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Question about power switch in Fisher Models 90T and 500C

artdecoradio

artdecoradio

Active Member
I have Fisher Model 500C that has a broken power (On/Off) switch which is a common problem. Last week someone gave me a Fisher Model 90T tuner/preamp which I'm going to part out since it's in bad shape cosmetically (rusted/corroded faceplate and other issues). It has a treble control (potentiometer) with a switch on it and a nice long shaft. This control is 500K and so is the control on the 500C (both 500K). It also looks like both also have a double pole switch. Check out the pictures below of the parts information and schematics for both models. The first 2 pictures are for the 90T and the remaining pictures are for the 500C. Will the pot/switch from the 90T work as a drop-in replacement for the broken pot/switch in the 500C? The pot/switch in the 90T is in good shape and works. What do you think? Thanks
 

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The volume control pot with the switch on the 500C is tapped for the loudness control circuit. If the 90T treble pot with the switch is similarly tapped, it may be a drop in if physically the same size. If it's not tapped you won't have a loudness function even if the nominal control volume is also 500K. Also, the 500C volume pot is audio taper, but you need to check that the treble pot is as well--I can't recall off the top of my head.

If the switch on the treble pot is exactly the same as the one on the 500C volume control, you might be able to 'transplant' it, but that job is a bit fussy.
 
Dave451 said:
The volume control pot with the switch on the 500C is tapped for the loudness control circuit. If the 90T treble pot with the switch is similarly tapped, it may be a drop in if physically the same size. If it's not tapped you won't have a loudness function even if the nominal control volume is also 500K. Also, the 500C volume pot is audio taper, but you need to check that the treble pot is as well--I can't recall off the top of my head.

If the switch on the treble pot is exactly the same as the one on the 500C volume control, you might be able to 'transplant' it, but that job is a bit fussy.
Click to expand...
Hmmmm. I didn't consider the tap issue. Provided below are pictures showing the schematics involving the pots in both the 90T and 500C. It looks like the 90T pot has a tap but do you think it would be a drop in replacement for the pot in the 500C?

By the way I would be very surprised if the 90T pot was not audio taper. I can't imagine that it would be linear taper. Swapping switches is also an option but as you said, it is a major hassle. I tried to do this once and it didn't turn out well.
 

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notdigital said:
Being a simpleton I’ll ask: why not just replace the switch?
Click to expand...
Hi. Yes, replacing the switch is an option but is very difficult for many reasons including the way the switch is attached to the pot along with the very distinct possibility of damaging the switch during the process (I know this from first hand experience having broken a switch while trying to remove it). Also to consider is the maze of wires involved. It's just easier in my view to remove the entire switch/pot combination and then insert it as a whole into the chassis that is being worked on.
 
Time to hit up barter town for a used spare whole pot ! I thought I had one lying around. I looked but now I think I gave it away a few years ago. I will ask the guy who got the freebies
 
artdecoradio said:
Hi. Yes, replacing the switch is an option but is very difficult for many reasons including the way the switch is attached to the pot along with the very distinct possibility of damaging the switch during the process (I know this from first hand experience having broken a switch while trying to remove it). Also to consider is the maze of wires involved. It's just easier in my view to remove the entire switch/pot combination and then insert it as a whole into the chassis that is being worked on.
Click to expand...
A much better, detailed, and accurate description of what I called "a bit fussy."
 
In addition, even if you get the switch off of the "old" pot, it is easy to damage the switch when attaching it to the "new" pot.

Regarding the tap on the pot used by the 500C, the pot on the 90T also has a tap (at 250K). So, would the 90T pot/switch be a direct drop-in replacement in my 500C without needing any additional modifications?
 
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