Lexicon PCM42 B10k pot repair

[DrGonz78]

This is the older early 80's Lexicon PCM42 and I am trying to repair a bad pot. Why repair and not replace? I don't think I will be able to find this pot anywhere... The pot has a hairline crack just right where the tracks meet the connection points to the pins and I think it can be mended somehow. So in other words I am wondering if I dab a bit of conductive glue (i.e. graphite glue) or something if it possible to bridge the connections again? I have a attached a pic of the pot. I have another pot that I can test this on to see if it would work first. Anyone have some experience doing something like this?

[Enzo]

Well, good luck.

I used to have some Effecttron units that used a similar pot. I used my last one a while back. Is this some sort of balance control?

Did you call Lexicon on the off chance they have one?

[DrGonz78]

Yeah there are two pots in this unit just like this one. This one is for the Manual Delay Mult. function and it creates an automatic toggling effect between two different delay settings. The other pot that is like this on the unit is a mix pot for the delay. These pots are so prong to crack due the age of the unit. Good thing about the crack is that it is hairline and not just snap crack. I will try calling Lexicon but my guess is that these things are about as obsolete as it gets. I think I will try using a graphite type paint/glue to bridge the crack and see what happens. Will let you know how it goes, thanks.

[Mick Bailey]

Wonder if a Circuitworks PCB pen would work?

[MarkusBass]

What is missing here is a photo of the shaft and a photo of the circuit where the pot is fitted. Most probably such a pot is not available but very similar pots are still on the market. I mean stereo pots with fourth lug. And they are very cheap (about $1 in Poland). You can just ignore one section and connect the pot with wires to the PC board (if there is enough space in the enclosure). That's why a photo of the PC board would help.

Mark

[Enzo]

If this is what I think it is, the fourth leg goes up to a center tap. Half a stereo won't cover that.

[DrGonz78]

Enzo is right on the money. The fourth leg is center tapped to the main path of the wiper circuit. Off to get some graphite glue as I feel it might be the only thing that will hold up. I have some other crazy ideas to try but this one first lol. I don't think the conductive pen would hold up well but still a thought I had too, thanks. I will take pics of the board later, but here is the front part with the shaft.

[Jazz P Bass]

Not too sure of the CT value but try this: https://syntaur.com/Items/3750.html

[g1]

That one Jazz posted looks like it, worst case the knob pointer may be 180 deg. out. Looks like the shaft is off center the same as original.
Another option is just using a standard pot. From the sounds of it, the tap is not critical: PCM-42....where to find some 16mm 10KB center tapped pots ,

[Jazz P Bass]

Lexicon PCM42 Schematic

Here is the schematic & board layout.

[DrGonz78]

Thanks Jazz and G-One! Yeah so it is not that important to have the fourth leg but still I think I will just buy that part anyway. Thanks Jazz especially for finding a source for the part! I will look closer at the schematic in relation to that fourth pin. I already had the scheme since I had to fix other problems early on.

Anyhow I messed around with some of that crappy graphite glue and brushed some on a piece of paper. After drying you get an impressive 28k resistance!! Do not buy that stuff it is better to make your own and get the resistance way down. Well thanks again everyone that posted, you guys are great!

[DrGonz78]

Just reporting back on this guy. First to note my brain was not working while I was posting on this thread a week ago, not the first time . I really meant the pot was for the wave form part of the circuit and it was sine to square wave adjustment, R127. Either way the new one came and it installed to fix the problem.

Notes: The D-shaft is 180 degrees out of rotation to the original pot. I was able to make a knob adjustment to make min and max margins point in the right way as the pot is turned. Also, the threaded part of the shaft would not fit through the hole of the face plate. I had to drill and enlarge the hole a bit to get it to fit. Other than those obstacles the unit is working great and I thank you all again.