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Old 15th May 2019, 10:09 pm   #1
Keith
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Default Fenman II VHF tuning hysteresis

Owners of this set will know that the VHF permeability tuning is implemented by a spring loaded plate in contact with a cam on the tuning shaft. Despite numerous attempts at cleaning/re-greasing, mine has always suffered from hysteresis (i.e. backlash), making tuning difficult. I wonder if other owners have experienced this and if a fix has been found.
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Old 19th May 2019, 4:22 pm   #2
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Default Re: Fenman II VHF tuning hysteresis

Looks like I'm the only one experiencing this problem!
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Old 19th May 2019, 7:09 pm   #3
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Default Re: Fenman II VHF tuning hysteresis

Just noticed your thread Keith, I also have noticed the same issue. I bought the set as "restored" and assumed that the drive cord was just a bit loose. I didn't realise that it was permeability tuning. I find your have to go past the tuned spot and then back up, a bit of a nuisance.
Peter
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Old 19th May 2019, 7:51 pm   #4
Radio_Dave
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Default Re: Fenman II VHF tuning hysteresis

All of mine do the same. One in particular is really bad and although it's never bothered me enough to investigate I've always suspected it was the cores binding inside the permeable tuner?


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David
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Old 19th May 2019, 8:07 pm   #5
Nickthedentist
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Default Re: Fenman II VHF tuning hysteresis

Mine too! Often little happens when the tuning knob is rotated, then "clunk!" and something in the permeability tuner frees itself and normal service is resumed.

Not got to the bottom of it yet...
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Old 26th May 2019, 3:33 pm   #6
Takapuna
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Default Re: Fenman II VHF tuning hysteresis

It must be a design feature! The cam follows the tuning knob without any backlash and the plate to which the cores are attached appears to do the same but I still have to "home-in" on the best tuning point.
Holding the plate against the spring at its high travel limit exposes the top end of all cores and they are a very loose fit inside their formers in my radio. A gentle flex of the rods to which they attach shows the cores can be moved across the inner diameter of the coil formers.

Phil.
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Old 26th May 2019, 4:52 pm   #7
Keith
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Default Re: Fenman II VHF tuning hysteresis

Thanks for your replies. I can't see any reason why the plate shouldn't follow the cam. In fact I have moved the spring mounting to give more pressure, without any effect. I hadn't thought of the possibility of the cores moving "across" the coil former but this would presumably affect the inductance, so may be the mechanism. I shall investigate further, as the problem detracts from what is otherwise an excellent receiver.
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