Valiant meter shunt?

[K6RAD]

Anyone know how to solder the nicrome wire meter shunts?  I replaced an open resistor on the same termlnal lug and now I can not get the meter shunt wire (R58) to take solder even if I scrape it with a razor knife.  The value in the schematic is .202 ohms, I was thinking about replacing it with two .5 ohm 10W resistors in parallel.  Are these the meter shunts that go bad in the Valiants?  There are also 3 resistor meter shunts in both the Valiants I am restoring, this one wire end is the only one I am having trouble with all the other ohm out good.  I can get solder to kinda stick and it ohms good but it looks cold.

[WQ9E]

I replace them with precision resistors from Mouser, usually just the final and modulator shunts.  The original units are often wildly inaccurate and were dependent upon the skill of the original builder.    If you want to use the original I have found a very small file is useful for cleaning the point to make a connection.  Realize that these are very short shunts so as you tin a larger area you significantly decrease the value of the shunt.

It is very important when working on a Valiant to loosen the screws on the insulating coupler in the VFO drive.  Otherwise this coupler (with different input and output shaft sizes) will break when the front panel shifts under the weight of the chassis.

If this is an early Valiant, carefully check the wiring from the LV transformer to the 866A cathodes.  Because HV appears on these they must be properly insulated for HV and the original wiring was not the proper type.  When it fails it will often destroy the LV transformer in the process.

Early Valiants were also built with a single 500 pf output DC blocking/coupling cap.  This cap is not sufficient for 160 and output will suffer.  Later units were built with either two 1,000 or 1,200 pf units in parallel. 

[Pete, WA2CWA]

I generally make my, in a pinch, meter shunts with hookup wire. For .202 ohms: Using 24 gauge solid wire, you need 94.429 ~ 94 1/2 inches of wire. Using 28 gauge solid wire, you need 37.35 ~ 37 3/8 inches of wire. It's generally not pretty, but it works.

http://www.cirris.com/testing/resistance/wire.html

[WA1GFZ]

Tin the wire with acid flux then clean it well so it will take solder on the terminal strip.

[Jim, W5JO]

Make a solid connection with the wire to the terminal, then flow solder over the entire terminal, which is supposed to hold the connection solid.  Over the years as the solder ages it allows that wire to loosen from the terminal and, bingo, false readings.  Be sure that connection is pinched tight against the terminal.   

Don't spend a lot of time trying to get solder  to adhere to that wire, it is very difficult at best.  The best option is the precision resistor route.

[K6RAD]

Wow, thanks for all the great information. You guys are awsome.

I plan on using a DDS VFO that I built from a kit and perserve the inboard VFO. Does anyone have information on doing that to a Yaliant?  I had it working on a Heathkit DX-40, untill its power transformer shorted out. 

 
Kurk
K6RAD

[KA2DZT]

I have a large supply of low resistance 3 and 5 watt WW resistors that I use for shunts. I think I have 0.24ohm resistors.  I also have hundreds of 1.0ohm 1W resistors.  Five of those would make 0.2ohm.

I would replace the shunt with something like I mentioned above instead of trying to solder the nichrome wire.  A little too long or a little too short and the shunt will be off the correct resistance.

If I need a super accurate low ohm resistor, I measure a number of them to get the most accurate value.  I also use a Wheatstone Bridge to measure them, sometimes the HP digital ohm meter may not be accurate at very low resistances.

You mentioned that the schematic shows a 0.202ohm resistor.  That, I think is rather wishful thinking.  Johnson probably never got any shunt that accurate, thats +/_ 1%.  They were lucky if the shunt was within 5%.  The meters movements were not that accurate.

Fred

[W3GMS]

I always use a shunt just slightly on the high side and then use parallel resistors to bring it right on.  I use a known calibrated MA meter in series with the shunt while trimming the shunt resistor as I have described.  That way all the variables are in the picture including the meter movement.  When both meters read the same your done!
Joe, W3GMS   

[The Slab Bacon]

I had a problem a few years ago with the plate current meter pinning on my
Invader 2000. It ended up being the meter shunt had opened up. This one, however was not in the chassis, it was inside of the meter movement. I got lazy on this one and instead of repairing/replacing the resistor, I used a low resistance wire wound pot across the meter for a shunt. Then I stuck a DMM in series with it and calibrated the meter to agree with the DMM. Good to go ever since.

(it's nice to have a massive stash of junk box parts)    

[WQ9E]

I used to wind replacements with the help of a Wheatstone bridge but now I just rely upon Mouser for new precision shunts. 

I also found the .202 spec humorous in the first Valiant I restored.  In the early kit built versions the builder was expected to carefully measure the shunt length and I doubt if many builders had anything better than a ruler marked in sixteenths and the width of one of those marks was a significant percentage of required shunt length.  Coupled with the questionable meter movement accuracy was more accident than likelihood.  As a good law abiding 14 year old novice in 1974 I carefully allowed for screen and grid input to stay within the then 75 watt novice limit.  Had I known how inaccurate the meter shunts were in my Valiant I would have been afraid to operate for fear of the feds breaking down the door. 

[The Slab Bacon]

Rodger (and others)
                           The accuracy of those meters usually ALWAYS leaves something to be desired. They should say "RELATIVE" on their scales. The biggest thing is just for you to be able to see peaks and dips when tuning, and any dramatic changes indicating there is a problem. I doubt if they were anywhere near +10-15% accuracy, even when they were new.

The ones that we repair are probably closer to correct than they ever were.

[kg8lb]

 Soldering Ni-Chrome wire into a shunt at home can be an exercise in gross futility .  To properly tin or "wet" the Ni-Chrome you should have the correct flux. Not just any acid type flux (though it does work--kind of) but a rather special stuff. You then need to abrade or scrape the Ni-Chrome just before tinning.  The tricky part is that the solder flow can make a huge difference in the ultimate resistance since you are usually working with a small length of wire , a little overflow means a major move.
  A look inside many precision analog instruments will often reveal the time honored spooled wire already mentioned.
Pretty easily done and very easily trimmed to near perfection.
 As also mentioned an open shunt can also open up a moving coil meter in very short order.