Permeability Tuning

[pwdrive]

You might be right Alan, I will check...

[pwdrive]

Not looking good Alan, I think a scrap VHF domestic is in order.

Lawrence.

[Mark Hennessy]

Just to add to the survey of permeability tuning, the Tandberg TP41 I fixed last night uses that for FM. You might not think that to look at it because there is a drive shaft emerging from the screening can, but when you venture inside, it's driving a rack and pinion arrangement that moves the ferrites along...

[pwdrive]

Hi Dr Mark, can you confirm the slug was ferrite or iron dust?

Lawrence.

[Yorkie]

Oddly, the Fenman I doesn't (on VHF), but the Fenman II and the HFT111 do. I don't know about the HFT113 but I think it does.
Was it down to economics or performance? It can't be size 'cause there's plenty of room in the Fenman I.

Alan

The Feman 1 does in fact use permeability tuining on VHF - leastways, the one that I restored does. It wouldn't tune, and the reason was that the cam which causes the cores to move in and out of the coils was seized. I'm not at home just now, but when I get a few mins over the next few days, I'll take a pic of the tuning arrangement as the chassis is out of the cabinet. Quite a neat bit of electro-mechaical engineering. Permeability tuning was used extensively on push-button car radios. The buttons weren't so much 'switches' as 'levers', which pushed the cores in or out of the coils by a pre-set amount chosen by the owner to tune to the desired stations. Primitive by current standards, but effective nevertheless.

[pwdrive]

I reckon this permeabilty tuning system is a work of art:

http://www.cryptomuseum.com/spy/sp15/fe8.htm

Lawrence.

[AlanBeckett]

Interesting. I wonder if there was more than one version.
Attached is the FM front end diagram, tuned by C5 and C14 which are part of the main gang.

Alan

   

[Joe]

Likewise on the manufacturer's:-

   

- Joe

[AlanBeckett]

David's got some sort of Doppleganger

Alan

[Mark Hennessy]

Hi Dr Mark, can you confirm the slug was ferrite or iron dust?

Hi Lawrence,

To be honest, I didn't look. That part of the radio was working, and I didn't want to risk causing more problems - it was a customer repair, and I'd spent enough time tracing a really obscure fault with the AM AGC circuits. Not helped by the fact that the only schematics I have are poor quality scans - impossible to read the component values or designations...

Cheers,

Mark