Topics

Variable Vacuum Capacitor (VVC) Maintenance


Immo, DL5KB
 
Edited

Hi All,
 
I see an increasing use of variable vacuum capacitors and subsequent reports of issues with driving them mechanically.
Main issue is according to my experience directly originated to the fact "NOS". These pieces are partly more than 30 years in their package and never moved since this time. The grease of the bearing and the thread is resinified due to age.
 
For renewing the lubricaion, the old grease has to be removed and substituted by new bearing grease.
 
Procedure:
- turn the capacitor to maximum capacity
- remove the srew by continuous turning ccw
- remove the bearing cap and the bearing
- clean the bearing by "bathing" in benzine or similar
- clean the thread with benzine soaked cloth and perhaps old teethbrush :-)
- relubricate with bearing grease
- reassemble the bearing, cap and thread
 
See and feel a new variable vacuum capacitor!
Any NEMA 17 stepper is able to drive it, even ungeared!
 
For information a scheme from american facturer Jennings, russian ones are similar.
 
Good success!

73, Immo


Immo, DL5KB
 

And another picture of COMET capacitors:


73
Immo


K4MJA
 

Mine does this...I don't know if this is normal. Supposed to be 4-100pF.


Immo, DL5KB
 

Hi Emir,
looks like 40 to 170 pF. After 10 turns, the "rotor" left the "stator" and capacity is only the system-capacity.
May it be, that the capacity of test setup has an own capacity of abt. 35pF? Then the VVC is 5 to 135 pF...
73
Immo
 


K4MJA
 

Yes Immo, I think you right. Not 35, but like 16 to 20 pF due to the test leads been in proximity to each other, so the actual capacitance is 5 to 155 pF.


Henri Bollen
 

Hi Immo,

I have a Jennings 12...500pF /7.5kV cap .Turning it with my stepper motor is very difficult and I like to clean it the way you did.
So far I removed the screw, but how do you take out the bearing? I don't want to destroy something and maybe i'm overly careful, but after all it is a 250 Euro cap..
What is your advice?
kind regards
Henri on4abx


Immo, DL5KB
 

Hi Henri,
I know, that Jennings has produced many models of VVC's during the last 40 years. All capacitors with glass body have a headplate, which is fastened by three or four screws to the body. You must remove these and than you can lift off the bearing. It is loose on the body, perhaps "fastened" by the old grease....
The picture shows the screws, marked with circles and arrows.


If your cap looks not similar, post a picture of yours. Perhaps I can help to disassemble.
73
Immo


Henri Bollen
 

hi Immo,
Indeed it is a different model.
P1100364.JPG

After unscrewing the top you see this:

P1100365.JPG

And on the outside of the blue top you can observe a bearing between 2 metal rings:


P1100366.JPG

What do you think?

73 Henri

Op do 1 apr. 2021 om 21:27 schreef Immo, DL5KB <dl5kb@...>:

Hi Henri,
I know, that Jennings has produced many models of VVC's during the last 40 years. All capacitors with glass body have a headplate, which is fastened by three or four screws to the body. You must remove these and than you can lift off the bearing. It is loose on the body, perhaps "fastened" by the old grease....
The picture shows the screws, marked with circles and arrows.


If your cap looks not similar, post a picture of yours. Perhaps I can help to disassemble.
73
Immo


Immo, DL5KB
 

Between the two plates is the bearing. looks a little worn. Get it out and clean it from old grease, even so the the two plates on top and beneath. If they are oxidized, polish them with sandpaper 800... I think that will do for good. 
Immo


Henri Bollen
 

ok Immo I will do that.
Thanks for the help.
I will let you know how it goes by tomorrow.
Have a good night,

73, Henri

Op do 1 apr. 2021 om 22:23 schreef Immo, DL5KB <dl5kb@...>:

Between the two plates is the bearing. looks a little worn. Get it out and clean it from old grease, even so the the two plates on top and beneath. If they are oxidized, polish them with sandpaper 800... I think that will do for good. 
Immo


Henri Bollen
 

i Immo,

I did clean the bearing, washers and screw per your directions.
P1100372.JPG

P1100371.JPG

P1100370.JPG

P1100369.JPG

Is it ok to use silicone spray to grease the bearing and is there something else I must clean?

73 Henri

Op do 1 apr. 2021 om 22:55 schreef Henri Bollen <henri.bollen3@...>:

ok Immo I will do that.
Thanks for the help.
I will let you know how it goes by tomorrow.
Have a good night,

73, Henri

Op do 1 apr. 2021 om 22:23 schreef Immo, DL5KB <dl5kb@...>:
Between the two plates is the bearing. looks a little worn. Get it out and clean it from old grease, even so the the two plates on top and beneath. If they are oxidized, polish them with sandpaper 800... I think that will do for good. 
Immo


Immo, DL5KB
 

hi Henri,
like new! Please use a grease especially for roller bearings, i.e. Bosch ft1 v26, the grease must reflow. The thread may be „oiled“. 
73, Immo


Henri Bollen
 

hi Immo,

The cap is turning a little more lightly now, but I think my stepper motor is not powerful enough for the Jennings cap.
The previous used cap was a Russian one and it turned very lightly.
I used this stepper motor to drive it:
I'll make a few tests today to assure the stepper is strong enough and doesn't slip.
Again many thanks for the time and friendly advise.

