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Discussion Starter #1
Hi Folks

I have a master volume pot on a Polytone amp I have that's getting wonky so I want to replace it.

There doesn't seem to be any markings on the pot but I have to admit I didn't take it out of the chassis yet.
But I did measure the ohms readings between a outside pole and middle pole with the pot on 10 ,
and this is what I got.

METER set to read 200K and the actual reading I got was 75.0.
So the question is what value pot do I need to get to replace the wonky one.?

any questions please ask.
G.
 

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If you can read schematics, scroll down to Polytone.
Schematics
 

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That's showing a 75k pot, which is unlikely. You're getting a false reading from something.
Everything I've looked at from polytone suggests it is a 10k pot, so best to check a schematic. What's the model of amp.
If you're reading is reasonably correct, most likely a 100k log pot.
FYI - When you check a pot, measure the two outside pins, and it usually doesn't matter where the knob is set.
 

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Discussion Starter #7
The amp in question is a Polytone Mini Brain head...
The pot in question is a Master volume pot and a reverb volume at the same time...it has a double set of flippers.
Here is a pic of the head.
G.
P Mini Brain Head.jpg
 

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Looks like it is 10k from the schematic.
The pot in question is a Master volume pot and a reverb volume at the same time...it has a double set of flippers.
How does that work. If you change the master you change the reverb? Odd.
That would be a dual pot.
I haven't found a mini brain schematic, but all the others I looked at had those two separate, and they were both 10k. I'd say log taper as well.
 

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Discussion Starter #10
Does the amp have two volume controls? (from your pic)
View attachment 283272 View attachment 283274
Actually the amp has 4 volume controls...
Front panel has a volume control for the main circuit and a volume control for the sonic circuit.
And a volume control for the reverb.

On the side panel there is a single master volume control for volume and reverb.
G
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It certainly would be nice to find a schematic for this amp. I have been searching and have had no luck...not even any promising leads.
 

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Discussion Starter #13
Problem Solved .....( I think).

OK....so I went with a hunch that the suggestion of a 100K pot was needed.
Dropped in to my favorite electronics store (Neutron Electronics) and asked for a 100K pot with double flippers ( sorry for the technical terms).
NO such thing at the store...BUT....they do have single flipper 100K pots.....so I bought 2...

I replaced the old pot and placed one of the wires on there....
For the second pot, I removed the "main Output" jack and taped it safely in the inside of the chassis.
Installed the second pot with the reverb wire on it and placed it in the now empty "main output" hole.

Thats it folks....works as it should only now I have separate Master and reverb controls...
Unless someone is going to tell me that those 100K pots are going to blow up my house....I'll keep it as is.

first pic is the wonkey pot. ( what was happening is that the amp volume was very low until I found a narrow sweet spot at about 8 or 9 and then all was normal).
thanks for all your help...much appreciated.
G.

DSC08351s.jpg
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DSC08347.JPG
DSC08353.JPG
 

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This seems to be a dual-ganged potentiometer, like a stereo pot. Often each circuit has it's own housing.

If it had two controls it would be a concentric pot, like you find on some guitars.

You can measure each ganged potentiometer, they are usually the same value, but sometimes you get one that is 10K and one that is 100K.
 

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Well done!

I admire you for working on an SS amp like that. Especially when you didn't have a new replacement for the original pot.

The wire in this pic looks like it might be shielded (coax) style wire.
Did you purchase it at Neutron?
I'm assuming the centre conductor is stranded....correct?
Is there a label marked on the outside of the cable?
What is the approximate outside diameter of the cable?
I'm asking as I dislike working with vintage braided shield wire and I am always looking for a good substitute.

Thanks for the info.


Screen Shot 2019-12-07 at 9.51.39 AM.png
 

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Discussion Starter #16
Well done!
I admire you for working on an SS amp like that. Especially when you didn't have a new replacement for the original pot.
The wire in this pic looks like it might be shielded (coax) style wire.
Did you purchase it at Neutron?
I'm assuming the centre conductor is stranded....correct?
Is there a label marked on the outside of the cable?
What is the approximate outside diameter of the cable?
I'm asking as I dislike working with vintage braided shield wire and I am always looking for a good substitute.

Thanks for the info.
View attachment 283410
First of all thanks Dave for even saying that I worked on a solid state amp... I don't really feel like I did cause I just change a "double flipper" pot to two single flipper pots.

The two wires you see in the pic are shielded coax with stranded center conductors. Both these wires where the original and soldered on the "double flipper " pot.
I didn't purchase any wires, I just un soldered them and then re soldered on the 2 pots a got.
The green tape you see , I put on there so that I can tell which is the Volume and which was the reverb wire.
I have no idea of the outside diameter but I guess I could measure it next time I open the amp up.
By the way, there is NO markings on the wires in question and if you look at the last pic of my "problem solved" entry, you can clearly see that the same wire is used on other parts on the circuit.

I hope all of this helps.
G.
 
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Thanks for all of the detailed info. Much appreciated.
Not to worry about trying to get more info re: the coax.

I found that removing parts (e.g., the stacked pot) from a PCB can be challenging.
 
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