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General => General Chat => Topic started by: RoGeorge on November 24, 2018, 01:14:28 pm

Title: When you realize finding *that* spare part won't be easy
Post by: RoGeorge on November 24, 2018, 01:14:28 pm
???  Broken potentiometer

(http://i66.tinypic.com/6i7ghu.jpg)

 :o
Title: Re: When you realize finding *that* spare part won't be easy
Post by: tautech on November 24, 2018, 03:00:26 pm
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 (http://www.potentiometers.com/select_ProAudio.cfm)
Title: Re: When you realize finding *that* spare part won't be easy
Post by: calexanian on November 24, 2018, 03:58:35 pm
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
Title: Re: When you realize finding *that* spare part won't be easy
Post by: Tac Eht Xilef on November 25, 2018, 12:21:28 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 (http://www.oldradioparts.net/). He sells/rebuilds/repairs vintage radio pots, & might be able to help or give some suggestions.
Title: Re: When you realize finding *that* spare part won't be easy
Post by: coppercone2 on November 25, 2018, 12:28:16 am
better learn how to make gears :'(
Title: Re: When you realize finding *that* spare part won't be easy
Post by: brabus on November 26, 2018, 01:21:06 pm
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.
Title: Re: When you realize finding *that* spare part won't be easy
Post by: Sudo_apt-get_install_yum on November 26, 2018, 03:14:20 pm
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
Title: Re: When you realize finding *that* spare part won't be easy
Post by: CatalinaWOW on November 26, 2018, 05:47:03 pm
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 (https://www.mouser.com/datasheet/2/427/p11sp11a-239880.pdf) 
Title: Re: When you realize finding *that* spare part won't be easy
Post by: coppercone2 on November 27, 2018, 01:15:49 am
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.
Title: Re: When you realize finding *that* spare part won't be easy
Post by: RoGeorge on December 08, 2018, 04:55:10 pm
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.

(http://i68.tinypic.com/33bztcx.jpg)

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:

(http://i63.tinypic.com/2unwodh.jpg)

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:

(http://i64.tinypic.com/126bgbq.jpg)

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

(http://i68.tinypic.com/1z1d4rn.jpg)

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

(http://i66.tinypic.com/2eozfdj.jpg)

 :)
Title: Re: When you realize finding *that* spare part won't be easy
Post by: German_EE on December 08, 2018, 08:07:21 pm
Well done  :clap:
Title: Re: When you realize finding *that* spare part won't be easy
Post by: CatalinaWOW on December 08, 2018, 08:22:22 pm
Very nice work.
Title: Re: When you realize finding *that* spare part won't be easy
Post by: RoGeorge on December 09, 2018, 01:15:08 am
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
Title: Re: When you realize finding *that* spare part won't be easy
Post by: t1d on December 09, 2018, 02:36:42 am
Not to detract from your excellent work, but maybe another option is to print a new sanded cap, on a 3D printer?
Title: Re: When you realize finding *that* spare part won't be easy
Post by: RoGeorge on December 09, 2018, 07:25:45 am
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.