The PHILCO Phorum

Hello all,
I am hoping for some assistance with my Model 16 B (121 ) QAVC Potentiometer.
Please excuse any rambling as I am lost on this issue.
I've owned my 16B (121) since the mid 1970s.
Re-cap in 2010, and as you would expect, lots of smaller repairs over 45 years.

I have basically always let the set tune "wide open" with the QAVC Toggle Switch in the down position.
From time to time over the years I would at least check if the QAVC adjustment was still working.
And it was.

I checked it the other day, and it no longer works.
With the Toggle Switch Up, the adjustment does nothing until I reach total Counter Clockwise position.
Once I get to total counter clockwise, there is a quite loud THUMP from the set, and then NO sound at all.

Turn it ever so slightly Clockwise, and it springs back to life! And I do mean ever so slightly turn the knob.
I did check/test related tube(s) and even swapped them with other spare known good tubes.

Next I decided to remove the PN 33-5015 to test with an ohm meter.
With it laying flat, knurled knob facing up, three solder tabs at around 6PM position, here is what I get:

- Ohm Meter connected to Left tab and Center tab:
   Knob turned totally Counter Clockwise reading briefly hits as high as 150 to 300 Ohms.
   (I have watched it on occasion read some numbers into the "K" range but ever so briefly then falls back to 150 to 300 Ohms)
   Now I just begin to turn the knob Clockwise and the digital meter goes to "O.L", no reading.
   The meter continues to read no continuity as the knob is turned all the way Clock Wise.
    I can only get a reading again by once again turning the knob all the way Counter Clockwise and I am back at approx. 150 to 300 Ohms

- Ohm Meter connected to Right tab and Center tab:
    Fully Counter Clockwise reading reads "O.L.", no reading
    Just barely start turning knob Clockwise and first readings varies between 18.5K to sometimes 21K ohms
    Slowing turning the knob clockwise, meter readings continue to decline until around 200 ohms in fully Clockwise position

- Ohm Meter connected to only the two outside tabs:
    NO reading at all in any position

My questions would be for anyone else who can recall ever trying to troubleshoot the QAVC Potentiometer.
1/ Does anyone know what the good ohm readings should be on the control?

2/ Has anyone ever successfully taken apart the control to clean it and get it working again?
    If YES: do you remember how you were able to take it apart without destroying it's contents?

3/ Has anyone successfully replaced the original PN 33-5015 with available aftermarket Potentiometer?
    If YES: do you remember what company you bought the part from, and hopefully what was the new part number and Specs?
    And if YES: do you recall any issues connecting the new part into the QAVC circuitry, as in: just connect to the same lugs, etc.

If you've made it this far into my question, thank you so much.
Hopefully someone from our Philco community can offer some of their experience and knowledge on this QAVC issue.
Rod

The pot should read 10K total end to end and the wiper should pick off any resistance between. I have a 16B 121 tombstone on the bench as we speak. Dumbie me broke the band switch front wafer. It's doing well and should be able to wire it back together today.

I checked the pot in mine and it seem have linear taper. This means when the wiper is at the center of the carbon track the resistance is equal from the wiper to each end of the terminals. Volume controls aren't, those known as audio taper. There's a bunch of different tapers for different circuits.

The only question I'm not sure about is the wattage. I'm think about 5w but that's just based on size so I could off. 10k linear taper is a pretty common value,you shouldn't have a problem finding one. Maybe Mike or Rod could pop in advise on the wattage. https://philcoradio.com/library/download...l.%201.pdf Mike has a 16B 121 cathedral

In terms of cleaning. You could but I would pop the back off and inspect the carbon track. Look for wear and cracks. If it's worn thru or has cracks it needs to hit the bin. I guess that it's more likely that it's going to have cracks rather wear. It's kind of a set it and for get it control. On my other 16B I never use it. It's more of a novelty.

If we can sort the wattage I may have something for you.

Hi Rod,

Looking at the schematic for a Model 16, I believe this is part #82. It is a 10,000 ohm potentiometer. It should read about that when putting your ohm meter across the two outer connections. It should also vary from 0 to 10K from the center to each side, depending on the direction being turned.
If it is showing open across the outer terminals, it might be burned out.
Have you tried spraying it with some control cleaner? That won't fix the outer contact open problem, but may help you to get a cleaner reading when rotating and measuring from the center post to an outer post.

Bill

Thanks for the reply. I figured it was shot with those bizarre readings. 
I tried prying off the back a couple days ago. I didn't want to completely destroy it in case I have to reinstall until I find a suitable replacement.
I did see all the cheap 10k pots available with most rated at .25 watt or so. 
I'm hoping another Phorum member is out there that did find a capable replacement and can assist with vendor and part number.

Agree with your novelty comment except I've actually used the feature for decades.
It's whenever the Mrs. is within earshot when I'm tuning around the world.
She always hates the "noise" and with the working QAVC I could back it way down. 
I always find the in-between station noise part of the charm!

Good luck with your latest 16B build, I'm sure it will be another great one.

Bill, thanks for the reply. 
Yes you are correct on the schematic #82 part.
I figured it was shot by the bizarre readings but never take anything for granted anymore.
I did try to get some Deoxit to work its way into it as an easy fix.
Thus far I haven't been able to pry off the back and hate to do more damage by continuing to do so.
Still hoping there's another member that found a suitable replacement and can help with vendor/part numbers.

Hi Rod,

If you want to get a handle on the wattage required, temporarily insert a 10K resistor in place of the pot and put an amp meter in series with the 10K resistor to measure the current. Then take the wire attached to the center post, touch each side of the 10K resistor and note the maximum current shown. This can then be used to calculate the required wattage.
Power (wattage) = Current squared x Resistance. You would typically increase the calculated amount by some factor to the next higher available size.

Bill, thanks for the suggestion! I may need it in the future.
For now,  I was able to keep working on the back cover as gently as possible and got it to finally pop off.
I cleaned up the inside and did give it a shot of Deoxit.
Next dried it out with a hairdryer and was surprised to see more normal ohm readings.
I decided to also insert a burnish tool against the spring loaded wiper for a couple of swipes.
End result is the original is working at maybe 75% with a couple of dead spots as I adjust it.
I was lucky enough to find a working sweet spot where it is knocking back enough of the inter-channel noise to make everyone happy.
For now I will leave it right where it is.
Hopefully another member can still suggest a known good after market replacement part number. After that I can perform a more permanent fix. 
Thanks again for the suggestions from all.