Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jun Sat 06, 2020 11:27 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 34 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: Knight VTVM model 83 F 120 won't cooperate!
PostPosted: Apr Tue 07, 2020 3:40 pm 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 9490
Location: Long Island NY
I think you answered your own question already. Meters should not stick; if they do, it usually means something is wrong. The stand-in Japanese panel meter seems to work fine and have full range so there's probably not an electrical/electronic problem.

Now as for what to do about a meter that sticks, it depends on the meter and how deep you want to get into it. First off, does it always stick in the same place(s)? If so, check for a scale card that has come unglued and is bulging, or a pointer that is bent backwards following a hard fall. Bent pivots or a cracked jewel can also cause this problem but you'd have to tear the meter down and put the armature on knife edges or look at the jewels under a microscope to find it--and if it's a jewel, good luck finding a new one anywhere. Probably easier to dump the VTVM and find another one if you arrive at this point.

If the meter seems to stick at random places, check for jewels that are too tight (happens sometimes when the meter frame is made of die cast zinc or pot metal which expand as they age). Jewels in most meters are adjustable; if they are tight you can loosen the front or the back and adjust the end shake. It should be barely visible, less than the thickness of a hair. Other possibilities are debris in the gap between the armature and the magnet poles, or rust on the pivots. Debris can be removed with a sliver of adhesive tape used as a "wand" in the gap. Rust can often be exercised out by connecting the meter to a function generator and operating it back and forth with an appropriate frequency, amplitude, and DC offset.

Under no circumstances should you spray anything into a meter movement or use compressed air on it. This can cause the delicate coil springs to tangle and damage the armature. The pivots of meters are meant to run dry and should never be lubricated.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: Knight VTVM model 83 F 120 won't cooperate!
PostPosted: Apr Tue 07, 2020 5:00 pm 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 3960
Location: Florida
If you open the movement don't go poking around with anything magnetic. The magnet can grab it and cause you to damage the hairsprings or bend the pointer.

RRM


Top
 Profile  
 
 Post subject: Re: Knight VTVM model 83 F 120 won't cooperate!
PostPosted: Apr Tue 07, 2020 10:22 pm 
Member
User avatar

Joined: Apr Thu 24, 2014 4:12 am
Posts: 389
Location: Sugarloaf PA.
Okay. Isn't static, cause I had to remove the plastic cover to be able to zero it! ( and I was careful) The little mechanism to zero it with the plastic cover on wouldn't go far enough to zero it!! Guess I should have know that it was the meter! So after I zeroed it, I gave it the test, and Yeah, the meter is no good! I'd really like to keep this stupid VTVM though! I recapped , changed all of the resistors, installed a 1n4007 in place of the selenium rectifier. cleaned the pots, checked tubes, cleaned and waxed the cabinet! Anyone know where I could get a spare meter for this Knight? Oh, I didn't see any Jewels inside, the whole needle mechanism seems to float around a coil.


Attachments:
If meter was zeroed reading @ 1ma.JPG
If meter was zeroed reading @ 1ma.JPG [ 111.04 KiB | Viewed 219 times ]
Top
 Profile  
 
 Post subject: Re: Knight VTVM model 83 F 120 won't cooperate!
PostPosted: Apr Wed 08, 2020 1:56 am 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 3960
Location: Florida
The jewels are at the coil ends, at the center of the hair springs. They are small, like the ones you might see in a good pocket watch.

If the front zero adjustment doesn't have enough range you can fix that by centering it and then moving the pointer with the one on the back. You have to take the motor completely out to do this.

To me it sounds like it's binding up. I'd try loosening the bearings a little. Look closely and you'll see a screw and jam nut on the front of the movement. Then I'd check for trash in the gap between the coil and magnet. It's common for some of the plating on the magnet to flake off and get caught.

Now, as for worrying about buggering it up, it isn't doing you any good like it is. If you fix it, it's fixed. If not, you're no worse off and have learned something for the next one.

RRM


Top
 Profile  
 
 Post subject: Re: Knight VTVM model 83 F 120 won't cooperate!
PostPosted: Apr Wed 08, 2020 12:27 pm 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 9490
Location: Long Island NY
The jewels are not visible when the meter is assembled. They are in metal cups or holders at each end of the armature. The holders are threaded and slotted so they can be adjusted in or out as needed. They have lock nuts which keep them from turning or loosening through normal handling. You are supposed to put a screwdriver in the slot so the holder doesn't move, loosen the lock nut, then adjust the holder. Keep it from turning with a screwdriver when tightening it. BTW, in a few meters the jewels really are sapphire or garnet, but this is quite rare. Most of the standard ones are glass.

