Hi all,
I had to repair the primary winding of the MW broadcast rf transformer from the HMQ and was puzzled by an extra length of wire that's connected to the rf plate side of the primary and lays over the secondary, seemingly to give some small amount of capacitive coupling between the two.

I've not seen this before, all comments welcome.

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Currently restoring Astor HMQ 32vdc
Completed and working great!!

Have a feeling I have repaired a couple of those sets, but not an RF coil, which is I presume in this case an aerial coil.

It was not unusual to have a 1-3pF cap across from primary to secondary, or sometimes a cap made by wrapping wire over another or twisted together. to give a small amount of coupling (external)

It is not unlikely this may serve a similar purpose Do put it back the way you found it or it may upset something.

I have just had the fun of repairing both the oscillator & aerial coil on a Philips due to rodent chewing them.

Marc

Hi Marc,
This is the coil that couples the rf valve plate to the mixer grid, not the aerial coil.
The radio still worked, as there's also a 25k feeding B+ to the rf plate, though I did find the alignment behaviour a bit odd
This coil had some damage on one spot and was broken in about 3 places.
Had to sacrifice about 15 turns before I had a single end that had continuity to the other end. And yes, I have waxed that coupling wire back in exactly as I found it.

The aerial coil in this set has a 8pf cap from primary to secondary.
I've played with some taxi two way valve radios that had a pair of insulated [winding wire] wires twitched around each other forming a few pf to couple stages together, just as you described.

Yes, it is "fun" fixing fined wire wound coils! Hope the set was turned on when the rodents bit into the coil!! Would have been a new life experience for them!!

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Currently restoring Astor HMQ 32vdc
Completed and working great!!

What you are describing sounds like its on a 1M5 or similar, coupling the first RF to the converter.

A common arrangement on battery farm sets and more upmarket SW sets.

That stage is TRF as the first valve is merely an RF amp. and the coil at the frequency of the band. If you throw the coil off frequency that can be trimmed, if you alter its bandspread, that's a whole new ballgame.

The oscillator coils will likely only occur in the screen and or cathode grid elements.

Marc

A thought - RF transformers can be limited in the higher frequency range due to hysteresis losses; maybe these wires provide some capacitive coupling to minimise these losses (???????)

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Cheers, Ian

Marcc
Yes, it's a 1M5 rf, 1C7 mixer and 1M5 if with the oscillator coil on grid 1.
I do note other brands with the same valve compliment, but arranged as antenna to 1C7 mixer, 1M5 if, 1M5 if

QUOTE: A thought - RF transformers can be limited in the higher frequency range due to hysteresis losses; maybe these wires provide some capacitive coupling to minimise these losses (???????)

That's a theory, the higher the frequency, the more signal is coupled via the wire compensating for core loss.

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Currently restoring Astor HMQ 32vdc
Completed and working great!!

I cannot see any form of slug in those formers. Clearly the material within the "ferrite" (if used) will influence all aspects of it.

These being MW coils are not overly high in frequency. We must assume that whoever built transformer, got it right and in order for it to continue being right, making structural repairs to it that will physically change it, may have serious ramifications electrically.

As that wire is within the magnetic field of a coil, I would expect some (albeit small) amount of coupling.

Marc

Hi Marc,
They do have a rather 'hefty' ferrite in all 3 Here's an end-on pic showing the slug
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The other smaller coils you can see in the pic are for the 3 shortwave bands.

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Currently restoring Astor HMQ 32vdc
Completed and working great!!