a secret life of pots question

[newperson]

Hi,
I am debugging an 1960's stereo amp.  I took out the pot that controls the balance (right to left channels).  It is an old clarostat marked RCV-1MST-3F   6140.  So I am guessing that it is from 1961 which dates correctly to the amp.  

I was wondering why the normal carbon inside ring around the pot is connected to the middle lug?  The lugs are wired 1 to right channel, 2 (center lug) to ground, and 3 to left channel.  The far tap lug is wired to ground and the center (2nd lug).  

I am attaching some images and a link to more images.  

Thanks to anyone for any education on this.

https://sites.google.com/site/clarostatpotentiometer/home

[tubelectron]

Hi,

It is a center-tapped pot, and the center tap is grounded. When you turn the pot, the grounded wiper travels to the channel to be attenuated, the other channel remains unaffected, just grounded by the center tap. It's a good balance control system, but if the pot is to be replaced, finding another one may be scarce... If it's not possible, a classical pot (lin) would be OK, then you neglect the inexisting center-tap and just ground the wiper. The only difference is that when adjusting balance on one side or another, one channel will be attenuated and the other one will be reinforced a bit instead of remaining at the same level. Typical value would be 1MB (lin) on a tube amp.

A+!

[newperson]

Hi,
Thank you for the answer.  If you don't mind, I still do not follow why one channel is connected to the center ground lug?  It seems like that channel would just be pulled to the ground regardless where the wiper is located on that side of the pot.  Does my question make since?  Did my posted image pull up for you to see?

Thank you again,

[tubelectron]

Hi,

Your images are not displayed, so I tried to give you understandable explanations... And it's not so easy !
I also try to understand your description... And its not so easy to for a Frenchman !

Let's consider the pot you mention. It is a 4 lug balance pot : Right ? Let's assume that it is right...

So the correct balance wiring is :
- one end is to one channel (say the left).
- the other end is to the other channel (say the right).
- the wiper (moving) is to gnd.
- the center tap (fixed) is to gnd also.

when the wiper goes to the end of the left channel, the left channel is attenuated gradually to zero, depending on how far the wiper is. The right channel remains unaffected, grounded by the center tap.

conversely :

when the wiper goes to the end of the right channel, the right channel is attenuated gradually to zero, depending on how far the wiper is. The left channel remains unaffected, grounded by the center tap.

so :

The role of the center tap is to give a separation between the 2 channels, and so make one channel unaffected when the other is, and conversely.

and :

The role of the wiper is to give the balance adjustment by attenuating one or another of the 2 channels, depending on its position.

finally :

If your pot hasn't the center tap, one channel will be attenuated and the other one - instead of staying to a constant level due to the center tap - will increase slightly, and conversely. There will be no major difference in the balance function, just a little bit less smoothness...

I Hope I am clear, and that I understood your purposes !

A+!

[PRR]

The schematic diagram is a maze. Stuff jammed together, connections crossing...

Re-draw it so you can see it:



Works as Bruno says. The schematic does not show the 4th lug or track center-tap, I've added it in color. The amp will work without the center-tap, but it may not have very-very-good stereo separation measurements.



WOW! Yes, I agree. The pot appears to have been made WRONG.

If it has been bad since 1961, why wasn't it fixed when new, or some time in the last 49 years ? ? ?

MAYBE it just looks like it connects. Have you used an ohm meter? Is it shorted from one end to wiper?

A simple LINear pot will replace it. Stereo separation will be near 40dB. The 4th terminal would give a better number, but not a better sound image.

You could try scraping or sawing the phenolic tab between wiper terminal and end terminal. That would fix this problem, if you can get the pot to go back together perfectly.

> Your images are not displayed

They are deep in an HTTPS link. Apparently this forum board's display code does not believe this is a valid image link. If I open his plain link https://sites.google.com/site/clarostatpotentiometer/home I see his images, click for a better view. If you are not seeing that, maybe France and Google are unhappy with each other today.

[newperson]

Hello again,
First off I really like France.  I got to live their for 3 months once and had a great time and met many nice and giving people.  You also have very nice English skills!

I understand your description of the functioning of the pot and the separation of the signal.  What I am missing from this is if you understand my question of why lugs 1 and 2 are attached directly within the inside of the pot.  I think the picture should help a lot.  Try this link,
https://sites.google.com/site/clarostatpotentiometer/home
It is the first picture.  I drew an arrow pointing to my confusion.  If that link does not work for you, perhaps I can send the picture by email to you if you don't mind.  

Thank you again for your patience.  In the meantime I will reread your last post to make sure I am not overlooking something that you are saying.  I will also look to see if I can figure out way the pictures I posted are not coming up.

[newperson]

Thanks for the reply and fixing the image.  I don't know if that amp ever worked.  It has a big "X" on the A channel.  Perhaps they only used it as a mono amp?  I need to read your post again for a proper reply.  I did not want to break the trace before I knew it really was a faulty pot.  Or I found another pot and replaced it to see if it would fix my issue, which is - the sound is only coming out of one channel.

[PRR]

> the sound is only coming out of one channel.

Disconnect the Balance pot (you already did).

Try the amp. It should work just-fine, both sides.

Without the balance pot, gain will be insignificantly higher (but you need careful measurement to prove it), and the gain probably won't be _exactly_ the same both sides. Parts tolerances will give a slight difference between channels. Only careful listeners exactly centered between speakers will be able to tell.

If one side is still dead, or very-weak, then it isn't the balance pot. Can you meter the DC level on each plate? Should be the same L or R. Can you feed a test-tone and meter or 'scope the level at various points in the circuit? (Do NOT get shocked!!)

Stereo tube amps are a PAIN to work inside, the saving grace is that if one side is sick, you can compare to the other side and narrow the problem.

[newperson]

Thank you for the suggestions.  I cleaned out that pot and put it back in after testing there is no connection between the two points.  I then soldered it all back together and found it was testing to ground again.  From that knowledge I found the connecting wire to it has a short somewhere within it causing it to go to ground.  It is a type of old coax wire with the hard wire within a plastic sleeve  and a flexible ground shield wire wrapping around the plastic.  Somewhere that ground wire is touching the main wire within it and when I soldered the flexible ground wire within the coax it was shorting the signal to ground. 

It's alive now!  I have some more caps to replace before I am finished, but now I am getting sound out of both channels.  I found the signal ending at that point from R.G's audio probe and measuring voltages on both sides.  You are right about stereo amps being nice if one side works so one has voltages to compare with.

If you would like to know, this is an old kit amp from Scott.  So there is a fair chance that channel never worked.  The amp and paperwork are marked in a way I can tell that whomever built it was trying to find the reason the channel was not putting out sound.  It is not my amp though.  I am doing the work for a friend.

Thank you both for the suggestions, fixing my attachments, and the education. 

[tubelectron]

Ah ! Yes...

With the picture now I understand what was the problem I couldn't figure well : an impossible thing in a factory assembly line quality control schedule; a somewhat unusual but possible bug in a kit amplifier !

Just for the occasion, I also have a 1961 HH SCOTT model 222C in my collection - but this is a factory-built unit :




A+!