One other word to the wise, never tighten the holders up on the pivots. This can easily damage the pivot points or break the jewels, then you really will need a new meter. Unfortunately VTVM meters were nearly always OEM'd for the specific make and model of instrument they were used in. You probably won't find an exact match unless you find another Knight 83F120 or another Knight VTVM model that used the same meter. It may be possible to find something similar that would work electrically from a different VTVM but it won't have the same fit or look, the scales may be a little different, etc.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: Knight VTVM model 83 F 120 won't cooperate!
PostPosted: Apr Fri 10, 2020 3:51 am 
Member
User avatar

Joined: Apr Thu 24, 2014 4:12 am
Posts: 389
Location: Sugarloaf PA.
So, I tried to fool with this meter and let me just say that I hunked it up! I ordered another 1ma meter from e bay with the anticipation that I would screw the old meter up! The meter face plate wasn't as big as the old vtvm but after a few hours scratching the head and using the dremel I was able to adapt this new meter on to the old face plate! I really wanted to preserve this old VTVM, especially after putting in the time recapping, etc. that I did! It seems to work fine... so far. The only issue is that the needle is a little short! But I'm looking to mainly use this VTVM to align my FM tube radios and their stupid discriminators coils.


Attachments:
New 1ma meter.JPG
New 1ma meter.JPG [ 122.35 KiB | Viewed 183 times ]
Top
 Profile  
 
 Post subject: Re: Knight VTVM model 83 F 120 won't cooperate!
PostPosted: Apr Fri 10, 2020 11:06 am 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 9490
Location: Long Island NY
This is one instance where you don't want to let perfection be the enemy of good enough. In order to track linearly across the scale with the face vertical, a meter movement has to be in static and dynamic balance. That's what the three small weights on the arms in front of the armature are for. Problem is, if you try to lengthen the pointer, the balance will be thrown off, and it may be hard or downright impossible to get it back.

Fortunately that style of meter was used in a variety of things so you may at some point come across a closer match, or even an exact replacement. At least the VTVM is usable now.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: Knight VTVM 83 F 120 won't cooperate! Now calibration h
PostPosted: Apr Fri 10, 2020 6:52 pm 
Member
User avatar

Joined: Apr Thu 24, 2014 4:12 am
Posts: 389
Location: Sugarloaf PA.
Yeah, I'm not going to try to lengthen the pointer, I'm going to leave well enough alone! My next dilemma is the calibration, I can't find information anywhere for this one. There's a DC+, a DC- and an ACV cal pots. seems like the ACV pot is for AC volts and I think I understand this but connecting to a battery and trying to make sense of the dc+ and DC- readings, what I think they should be isn't adding up for me!


Last edited by ychris on Apr Fri 10, 2020 7:09 pm, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: Knight VTVM 83 F 120 won't cooperate! Now calibration h
PostPosted: Apr Fri 10, 2020 6:59 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 4589
Location: Littleton, MA
ychr wrote:
but connecting to a battery and trying to make sense of the dc+ and DC- readings, what I think they should isn't adding up for me!

What readings are you getting? What type of battery? What scale is the meter on? Do you have the correct DC probe - one with a 10 MΩ resistor in it?

_________________
Steve Byan https://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Knight VTVM 83 F 120 won't cooperate! Now Calibration h
PostPosted: Apr Fri 10, 2020 7:26 pm 
Member
User avatar

Joined: Apr Thu 24, 2014 4:12 am
Posts: 389
Location: Sugarloaf PA.
I'm using a 1.5 volt "C" battery, I do have the 10meg adapter, in my schematic it says it's the DC V probe, Thank you, I'm now using this for DC. The DC plus switch setting and DC+ adjustment pot seems to make sense now! But I'm still confused with the DC minus switch setting! The meter doesn't do anything if I connect the leads up to the battery backward and put it on the DC- switch setting, even if I adjust the DC- pot?


