How to clean my tuning capacitor?

[Bill Bowden]

It's an old (1960s AM radio) air dielectric, planetary drive with ball
bearing 6X reduction). It's a very small mechanical cap measuring
about 9/8 inch square by 11/16 height with closely spaced plates
(150pF and 100pF). Problem is, the oscillator section jumps about 5KHz
out of 1Mhz or maybe 0.5% with small adjustments to the cap. It
appears it may be losing a ground connection (or high resistance) to
the moving plates, or possibly some dust between the plates. I assume
the ground connection is made through the ball bearings. I tried
spraying some contact cleaner on the ends of the shaft but it didn't
help. What would you suggest to restore the capacitor to it's original
condition? I thought about soaking it in alcohol, but maybe that's not
a good idea.

-Bill

 

[amdx]

It's an old (1960s AM radio) air dielectric, planetary drive with ball
bearing 6X reduction). It's a very small mechanical cap measuring
about 9/8 inch square by 11/16 height with closely spaced plates
(150pF and 100pF). Problem is, the oscillator section jumps about 5KHz
out of 1Mhz or maybe 0.5% with small adjustments to the cap. It
appears it may be losing a ground connection (or high resistance) to
the moving plates, or possibly some dust between the plates. I assume
the ground connection is made through the ball bearings. I tried
spraying some contact cleaner on the ends of the shaft but it didn't
help. What would you suggest to restore the capacitor to it's original
condition? I thought about soaking it in alcohol, but maybe that's not
a good idea.

-Bill

Search the archives of the crystal radio section here,
http://theradioboard.com/rb/
These guys have cleaned a lot of air caps.
Mikek

 

[Phil Allison]

"Bill Bowden"

It's an old (1960s AM radio) air dielectric, planetary drive with ball
bearing 6X reduction). It's a very small mechanical cap measuring
about 9/8 inch square by 11/16 height with closely spaced plates
(150pF and 100pF). Problem is, the oscillator section jumps about 5KHz
out of 1Mhz or maybe 0.5% with small adjustments to the cap. It
appears it may be losing a ground connection (or high resistance) to
the moving plates, or possibly some dust between the plates. I assume
the ground connection is made through the ball bearings. I tried
spraying some contact cleaner on the ends of the shaft but it didn't
help.

** What did you use - Freon ?

A few drops of WD40 on the bearings at each end of the moving plates usually
fixes it. Do not spray, use a syringe or the tip of a small screwdriver to
apply the liquid.

Anecdote:

A customer decided to fix his FM tuner ( Quad FM3) with the same problem.

He sprayed WD40 all over the gang and nearby PCB - the oil layer left
behind offset the local oscillator frequency by about 30 MHz, so right off
the FM band completely.

After a lot of rinsing with turps and drying with hot air, I got it back.


.... Phil

 

[amdx]

Search the archives of the crystal radio section here,
http://theradioboard.com/rb/
These guys have cleaned a lot of air caps.
Mikek

Don't know if you tried my link, but here's more.

http://antiqueradios.com/forums/viewtopic.php?t=73596
and
http://theradioboard.com/rb/viewtopic.php?f=2&t=66&hilit=cleaning+air+variables


If you use the dishwasher method, I would check the detergent on
an aluminum pie plate to make sure it doesn't react badly.
My wife changed detergent when she found the one I said got a best
value rating in Consumer Reports was eating away her pie plates.
These were the thin pie tins you get when you buy a crust.
Mikek

Mikek

 

[Bill Bowden]

   Don't know if you tried my link, but here's more.

http://antiqueradios.com/forums/viewtopic.php?t=73596
andhttp://theradioboard.com/rb/viewtopic.php?f=2&t=66&hilit=cleaning+air...

If you use the dishwasher method, I would check the detergent on
an aluminum pie plate to make sure it doesn't react badly.
My wife changed detergent when she found the one I said got a best
value rating in Consumer Reports was eating away her pie plates.
These were the thin pie tins you get when you buy a crust.
                       Mikek

         Mikek

Nice pictures of ultrasonic cleaning results. Look like new.

-Bill

 

[Bill Bowden]

There's usually a spring blade contact pressing against a turned shoulderon
the shaft just before it enters the bearing. The contact is often silver
plated, which can develop a heavy black tarnish. AFAICR this has never
caused a problem - YMMV.

Yes, there is a spring blade contact at the opposite end of the shaft
as the ball bearings. Didn't see any silver, the blade appears copper
or brass. I soaked the cap in alcohol for a day and moved the plates
many times. Then I dried it the sun and added a few drops of WD40 to
both ends as Phil suggested. I removed the mica insulator and cleaned
it separetely. Cap works much better now, but still a little eratic
near the bottom of the band where the plates are near fully closed.
Might need some more WD40, or maybe some thin sewing machine oil?

