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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 5th May 2021, 6:33 pm   #1
DonaldStott
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Default Variable Capacitor - Scratchy

What's the best cure for a Variable Capacitor that is scratchy/crackly at various points in its rotation?

I'm assuming some dust, rust or some other debris on the vanes?

No evidence of the vanes touching.
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Old 5th May 2021, 6:41 pm   #2
Cruisin Marine
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Default Re: Variable Capacitor - Scratchy

Blow it through with compressed air first.
If still trouble (and assuming it an open type and not the modern miniature enclosed type)
you will have to determine where the partial short is taking place by placing a thin insulator and move it along vane by vane to isilate where the problem is. Then you will have to straighten it if possible.
The really old one's (not ali) are a to straighten as they are very hard metal and contain magnesium I have been told, so don't use much heat.
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Old 5th May 2021, 6:52 pm   #3
paulsherwin
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Default Re: Variable Capacitor - Scratchy

There are two potential problems. The first is that the vanes may be misaligned, bent or contaminated with conductive material, the second that the contacts connecting the rotor vanes shaft are dirty, damaged or worn. The second issue can sometimes be addressed with a squirt of Servisol.
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Old 5th May 2021, 7:07 pm   #4
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Default Re: Variable Capacitor - Scratchy

Once you have tried the above it’s worth putting a test meter on each section to see if it’s shorting ( disconnect the section From the radio )
If it is you can use a thin piece of plastic ( smart phone screen protectors work well )
Slide the plastic between the vanes to find the culprit
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Old 5th May 2021, 7:51 pm   #5
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Default Re: Variable Capacitor - Scratchy

If it is dirt causing the problems then personally I like to use iso-propranolol spray to thoroughly clean them.
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Old 5th May 2021, 8:51 pm   #6
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Default Re: Variable Capacitor - Scratchy

I check the vane alignment first and make any adjustments (gently bending any offending vanes). The if the problem of noise persists, then I disconnect the wires to the tuning gangs and temporarily connect the HT in their place. A couple of rotations back & forth - accompanied by some fireworks/sparking - will clear any crud lodged between the vanes.
Job done and it works 100%. Don't forget to reinstate the wires and disconnect the HT :0
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Old 5th May 2021, 9:10 pm   #7
60 oldjohn
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Default Re: Variable Capacitor - Scratchy

The outer vanes may be bent, as long as they are not touching do not straighten them. They have been bent so the tracking of the tuning scale agrees at both ends.

John.
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Old 5th May 2021, 9:22 pm   #8
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Default Re: Variable Capacitor - Scratchy

Audiomm thatís interesting, never tried that method before.
John
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Old 6th May 2021, 8:12 am   #9
DonaldStott
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Default Re: Variable Capacitor - Scratchy

Here's a short YouTube video that hopefully lets you hear the problem?

Very scratchy/crackly at one point and when vanes are fully closed.

https://youtu.be/vkomrlIly4M

I've tried compressed air through the vanes and Servisol on the rotor vanes shaft but no joy!
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Old 6th May 2021, 9:06 am   #10
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Default Re: Variable Capacitor - Scratchy

Disconnect the stators from circuit and check for shorts/leakage with a meter set to measure high resistance, I used to find an analog meter was best, might be the camera shot but the outer rotor plate on the LH section looks a bit to close to the stator plate?

Lawrence.

Last edited by ms660; 6th May 2021 at 9:12 am. Reason: extra info
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Old 6th May 2021, 10:37 am   #11
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Default Re: Variable Capacitor - Scratchy

Just watched the video. Yes, had this several times in the past. Cleaning with Iso did the trick; I find servisol is not suitable as it leaves a very fine film and can actually cause this problem.
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Old 6th May 2021, 11:02 am   #12
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Default Re: Variable Capacitor - Scratchy

Difficult to see, but are the rotating vanes central in the static vanes?
If they are out, check there is no end float in the shaft first. Pull / push the shaft back and forth, set off of course.
Carefully check the grounding springs on the rotor shaft are clean and still have good pressure against the shaft.
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Old 6th May 2021, 11:38 am   #13
DonaldStott
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Default Re: Variable Capacitor - Scratchy

With my loupe I've checked that the rotating vanes all sit centrally relative to the static vanes and that there is no play on the shaft.

No obvious problems with the grounding springs either.

Using my cheapo MK-328 tester I'm measuring 537pF when fully open and 1095pF when rotated to fully closed/engaged.

I'm tending towards gunk, rust or other debris so a fine spray of IPA may help?
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Last edited by DonaldStott; 6th May 2021 at 11:59 am.
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Old 6th May 2021, 11:53 am   #14
stevehertz
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Default Re: Variable Capacitor - Scratchy

Sounds like a straightforward case of dirty contacts to me. I'd just hit the contact sliders hard with Servisol. It may affect the tuning for a while until it 'burns' off.
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Old 6th May 2021, 6:46 pm   #15
DonaldStott
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Default Re: Variable Capacitor - Scratchy

Applied a further liberal dose of Servisol as suggested by stevehertz and worked the variable capacitor back and forth several times.

Left everything for an hour or two and went back to a set with no scratching/crackling when tuning on both LW and MW.

Thanks for all the advice which helped with another successful outcome! One day I hope that I'll be brave enough to try the fix suggested by audiomm!
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Old 6th May 2021, 10:34 pm   #16
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Default Re: Variable Capacitor - Scratchy

Well done Donald!
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