Calling all 500C Owners!

[Vinylcafe]

There have been references in this thread to the loudness tap being both approximately 150 ohms, and approximately 140,000-150,000 ohms.

Make sure you state it correctly if you have this custom made.

Got it. Should be 150K, with the emphasis on the "K",:thmbsp:

 

[Vinylcafe]

I also presume it's a log taper we are looking for here, correct?
The only other question I have is around shaft length.
They spec it from the mounting surface, will that work, as opposed to measuring it from the end of the bushing?

 

[Moko]

I would be interested in a new on/off volume pot as the on/off part no longer works on mine.

Why not contact some of the Fisher repair and kit people as they would probably bulk buy enough to get you up to the numbers required.

 

[larryderouin]

Vinyl; Measurement on mine is 1-1/2" measured from the mounting surface where the threads start to the end of the shaft, as State Electronics measured theirs. And I edited my measurements on the tap in the earlier post.

Larry

 

[audiodon]

Is audio taper the same as log taper?

 

[derekva]

Is audio taper the same as log taper?

Yep.

-D

 

[Vinylcafe]

They are still investigating an off center tap to hit the 150K ohms.
Might not know until next week.

 

[Vinylcafe]

No Joy. ..

Turns out the centre tap has to stay at the centre.
Can't be offset in any way, so no way that one can get the tap at 150K ohms.
For this to be worthwhile it has to be exactly the same; close is not enough.
My apologies for raising any false hopes.
Not to be

 

[dsndblm]

Too bad. Thanks for the effort.

 

[audiodon]

Well . . .
I just received a Fisher 500-B volume pot back from Mark Oppat, who rebuilt it.
When it gets installed, I'll try to remember to update this thread with the results of the rebuild.

 

[bhamham]

Turns out the centre tap has to stay at the centre.
Can't be offset in any way, so no way that one can get the tap at 150K ohms.
For this to be worthwhile it has to be exactly the same; close is not enough.
My apologies for raising any false hopes.
Not to be

Is there any way you could come off that center tap (assume it's 250K) and reduce it to 150K? I don't know of any but just thinking out loud

 

[Vinylcafe]

According to the manufacturer with a log taper the centre tap comes in at about 50K, a far cry from the 150K (approx), we've measured on our original pots.

 

[bhamham]

According to the manufacturer with a log taper the centre tap comes in at about 50K, a far cry from the 150K (approx), we've measured on our original pots.

OK, that makes it easier - just add a 100K resistor in series with the tap. Wouldn't that work? (schematic is Fisher 400)

 

[Chappy]

OK, that makes it easier - just add a 100K resistor in series with the tap. Wouldn't that work? (schematic is Fisher 400)

I have a possible suggestion that would be easy to try and see if it works. Looking at the schematic above try the following:

- replace pot R98A with a normal 1 Meg audio pot.
- add a 1 Meg trimpot in parallel with the audio pot and attach the wiper to the contour circuitry. Adjust the trimpot to get the required resistance split such that the loudness knob works as it did before.
- repeat above circuitry for the other channel.


The 1 Meg trimpot in parallel with the 1 Meg audio taper pot will give an overall input impedance of 500K as seen by the signal entering via C83. The trimpot can be fiddled with and then be permanently replaced by two fixed resistors.

This suggestion might offer up a less expensive way to replace the pot. I'm not sure if it will work well enough since the interaction between the original volume pot taper and the contour circuitry will be lost , but it shouldn't cost too much to give it a shot as a standard dual 1Meg pot shouldn't be that expensive.

Steve