added, it's probably copper but it could be silver paint, I can't remember. Then the carbon tracks (red) and soldermask (light blue) are added. On the real board, the soldermask looks dark green. Once the original wiring is covered in soldermask, a layer of silver paint (pink) is beadded on top of that. You can see how the outer silver track stubs off to connect to pin 1, and pin 5 has a trace that runs up to the inner silver track. The last two pics show the carbon, silver tracks and soldermask on their own for clarity

 

[Eric]

Who would guess so much engineering would go into a potentiometer. Thanks for the detailed drawings, that helps visualize the connections and what is going on as the shaft is turned. Frustrating these are no longer available. With everything electronic/digital the need for these types of controls makes it cost-prohibitive to produce. The details that went into producing this simple yet complex volume control would not be done in today's cost-cutting builds. Control is now done electronically with a circuit opening/closing a switch -I'm sure it's cheaper.
I injected a 1k signal into the board on the left side and I had a signal on both sides of the balance control connection on my scope. I noticed if I moved the treble just a certain way the signal would totally disappear on the right side and was fine on the left (how it should be). I pulled the bass and treble pots, and sure enough, the upper wipers were broken in each, neither one was damaged. Super glue tonight. Update you all tomorrow.

 

[caution]

It's just a cheap pot. A better one would have a dedicated stage for each channel, have wires coming off the top for the taps, shafts made of metal, more robust attachments for the wipers..... overall a much better chance at fixing any issues. One could tap a normal volume pot with the right tools and skills, but can still be difficult to get right.

Here's the pinout in a little more detail

 

[Eric]

Everything is working normally after repairing wipers on volume, bass, and treble. The inner wipers were broken on each pot. Super glue seemed to work the easiest without worry for thickness. There is not much room between the trace board and the back of the shaft in order to glue the wipers back into position. My primary symptom was balance control had to be centered to get sound and no stereo output, hooking up one line inputs would produce sound in both channels. stereo LED would work on radio, but flipping the switch between stereo and mono did not change the sound (still sounded mono). Thanks for all the data caution that you provided. It does indeed seem these will be a point of failure as time goes on.

 

[caution]

Yeah, the loose wipers in the balance pot probably shorted the channels together, and the stereo/mono switch had no effect because it sits behind it in the audio path.