Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Jennings Vacuum Variable




 
The AM Forum
August 11, 2022, 05:20:56 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Jennings Vacuum Variable  (Read 1445 times)
0 Members and 1 Guest are viewing this topic.
KD1SH
Member

Offline Offline

Posts: 426



« on: May 08, 2022, 08:17:46 PM »

   Picked this up at Nearfest last year, really cheap, $20 as I remember. Assuming it's good, it was a great deal. But, looking at it closer now, I realize why it was so cheap: the top "thrust bearing" thingie is missing. The copper colored cylinder at the top pulls out by approximately one inch, under spring tension, to decrease the capacitance. The female thread in the plunger looks to be maybe M6. The threaded shaft that would normally go into the threaded hole needs only to have a shoulder to bear against the thrust bearing cone that's supposed to sit on top.
   I can probably kluge up something on my lathe that would take the place of the missing parts, but - for anyone who's worked with more of these than I have - what holds the thrust bearing/cone onto the main body? Is it just held on by the inward-pulling spring tension, so that if you were to completely unscrew the shaft the cone would come right off?


* Jennings_vac1.JPG (83.3 KB, 1024x768 - viewed 117 times.)
Logged
W1ITT
Member

Offline Offline

Posts: 513


« Reply #1 on: May 08, 2022, 09:01:57 PM »

If you can imagine a heavy metal shot glass...it would be a man-sized shot...inverted over the top end, reaching down to the shoulder of the capacitor, and if that shot glass had a hole in the center for a bolt to bear against, that would be the concept.  They are usually referred to as "turning heads" for vacuum variables.  Test a few bolts in it and determine what thread fits into the end.  When you find what goes in nicely, grab onto it and attempt to pull.  If there's plenty of resistance, that indicates the the capacitor still has a good vacuum.  Turning heads aren't always easy to find, but if you can get one, and if the vacuum is good, you scored a fine prize for the money.  Turning heads occasionally come available when a capacitor rolls off a table, and they often have tear stains on them.
73 de Norm W1ITT
Logged
KD1SH
Member

Offline Offline

Posts: 426



« Reply #2 on: May 09, 2022, 09:15:27 AM »

   Thanks for the good description. I actually confirmed that by comparing it to another Jennings that I have - one with the glass body - and it's exactly as you describe (I should have thought of that before - duh!). I can grasp the brass cylinder with my fingers and pull it out about an inch, and it's got some good resistance, so the vacuum is probably okay.
   Wish I had a hipot tester. I passed on one at Nearfest a few years ago.

If you can imagine a heavy metal shot glass...it would be a man-sized shot...inverted over the top end, reaching down to the shoulder of the capacitor, and if that shot glass had a hole in the center for a bolt to bear against, that would be the concept.  They are usually referred to as "turning heads" for vacuum variables.  Test a few bolts in it and determine what thread fits into the end.  When you find what goes in nicely, grab onto it and attempt to pull.  If there's plenty of resistance, that indicates the the capacitor still has a good vacuum.  Turning heads aren't always easy to find, but if you can get one, and if the vacuum is good, you scored a fine prize for the money.  Turning heads occasionally come available when a capacitor rolls off a table, and they often have tear stains on them.
73 de Norm W1ITT
Logged
Detroit47
Member

Offline Offline

Posts: 611



« Reply #3 on: May 09, 2022, 11:25:42 AM »

Looking at that picture. I would say that cap is probably bad. That piston should be flush with the end when not pulled by the drive. Check it for shorts with an ohm meter. Also a quick check with a cap meter should show about 500 pf.

Johnathan N8QPC
Logged
KD1SH
Member

Offline Offline

Posts: 426



« Reply #4 on: May 09, 2022, 07:56:38 PM »

  I had hope for it, but I've come to the conclusion that it's a paperweight. It's supposed to be 12pf to 500pf, but it sits right around 20pf with very little change no matter how I push or pull the piston. Push it in as far as it will go, and it reads shorted at the point where it stops. Pull it out as far as it will go, and it reads around 30pf. Oh, well - it was worth a shot for $20.

Looking at that picture. I would say that cap is probably bad. That piston should be flush with the end when not pulled by the drive. Check it for shorts with an ohm meter. Also a quick check with a cap meter should show about 500 pf.

Johnathan N8QPC
Logged
W1ITT
Member

Offline Offline

Posts: 513


« Reply #5 on: May 09, 2022, 08:09:13 PM »

If that bottom flange is a clamp-on rig, remove it and save it.  They don't always come with the units, and you may get lucky on your next one.  If it's silver soldered on, that just makes it a more stable paperweight.
73 de Norm W1ITT
Logged
KD1SH
Member

Offline Offline

Posts: 426



« Reply #6 on: May 09, 2022, 08:14:32 PM »

It is a clamp-on; I will definitely hang onto it.

If that bottom flange is a clamp-on rig, remove it and save it.  They don't always come with the units, and you may get lucky on your next one.  If it's silver soldered on, that just makes it a more stable paperweight.
73 de Norm W1ITT
Logged
W1ITT
Member

Offline Offline

Posts: 513


« Reply #7 on: May 09, 2022, 09:44:00 PM »

https://mgs4u.com/product-category/rf-components/vacuum-capacitors/capacitor-accessories/

Check out the prices of the various mounting flanges.  For 20 bucks, you did OK.  Sooner or later, somebody will need one.

73 de Norm W1ITT
Logged
WB3JOK
Member

Offline Offline

Posts: 624



« Reply #8 on: May 09, 2022, 11:18:03 PM »

  I had hope for it, but I've come to the conclusion that it's a paperweight. It's supposed to be 12pf to 500pf, but it sits right around 20pf with very little change no matter how I push or pull the piston. Push it in as far as it will go, and it reads shorted at the point where it stops. Pull it out as far as it will go, and it reads around 30pf. Oh, well - it was worth a shot for $20.

I'm trying to figure out what exactly is wrong internally, if the piston moves but the capacitance barely changes... if the moving part of the cap was broken off the piston, wouldn't that be audible or at least change when the cap is inverted?  Huh
Logged
KD1SH
Member

Offline Offline

Posts: 426



« Reply #9 on: May 10, 2022, 07:33:35 AM »

  It is indeed quite a puzzle. looking at diagrams of the typical Jennings cap, it looks like the normal, "at rest" state of the bellows is fully extended/expanded, so that combined with the vacuum would tend to hold the plates together - fully meshed. Yet the piston's at rest position is slightly short of fully inserted - you've got to give it a little pressure to take up the slack. There's considerable resistance to pulling the piston out - apparently either the spring tension of the bellows or the force of the vacuum - and it pulls itself back in when released, but not quite all the way. I'm thinking that it took a good impact and got scrambled inside. But how? If the plates were physically jammed together, I wouldn't be able to move the piston at all, and it would read shorted all the time. If the bellows somehow came disconnected from the plates? Then I could move the piston with no effect on the capacitance, which is what's happening.

I'm trying to figure out what exactly is wrong internally, if the piston moves but the capacitance barely changes... if the moving part of the cap was broken off the piston, wouldn't that be audible or at least change when the cap is inverted?  Huh
Logged
Detroit47
Member

Offline Offline

Posts: 611



« Reply #10 on: May 10, 2022, 01:40:08 PM »

From unpleasant experience I would say that the seal is compromised.  That leading to a loss of vacuum. The plates are usually held in because of the vacuum. So at rest you would see max capacitance. On some capicators there is an adjusting screw to keep the piston from bottoming out. Causing damage to the plates.

Johnathan N8QPC
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.071 seconds with 18 queries.