Rgd rr223

[crackle]

Hi
I have been having a lot of trouble with this radio but have now finally got it working with a replacement ferrite rod and coils from a similar KB KR023 radio.

When the radio arrived the ferrite rod had jumped out of its mounts and was loose in the case. The coils appeared OK, and the rod looked OK as well, but later I found that MW was very quiet and rather intermittent when I was adjusting the MW coil for the low frequency alignment.
I checked and there was a small amount of flexing in the rod.
The rod is covered in what looks like a heat shrunk plastic sleeve and the clean break in the ferrite was also under several layers of original looking tape.
I removed the tape and made a ring cut around the plastic sleeve about an inch from the now visible break. It was was then possible to pull the 2 halves of the rod apart and add a couple of drops of superglue to the joint and realign it in the tube. This should have worked fairly well, but this was when I found that there were also hairline damage to the Litz wire on the MW coil.
The coil is supposed to measure about 2 ohms but my damaged one was measuring about 11 ohms, so although it had continuity some of the 5 separate strands of the litz wire were damaged. MW was working but it was quiet and difficult to align the radio.
This leaves me with the task of repairing the ferrite rod or finding a replacement one with MW coil. (I have put a request in the wanted section)
Its early days yet, but if nothing turns up I will have rewind the MW coil myself.

Finally I come to my question, what difference will it make if I rewind the coil with ordinary enamelled copper wire of the same overall diameter as the litz wire, 0.7mm?

Thanks
Mike

[Herald1360]

Probably not a lot.

[crackle]

OK then I will have a go.
Somehow I got my numbers confused, the overall diameter of the original litz wire was 0.21mm each of the 5 strands was 0.07mm.
Anyway I have unwound an old choke and got the solid enamelled wire off, it is .19mm diameter and the closest I can find.
I have wound on 53 turns with the taps at 4.5 and 6 turns. I have put a dab of nail varnish on the end of the winding to help anchor it.
When it is dry I will spread the winding out more evenly so it is the same length on the former as the original, When I wound it it ended up shorter as the wire is thinner.

Fingers crossed.

Mike

[Herald1360]

Do you have anything to check the actual inductance of the two windings, Litz and plain? They need to be the same to maintain tracking etc for the set.

[crackle]

My DMM has inductance measurement and the original coil came up as 0.174mH. The new coil measures 0.019 in open air to 0.2mH when fitted on the ferrite rod.
Attached are the details just in case anyone else ever needs them.

The new coil works just as well as the original one and peaks nicely when adjusted on the rod for the low frequency trimming and also peaks well on the trimmer cap for the HF trimming.

This has been a difficult repair, one thing after another seemed to go wrong, gut it is all working very well now.
Thanks for the advice on the coil.
This is the RF circuit, the top left coil is the MW aerial one.


Here is the finished ferrite rod.


Here is a photo of the radio.

More photos here.

Mike
Mike

[Mike. Watterson]

Yes, adjusting coil position near end of rod and in some cases LW <-> MW coil spacing affects the inductance quite a bit and often is part of the LF alignment, often I think 600 KHz and 175 KHz rather than extreme end of the band.

Good job.

I've found that often there is what LOOKS like Litz on pile wound LW which is single core and the reason is so the wire doesn't slip during winding.


In theory Litz gives better Q because of skin effect on a single core, except really LW and MW aren't really high enough frequencies for much skin effect. So I've concluded it's to aid flexibility, lower self capacitance, aid winding and have more consistent inductance. I've seen also MW coils that are actually fabric covered single core enamel covered wire.

I've wound quite a few L.O. Coils in battery valve sets and the only difference I've seen is that the actual Litz or fabric covered wires are easier to wind.

Some HF transmitters / receivers from the 1970s onward use multiple enamel wires twisted like rope to reduce skin effect. They don't have a lot of turns. So I'm thinking the real reason for Litz on LW & MW aerial and L.O. coils is to reduce the self capacitance.

[Herald1360]

A good result!

The new coil may be higher inductance at the same point on the rod, but of course moving it a bit further out will sort that during alignment.

[Hybrid tellies]

A nice repair Mike. Those new windings must have been quite a tricky job.

[crackle]

Yes I would say so, I had to do it under a 8 inch illuminated magnifier glass and wearing 2 pairs of spectacles.

Mike