Hallicrafters SX-25 BFO - Need repair guidance

[IsquaredR]

I've got a vintage Hallicrafters SX-25 and am trying to restore its beat frequency oscillator. Perhaps someone on this forum can help me understand what is needed to allow the 455 KC
feedback to return so I can hear CW signals.

The problem started with a defective BFO coil which was damaged. When I started working on the BFO I measured (as best that I could) the inductance of the primary and secondary windings (which more accurately seems to be a tapped extension of only one coil although it it situated inside the larger coil)
on the coil. The primary winding consisted of 114 turns of AWG 36 wire on a 3/8-inch coil form.
The secondary winding consisted of 55 turns of AWG 36 wire wound inside of the primary winding and nearest to the coil form. The primary winding measured 181 microhenries and the secondary measured 49 microhenries. This is complicated by the fact that the coil is a variable coil with a threaded rod which moves a tuning slug back and forth. The tuning slug is 0.374-inch in diameter and 0.532-inch long. This ostensibly is to allow the receiver listener to vary the pitch of the beat note to a pleasing tone.

Due to the damage to this coil I attempted to rewind the coil but am not sure I understand how its generating its feedback.

In the attached photo I have expanded the BFO section. I have also uploaded a full schematic to
****************/HY7LDrd which is not a compressed file so you should be able to clearly see all of the nomenclature.

As you look at the expanded view of the BFO here are the key components:

R42 20,000 ohm resistor
C46 500 picofarad mica capacitor
C45 100 picofarad mica capacitor
R41 50,000 ohm resistor

L2 is the feedback coil with which I am having trouble.
6J5 is the BFO valve.

At first I thought the 500 picofarad capacitor across the primary winding would create an LC
combination resonant to 455 KC. But then to what should the secondary winding be resonant?
Am I incorrect in assuming this coil is supposed to be resonant at 455 KC? I could understand it
better if the LC combination of the coil plus 500 picofarad capacitor yielded 455 KC and that was the end of it. But what does the 100 picofarad capacitor and the smaller value of the secondary do? Obviously the oscillation of a 455 KC signal goes into the detector where it beats against the incoming RF to create a beat note. But somehow I have missed how this coil and handful of capacitors generate an LC resonance at 455 KC between the two windings split with the 500 and 100 picofarad capacitors.

I really appreciate any help you good people can lend to help me understand this aggravating coil.

[ms660]

The BFO is a shunt fed tuned grid oscillator (aka a Meissner or Armstrong oscillator) The tuned circuit is formed by the 500pF capacitor and the inductance that's in parallel with it, the shunt fed feedback inductance doesn't form part of another resonant circuit as such, the 100pF capacitor and the grid resistor ensure that the valve/tube operates in Class C which is normal for oscillators of this type.

Lawrence.

[IsquaredR]

Thank you Lawrence. You've provided the information I need to re-make the coil. As I interpret it, the primary coil in parallel with the 500 pF capacitor should resonate at 455 KC. My only remaining question is that tuning slug location. When I dismantled the oscillator coil assembly it appeared that the tuning slug was not near the coils. Should not the tuning slug be set directly inside the windings when the primary winding (with a 500 pF capacitor) is resonant at 455 KC? (Using a grid dip meter).According to my LC calculations a combination of 500 pF and an inductance of 245 microhenries should achieve this. So if I wind another coil with that inductance it should dip the grid dipper at 455 KC so long as the tuning slug is in the right place; but I am not sure where the "right place" is for that slug.

I really appreciated your response, Lawrence. Thanks.

[ms660]

No problem.

Providing there's no other damage and the tuning slug adjuster works as it should I would wind the new coils as best as possible to the original spec. make sure the tube is ok, replace the three capacitors and check that the plate and grid resistors are ok then take it from there.

Lawrence.

[Jon_G4MDC]

Resonance on 455 kHz with the slug about half way into the winding would be one way to think of it. But that might be too "tweaky" for setting the few kHz of offset wanted on a BFO control.

It might be why you found the slug appeared to be some way out from the coil. It only relied on approaching the winding not being right inside it. That would give a less sensitive freq vs rotation law.

It's always difficult with tapped coils or those with an off centre link winding to know which end the slug should be positioned. At VHF I have known this to make significant difference to the performance, there are two resonant points but one is better than the other. Some sets included where the slugs should be positioned in the alignment instructions.

[IsquaredR]

Thank you Jon and Lawrence for your insights and taking the time to comment. I have the coil re-made and it dips at 455 KC. I needed to do some clean-up finish work on the coil requiring epoxy so today I should see how it works in circuit. Jon, "would-that-it-were," if Hallicrafters had provided such information about this coil it would have been wonderful. But their original service manual is rife with errors and omissions and, quite frankly, not very well written to begin with. As a result I find myself "reverse-engineering" what I need on this receiver. Ernie

[Ed_Dinning]

Hi, be careful when using epoxy with coils as some types have a high ue (permitivitty) that will alter the self capacitance of the coil.
Spray varnish or paraffin wax may be a better option, but expect a frequency shift of some sort from the impregnated coil

Ed

[FrankB]

It is my experience that if the "mica" caps are paper mica, they are bad and need replacement. The paper is breaking down and causing lots of problems.
I do not even bother to test them any more, just replace with new epoxy coated ones.

I have run into at least 2 cases so far, that the caps will test just fine capacitance and voltage wise on several testers, but even NOS ones are bad. Replacement cured the problems in the sets. Not sure why they tested good on an ESR tester, eye type tester. and an Sencore LC tester, and ohmmeter test.

[IsquaredR]

Thank you Ed and Frank. Ed I have a bottle of "Q-Dope" which I was planning to use once I get the coil to oscillate. Obviously at this point I do not wish to fix anything with dope until I am sure the coil functions.

Frank: We who dabble with vintage equipment are persuaded to believe that one should never change mica capacitors because they do not decline. I too have experienced paper micas faltering so your advice is right on the mark.

Jon: I believe my problem is the slug. I get a very weak dip on my grid dip meter at 455 KC, which is where I want it. However when I tighten the nut holding the coil in its mounting bracket it physically pulls the slug forward and it is enough to shut off resonance. So my task is now to redesign my coil to obviate this from occurring. I also am wondering if the slug needs to be near but not abeam the coil for the reasons you stated. When I finish the next coil I am going to try positioning the wire "near" but not abeam the coil and see what happens.

Thanks to all for taking the time to jump in here on my behalf.

Ernie