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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sat 13, 2018 6:27 pm 
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To figure out which terminal is which on an RF or IF transformer, there are a couple of methods to use:

1. Study the schematic and trace the wiring, so you can identify the terminals of the transformer by looking at what they are connected to.

2. Use an ohmmeter to measure the resistance between pairs of terminals on the transformer. In a typical transformer with an untapped primary and secondary coil, you'll find that there are two pairs with a resistance of a few ohms to at most a few tens of ohms, and all other combinations show open circuit (or higher resistance, depending on connections to other circuitry).

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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sat 13, 2018 7:39 pm 
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yep traced and also compared to other fubar model 20 chassis which apparently has a good coil in that spot. Can I rewind it, not sure yet. Hoping its the top coil winding as its wound on top of the other winding.


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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sat 13, 2018 7:43 pm 
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The primary winding on those coils is not particularly critical, so if you end up with a little different arrangement of the turns, or the number of turns is slightly off, it will still work.

On the other hand, the secondary winding, which is tuned by the tuning capacitor, is quite critical, and must be rewound very close to the original arrangement.

So if your primary coil (the one that's burned out for you - the one going to the plate of the prior tube) is on top, you can rewind it. If it cannot be rewound without disturbing the secondary coil, I'd hesitate to try it.

If you have another "fubar" Philco 20 chassis, how about swapping the good coil from that chassis into the one you're working on now?

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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sat 13, 2018 9:03 pm 
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I agree on the fubar chassis but I only say fubar because somebody else has over capped it with a motor start cap, among other things, yes really a huge motor start cap in parallel with the cap can. I might be able to save it. Not sure yet. Yes its the outer coil thats messed up open. I have 32, 34, 36 and maybe 38 guage. Measured with calipers, the original winding without cloth is .004 inches and .007 with the cloth around it. Not sure yet which wire to go with other than its 46 turns around form.


Last edited by quartermilecamel on Jan Sat 13, 2018 9:19 pm, edited 1 time in total.

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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sat 13, 2018 9:13 pm 
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Wire size doesn't have to be exact. Go with one that's close, and try to wind it approximately the same as the original coil. Since this is the untuned primary, it will probably work OK. Not so critical to get the winding to exactly match the original.

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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sat 13, 2018 9:29 pm 
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wow just found the 38 guage which is .004 according to calipers it measures the same. I can see why philco had so many issues with wire when they put this weird fine cloth over the wire, which made it rot. Thought about unwinding the original and fixing it till I kept on unwinding it kept on breaking. clearance for take off with 38 guage.


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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sat 13, 2018 11:17 pm 
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2nd rf 24 tube now has b+ on it 223 volts(insert fist pump here). Measured resistance of fubar chassis coil and it was 10.3 and this newly wound coil has 12 ohms and roughly 46 turns of wire.


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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sat 13, 2018 11:42 pm 
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trying to figure out why the detector has double the plate voltage on it. when I measure resistor 18 which is supposed to be 100k it is......with one lead not connected to the radio. when I connect it, I read 976k ohms. Not sure how thats even possable measuring a much larger value with the resistor installed both ends of it in the circuit


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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sat 13, 2018 11:52 pm 
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On the detector, check resistors 12, 15, 17, and 18. If all OK, and the tube tests OK, then don't worry about the higher plate voltage you are seeing. In the old days, the type of meter used to measure voltage would have loaded that point down and shown a lower voltage than the radio really has when operating. Your modern meter loads it down very little, and reads closer to the true operating voltage.

Sometimes if you see a very strange resistance reading when a resistor is in circuit, it is because there is some current still flowing in the radio. Capacitors hold some charge, and tube filaments may still be a little warm. That's enough to mess up your resistance reading. When you see this, try reversing your meter leads. If the resistance reading one way is different than the other way, there is current flowing in the resistor, and you cannot trust either of the measurements. Usually, just letting the radio sit for a few minutes is enough to discharge everything. Or, just disconnect one side as you did. Then you can trust the measurement.

Any sign of life from the radio at this point?

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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sun 14, 2018 12:05 am 
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havnt checked to see if it plays other than to test voltages. Resistors 17 and 20 were reading 365k instead of 500k.........leaky cap between them, replaced it, now ohms are near 500k. I havnt messed with the caps inside the flat retangular sheetmetal cans yet either. also I do have the variac set to about 90 volts to keep from overvolting the chassis since I have 2 10ufd caps in place as I dont have the equivalent replacements for the huge cap can next to transformer that I havnt figured out how one can just unbolt the thing, snip wires and solder caps between tar and terminal strip unless you have to pry open the canister.


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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sun 14, 2018 12:24 am 
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detector tube plate now down to 48 volts.


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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sun 14, 2018 1:04 am 
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square can caps also leaky.....replaced. Now listening to local basket ball game on it!!!!!!!!!!!!!!!!!!!!!!!!


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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sun 14, 2018 1:22 am 
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Looks like it's been a very good day in your workshop.

Nice job on rewinding that coil. That's getting into more "advanced" restoration!

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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sun 14, 2018 2:32 am 
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are the other 2 primarys of the round coils this same style of design? Top wound coil has the crappy cloth covered wire and the coil underneath it they didnt use cloth?


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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sun 14, 2018 2:48 pm 
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reading nostalgia airs schematic and I thought there would be an alignment procedure, but I guess not. I tried messing with the cap adjustments on the side of the tuning condenser, but not sure yet how to go about tweaking them up.
Aligning philco recievers by john f rider great, it will be there. Nope! They skip model 20???


Last edited by quartermilecamel on Jan Sun 14, 2018 4:03 pm, edited 1 time in total.

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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sun 14, 2018 3:59 pm 
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http://www.silverslingshot.com/PDFFILES/aligningphilcoreceivers_1937.pdf


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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sun 14, 2018 4:06 pm 
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Oh ok it is there at the bottom of model 3


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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sun 14, 2018 6:25 pm 
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if memory serves me correct a 200mmf cap is really 200pf?


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 Post subject: Re: Model 20 philco troubleshooting, speaker testing issues
PostPosted: Jan Sun 14, 2018 8:18 pm 
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Yes, 200 mmf = 200 pF.

Since this is a TRF receiver, alignment is particularly easy. Tune in a weak station about 2/3 the way up the dial (say around 1000 - 1300 kHz, exact frequency not critical at all). Then adjust the trimmers on the tuning capacitor for maximum signal strength. And that's it -- you're done.

I see the Rider document above recommends aligning at 1400 kHz. End result will be about the same. I like aligning a TRF a little lower on the dial (1000-1300 kHz), as a compromise to get things working well over the whole dial. But the difference may not even be noticeable.

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