Author Topic: When you realize finding *that* spare part won't be easy  (Read 910 times)

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Online RoGeorge

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When you realize finding *that* spare part won't be easy
« on: November 25, 2018, 12:14:28 am »
???  Broken potentiometer



 :o

Online tautech

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Re: When you realize finding *that* spare part won't be easy
« Reply #1 on: November 25, 2018, 02:00:26 am »
That is a curly one !  :scared:
Here they list up to 8 gangs with a switch but not twin shafts:
http://www.potentiometers.com/select_ProAudio.cfm
Avid Rabid Hobbyist
 

Offline calexanian

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Re: When you realize finding *that* spare part won't be easy
« Reply #2 on: November 25, 2018, 02:58:35 am »
Wow. And that is the kind you can’t really open up and clean out.  Yeah that’s a mission impossible right there. I would go the try to buy another one route or a parts unit kind of thing first. Then if it’s an unreplaceable kind of equipment design a replacement with available components. Fortunately it appears to be on its own carrier board therefore room to design a new custom board.

Rough example right there
Charles Alexanian
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Offline Tac Eht Xilef

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Re: When you realize finding *that* spare part won't be easy
« Reply #3 on: November 25, 2018, 11:21:28 am »
???  Broken potentiometer
Fortunately it appears to be on its own carrier board therefore room to design a new custom board.

Agreed, though the fact that the back couple seem to be tapped may make it harder. But before you go down that route, have a chat to Mark Oppat at Old Radio Parts. He sells/rebuilds/repairs vintage radio pots, & might be able to help or give some suggestions.
 

Offline coppercone2

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Re: When you realize finding *that* spare part won't be easy
« Reply #4 on: November 25, 2018, 11:28:16 am »
better learn how to make gears :'(
 

Offline brabus

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Re: When you realize finding *that* spare part won't be easy
« Reply #5 on: November 27, 2018, 12:21:06 am »
I would say it's a rather classic switch/volume/tone Potentiometer from a car radio. The thread and knob fitments seem to be quite Standard; I should even have some samples in my lab!

Of course the footprint will be slightly different, but you can solder some wires and make it fit.
 

Offline Sudo_apt-get_install_yum

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Re: When you realize finding *that* spare part won't be easy
« Reply #6 on: November 27, 2018, 02:14:20 am »
I once had to replace a ceramic potentiometer for a job. The new potentiometer cost $70USD and was still being manufactured. This will cost a fortune if you are able to find one to begin with :P
 

Offline CatalinaWOW

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Re: When you realize finding *that* spare part won't be easy
« Reply #7 on: November 27, 2018, 04:47:03 am »
Ginning up a replacement shouldn't be impossible.  The key search term is modular.  Here is one example.

https://www.mouser.com/datasheet/2/427/p11sp11a-239880.pdf 
 

Offline coppercone2

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Re: When you realize finding *that* spare part won't be easy
« Reply #8 on: November 27, 2018, 12:15:49 pm »
I thought it was non-linear and used gears to adjust the turns ratio ?

what circuit uses tracking resistors? I would imagine there is at least a linear offset.
 

Online RoGeorge

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Re: When you realize finding *that* spare part won't be easy
« Reply #9 on: December 09, 2018, 03:55:10 am »
Couldn't find a replacement, but decided to restore it only for nostalgia reasons (it was an unobtainable item while I was a kid).  ^-^

The potentiometer is indeed the stereo volume/balance/tone/on/off from a Romanian auto radio-cassette player designed during the communist era, about 30-40 years ago.

It took me acouple of hours to dismantle it, find a way to fix it then put the thing back.  Even so, the time spent fixing it was less than the time wasted online searching for a (compatible) replacement.  The potentiometer defect was that it was freely turning (it was supposed to have a limited turn angle) and the on/off was not switching at all.  This is a pic with 3 out of 5 sections dismantled. The bronze lever on the axle was supposed to rotate various sections.



The bronze flattened part was suppose to come in touch with a small cap (made probably from ABS) that was turning the volume and on/off assemble.  The middle of the cap was sanded with time by the bronze lever, so it was not possible tu turn the cap any more, that's why the free turning knob:



A small metal insert cut from a Champagne metal bottle cap was melt into the ABS sanded cap in order to restore the full functionality:



And the whole assembled thing former unobtainium now looks like this:



Decided to use it as a dedicated audiobook player in the kitchen:



 :)

Offline German_EE

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Re: When you realize finding *that* spare part won't be easy
« Reply #10 on: December 09, 2018, 07:07:21 am »
Well done  :clap:
Should you find yourself in a chronically leaking boat, energy devoted to changing vessels is likely to be more productive than energy devoted to patching leaks.

Warren Buffett
 

Offline CatalinaWOW

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Re: When you realize finding *that* spare part won't be easy
« Reply #11 on: December 09, 2018, 07:22:22 am »
Very nice work.
 

Online RoGeorge

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Re: When you realize finding *that* spare part won't be easy
« Reply #12 on: December 09, 2018, 12:15:08 pm »
I didn't expected all the appreciation, but thank you.  :)

The repair was posted mostly because during the online search for the spare part I found out that I was not the only nostalgic about that model, and there were a couple of other second hand RALLY RC 2341 for sale, yet all of them had the same defective potentiometer.  Now they can be repaired properly.  Seems like that potentiometer had a bad design, and so it became the main reason of failure.  Otherwise the radio-cassette is doing just fine, no recap required, even the motor belt and the rubber rollers were still OK after so many years.  The radio receiver was also working, but in the meantime Romania dropped the eastern UUS FM band (64-73MHz), adopted the western FM band (88-108MHz), and decommissioned most of the AM stations in the LW band, but that's another story.  :P

Offline t1d

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Re: When you realize finding *that* spare part won't be easy
« Reply #13 on: December 09, 2018, 01:36:42 pm »
Not to detract from your excellent work, but maybe another option is to print a new sanded cap, on a 3D printer?
 

Online RoGeorge

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Re: When you realize finding *that* spare part won't be easy
« Reply #14 on: December 09, 2018, 06:25:45 pm »
That was my first thought, too, to 3D print another one, but I was afraid the failure will probably happen again with time:  the bronze flat part will sand or crack the new 3D printed part, transforming the junction slit into a circular pit, like it happened with the original part from the second pic.  I hope this wouldn't happen any more now, because the junction slit is made out of metal, too.


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