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 Post subject: Philco Model 20 need suggestions.
PostPosted: Nov Sat 09, 2019 5:04 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Rome New York USA
I have replaced all the caps, and bad resistors, subbed tubes. the problem is I get a station with low volume only at one location on the volume pot. If I move off the pot location I lose the station. The pot checks good ohmic wise. This chassis has been a nightmare, so many bad components. Any suggestions where I should check?

Frank

All the voltages look good, been recapped, all resistors replaced, all were out of tol., tubes have bee subbed, the wiring has be checked and double checked, no problem found, the xformers and coils check ok. Still only getting one station, as I adjust the volume control either increase or decrease, I lose the station. Any suggestions where I can check


Last edited by Frank Giardino on Nov Thu 14, 2019 6:09 pm, edited 1 time in total.

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 Post subject: Re: Philco Model 20 Volume control problem.
PostPosted: Nov Sat 09, 2019 6:13 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3256
Location: Lexington, KY USA
The NA Rider schematics for the various versions of the Model 20 show a dual gang pot for the volume control. Is this what you have in your chassis?

http://www.nostalgiaair.org/Resources/download.asp?FN=\M0029582.pdf

Is one of these schematics correct for your radio? Which one? If not, can you give us a link to a correct schematic?

If I move off the pot location I lose the station.

It does sound as if there is a volume control problem.

The pot checks good ohmic wise.

How was this check done? Were both gangs checked? Did both wipers have good continuity over the full rotation?

Depending on how the control was checked, the test current may have been very small. In the set, the wiper contact may fail at the higher currents encountered there.

Something to check: Monitor the cathode voltage on one of the 24 RF amplifier tubes as you run the volume control from minimum to maximum. Does the voltage change smoothly as the control is turned? Is the voltage around 2 to 3 volts when the control is fully clockwise?

Ted


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 Post subject: Re: Philco Model 20 Volume control problem.
PostPosted: Nov Sat 09, 2019 6:21 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3256
Location: Lexington, KY USA
You might take a look at this topic:

https://www.antiqueradios.com/forums/viewtopic.php?f=6&t=366978

Some guy named Frank Giardino seems to be having a similar problem with what may be the same radio, but he says his set has IF stages, of which a normal Model 20 has none.

Ted


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 Post subject: Re: Philco Model 20 Volume control problem.
PostPosted: Nov Sun 10, 2019 1:18 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Rome New York USA
Thanks Ted,
That is my post on the Philco 20, didn't get any replies, and it was another problem, so I thought I would start a new one. The philco is a model 20 with 2-volume controls, and the schematic on Nostalgia is the correct one. I checked both pots with my ohm meter, and they do run smoothly thru the entire pot, dose not look like the wiper lifts off. I will try the cathode voltage check, that would be a better check. Thanks again for the reply.

Frank


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 Post subject: Re: Philco Model 20 Volume control problem.
PostPosted: Nov Sun 10, 2019 8:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Rome New York USA
I checked the volume control with voltage applied, voltage varies smoothly to the cathodes of the 24's. Next question, should I be getting a varying DC voltage reading on the 24 detector tube?

frank


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 Post subject: Re: Philco Model 20 Volume control problem.
PostPosted: Nov Sun 10, 2019 10:29 pm 
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Location: Livermore, CA
Frank

No voltage is constant on the detector (3rd #24 tube). Volume control only adjusts voltage to the first two tubes.

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Norm


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 Post subject: Re: Philco Model 20 Volume control problem.
PostPosted: Nov Mon 11, 2019 10:02 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Rome New York USA
Thanks Norm,
I could see how I was getting a constant 50VDC on the cathode, and you confirmed it, still cannot find how it gets there, all the components have been changed out, and double checked, now going thru the wiring to see if it's not wired correctly.

Thanks, Frank


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 Post subject: Re: Philco Model 20 Volume control problem.
PostPosted: Nov Mon 11, 2019 10:54 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Rome New York USA
This is what I have on the cathode of the Detector; Approx 15VDC, and its variable with the volume control, replaced the 24A back to a 24, no change, 0- voltage read on cathode side of C14, 0-volts read on gnd side of R14, 15VDC read on the cathode side of R14. Removed the lead of C13, voltage is still there on the cathode. Visibly inspected the tube socket, nothing is out of place. Any suggestions where i can look, really got me puzzled.

Frank


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 Post subject: Re: Philco Model 20 Volume control problem.
PostPosted: Nov Tue 12, 2019 12:36 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 37563
Location: Livermore, CA
Frank

Voltage is on cathode of a tube when it draws current. It's effectively plate current to cathode of the tube.

