Model 20 output trans "surgery", and questions. - Printable Version

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+--- Thread: Model 20 output trans "surgery", and questions. (/showthread.php?tid=931)

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Model 20 output trans "surgery", and questions. - gary rabbitt - 01-18-2008

Hi all,

I recently found a very nice Philco 20 De Luxe with the bird's eye maple arch, and just starting the process of doing the chassis.

On a preliminary inspection, and familiarization of the circuit, I found the output trans to have one side of the center tap open, The other side had 322 Ohms.

http://www.nostalgiaair.org/PagesByModel/582/M0029582.pdf

I decided to disassemble the trans hoping it was a broken wire at the inside terminal. No luck.

I then prodded around the top layer of windings and still did not find any continuity. I then cut a little of the top layer off, went to the second layer, carefullt scraped about 2 turns of wire, I got a reading of 316 Ohms.

Soldered a thin wire to the winding, then added back the heavier lead to the terminal, added the insulation, and a little JB welld for stability, and reassembled the trans.

My question is, will the difference on 6 ohms make any difference in the operation fo the push pull output? I say no, but want to make sure. I suspect there is enough vairence in other parts to make this 6 ohms not matter at all, except maybe on a scope under test conditions .

The second question on this model is what is the voltage rating on the filter caps? I suspect that the 630v caps we use will work fine. It does seem odd to have such a large filter can to hold such small vaule caps.

There are two 1uf caps, a 1.5uf, and a 1.3uf. I have not taken the can off yet to see if there is room in the can for the replacements. I don't want to empty the can, but want the underside to look original. The chassis has never been worked on before.

Thanks in advance, and take care,
Gary [Image: http://www.grabbittradios.com/radio/emoticon.gif]

[Image: http://www.grabbittradios.com/radio/transrep1.jpg]

[Image: http://www.grabbittradios.com/radio/transrep2.jpg]

[Image: http://www.grabbittradios.com/radio/transrep3.jpg]

[Image: http://www.grabbittradios.com/radio/transrep4.jpg]

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- Chuck Schwark - 01-18-2008

That slight impedance "mis-match" will not affect anything.

I notice you said that the top half of the winding was "open", but your note on the schematic says "0" ohms!! Not the same thing obviously!! Zero ohms is a SHORT, not an OPEN which is infinite ohms...

Just be aware that there may be more corrosion spots waiting to fail in there ....

Use 630's for the electrolytics. You can use non-polarized yellow/mylar/poly types which will fit back inside the can. They do not have to be electrolytics. If you want to use electrolytics, then use 2.2, 3.3, or 4.7uF max to hold down the turn-on current strain on the rect and transformer.

Older constuction and materials in the mid-30s dictated physical sizes then due to being non-polarized wax-paper types many times. Paper and wax have low dielectric constants (hence needing more surface area per microfarad) that does mica, mylar, poly, etc.

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- gary rabbitt - 01-18-2008

Thanks Chuck,

"notice you said that the top half of the winding was "open", but your note on the schematic says "0" ohms!! Not the same thing obviously!! Zero ohms is a SHORT, not an OPEN which is infinite ohms... "

OMG thanks for pointing that "Brain Fade" moment out to me Chuck


The trans itself looked pretty good, and the insulation was not flaking or I didn't see any corrosion spots inside. The 'mess' you see was where I had 'picked' the layers carefully with an Exacto knife.

The set looked very good otherwise, no rust or anything like that other than a lot of dust bunnies. I will need a new dial though as this one's seperated. Looks like two scales on top of each other. Mike at Radio Daze has them.

I will probably go with the electrolytics to ease the turn on surge of the set. I assume they will be the 450 volt rating?

Understand what you say about the construction on the older caps and their physical size.
Thanks again Chuck.
Gary.

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- 49Stude63 - 01-18-2008

Gary on many of the Philco PP trans that I have checked on the early Philcos the resistance varies anywhere from 5-15 ohms so I figure this is the way they were and don't worry too much.

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