Clansman 'Roller Coaster'

[dotsndashes]

Came across this item some time ago, nice solid component. Question .! there are two 'contacting wheels', on the rotating former, not at all sure why two ?
I assume! rightly ,or wrongly, that one is used to short out unused 'turns' !
Would greatly appreciate a possible answer, .. Oh ! by the way, how should the 'contact wheels' be aligned ?? Thanks for any knowledgeable answers ...

[Julesomega]

A friend has a couple of working Clansman, I'll get some photos next time I go over

[G6Tanuki]

Roller-coasters, like all tapped/adjustable coils, can sometimes suffer an effect called 'suck-out', where the unused part of the coil happens to have a self-resonance at the working frequency: if the coil is a transmitter 'tank' you can get some seriously-high circulating currents as a result, causing loss of output-power and much local heating.

One way round this is to short-circuit the un-needed part of the coil, but again this can mess-up the 'Q' and reduce the inductance.

Short-circuiting _part_ of the unused winding - the part furthest from the part-in-use - helps reduce these unwanted effects. One wheel for the tap, the other as a moving-short?

See http://g3ynh.info/comps/Vari_L.html and note mention of the MFJ "Self-resonance killer"
that shorts different bits of the coil under different circunstances.

[dotsndashes]

Hi ,Julesomega.... Thank s for your reply and your offer greatly appreciated

In the meantime Thanks to you also G6Tanuki for your input Yes I appreciate all, the remarks re the circulating currents,That is indeed, an old problem with excess and sometimes unnecessary turns on a given inductance.!
Becomes , somewhat difficult with large coils . and small gauge wire..
Hence my query re the, two, 'roller' contacts in a sound ! design like the Clansman !

[Oldcodger]

Dots & dashes- which model Clansman. I ask as I spent a few years on 353 production and customer return repairs at MSDF Fife. If it's a 353, then from somewhere in the dark depths I might be able to bring up an answer.

[dotsndashes]

Hi Oldcodger so sorry for the delay i replying to your input, been away fro a few days,,,...To answer your query re the Clansman model No. , unfortunately, I have no idea . It was just the variable inductor ('Roller Coaster') I obtained .
I assumed , the item came from a normal HF range, tcvr, not the VHF type. ??
Sorry can't help with details, but many thanks for the interest ( Another 'oldcodger. hi..

[Julesomega]

Called on my friend Brian this morning: he says his Clansman are PRC-320 which use slug-tuned and switched inductors for antenna tuning.

By comparison the PRC-321 has an external ATU known as the TURF which he thinks does not use a roller-coaster, but the 321 also has an external tuner known as the SURF or Selector Unit RF. Brian has not had one of these but the pictures show that they probably had a roller-coaster.
If you can give more details or a photo, I expect someone can advise further.

[Stockden]

The attached photo shows the TURF 25W used with the VRC-321. As you can see, it has a "Turns Counter" dial at the LHS, which presumably is driving a roller coaster inductor. The picture towards the bottom of this page would seem to support that.

Hugh

[dotsndashes]

First of all, may I offer my sincere apologies for my 'oversight ' in not replying
sooner, to both ,Stockden and Julesomega:
You both seem to arrive at the same conclusion, that the 'roller coaster' in question, is indeed from the Clansman TURF-25 watt tuner , and since I have no knowledge whatsoever on this ex-gov equipment , then I gladly accept your views, and thank you both, very much , for this important (to me) information.
I know , this does not answer my original query, but at least it has answered , 'which relevant' piece of 'gear ' the item came from..
Once more my apologies, and my sincere thanks for your interest

Best wishes to you both; and all the other members who contributed