[Daniel Bingamon]

What are the proper capacitances for a Miller I3-PC1 IF Transformer?   It's NOS, never installed in a set and apparently has Sliver Mica diseases (arcing).

[Radioroslyn]

You can browse thru the Miller catalog  https://worldradiohistory.com/Archive-Ca...log-59.pdf  but I doubt it's going to disclose that information. The general consensus is about 100 mmfd. The proper value is determined by trial and error.

[Chas]

One can try to measure the capacitance with an analog bridge like a Heathkit IT-11 Capacitor Checker. Lift one wire of each coil so only the capacitor is in the bridge measurement. The bridge does not place any high voltage so there would be no arcing. Use short leads well separated and deduct about 5pf for stray.
Chas

[Daniel Bingamon]

I measured the caps after disconnecting the coils.  Here I thought they were 1:1 but one has 137pF and the other has 100pF.  Once measured it also obvious that the two are different, one has a small tuning slug.

[Chas]

I think your in the ball park with those values.

Though tedious, make a temporary installation of the values found. Isolate the primary with a 10k resistor on the plate side then to a RF oscillator. Common connect the B+ and AVC connections to the IF, generator and the VTVM/scope. Use either the scope or a VTVM at high impedance and measure the RF as the generator is slowly swept through the IF pass band. If the values are correct then the slugs will not have to be moved.

Try other values if they get closer to the resonance if need be.

I have done similar to identify the resonance of unknown IF's from the junk box as well as pre-set IF's. They will be very close but not exact as the stray capacitance in the test procedure and in the radio is just a guess... Unfortunately, this test will not qualify a junque box IF as to having "SMD" or not. Must have B+ to do that...

I am in favor to not moving the slugs if at all possible, the IF passband shape may not be the same if the slugs are much closer or further apart to achieve resonance.

YMMV  Chas

[Daniel Bingamon]

From what I understand, the mfg would pick and choose capacitors to go with the coil. Even though the coils are wound on machine no two are identical -- so, yes it is best to keep the slugs at their original position if possible.

I found this youtube video at: https://www.youtube.com/watch?v=8U0qBLMWgoc
Even though it will change some in the circuit, this is a good way to test if the transformer is working to some extent - although it won't identify a cap that is breaking down at HV.

[Daniel Bingamon]

So, I replaced the caps in them as close as possible, yet I have another question.
This is radio I built from the ground up. I even designed the circuit board. I chose to use the generation of 12SA7, 12SK7 and 12SQ7 set.
Is anyone familiar with the impedances these circuit require?
I have an input transformer for the 1st stage and an output transformer for the 2nd stage. The input transformers caps are 107 pF one side and 137 pF on the other. The output transformer caps are 110 pF on one side and 138 on the other.
The question is on each set, which way does the side with the lower value capacitor go? Would the higher impedance go on the side that the output tube plate is connected to and the lower impedance on the grid side (1st stage) or detector side (2nd stage)?

Since a tank circuit balances out to lower capacitor/higher inductance or higher capacitor/lower inductance both balancing out to 455kHz, my gut feeling is that the low capacitor/higher inductance would go with the higher voltage side of the circuit. I just don't yet know enough about IF transformer impedances.