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model 91 volume control issues - Printable Version

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model 91 volume control issues - qrper - 01-16-2016

Boys and girls,

After a recap, a few resistors replaced, and a few tubes, the radio is working.

yet a problem remains.

I can't seem to turn the volume down. Now, it does work, but fully counterclockwise, there's plenty of volume. And at that level, the control acts like a tone control, lowering the pitch of the audio.

Tone control works fine

I can't do much more for another week on this as while I was working on the radio, the 80 rectifier tube decided to call it quits. Got another one on order, expensive little buggers...

Anyone have a clue about the odd volume control?

I'm Mike, WB8VGE

RE: model 91 volume control issues - Radioroslyn - 01-16-2016

Hi Mike,
Measure the resistance from the volume control wiper to ground. Turn the control all the way down should see 0  ohms or a very low resistance. If not check the controls gnd connection, try a little WD-40 to clean innards if that doesn't do it replace the volume control. It's 350k w/a tap.
http://www.nostalgiaair.org/pagesbymodel/884/M0013884.pdf

Terry

RE: model 91 volume control issues - qrper - 01-16-2016

(01-16-2016, 09:21 PM)Radioroslyn Wrote:  Hi Mike,
Measure the resistance from the volume control wiper to ground. Turn the control all the way down should see 0  ohms or a very low resistance. If not check the controls gnd connection, try a little WD-40 to clean innards if that doesn't do it replace the volume control. It's 350k w/a tap.
http://www.nostalgiaair.org/pagesbymodel/884/M0013884.pdf

Terry

Terry,

the schematic shows a tap, but the control in this radio doesn't have it. t\

There are only the three usual connections, the wiper, and two on either side. I took a bright light and with its help, ran a fingertip around the control and didn't fine or saw the tap.

Mike

RE: model 91 volume control issues - Ron Ramirez - 01-16-2016

Only the later 91 models (those with two bands, AM and the low SW or "police" band) had the tap on the volume control. Early models used a 500K volume control without a tap.

Early 91: http://philcoradio.com/tech/images/91.jpg
Late 91/early 14: http://philcoradio.com/tech/images/1491.jpg

RE: model 91 volume control issues - Radioroslyn - 01-16-2016

Ok Let's try this:
http://www.nostalgiaair.org/pagesbymodel/657/M0029657.pdf
So Mike is the gnd side of the volume open??

Terry

RE: model 91 volume control issues - qrper - 01-17-2016

(01-16-2016, 11:58 PM)Radioroslyn Wrote:  Ok Let's try this:
http://www.nostalgiaair.org/pagesbymodel/657/M0029657.pdf
So Mike is the gnd side of the volume open??

Terry

Ok,

that makes sense about the control now.

the ground side is okay. I measure about 500k from outside lugs. (with wires disconnected)

Mike

RE: model 91 volume control issues - Radioroslyn - 01-17-2016

But what happens if you connect an analog ohm meter to the wiper and the grounded side of the volume control and then turn it slowly from end to end? Does the needle jump around as up turn the control?
If not then methinks that you've got a wiring error or a shorted terminal to something else other than to ground near 32 or 37.

Terry

RE: model 91 volume control issues - codefox1 - 01-17-2016

Either you have mis wired something or the pot is bad. If you have a spare, sub it and see what happens. Probably ground end or connection is faulty.

RE: model 91 volume control issues - qrper - 01-18-2016

Gang,

It hurts to admit, but I had a screwup. one of those capacitor blocks I re-stuffed appeared to have been re-stuffed incorrectly. Basically, I had a .01 cap going to the wrong lug.

I disconnected the volume control and i STILL had plenty of audio.

turned out I had a .01 cap routing audio directly into the audio pre-amp.

The 80 tube was replaced and when powering up, the radio did not play. I monitor ac current and my meter pegged as the radio warmed up.

That problem tuned out to be a bad 42 audio amp.

new tube, new rectifier and the correct capacitors, the radio is playing like it should. (shadow box still not working, but I can live with that.)

thanks for all the input guys,

I'm Mike, WB8VGE