-Bill

 

[Bill Bowden]

It may be that there's some corrosion on the rotor shaft where the
capacitance change is erratic, or it might be that the spacing between
the plates isn't constant at the high-capacitance end of travel.

The cap got more erratic today, so I applied more WD40 and then lost
all signals except one 50KW station about 7 miles away. Everthing
appeared normal, oscillator and antenna circuit both on correct
frequency. It appeared the Q of the antenna circuit degraded
drastically due to WD40 which may have spilled on the capacitor
plates. So, I tried measuring the resistance of some WD40 on the table
and it measured over 20Megs (limit of the meter). A few hours later,
the cap started working again, so I connected a 10 Meg resistor across
the antenna capacitor plates which made very little difference. So,
it's still a mystery how WD40, with such high resistance, effects the
Q of the capacitor. Maybe it caused some corroded point to get worse?

I saw a 16 ounce (45 watt) ultrasonic cleaner at Harbor Freight for
$29.99. I might invest in that if I can think of some other uses. I
hate to buy it just to use it once.

-Bill

 

[Jamie]

The cap got more erratic today, so I applied more WD40 and then lost
all signals except one 50KW station about 7 miles away. Everthing
appeared normal, oscillator and antenna circuit both on correct
frequency. It appeared the Q of the antenna circuit degraded
drastically due to WD40 which may have spilled on the capacitor
plates. So, I tried measuring the resistance of some WD40 on the table
and it measured over 20Megs (limit of the meter). A few hours later,
the cap started working again, so I connected a 10 Meg resistor across
the antenna capacitor plates which made very little difference. So,
it's still a mystery how WD40, with such high resistance, effects the
Q of the capacitor. Maybe it caused some corroded point to get worse?

I saw a 16 ounce (45 watt) ultrasonic cleaner at Harbor Freight for
$29.99. I might invest in that if I can think of some other uses. I
hate to buy it just to use it once.

-Bill

You created a coating that increased the dialectic constant for the
plate that tune's the antenna..

THen again, could be the differences between G and E line effects..

Jamie

 

[Phil Allison]

"Michael A. Terrell = Trolling Psycho"

WD40 isn't suitable as a lubricant for the bearings or grounding
plate.

** The OP needed a contact cleaner FIRST.

WD40 is that and then it leaves a thin, oily film the lubricates and
protects against moisture.

You want GC Tunerlube,

** Much the same as WD40.

white lithium grease or even Vaseline in
a pinch.

** ROTFLMAO !!

Using any sort of grease is never going to help a bad contact.

YOU FUCKWIT, RABID NUT CASE !!!!!!!!!!!

WD40 hardens into a hard to remove, non-conductive coating.

** Not unless there is a LOT of heat involved and no moving parts.

Look at the MSDS and you'll see that it is classed as a solvent.

** Which makes it a good surface cleaner AND totally contradicts everything
you just said.

BTW:

FOAD you stinking TROLL !!!!!!!

 

[Phil Allison]

"Bill Bowden"


The cap got more erratic today, so I applied more WD40 and then lost
all signals except one 50KW station about 7 miles away. Everthing
appeared normal, oscillator and antenna circuit both on correct
frequency. It appeared the Q of the antenna circuit degraded
drastically due to WD40 which may have spilled on the capacitor
plates.


** More likely rust or dirt particles from the bearings made it between the
plates - carried by the EXCESS of WD40 you applied.

I have used WD40 successfully on intermittent tuning gangs in FM receivers
many times, never any problem.

BUT I used it sparingly and carefully !!!

You may need to use an evaporative cleaner on the plates now - ie stuff
that smells nice and disappears in seconds if you spray it on your hand.




.... Phil

 

[Bill Bowden]

"Bill Bowden"

The cap got more erratic today, so I applied more WD40 and then lost
all signals except one 50KW station about 7 miles away. Everthing
appeared normal, oscillator and antenna circuit both on correct
frequency. It appeared the Q of the antenna circuit degraded
drastically due to WD40 which may have spilled on the capacitor
plates.

** More likely rust or dirt particles from the bearings made it between the
plates  -  carried by the  EXCESS  of  WD40 you applied.

I have used WD40 successfully on intermittent tuning gangs in FM receivers
many times, never any problem.

BUT  I used it sparingly and carefully  !!!

You may need to use an evaporative cleaner on the plates now  -  ie stuff
that smells nice and disappears in seconds if you spray it on your hand.

...  Phil

Yes, I think you are right. I disconnected the cap and connected an
ohmmeter across the plates and read many points of low resistance. So,
I soaked the cap in a cup of isopropy rubbing alcohol and rotated the
plates every few minutes and dried it out in the sun and then applied
a few drops of sewing machine oil and it seems ok now. I found
someone who has a ultrasonic cleaner to loan and will try it out next
week.

-Bill