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Author Topic: VTVM with zero and infinity setting problem on R X 1 scale  (Read 2672 times)
KC5LY
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Posts: 17




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« on: June 08, 2014, 07:13:44 PM »

I have a Knight Kit KG-625 VTVM that I acquired several years ago. It has developed an issue with the R X 1 scale where if the probe leads are shorted to set the zero point it is quite a lot off from the R X 10 and higher R scales plus the infinity ohms adjustment on the R X 1 scale will not adjust to the infinity symbol. I have a new battery in the battery holder and the contacts of the holder were cleaned. I replaced the tubes, cleaned the range and function switches with Deoxit. Calibration adjustments seem good on all functions except for the way the R X 1 scale acts.

I found one wire attaching to a solder lug of the function switch which had some sticky green goo on the wire. It does come off on the fingers as a sticky substance. I have seen this before - it seems to be some sort of reaction with air and the insulation of the wire. I began to look more closely and I think I see something similar happening to every staking bar on each switch contact on the function and range switches. Each spring loaded contact is staked to the bakelite insulation with a rectangular bar that passes through the contact lug and is staked on both sides of the bakelite to hold the contact in position. Some older switches I have seen had brass eyelet type rivets holding contacts to the bakelite insulation. I am wondering if this is also some sort of corrosion that may be involved with humidity and air contaminants. However the discoloration on the staked contacts appears dry, not sticky like the one wire end. I may try some lighter fluid or alcohol on a Q-tip at one contact to see if either solvent dissolves the coloration I see and provides any cleaning.

The contacts make firm contact with the moving and stationary elements of the switches. The wires that solder to them have good solder joints as far as I can see. The discoloration on the contact lugs does not seem to have any effect on the resistance from one end of the contact to the other, so this may not have anything to do with the problem the meter exhibits.

I think I do see one possible cold solder joint to a brass ground lug at the 12AU7 circuit. Tomorrow I plan to disconnect the parts from it, remove from the meter and scrape it until I can get a nice clean surface to solder to. All the other ground lugs in the meter are more typical plated types with other solder lugs on a bakelite strip. This one is just a ground soldering lug mounted with screw, nut and star-washer.

Any ideas or suggestions?

Joe
KC5LY
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KA5PIU
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Posts: 446




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« Reply #1 on: June 08, 2014, 08:05:21 PM »

Hello.

Tubes?! the little round things with lights!?
http://bama.edebris.com/manuals/knight/kg625/
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KC5LY
Member

Posts: 17




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« Reply #2 on: June 09, 2014, 04:08:45 AM »

Yes, I have the complete construction and owner's manual for the unit. It has a nice large analog meter movement that is easy to read. These are excellent for watching peak alignment of tuned circuits in receivers using test points in the receiver or using a detector probe.

I will be at Ham Com later this week and may be looking for another one for parts.

Joe
KC5LY
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K8AXW
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Posts: 3956




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« Reply #3 on: June 09, 2014, 09:22:17 AM »

Have you tried different probes or a short piece of wire?  You're talking about a VTVM that has a white beard two feet long!! 

Suspect everything and after the process you just completed, I would have to suspect the probe leads.  If the probe has components or a switch, then the suspicion become even greater.

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KB4QAA
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Posts: 2443




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« Reply #4 on: June 09, 2014, 12:47:14 PM »

The green material is an oxide of copper, ordinary corrosion.  The ancient term is Verdigris corrosion.  

I recommend you spend some time cleaning all points of corrosion in the meter.  You can use ordinary contact cleaner (without lubricant) or rubbing alcohol.  Start with a soft nylon brush or tooth brush, proceed to a brass brush applied lightly if necessary.  

Standard boat anchor cleaning methods can be used (protecting the meter and any water vulnerable parts.

Follow up by applying a preservative like Deoxit, or tuner cleaner with lubricant.  Be careful not to allow them to soak into phenolic boards or wafer switches.

A drop of liquid flux will help make soldering/desoldering old joints easy and reliable  Also great for coax soldering.
http://www.jameco.com/1/1/189-10-4202-liquid-solder-flux-2-fl-oz-bottle-miscellaneous.html

Keep the old gear operating!  bill
« Last Edit: June 09, 2014, 12:51:09 PM by KB4QAA » Logged
DL8OV
Member

Posts: 134




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« Reply #5 on: June 09, 2014, 01:09:07 PM »

Nothing wrong with the "little round things with lights", any ham who's smelled a shack full of tube gear will know what I mean.

Peter DL8OV
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G3RZP
Member

Posts: 4830




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« Reply #6 on: June 10, 2014, 03:57:28 AM »

Many are the times they prove useful where large RF voltages need handling
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KA5PIU
Member

Posts: 446




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« Reply #7 on: June 10, 2014, 04:08:08 AM »

Hello.

Yes, but they can get hot!, and shock!, and are easily broken!
Glass container and all?! everybody knows bottles are made out of Plastic!
Anyone remember the Nuvistor?
http://en.wikipedia.org/wiki/Nuvistor
Compactron?
http://en.wikipedia.org/wiki/Compactron
Yes, the tube, but just like modern tires, we have all gone tubeless!
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G3RZP
Member

Posts: 4830




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« Reply #8 on: June 11, 2014, 01:10:35 AM »

>Yes, the tube, but just like modern tires, we have all gone tubeless!<

Speak for yourself! I still have a transceiver with tube PA and driver as well as a tube amplifier! Plus several very good phase noise tubed signal generators....
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KC5LY
Member

Posts: 17




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« Reply #9 on: June 18, 2014, 04:54:42 PM »

Yes tubes usually are glass, but the nuvistor had a metal shell and ceramic bottom where the pins came out to plug into the mating socket. Then there was a period in the late 1930s and well into the 1940s in which many radios and some television tubes were metal and therefore needed no external shield. These were very rugged tubes and served the military well during the war.

I did finally receive new capacitors and especially the electrolytic. The electrolytic was changed and I spent time cleaning the range and function switch. I changed the 9.1 ohm resistor in the R X 1 scale. That helped some, however there was still the problem getting that scale to zero  and infinity set properly. I made a check for resistances on grounds throughout the unit. I had already spotted a potential bad ground at a ground lug near the 6AL5. I had to clean the bottom and top of the part of it that mates with the chassis metal. I also cleaned the plated bracket it mounts to. Then I added some Penetrox-A to insure a good ground and prevent future corrosion and reassembled with the original screw, starwasher and nut. At that point I had a zero ohm connection for that ground lug to the metal bracket. I then discovered that the bracket also needs to ground to the aluminum front panel. Some corrosion at the 4 points where the meter movement screws attach to the bracket and front panel was causing about a 2 ohm resistance there. All together there had been about 4-5 ohms of resistance in the various grounding points. I cleaned tho bracket flange where it touched the front panel aluminum and also the aluminum with a light abrasive to get rid of corrosion. Then some more Penetrox-A was used and the bracket reassembled to the front panel using the nuts and starwashers that held the meter to the front panel and bracket that held the tubes and other components.

When I finished there was a zero ohm resistance from any ground lug on the bracket that holds the tubes and sockets plus all their nearby components to the bracket metal and from the bracket metal to the front panel. A final re-calibration sequence was followed and the result was that the proper operation of the R X 1 scale was obtained.

I was aided by the use of a second ohm meter to test the connections in the unit being repaired. Some random tests of resistors from my parts bins showed reasonable resistances for the parts tested. Voltages look very close to two other meters on various scales and voltage points in a receiver.

Joe
KC5LY
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