The PHILCO Phorum
RF Choke and Condenser -42-PT-7 - Printable Version

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RF Choke and Condenser -42-PT-7 - Masonparker - 11-11-2024

I'm working on a 42-PT-7 and I have some questions on the RF Choke and condenser

Based on the schematic  Item 7 should be .2 mdf part no. 76-1161  that condenser that is currently installed is a 2 mdf   part No. 30-4594

Looking to find out if there was any updates that made the change of should I install the .2 per the original schematic 

Also the currently installed condenser has 7 windings around the foil side.  Looking for confirmation that these windings are not required

RE: RF Choke and Condenser -42-PT-7 - morzh - 11-11-2024

OK, well,

This seems like a wave trap and is not even important. You could simply remove it.
As for the value of the cap:

Let's assume 2cm diameter (if it is 2uF, it is large), and 1cm the length of the winding.
We will also assume air core.
It is just the first order calcs:

The inductance is:

Mu-zero (air dielectric const) * A (area) * N-square (turns) / l (length)

L = 4*3.14 * 10^-7 * 49 * 3.14 * 10^-4 = 2uH

Now the resonant frequency for 2uF is:

1/ (2 * Pi * SQRT (LC)) = 1/(6.28 * SQRT (0.000002 * 0.000002)) = 79 kHz.

For 0.2uF:

We will also assume the cap is 1/2 the other one's diameter, which will decrease L by 4.
But in the Frequency it is SQRT, so it is factor of 2.

So it is 500kHz.

It is pretty close to 455 kHz IF frequency.


So, my vote is for 0.2uF.


----------------------------



And, again - this is not important. This LC could be removed.

RE: RF Choke and Condenser -42-PT-7 - RodB - 11-11-2024

I agree Mike, I see 0.1 or 0.2 mfd for those. Come to think, the 0.1 caps have twice the coils of wire. And even more evidence is that if it's a paper wax capacitor it's 0.2mfd.

RE: RF Choke and Condenser -42-PT-7 - morzh - 11-11-2024

Rod

I would also assume (had that happen just very recently with, I think, 38-14) that this was 0.2uF with the dot missing, but because the Philco P/N quoted by Mason was different from the listed in the documents, I decided to do the quick calculation, to see if there indeed was a factory installed 2uF.
This said, this part today is non-functional anyhow.

RE: RF Choke and Condenser -42-PT-7 - MrFixr55 - 11-12-2024

Hi Folks,

If you are talking about Part #7, this is the cap that capacitively grounds the physical chassis to B- and is important. It should be 0.2uF and the coil portion is likely not urgent, just good engineering on Philco's part.

The likely proper values should be 0.2UF, similar to other Transitones, like the 42PT94.

Some people call this the "Death Cap" because of the following:

For some reason (possibly to reduce hum), the power switch is in the B- and filament return leg of the power line. This is the point where the cap in question connects B- to the chassis for RF coupling of the tuning cap, shields, etc., making this a "warm chassis" set. This is as opposed to a hard wire connection between chassis and this point making this a "hot chassis" set. The problem is that if this leg is connected to the neutral pin of the power cord, the chassis is at ground potential with the power ON, making it "safe". However, with the power switch off, the low cold resistance of the tube filaments make the chassis "hot", even though the cap. The cap will limit current (warm chassis) unless it shorts, then there is no limiting, making the thing a death trap.

So, the issue is that with the way the set is built, that the chassis will be hot depending on the position of the power switch and the polarity of the plug. With the plug in one direction, the chassis will be at ground potential when the switch is on but hot with the switch off. With the polarity of the power cord reversed, the chassis will be at ground potential with the switch off, but hot with the switch on.

Several restoration sites recommend moving the switch to the "hot" side (Input to rectifier and rectifier end of filament string) and using a polarized cord to prevent this condition.