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 Philco, Model 38-12

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lovetuberadios
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Join date : 2015-11-19

PostSubject: Philco, Model 38-12   November 19th 2015, 11:33 am

Good afternoon!
                          I am currently working on a Philco, model 38-12 chassis. I removed the power transformer and repaired the broken wiring.
I then measured the windings resistance and all were found good. I then applied 120 VAC to the primary and measured the secondary voltages.
The tube filament voltage read 6.3 VAC, its resistance measured less then an ohm like the schematic showed.
Installed the transformer into the chassis and wired it. Rechecked the resistances without the tubes. Here is my question: The tube filament between pin 1 of the 84 rectifier tube and to ground(chassis) is now reading zero ohms. I want to use the variac to apply power to the chassis without the tubes to make sure the voltages are at the correct locations before I install the tubes. Should the tube filament resistance be what I measured when the transformer was out of the chassis, about 0.6 ohms? The schematic shows one tube filament wire connected to pin 1 of the 84 rectifier tube and the other wire connected to ground. Any ideas I would be very greatful.
thank you kindly for your help.
lovetuberadios
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Cliff Jones
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Join date : 2010-11-22

PostSubject: Re: Philco, Model 38-12   November 19th 2015, 3:38 pm

The schematic shows .1 ohms or less, you indicate .6, it maybe OK but, that is the winding not the filament inside the tube itself. Just check your solder connections, since it's a rectifier, the solder connections that are connected have to tolerate a lot of heat, so when you turn off the radio, the solder is being subject to temperature changes, that causes the solder to fracture and over a long time it will develop what is called a cold solder joint, so it's best to use fresh solder just to make sure.
If you have a cold solder joint the resistance increases, maybe that's why you're reading .6 ohms. 

And yes you should read 0 ohms on the socket pin to chassis ground, which means that one side is good as far as continuity.

I would suggest you use a dim bulb tester in series, so it will limit the current, and not exceed your variac current. You can also check the filaments of each tube, just to make sure by doing a continuity (ohmage) test.

Have you replaced the power filter capacitors???
If not you may burn out the transformer. If you had to resolder the transformer, that is a concern of a previous short being developed by the caps as mentioned. But my not being there is a guess as to why the repair.

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FrankB
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PostSubject: Re: Philco, Model 38-12   January 5th 2016, 5:24 pm

Your meter may also be lying to you.
Most meters wont measure that low. They will also include the test lead resistance and the plug to socket resistance of the test leads too. .6 looks fine from what I have seen in the past.
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