Attachments:
4_10_20.JPG
4_10_20.JPG [ 137.9 KiB | Viewed 159 times ]
Top
 Profile  
 
 Post subject: Re: Knight VTVM 83 F 120 won't cooperate! Now Calibration h
PostPosted: Apr Fri 10, 2020 7:32 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 4589
Location: Littleton, MA
A fresh alkaline cell should measure about 1.6 volts. A "heavy duty" zinc chloride cell will be about 1.55 volts.

As for the DC- function, check the wiring of that position of the function switch, clean the DC- function switch contacts, and clean the DC- cal pot. If it works properly on DC+, then it should work on DC-.

_________________
Steve Byan https://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Knight VTVM 83 F 120 won't cooperate! Now Calibration h
PostPosted: Apr Fri 10, 2020 10:03 pm 
Member
User avatar

Joined: Apr Thu 24, 2014 4:12 am
Posts: 389
Location: Sugarloaf PA.
It seems to work on the dc plus! but when I connect my positive lead (the one with the 10 meg resistor adapter on it) to the negative terminal of a C battery and the negative lead to the positive of the C battery the meter barely moves at all? and the DC- cal pot doesn't do much of anything! I checked the DC neg cal pot and it ohms out good (from zero to 3K) all of the switches are good (no resistance across the connection when switched on DC minus. ?? Should this meter go up when switched on DC minus and my meter leads swapped ( positive with the 10meg adapter on the neg of the battery and the negative meter lead on the positive of the battery) ???? Here's another pic of the meter on DC minus....and the meter barley moves off of zero!


Attachments:
DC minus.JPG
DC minus.JPG [ 224.6 KiB | Viewed 150 times ]
Top
 Profile  
 
 Post subject: Re: Knight VTVM 83 F 120 won't cooperate! Now Calibration h
PostPosted: Apr Fri 10, 2020 10:14 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 4589
Location: Littleton, MA
ychris wrote:
I checked the DC neg cal pot and it ohms out good (from zero to 3K) all of the switches are good (no resistance across the connection when switched on DC minus.

Then check the wiring. Maybe a broken wire or bad solder joint, or a misconnected wire?

ychris wrote:
Should this meter go up when switched on DC minus and my meter leads swapped (positive with the 10meg adapter on the neg of the battery and the negative meter lead on the positive of the battery)?

Yes

_________________
Steve Byan https://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Knight VTVM 83 F 120 won't cooperate! Now Calibration h
PostPosted: Apr Sun 12, 2020 1:05 am 
Member
User avatar

Joined: Apr Thu 24, 2014 4:12 am
Posts: 389
Location: Sugarloaf PA.
Alrighty! I'm not sure what happened but now the DC neg is working! I inspected the whole thing, cleaned everything again, made sure there wasn't any loose connections, etc. and now it's working? So it's not that difficult to calibrate, I mechanically zeroed the meter, turned it on, waited a few minutes, switched it to AC, zeroed the meter with the zero pot, connected it to my house voltage 122vac, adjusted the acv pot so the meter reads 122 volts, switched it to dc plus, put my 10 meg adapter on the positive meter lead, zeroed the meter with zero pot, connected it to my "C" battery, adjusted the DC plus pot so it reads 1.5 vdc on the meter, then I switched the meter to DC minus, zeroed it with the zero pot again, switched the meter leads (positive lead to the negative of the battery and the negative lead to the positive of the battery) and I adjusted the DC minus pot to read 1.5vdc. I hope this might help someone in the future, It was frustrating adapting a new 1ma meter from ebay to work on the old dial face, it took a few hours but I think it was worth it! It seems like whenever I switch the meter to say AC to DC plus or DC minus or capacitor or resistance, I have to zero it every time I switch to a different setting. The only difference I see with my new meter guts is it ohms out to 150 ohms where the original meter says it is 100 ohms in the schematic. I put a 300 ohm resistor across to get 100 ohms but it didn't make that big of a difference, so I took it out! Should I have left the resistor in? The donor meter is on the side in this pic.


Attachments:
DONE!.JPG
DONE!.JPG [ 151.76 KiB | Viewed 137 times ]
Top
 Profile  
 
Post New Topic Post Reply  [ 34 posts ]  Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: Barry H Bennett, bob91343, craterranch, pauls.ironhorse and 12 guests



Search for:
Jump to:  


































-->


Privacy Policy :: Powered by phpBB