[Stonne]

I am working on a 51-532 and i am attempting to rebuild the IF xfmrs. I'm stuck on the 2nd IF. Can anyone tel me the correct values of the Pri and Sec micas. The online calculator tells me that the Pri mica should be 172pf and the Sec mica should be 120pf. I think these numbers are correct but what about the other mica caps that are attached to the bottom of the Sec tank? Are they there to bypass any remaining RF to ground? What value will work? Should the also be about 100pf?
Any suggestions will help.

[Ron Ramirez]

I can't answer your question about the primary and secondary capacitors, but the extra two caps (between terminals 2 and 5 and between 5 and 6) are likely 100 to 110 pF each. They are there to help remove the RF component from the audio signal.

What is this "online calculator" you speak of? Link please?

[morzh]

Since you seems to have removed the resonant tank caps (else why would you ask about their values; micas are rarely need replacement, and in 50-s radios they are rarely subject to the SMD), unless they were destroyed, just measure them wigth a good C-meter (not with a DMM, they are not accurate in the units of pF, though you could try to press "delta" if it is a Fluke) and see if they are good.

In case the caps are bad (destroyed, broken during removal etc):

1. If the coils are good, measure L at the frequency as close to the IF as possible (my Stanford allows 100kHz which is better than 1kHz or 10kHz). Leave the slug in the position it was originally of not disturbed. If it was out, put it more or less in the middle before the measurement.
2. If the coils are bad, rewind as close as possible to the original. Then do the measurement.
3. When knowing your L, calculate the C from the standard tank formula (1/(6.28 * sqrt(L*C)))
4. Find a mica cap closest to the calculated value, then solder it in and using the generator, measure the resonant frequency (either by observing the V or I), see if you could put it in 455kHz moving the slug.
If the frequency is out of range, change the capacitance.
5. Do that for the second coil.

[Stonne]

Thanks Ron and Morzh,

The link for the calculator is:
https://www.omnicalculator.com/physics/r...equency-lc

I have replaced the mica sheets in other IF tanks before with good success but have not run into the type with the two extra caps at the bottom of the secondary tank.

Thanks Ron I will try 100pf's and see how they do. Maybe even 220pf might work.

[morzh]

I see.... this calculator uses the standard formula, same that I just listed.

I thought it would be some geometry-based calculator.
This one will not help you anymore than the one I have listed above.

You need to know your iunductance and capacitance, which is what I listed in the algorythm.

[Ron Ramirez]

> Maybe even 220pf might work.

No, you don't want to go much above 110 pF. Smaller is ok (again, to a point). Best practice: Keep the values within the 100 pF ballpark.