500K vs 100K Potentiometer

Jim 2018

Active Member
So on my 290-T chassis, I have issues with the power switch on the volume control. I got brave and took the pot apart to see if I could fix the switch. In the process, I broke the PCB of one of the channels. Which is just as well because no matter how much I cleaned it, I couldn't get static totally out of it. Since this part is rare and hard to inexpensively replace with an original part, I have some questions.

I happened to have in my workroom a dual gang 100K ALPS 4 lug audio pot. I expected that it wouldn't work correctly because of the impedance difference. However and to my surprise, it seems to work fine in the Fisher!

Long term I need to source another pot because the shaft is too short and narrow for when the chassis is returned to the console. It seems sourcing a brand new dual gang 4 lug 500K 35mm x 6mm shaft with a D end with a rotary power switch capable of switching AC power is not going to happen.

However, it seems that if I could get away with a 100K or a 300K pot, I might be able to do it (though without a power switch). Also, pots with 4 lugs are rare. I'm not exactly sure what it is for but I get the impression it is probably for the "loudness circuit" which in my system consists of a resistor and a couple ceramic capacitors. Do I really need that?

If that loudness circuit is something one should retain and if I could not find a 4 log pot, I noticed the bass and treble controls use the same pot as the volume pot sans the power switch and sans use of the 4th lug. If I could install say a brand new 3 lug 100K or 300K pot for the bass and treble, then I'd have a factory original for volume available with the 4th lug and a spare pot to boot. Any thoughts?
 
Jim -- The 100K control will certainly work in a pinch for sure, but not without its consequences, which would be three things:

1. The total gain of the preamp section will be reduced somewhat due to greater signal drop across the balance control resistors that the volume control works in tandem with. If you had enough gain before this likely isn't a problem, but a fact none the less.

2. The low frequency response will also be attenuated somewhat on top of the general attenuation mentioned in #1 above. Again, depending on the speakers you're using, you might never miss it.

3. The characteristics of the loudness circuit have now changed, since the specified components were designed to work with a tap impedance of about 125K, while in all likelihood, the tap on your new control is located at about 20K. The net result is that the loudness circuit no longer provides the LF boost it used to.

As well, the volume control response is now likely different as well due to the mis-match between the new control and the loudness components.

There are folks working to produce a quality replacement volume control for Fisher gear, but until that comes to fruition, I'm sure your 100K control is a nice substitution piece.

Dave
 
Jim -- The 100K control will certainly work in a pinch for sure, but not without its consequences, which would be three things:

1. The total gain of the preamp section will be reduced somewhat due to greater signal drop across the balance control resistors that the volume control works in tandem with. If you had enough gain before this likely isn't a problem, but a fact none the less.

2. The low frequency response will also be attenuated somewhat on top of the general attenuation mentioned in #1 above. Again, depending on the speakers you're using, you might never miss it.

3. The characteristics of the loudness circuit have now changed, since the specified components were designed to work with a tap impedance of about 125K, while in all likelihood, the tap on your new control is located at about 20K. The net result is that the loudness circuit no longer provides the LF boost it used to.

As well, the volume control response is now likely different as well due to the mis-match between the new control and the loudness components.

There are folks working to produce a quality replacement volume control for Fisher gear, but until that comes to fruition, I'm sure your 100K control is a nice substitution piece.

Dave
So what do you think about a 300K, would that be a "lesser evil"? It seems that might be easier to get. I need to replace the 100K because the shaft is too short to connect with the volume knob.
 
Try an auction site and search for fisher volume pot on off switch, there is a person who sells upgraded switches. He seems to know his business. I found this person while looking for a balance pot for my Fisher X-101
 
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If you put this text into an eBay search, quite a lot comes up: fisher volume pot

Thomas - I was told last week that we are not allowed to put eBay addresses in forum posts. Refer to the part name or auction number instead.
 
If you put this text into an eBay search, quite a lot comes up: fisher volume pot

Thomas - I was told last week that we are not allowed to put eBay addresses in forum posts. Refer to the part name or auction number instead.

Thanks, I will edit the post
 
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