73 Henri


Op vr 2 apr. 2021 om 11:55 schreef Immo, DL5KB <dl5kb@...>:

hi Henri,
like new! Please use a grease especially for roller bearings, i.e. Bosch ft1 v26, the grease must reflow. The thread may be „oiled“. 
73, Immo


Immo, DL5KB
 

Henri,
you're welcome. Your stepper won't turn your vac-C without problems. A good solution is a geared 1:3 or 1:5 NEMA17. This also makes a better resolution per turn and more torque. 1:3 makes 600 steps a turn, 1:5 makes 1000 steps. You can test the affordable torque for the cap with a ruler and a scales. Fix the ruler on the axis and press the scales with the other end, till it starts to move. Wight x length of ruler is the torque which is needed.(kg/cm)  Then you can decide, which gear and stepper you need to get the affordable torque.
73, Immo


Henri Bollen
 

Hi Immo,
Splendid idea to measure the torque.
I did it wit a ruler of 10cm. The scale gives 100g
So the torque in kg/cm should be 10cmx0,1kg= 1kg/cm
If this is right the steppermotor I was using (5kg/cm) is perfectly able to handle the Jennings cap.
I have another stepper laying on the shelf, but this one has a 13.6:1 planetary drive, and it would be too slow.
So, Immo, I think I'm going to mount the 5kg/cm stepper and see what it does.
I let you know.
73, Henri

Op vr 2 apr. 2021 om 13:50 schreef Immo, DL5KB <dl5kb@...>:

Henri,
you're welcome. Your stepper won't turn your vac-C without problems. A good solution is a geared 1:3 or 1:5 NEMA17. This also makes a better resolution per turn and more torque. 1:3 makes 600 steps a turn, 1:5 makes 1000 steps. You can test the affordable torque for the cap with a ruler and a scales. Fix the ruler on the axis and press the scales with the other end, till it starts to move. Wight x length of ruler is the torque which is needed.(kg/cm)  Then you can decide, which gear and stepper you need to get the affordable torque.
73, Immo


Henri Bollen
 

Hi Immo,
I first wish to excuse me for the time I did confiscate to solve a problem wich in fact was none.
I bought a new vacuum cap with a bigger range (Jennings 12-500pF) so I could also use the lower bands upto 80m.
My previous cap was a Russian KP-4 10-100pF and it was easily turned by the stepper. When I replaced the Russian cap with the Jennings, I observed the stepper was slipping constantly. So I tought the stepper was not strong enough. After cleaning the bearings per your instructions, the cap was turning a little better, but the stepper kept slipping.
P1100374.JPG
BUT....I forgot one thing: When installing the Russian cap, I trimmed the current of the driver to be just enough to turn it without slipping (See trimpot inside red circle). you probably can guess what happened: When the current was adapted to the new cap, the stepper turned the Jensen with ease.😱
To end a weird story well: Thanks to you I now have a well oiled and smoothly working Jensen Cap.
Many thanks  for your patience and the expert advice.

73 from Henri on4abx

Op vr 2 apr. 2021 om 16:32 schreef Henri Bollen <henri.bollen3@...>:

Hi Immo,
Splendid idea to measure the torque.
I did it wit a ruler of 10cm. The scale gives 100g
So the torque in kg/cm should be 10cmx0,1kg= 1kg/cm
If this is right the steppermotor I was using (5kg/cm) is perfectly able to handle the Jennings cap.
I have another stepper laying on the shelf, but this one has a 13.6:1 planetary drive, and it would be too slow.
So, Immo, I think I'm going to mount the 5kg/cm stepper and see what it does.
I let you know.
73, Henri

Op vr 2 apr. 2021 om 13:50 schreef Immo, DL5KB <dl5kb@...>:
Henri,
you're welcome. Your stepper won't turn your vac-C without problems. A good solution is a geared 1:3 or 1:5 NEMA17. This also makes a better resolution per turn and more torque. 1:3 makes 600 steps a turn, 1:5 makes 1000 steps. You can test the affordable torque for the cap with a ruler and a scales. Fix the ruler on the axis and press the scales with the other end, till it starts to move. Wight x length of ruler is the torque which is needed.(kg/cm)  Then you can decide, which gear and stepper you need to get the affordable torque.
73, Immo


