Replacing a ceramic spindle - UK Vintage Radio Repair and Restoration Discussion Forum

Silicon · 1st Jul 2019, 10:07 pm

The ceramic spindle of a four gang variable capacitor is broken.

It is part of a Marconi passive preselector for use with HF receivers.
The length of the spindle is about 200mm and the diameter is approximately 11mm.

I assume that a replacement ceramic spindle will be costly and rare.
Since it will only be used personally, I want an economic repair.

I can buy a Delrin (POM Acetal) rod which I believe is one of the more rigid plastics.
A glass rod is another option.

Has anyone attempted a repair like this? Do you have any better suggestions?

Radio Wrangler · 1st Jul 2019, 11:13 pm

I think I have one.

I built a preselector some time ago and obtained a rather nice 4-gang capacitor. My circuit design needs all four rotors earthed, so it made sense to turn up a brass shaft for it. So the ceramic shaft may be left lurking in a drawer somewhere.

I'll have a look when I have some time.

I think a number of these capacitors came on the market in the mid 80's.

David

Silicon · 1st Jul 2019, 11:46 pm

The four rotors in this variable capacitor are all wired to chassis. Maybe a steel or brass spindle would be just as good, for my needs.

I initially assumed that they used a ceramic spindle to electrically isolate the rotors.
However there may be other advantages such as thermal stability.
Some ceramics are 70% more rigid than steel.
Perhaps it is the material of choice for the perfectionist.

Radio Wrangler · 9th Jul 2019, 4:09 am

I've had a search..

I still think I've got one rolling around, somewhere.

What I have found, is an entire capacitor. 4 sections, big ceramic shaft.

David

turretslug · 9th Jul 2019, 10:37 am

I wonder if these were B40 spares- I'd heard that it wasn't so unusual to find that the gang capacitor shafts were broken, probably not helped by cramped below-decks situations, brusque handling and considerable weight! If there was noticeable attrition, perhaps stock-holding was generous in the way that there used to be plenty of NOS AR88 output transformers around- until there weren't...

1st Jul 2019, 10:07 pm	#1
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,163	Replacing a ceramic spindle The ceramic spindle of a four gang variable capacitor is broken. It is part of a Marconi passive preselector for use with HF receivers. The length of the spindle is about 200mm and the diameter is approximately 11mm. I assume that a replacement ceramic spindle will be costly and rare. Since it will only be used personally, I want an economic repair. I can buy a Delrin (POM Acetal) rod which I believe is one of the more rigid plastics. A glass rod is another option. Has anyone attempted a repair like this? Do you have any better suggestions?

1st Jul 2019, 11:13 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,868	Re: Replacing a ceramic spindle I think I have one. I built a preselector some time ago and obtained a rather nice 4-gang capacitor. My circuit design needs all four rotors earthed, so it made sense to turn up a brass shaft for it. So the ceramic shaft may be left lurking in a drawer somewhere. I'll have a look when I have some time. I think a number of these capacitors came on the market in the mid 80's. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

1st Jul 2019, 11:46 pm	#3
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,163	Re: Replacing a ceramic spindle The four rotors in this variable capacitor are all wired to chassis. Maybe a steel or brass spindle would be just as good, for my needs. I initially assumed that they used a ceramic spindle to electrically isolate the rotors. However there may be other advantages such as thermal stability. Some ceramics are 70% more rigid than steel. Perhaps it is the material of choice for the perfectionist.

9th Jul 2019, 4:09 am	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,868	Re: Replacing a ceramic spindle I've had a search.. I still think I've got one rolling around, somewhere. What I have found, is an entire capacitor. 4 sections, big ceramic shaft. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

9th Jul 2019, 10:37 am	#5
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,394	Re: Replacing a ceramic spindle I wonder if these were B40 spares- I'd heard that it wasn't so unusual to find that the gang capacitor shafts were broken, probably not helped by cramped below-decks situations, brusque handling and considerable weight! If there was noticeable attrition, perhaps stock-holding was generous in the way that there used to be plenty of NOS AR88 output transformers around- until there weren't...