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Philco 37-89 questions?? - Printable Version

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Philco 37-89 questions?? - Richard Moore - 06-07-2011

This is my first post on the Phorum.

I have been working on a Philco 37-89F and have a couple of questions. I have been using the schematic I got from Nostalgia Air.

The schematic shows the grid of the 6K7 1st IF amp connected thru the secondary of the 1st IF transformer thru a brown wire to the AVC bias line. However, my radio has the brown wire going to chassis ground and it looks like it came from the factory that way. Can you tell me which is correct?

The second question I have is about a replacement for the RF choke, part #11 in the schematic. When I originally measured it I read 85 ohms instead of 9 ohms from the schematic. However, I now show that it is totally open. I see that Antique Electronic Supply has an rf choke part number P-C1535B that is also 9 ohms. Will it work as a replacement?
Thanks for any info. Dick

Re: Philco 37-89 questions?? - Radioroslyn - 06-07-2011

Hi & Welcome to the Phoum!
Don't know about the agc line. Ron or Chuck would be in the know on that. As for for your rf choke it is NOT a RESISTOR.Rf chokes are design to limit the flow of rf energy. The amount of inductance will determine the frequency it will limit or block. Sorry lately I've had a few folks that think that chokes are measured in resistance. They are not! They are measured in MH or mh not ohms. Most schematics use a resistance measurement so that the repairman can tell if it is shorted or open. What you need to know it the inductance of the choke. Once that is known then you can find a suitable replacement. A very common value for SW radios is 2.5mH. The current rating isn't very critical as rf amp doesn't use much current. Back in the old days used to make my own. 300 turns of small magnet wire on a wooden thread bobbin.
Good luck w/it
Grumpy Terry

Re: Philco 37-89 questions?? - Richard Moore - 06-07-2011

Terry: Thanks for the interest in my question. I understand that chokes are measured in millihenries of inductance, I just don't have that info from the schematic. That's why I wanted to ask the Philco experts if the 2.5 mH choke from AES that I referenced would work.

Thanks again. Dick

Re: Philco 37-89 questions?? - Ron Ramirez - 06-07-2011

Hello and welcome.

Having never owned a 37-89, I looked at the schematic just now and I find the use of the choke quite interesting. Looks like this is how B+ is fed to the plate of the RF amp tube, as the signal goes through a 1 pF cap to an intermediate (RF) coil, and then on to the signal grid of the 6A8G. So...I would guess the 2.5 mH choke would be fine for your application.

As for the AVC line or lack thereof in your 37-89's IF circuit, I would connect it back as shown in the schematic. If the brown wire is connected to ground, the IF amp will not receive any AVC voltage, and that stage could go into oscillation as a result.

There were some changes in the 37-89 circuitry; you can find these changes in the "Changes in Models" PDF I have made available to Philco Phorum members.

http://www.philcoradio.com/phorum/showthread.php?tid=215

Re: Philco 37-89 questions?? - Richard Moore - 06-12-2011

I have replaced the bad rf choke and am now re-capping the rest of the radio, which leads me to more questions.

What should I do with the two bakelite block condensers, #44 and #54?

Can I replace #44 in the tone control circuit with a regular cap of a similar or close value?

Can I replace the #54 line-to-gnd noise bypass cap with a regular, say .01 mf cap 250V, or do I need to go with a more expensive safety cap?

Thanks for any suggestions
Dick