The PHILCO Phorum

Pulled the 20 back out. Seems to work O.K., but with one exception: volume control.

Both 250 and the 5K run smooth. Starting at 5K and zero volume, I turn the knob. Still have no volume until it hits 3.5K. Then it increases until 2.5K. It then decreases slightly until I hit 1.5K, then nothing.

Took resistance measurements with power off, since my meter has effect on volume.

Greatful for reply,

Pat

The dual section volume control on the model 20 and 70's work like this. The 5K section shorts the rf signal to ground coming down the antenna as the control is turned downed down. The 250 ohm section controls the gain of the rf amp stages by lowering the cathode resistance to ground. This increase the gain. The trick is that if you just use the 250 ohm pot to vary the gain you can't get it low enough so you can't hear any stations. That's why the 5k pot section is used as a backup to killoff the signal.

Folks that sell replacement volumes controls for these sets what they consist of is a a fixed resistor and a 250 ohm pot. The 5k section isn't particularly important in terms of the set working or not. If it was shorted at the low end that could be a problem.

I reread your question and I think the answer is that you can't measure the 5k pot in circuit as the ant coil is in parallel with it. It's a much lower resistance.

Could it be that one half of the control is wired wrong, causing half of the control to increase while the other half is decreasing?

Steve

Has anyone re-wired the volume pot? If not, it is probably OK.
Also keep in mind, it is mounted on a fiber washer and touching the shaft EVEN with the knob on (due to capacitance) causes increase in volume.
And as Terry said, 5K section only could be checked out of circuit. In-circuit it will show a short between the wiper and chassis.

Patrick 
Yes it would be nice to separate the dual pots for testing. Tune in a station and listen whilest turning separate controls. 

And you could do the three card monte with the 24 tubes. 

What Radioroslyn said the bias section of the pot can't lower the gain enough to mute the radio. So the 5k section of the pot attenuates the input signal from the antenna at the same time. You should be able to measure the 5k pot between the chassis ground and the antenna connector. Don't forget to turn the pot while measuring. 

On the other part try watching the bias voltage to see if it changes smoothly or full of glitches. 

You could sneak in a germanium diode in and make an AVC control. 

I just got the part about cross wired the pots so one is going up while the other is going down. Yea they need to work together.

>>You should be able to measure the 5k pot between the chassis ground and the antenna connector.

No he won't. Unless he disconnects the antenna coil.

Are we looking at the same drawing? I am looking at the one above. With the pot turned to ground can't you just measure the antenna side of the pot at the connector?

Yes you can measure end-to-end. But if you want to see the wiper behavior (which is what often is the problem), the wiper will always be shorted.

Yes the wiper is shorted thru the coil to ground. But if the 5k pot functions won't the grounded wiper cause a varying ohms reading at the antenna?

I learned that trick from Ron.

Thanks for all the replies. I'll play with it this morning and report what I find. Complicated little bugger, huh?

OZ4


I just re-thought that....yes, you are right, if you measure end-to-end and then turn the resistor up and down the resistance should change from 5K to almost zero. And if there are skips, you will see the jumps from the partial to full 5K.

Fortunately, I had another 20 to check out. One that works. The pots were identical. So as OZ4 mentioned, I started swapping 24's. I first used the 3 good ones, and it worked. Then I put the 3 suspect ones in the good unit, and they worked!

Something else must be going on. But after going through about 10 tubes, I found a combination that worked. But the volume does level off at the top end, but no one listens full blast anyway, so I'm going to consider it fixed so I can move on. Not that curious. Life's too short.

Thanks to all.

Pat

Read where you can replace the volume with one 5k wire wound pot with a couple of 200 ohm 1 watt resistors to replace the dual volume original pot for smoother operation.
PHILCO 70, 7OA. ( same as 20 I assume) Replacing dual
volume control with single, tapered unit,
giving even smoother control. Circuit
is self-explanatory. The 200 ohm fixed
resistors are I watt types.

Basically you bridge the candohm with a 200 ohm resistor, tie the top side to the bottom of the pot, and at that point another 200 ohm resistor to feed the original cathode feed, the wiper of the pot to ground, and the top of the pot attaches the antenna wire feed, and also to the the antenna coil, that's it.

Found it in old Radio Service Shortcuts publication which I managed to find and download. Another member here clued me in on this, have not tried it. Also in that pub, one mentioned using an old number 1 lead pencil to smooth over the contact points on an original volume control, that was interesting as well.