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jrs
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Post subject: Volume pot question Posted: Feb Thu 09, 2012 6:11 am |
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Joined: Sep Tue 28, 2010 4:13 am Posts: 727 Location: Hamilton, Ontario
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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? 
_________________ John
Every expert was once a beginner
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glasdave
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Post subject: Re: Volume pot question Posted: Feb Thu 09, 2012 8:57 am |
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Joined: Feb Tue 24, 2009 8:20 am Posts: 7727 Location: Aurora Colorado
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When the control is all the way to the low position, do you still show some resistance? If so it may be normal
_________________ I move the world just one step on...
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radiotechnician
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Post subject: Re: Volume pot question Posted: Feb Thu 09, 2012 10:38 am |
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Joined: Sep Thu 23, 2010 6:37 am Posts: 3942 Location: Powell River BC
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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.
_________________ de VE7ASO VE7ZSO Amateur Radio Literacy Club. May we help you read better. Steve Dow ve7aso@rac.ca
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Oldbear
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Post subject: Re: Volume pot question Posted: Feb Thu 09, 2012 2:03 pm |
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Joined: Jan Thu 01, 1970 1:00 am Posts: 3784 Location: St. Louis, MO, USA
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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.htmThe 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 [ 47 KiB | Viewed 422 times ]
_________________ Terry Davidson
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Alfred Corbin
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Post subject: Re: Volume pot question Posted: Feb Thu 09, 2012 3:13 pm |
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Joined: Jul Sat 01, 2006 11:17 pm Posts: 1114 Location: Northwest Florida (Panhandle)
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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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Oldbear
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Post subject: Re: Volume pot question Posted: Feb Thu 09, 2012 4:39 pm |
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Joined: Jan Thu 01, 1970 1:00 am Posts: 3784 Location: St. Louis, MO, USA
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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.
_________________ Terry Davidson
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jrs
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Post subject: Re: Volume pot question Posted: Feb Thu 09, 2012 10:55 pm |
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Joined: Sep Tue 28, 2010 4:13 am Posts: 727 Location: Hamilton, Ontario
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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.
_________________ John
Every expert was once a beginner
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jrs
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Post subject: Re: Volume pot question Posted: Feb Sat 11, 2012 6:32 pm |
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Joined: Sep Tue 28, 2010 4:13 am Posts: 727 Location: Hamilton, Ontario
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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?
_________________ John
Every expert was once a beginner
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jrs
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Post subject: Re: Volume pot question Posted: Feb Sat 11, 2012 7:20 pm |
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Joined: Sep Tue 28, 2010 4:13 am Posts: 727 Location: Hamilton, Ontario
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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 anyway s 
_________________ John
Every expert was once a beginner
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Oldbear
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Post subject: Re: Volume pot question Posted: Feb Sun 12, 2012 12:48 am |
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Joined: Jan Thu 01, 1970 1:00 am Posts: 3784 Location: St. Louis, MO, USA
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4.7 meg at 2 watts? Zounds, I'm surprised you had one like that! 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! 
_________________ Terry Davidson
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jrs
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Post subject: Re: Volume pot question Posted: Feb Sun 12, 2012 4:18 am |
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Joined: Sep Tue 28, 2010 4:13 am Posts: 727 Location: Hamilton, Ontario
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_________________ John
Every expert was once a beginner
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Nelson in Winnipeg
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Post subject: Re: Volume pot question Posted: Feb Sun 12, 2012 5:47 am |
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Joined: Feb Fri 05, 2010 6:14 am Posts: 911 Location: Winnipeg, Canada
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Zounds! Is that thing chromed? Nelson
_________________ "Never get between electricity and where it wants to go." -Red Green
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jrs
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Post subject: Re: Volume pot question Posted: Feb Sun 12, 2012 7:03 am |
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Joined: Sep Tue 28, 2010 4:13 am Posts: 727 Location: Hamilton, Ontario
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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????????
_________________ John
Every expert was once a beginner
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Tube Radio
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Post subject: Re: Volume pot question Posted: Feb Mon 13, 2012 4:32 am |
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Joined: Jan Thu 01, 1970 1:00 am Posts: 7867 Location: Warner Robins, GA
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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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