Volume control only adjusts cathode voltage on the first two 24 tubes. Doesn't matter if you use 24 or 24A. In some Crosley radios a 24 is required as oscillator.

Does the radio operate? We may be looking at different revisions of Philco 20?

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Norm


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 Post subject: Re: Philco Model 20 Volume control problem.
PostPosted: Nov Tue 12, 2019 1:38 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Rome New York USA
Norm,
I pu a station only when the voltage on the cathode is around 15VDC, as I adjust the volume control above or below the 15v I lose the station. It is a Model 20, the radio chassis checks out perfectly with the schematic for 20 &20A.

Frank


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 Post subject: Re: Philco Model 20 Volume control problem.
PostPosted: Nov Tue 12, 2019 3:13 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 37563
Location: Livermore, CA
Frank

Screen grid (pin #3) must be positive but below plate (pin #2) voltage. Detector screen and plate are supplied through #15, #17 and #18. Are these resistor close to the proper values?

Are any resistors in the divider (item 25) open? Resistor values from left to right are 2470, 75, 182 and1400 ohms. They don't have to be exact but not open.

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Norm


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 Post subject: Re: Philco Model 20 Volume control problem.
PostPosted: Nov Tue 12, 2019 3:51 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Rome New York USA
Norm,
All resistors have been replaced, they were all out of tol.


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 Post subject: Re: Philco Model 20 need suggestions.
PostPosted: Nov Thu 14, 2019 8:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Rome New York USA
Still bewildered by this unit, read the original post what I have done, and appreciate some suggestions what else i should look at.

thanks


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 Post subject: Re: Philco Model 20 need suggestions.
PostPosted: Nov Fri 15, 2019 8:09 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3256
Location: Lexington, KY USA
What test equipment can you use to troubleshoot this set? Signal generator? Signal tracer? scope?

We will try to suggest things to try, but it won't help you much if we suggest using stuff you don't have.

Ted


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 Post subject: Re: Philco Model 20 need suggestions.
PostPosted: Nov Fri 15, 2019 2:45 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Rome New York USA
I have a signal tracer. I get a signal from the 1st 24 to the speaker, but now not getting any stations. Could the problem be in the 1st xformer, #2 on the schematic. Its not open, I have 300 ohms on the primary, and 5 ohms on the sec. seems out of balance to me.

Frank


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 Post subject: Re: Philco Model 20 need suggestions.
PostPosted: Nov Fri 15, 2019 3:21 pm 
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Joined: May Fri 01, 2015 2:33 am
Posts: 1462
Location: Roslyn,Pa 19001
Just as a rule of thumb the plate windings (pri) are going to be pretty low in resistance abt 4 or 5 ohm. The grid windings are going to be abt 25-30 ohms. As will the primary of the ant coil. 300 ohms seems very high and you've probably got some rot there. Look for green dots on the winding. If you don't see any it's still bad.
GL

Terry


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 Post subject: Re: Philco Model 20 need suggestions.
PostPosted: Nov Fri 15, 2019 3:27 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Rome New York USA
Thanks guys, gives me somewhere to look. Will let you know what I find later today.

Frank


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 Post subject: Re: Philco Model 20 need suggestions.
PostPosted: Nov Fri 15, 2019 3:57 pm 
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Joined: May Fri 01, 2015 2:33 am
Posts: 1462
Location: Roslyn,Pa 19001
You can measure it's resistance (ant coil primary) from the grid cap of the 1st rf amp (24A tube) to the chassis but it will vary a lot by adjusting volume control. If you get 5000 ohm with the control full up to coil is open. If the coil is good the resistance is going to be much lower and only going to change when volume control is turned down low, maybe the first 20% of the rotation.

Terry


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 Post subject: Re: Philco Model 20 need suggestions.
PostPosted: Nov Fri 15, 2019 7:04 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3256
Location: Lexington, KY USA
I get a signal from the 1st 24 to the speaker

This would seem to indicate that the speaker is the problem!

How much signal do you get at the speaker? A normal amount, for your signal tracer?

You would hear no sound, even hum, in this case.

Ted


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 Post subject: Re: Philco Model 20 need suggestions.
PostPosted: Nov Fri 15, 2019 7:39 pm 
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Joined: May Fri 01, 2015 2:33 am
Posts: 1462
Location: Roslyn,Pa 19001
I discounted the speaker as in the same post he mention part #2 which is the ant coil. It's not uncommon on Philco sets from 1928-35 to have coil issues. Mostly open primaries which are not difficult to rewind. Also not uncommon for the early K speakers (ones used in the Philco 20,21,70, and,90.) cone and spider to get stiff and diminish the frequency response.

https://philcoradio.com/library/images/schem/20b.jpg

Terry


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