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.

RE: RF Choke and Condenser -42-PT-7 - Arran - 11-13-2024

From what I remember from working on several transformerless Philco sets some have that choke on a proper cardboard form slipped over the body of the capacitor, but others have a length of 22 gauge wire wrapped around the outside of the capacitor shell itself. If Mike's calculation is correct, and likely is, then the purpose of the choke must have been to block power line noise from interfering with the IF frequency of the radio. Whether this was ever needed at the time, let alone now, is debatable since AC/DC sets produced by most other manufacturers chose to omit using a choke in this position.
Other then converting a true hot chassis set to one where the chassis "floats" electrically, I'm not sure that there are many mods one could make to make an AC/DC set safer short of operating the set through an isolation transformer. The end user is unlikely to ever come into contact with the chassis inside one of these sets anyhow, the original manufacturers usually issued these sets with cardboard or plastic backs, with push on plastic knobs, in some cases they even concealed the screws holding the chassis in place in one way or another. Realistically these sets are pretty much a novelty to anyone other then an antique radio enthusiast, and might be plugged in and played only occasionally, if at all. Even with my own collection there are one or two sets that I use semi regularly, the rest sit on a shelf and collect dust, i should really put them into rotation more.
Regards
Arrran