Help identifying this oddball Alps potentiometer - Page 1
Forum section: electronics / general

author: barbeque
author_rank: Contributor
timestamp: January 06, 2020, 07:48:01 pm
content: I finally figured out why my 1983 NEC PC-6001 doesn't have any sound output, and it took me an embarrassingly long time.It looks like the black plastic volume knob on this was sticking out from the computer's case when it was transported, and at some point in shipping it got punted pretty hard and snapped the substrate right off around the metal shielding.I tried holding the pot together really tight with one hand (cheaper than super glue) and turning the knob to see if I could get any resistance reading out of it, but the little traces have some visible breaks in them. Maybe this could get fixed with wire glue or a conductive pen? I probably can't solder to that black carbon-y material.I can always adapt another pot - and probably will in the meantime - but I'd prefer to keep at least a similar footprint. I don't even know what this kind of "tripod" arrangement would be called.The numbers on the top of this (supposedly 10K on the schematic; actually 3KΩ) Alps pot are "223a 3KB" and there are no other markings on the body. I haven't seen a lot of potentiometers with a big structural rear leg like this before, and judging from how it failed, it didn't help much with strain relief.If there are any pot heads out there, please lend me your acquired wisdom.

author: barbeque
author_rank: Contributor
timestamp: January 06, 2020, 11:58:09 pm
content: For now, I took it apart (just had to pry back the crimps) and super-glued it together. It's obviously not continuous, but I'm going to try some of that carbon-black conductive paint and hope for the best.Thought some other posters would like to see the inside of this thing. I really like the little wiper prongs.

author: German_EE
author_rank: Super Contributor
timestamp: January 08, 2020, 07:29:52 pm
content: For a volume control the value shouldn't matter too much, I've seen everything from 1K ohm to 1M ohm. It looks like a standard PCB mount pot so try heading off to the goodwill stores and looking at the old TVs and stereos.You may end up doing some creative engineering to get the knob to fit on the new shaft but that's the sort of thing that makes life interesting.

author: MT
author_rank: Super Contributor
timestamp: January 08, 2020, 08:09:51 pm
content: Looks like a custom oddbal in the standard P120 model size. Lots of air space to fit a P160, P170 RK09 types pot as replacement

author: andy3055
author_rank: Super Contributor
timestamp: January 08, 2020, 09:06:40 pm
content: Will this help?https://www.mouser.com/ALPS/Passive-Components/Resistors/Variable-Resistors/Potentiometers/_/N-blay9?P=1z13wb5&No=200&gclid=EAIaIQobChMIlMHb2_P05gIVkhx9Ch3jGwwXEAAYASAAEgJPlvD_BwE

author: Yansi
author_rank: Super Contributor
timestamp: January 08, 2020, 09:39:46 pm
content: Quote from: German_EE on January 08, 2020, 07:29:52 pmPretty invalid assumption, considering you (we) don't even know how the circuit looks around it!

author: andy3055
author_rank: Super Contributor
timestamp: January 08, 2020, 09:40:31 pm
content: Quote from: andy3055 on January 08, 2020, 09:06:40 pmI have these if you want. One 10K and one 22K. PM me if you want.

author: barbeque
author_rank: Contributor
timestamp: January 09, 2020, 06:07:52 pm
content: I ended up super-gluing the substrate back together, and then I picked up a tube of "electric paint" conductive paint.After a few coats of that paint, I was able to detect resistance between the two front terminals (initially 4.7kΩ, but as it dried over the next hour it lowered to 3.03kΩ - very strange and unexpected phenomenon), and then I reassembled the pot and tested it. Works great!Not sure how much longer it will last, but it seems to be working great now. I was really lucky it broke where it did because it gave me plenty of room to glob paint.Thanks for the help, everyone!Video of the PC-6001 working (just playing ABCDEF in a constant loop):https://youtu.be/jYUUjryPhl0