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 Post subject: Volume pot question
PostPosted: Feb Thu 09, 2012 6:11 am 
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Location: Hamilton, Ontario
I am just finishing up a Canadian Phonola and everything is recapped, out of spec resistors removed, aligned and thoroughly cleaned. The radio is working well but I still have low volume at zero volume level. At one point during the recap I know the volume control was fully functional. The pot has been cleaned, C34 has been replaced and R11 checks out alright.

Any ideas?

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 Post subject: Re: Volume pot question
PostPosted: Feb Thu 09, 2012 8:57 am 
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Location: Aurora Colorado
When the control is all the way to the low position, do you still show some resistance? If so it may be normal

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 Post subject: Re: Volume pot question
PostPosted: Feb Thu 09, 2012 10:38 am 
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Location: Powell River BC Canada
Did you change the bias cell ? It might be dried out and acting like a very small capacitor leaving
the grid of the 6FS5 open and subject to crosstalk from adjacent wires to the top of the pot from the detector.

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 Post subject: Re: Volume pot question
PostPosted: Feb Thu 09, 2012 2:03 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: St. Louis, MO, USA
I think Steve's on the right track; there's a high probability the trouble involves the bias cell.
This link explains them and gives a method to try to rejuvenate them:
http://www.philcorepairbench.com/biascell.htm
The circuit arrangement is a little unusual in that the bias cell is directly in the path of the audio signal.
If the cell cannot be brought back to life or made to operate satisfactorily, the usual thing to do is to eliminate it with a circuit modification as shown below. This relies on the grid to develop it's own bias through it's "contact potential."
There are other methods, but this one seems to be the most favored.
Attachment:
bias cell replacement.jpg
bias cell replacement.jpg [ 47 KiB | Viewed 491 times ]

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 Post subject: Re: Volume pot question
PostPosted: Feb Thu 09, 2012 3:13 pm 
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Location: Northwest Florida (Panhandle)
I don't believe the bias cell can cause that problem. If the resistance from the volume pot reads essentially zero at minimum setting, there should be no signal leakage through the cell.

Since you have recapped it, and this is quite a high impdance circuit, subject to pickup, I wonder if you might be getting some accidental capacitive coupling in your wiring. With the volume turned to minimum, can you change the audio level by poking around the wires in the vicinity of the VC?


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 Post subject: Re: Volume pot question
PostPosted: Feb Thu 09, 2012 4:39 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: St. Louis, MO, USA
I think Steve's idea was that with a dead bias cell, there would be zero bias on the grid. Then the gain of the tube would be very high and overly susceptible to stray signal pickup through accidental coupling.

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 Post subject: Re: Volume pot question
PostPosted: Feb Thu 09, 2012 10:55 pm 
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Location: Hamilton, Ontario
I have never dealt with bias cells before so I am going to have to do some reading first.

Occam's razor...........I will meter out the pot first.

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 Post subject: Re: Volume pot question
PostPosted: Feb Sat 11, 2012 6:32 pm 
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I metered out the 2 Meg pot here today and I am getting about 22 ohms at zero volume level from the pot. I removed it, recleaned it and reinstalled it and yet I am still getting the same ohm readings and low audio when the volume is at the zero level.

Can I therefore eliminate the pot as the problem and proceed to looking at the bias cell? If I go with Terrys idea of bypassing the bias cell with a .01uf and a 5 Meg what wattage resistor should I use?

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 Post subject: Re: Volume pot question
PostPosted: Feb Sat 11, 2012 7:20 pm 
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Problem solved! You guys are pretty smart

I removed the bias cell and modified the circuit as per Terrys diagram and the volume pot works perfectly now. I used a 4.7M 2W resistor; from what I have read that part of the circuit is lower volatge and current anyways :wink:

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 Post subject: Re: Volume pot question
PostPosted: Feb Sun 12, 2012 12:48 am 
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Location: St. Louis, MO, USA
4.7 meg at 2 watts? Zounds, I'm surprised you had one like that! :shock:
Current there is indeed very small. You could have used 1/2 watt, or even less. Sorry I didn't catch your post early enough to say so. At least you don't have to worry about it overheating now! :D

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 Post subject: Re: Volume pot question
PostPosted: Feb Sun 12, 2012 4:18 am 
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Location: Hamilton, Ontario
Its way too cold to do cabinet work here so I end up collecting completed chassis until the warmer weather. Here is a few pics of the finished Phonola Z40a65e. I still have to paint the top clam shell of the transformer but as I said it is just too cold here.

Image
Image
Image
Image
Image

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 Post subject: Re: Volume pot question
PostPosted: Feb Sun 12, 2012 5:47 am 
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Joined: Feb Fri 05, 2010 6:14 am
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Location: Winnipeg, Canada
Zounds! Is that thing chromed?
Nelson

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 Post subject: Re: Volume pot question
PostPosted: Feb Sun 12, 2012 7:03 am 
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Location: Hamilton, Ontario
To be quite honest, I don't know.

It was heavily oxidized before I cleaned it up so I would tend to think that it is just plated. The strange thing to me is, as you know, Canadian Phonola/Dominion/Electrohome are not considered to be high end sets so it makes me wonder why the manufacturer took the time/money to plate or chrome the thing????????

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 Post subject: Re: Volume pot question
PostPosted: Feb Mon 13, 2012 4:32 am 
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Location: Warner Robins, GA
Some Eastwood's Chassis black satin paint would look great on that transformer.

An example of how that paint looks on a transformer

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