Help with the volume control - model 70 - Printable Version
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+--- Thread: Help with the volume control - model 70 (/showthread.php?tid=7077)
Help with the volume control - model 70 - doverite - 07-21-2013
I recently had to replace the dual control on my model 70 with a single control and am experiencing a strange problem.
Starting from fully off, the volume increases as I turn the knob clockwise but, at a fairly low level, the volume decreases and completely disappears as I continue turning the knob clockwise!
I don't have a scanner to show the circuit which came with the replacement kit which I purchased but I replaced the 225 ohm section of the original dual control with a 220 ohm fixed resistor and, to quote the instructions a 270 ohm resistor is added to "supply initial bias on RF and 1st IF grids".
This problem wasn't present before the change and wonder if anyone on the forum might have an idea as to what is causing this problem.
My apologies for such a long question for my first posting!
Thank you for your help.
Joseph
RE: Help with the volume control - model 70 - Ron Ramirez - 07-22-2013
So, I take it, you originally had a dual volume control (250 ohms and 5000 ohms), and you used a 220 ohm resistor to replace the 250 ohm section of the original control, and you are now using a 5000 ohm single control in place of the original 5000 ohm section?
RE: Help with the volume control - model 70 - Radioroslyn - 07-22-2013
Try switching the wires at the outside terminals of the pot.
Terry
RE: Help with the volume control - model 70 - doverite - 07-22-2013
Hi Ron and Terry
Thank you both for taking time to reply to my question.
I will switch the outer leads on the volume control and see what happens.
To answer the question from Ron, I purchased a kit from Mark Oppat which, I understand was put together for the express purpose of replacing the dual control.
It came with the two resistors which I mentioned and a single volume control. I do not know the resistance of the control and wondered what value it should have for this usage.
The chassis is in the radio at the moment and when I take it out I will measure its resistance and let you know.
Thank you again for your help.
Joseph
PS Mark did supply me with a circuit diagram showing the changes and it look like the new circuit calls for a 10 M (meg?) volume control.
RE: Help with the volume control - model 70 - doverite - 09-07-2013
A belated thank you to those who answered my question about a new volume control on my Philco 70.
I now have things working again but it was more involved that just switching the outer connections on the control.
When I looked at the chassis in more detail, I realized that, at some point in the past, someone had added external capacitors to the bakelite blocks without removing those inside. This involved some changes to the wiring and this is what threw me at first and led to the volume control not functioning properly.
I restuffed the blocks, put the wiring back with the help of a good photograph of the underneath of the chassis and the volume control works perfectly.
Thank you again.
Joseph