I need a Potentiomter For a fisher ph470
the number on it is 971981 074M-50KG would like to replace both of them the one is working ok but the other one is not working right
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Posted 31 August 2016 - 11:50 AM
i did try a potentiometer out of a old receiver it's the knob kind it has 4 pins for each channel
but when i hooked all 4 pins up it made the left channel lower & when i unhooked it it played fine. so the 4th pin seems like it's not needed
but i don't know if i want to drill a hole the the boombox i would like to find the right size sliders for it if i can
Posted 31 August 2016 - 02:10 PM
What's wrong with it, and what's it for? You can't just use any ordinary slider to replace it, because it's not just the 50k that matters. You need to consider the taper and also the pin assignments look very unique on this. This pot does not look like it's stereo unless it's for balance?
As for another pot "sort of" working if you leave something disconnected, it might behave like that because the pin assignments or purpose is incorrect, so having the 4th pin connected might've created an undesired short.
Anyhow, it shouldn't be too hard for you to use a DMM and test the traces to see if you have proper continuity. I mean, you were able to successfully take that slider out and disassemble it. Correcting any broken trace issues should fix it.
Posted 31 August 2016 - 03:35 PM
it's for the volume control it has one for left & one for right channel
it's not testing on my mutimeter at the pins. but at the end of the carbon it is. and it looks like some of the carbon is worn off at the beginning
and the wiper that's on the slider are all corroded
IMG_0829.JPG 99.08K 15 downloads
Posted 01 September 2016 - 10:00 AM
Right. So because you have separate L/R volume controls, is why these are not stereo pots. Finding one with the 4th pin is going to be nearly impossible because that 4th pin is for loudness control, and modern audio doesn't rely on separate loudness compensation circuitry anymore, typically accomplishing this with digital manipulation or dedicated audio chips. Cheaper and easier to do this way. But I digress. In any event, because current demand for slide pots with loudness compensation is very low, therefore manufacturers have for the most part, abandoned them.
However, as you are already prepared to trash the current pot and replace it, drill a hole into the boombox even, why not just rebuild the pot and see? The danger is that after fixing, that you will have slight uneven behaviour between the left and right pots due to potential alteration of the resistance taper introduced by the repair, but it should work.
Since you say that the traces appear worn and have no continuity then the solution is to apply some trace repair compound. You can get them with small nail-polish type bottles with brushes. The silver ones are probably full conductive and the black ones are probably carbon that will vary resistance based on application thickness. For you, since the silver part is what is worn, then just apply the silver stuff ONLY until it contact the still conductive black portion. Now, this repair probably won't be as durable as the original factory coating so don't fool yourself and think you'll get as much life. Furthermore, it's probably prudent to add a little oil-like preservative (such as Deoxit gold D100) so it will wear better. Just a smidgen. Also, presumably at this point, as a collector, you also have a ton of other boomboxes available to you. So I don't expect that the repaired pot will receive nearly the type of use that a new unit 35 years ago would receive since, you probably like to rotate your boxes, and probably listen to a single volume range 99% of the time anyhow.
So as there's nothing else to lose, just get and apply some (sparingly or you'll ruin the control) conductive paint or epoxy and put it back together. I mean, realistically, finding a PH470 is hard enough. Finding a slider, in good condition, for the PH470 is.... well you get the idea.