Henri Bollen
 

Hi Immo,

Sorry to bother you again. yesterday I wrote the stepper turned the Jensen cap with ease, and it was the truth, on my workbench it had plenty of torque. Mounting it on the loop, the torque was not sufficient to drive the Jensen Cap reliably without slipping. Even turning the trimpot to maximum current was not enough. What I did not mention was the cable length is 65m shielded cat 7. Yes, I know, it is a lot of cable, but I could not place the 6m hinged mast on any other place. The loop of d=2m is placed on top of that mast. VERY quiet reception with minimal interference indeed.
So, I am sure the loss over that long line is due to the low torque of the stepper.
I see a few solutions:
1. Raise the motor voltage to 24V . The DRV8825 datasheet tells me it can be done.
2.Take de driver and common mode chokes and mount them on a separate pcb located near the loop with a local Power supply. The only question to be answered should be the influence of the 65m signal line on the data stream.
What do you think of it, Immo? Or should I open a new topic?
73 an happy Eastern,

Henri


Joe Scalet
 

Use something heavier than CAT 6!!!


On Sat, Apr 3, 2021 at 4:56 PM Henri Bollen <henri.bollen3@...> wrote:
Hi Immo,

Sorry to bother you again. yesterday I wrote the stepper turned the Jensen cap with ease, and it was the truth, on my workbench it had plenty of torque. Mounting it on the loop, the torque was not sufficient to drive the Jensen Cap reliably without slipping. Even turning the trimpot to maximum current was not enough. What I did not mention was the cable length is 65m shielded cat 7. Yes, I know, it is a lot of cable, but I could not place the 6m hinged mast on any other place. The loop of d=2m is placed on top of that mast. VERY quiet reception with minimal interference indeed.
So, I am sure the loss over that long line is due to the low torque of the stepper.
I see a few solutions:
1. Raise the motor voltage to 24V . The DRV8825 datasheet tells me it can be done.
2.Take de driver and common mode chokes and mount them on a separate pcb located near the loop with a local Power supply. The only question to be answered should be the influence of the 65m signal line on the data stream.
What do you think of it, Immo? Or should I open a new topic?
73 an happy Eastern,

Henri


Joe Scalet
 

or CAT 7

On Sat, Apr 3, 2021 at 5:10 PM Joe Scalet <joe.scalet@...> wrote:
Use something heavier than CAT 6!!!


On Sat, Apr 3, 2021 at 4:56 PM Henri Bollen <henri.bollen3@...> wrote:
Hi Immo,

Sorry to bother you again. yesterday I wrote the stepper turned the Jensen cap with ease, and it was the truth, on my workbench it had plenty of torque. Mounting it on the loop, the torque was not sufficient to drive the Jensen Cap reliably without slipping. Even turning the trimpot to maximum current was not enough. What I did not mention was the cable length is 65m shielded cat 7. Yes, I know, it is a lot of cable, but I could not place the 6m hinged mast on any other place. The loop of d=2m is placed on top of that mast. VERY quiet reception with minimal interference indeed.
So, I am sure the loss over that long line is due to the low torque of the stepper.
I see a few solutions:
1. Raise the motor voltage to 24V . The DRV8825 datasheet tells me it can be done.
2.Take de driver and common mode chokes and mount them on a separate pcb located near the loop with a local Power supply. The only question to be answered should be the influence of the 65m signal line on the data stream.
What do you think of it, Immo? Or should I open a new topic?
73 an happy Eastern,

Henri


Henri Bollen
 

Well Joe,
It worked for my Russian cap. I combined the 8wires to 4 pairs and never had an issue. But you probably are right: Thicker wire should have less loss. But then I must reroute the whole 65m and that's not easy.
73 Henri

Op zo 4 apr. 2021 om 00:10 schreef Joe Scalet <joe.scalet@...>:

Use something heavier than CAT 6!!!


On Sat, Apr 3, 2021 at 4:56 PM Henri Bollen <henri.bollen3@...> wrote:
Hi Immo,

Sorry to bother you again. yesterday I wrote the stepper turned the Jensen cap with ease, and it was the truth, on my workbench it had plenty of torque. Mounting it on the loop, the torque was not sufficient to drive the Jensen Cap reliably without slipping. Even turning the trimpot to maximum current was not enough. What I did not mention was the cable length is 65m shielded cat 7. Yes, I know, it is a lot of cable, but I could not place the 6m hinged mast on any other place. The loop of d=2m is placed on top of that mast. VERY quiet reception with minimal interference indeed.
So, I am sure the loss over that long line is due to the low torque of the stepper.
I see a few solutions:
1. Raise the motor voltage to 24V . The DRV8825 datasheet tells me it can be done.
2.Take de driver and common mode chokes and mount them on a separate pcb located near the loop with a local Power supply. The only question to be answered should be the influence of the 65m signal line on the data stream.
What do you think of it, Immo? Or should I open a new topic?
73 an happy Eastern,

Henri