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 Post subject: Shunt question
PostPosted: Nov Fri 13, 2020 5:38 am 
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Joined: Mar Wed 27, 2019 1:56 am
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Location: League City, TX 77573
I'm looking at a Johnson Viking II transmitter and I’m curious what they re referring to as a shunt like the SH4. I included a small section of the schematic were SH4 is located center and a pic in the radio located upper right and a pic of the parts showing the full description.

My question is what are they? How do you measure them and where can you acquire replacements?


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 Post subject: Re: Shunt question
PostPosted: Nov Fri 13, 2020 6:23 am 
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Those are meter shunts, used to calibrate the meter for the different ranges.

DM


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 Post subject: Re: Shunt question
PostPosted: Nov Fri 13, 2020 7:33 am 
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Location: Tucson, Arizona U.S.A.
They're just resistors but the value depends on the characteristics of the meter they are used with.

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 Post subject: Re: Shunt question
PostPosted: Nov Fri 13, 2020 2:47 pm 
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Location: Long Island NY
Ohm's Law is not a 1960s spaghetti western. It's a fundamental law of physics that relates current, voltage, and resistance. In this case, milliamps, millivolts, and ohms.

You're no doubt familiar with ammeters and milliammeters. They have to be inserted in circuits to measure current. Well, sometimes that just isn't very convenient or cost effective. In that case what one can do is put a resistor in the circuit and measure the voltage drop across it. To avoid dropping too much voltage the resistance can be kept small, and a meter that measures very low voltages (i.e. millivolts) is then used. Johnson referred to these resistors as 'shunts' which is the original term, but we'd call them current sense resistors today. Lots of electronic devices like battery chargers and products that contain rechargeable batteries, motor controllers, and power supplies with overcurrent shut-down use this technique to measure current today.

To be sure, for high accuracy you have to take the resistance of the leads from the millivoltmeter to the shunt into account if the leads have resistance that is sizable compared to the shunt resistance. Johnson probably assumed the meter test leads that would be used in this case are low enough in resistance to be insignificant.

Ohm's Law only tells you what to expect in general; i.e. 100 mV / 500 mA = 0.2 ohms. A correct calculation would need to know the sensitivity of the meter being used and the resistance of the connecting leads which are in parallel with the shunt. That's why the others are values like 5.1 ohms and 2.2 ohms. Perhaps in the service manual they tell you what kind of meter they expected you to use.

If they need replacement, you will either use a resistor like they did originally, or you may have to make something out of a piece of resistance wire, duplicating the original exactly. Do not fall into the trap of replacing carbon comps with metal films or metal oxide films; these are made of the same ceramics as disc capacitors and have their elements cut into spirals like RF chokes. In a ham transmitter that may operate as high as 10 M these factors can introduce unknown parasitics into the circuit and cause all kinds of headaches.

Edit: Math before morning coffee error corrected above.

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Last edited by Chris108 on Nov Fri 13, 2020 4:59 pm, edited 1 time in total.

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 Post subject: Re: Shunt question
PostPosted: Nov Fri 13, 2020 3:28 pm 
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Location: League City, TX 77573
First thank you everyone for the reply.
So would 100 mv / 500 ma = 50 ohms or mOhms. ?


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 Post subject: Re: Shunt question
PostPosted: Nov Fri 13, 2020 4:21 pm 
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Location: Dallas, TX
Ugctheaters wrote:
First thank you everyone for the reply.
So would 100 mv / 500 ma = 50 ohms or mOhms. ?

Nope! 0.2 ohm. Meter shunts are almost always very low resistance. Also the shunt accuracy effects the accuracy of the meter reading. Sometimes they can be tweaked.

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It's not the Destination, It's the Journey.


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 Post subject: Re: Shunt question
PostPosted: Nov Fri 13, 2020 6:51 pm 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
SH4 in the schematic has an RF bypass cap. It will see only DC. Any old inductive resistor will work fine as a replacement, as long as its accuracy and ratings are OK.

To check these low resistance shunts, you may need to apply a known current and measure the voltage right on the resistor.

They do make meters that work accurately for such low values, but a common multimeter can't do this.

If the original parts are wirewound and have continuity, they are probably OK.

Ted


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 Post subject: Re: Shunt question
PostPosted: Nov Fri 13, 2020 8:26 pm 
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Location: Long Island NY
Unless you have done an engineering phase analysis of the circuit and tested it, throwing "any old inductive resistor" into a circuit like this is a crapshoot. You can't count on the bypass capacitor being adequate when you introduce a 10-turn RF transformer winding, in the form of a spiral cut film resistor, into the grid circuit of a transmitter. Not saying it won't work, only that no engineers worth their salt would look at half a schematic and say for certain that there won't be any impact. SH-3 and SH-4 are shown on the parts list as 5.1-ohm, 5% resistors, and the pictorial makes it appear they are carbon comps. So it would be easy to measure them with just about any reasonably accurate ohmmeter, and leave them alone unless they're really off. SH-5 is listed as a 2.2-ohm, 10% resistor which most ohmmeters should be able to measure accurately.

As for SH-1 and SH-2, there may be no value given because they were made in the factory, probably in the form of wire loops.

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 Post subject: Re: Shunt question
PostPosted: Nov Sun 22, 2020 2:22 am 
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Location: Lincoln City, OR 97367
Gosh, Chris... lighten up!

I agree with Ted. Fifties vintage ham transmitters ain't rocket science. You've got a couple of watts of grid drive separated from the metering circuit by an RF choke, a resistor and a bypass cap. You could wind some coat-hanger wire around a pencil and stick it in there and as long as the resistance was correct, it would work fine.

I habitually use metal film resistors to replace carbon composition resistors in vintage ham gear (with the exception of a very few circuits such as parasitic suppressors or tuned circuits involving resistors) and I have had no problems whatsoever. This is fortunate, as carbon composition resistors are getting harder and harder to find.

I also use carbon film when I can't find a certain value in metal, but I prefer the metal film or metal oxide resistors; I'm not sure how a carbon film resistor will behave over time with respect to value stability.... and we all know how carbon composition resistors behave.

Regards,

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 Post subject: Re: Shunt question
PostPosted: Nov Sun 22, 2020 3:49 am 
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Location: Long Island NY
This ain't the radio police. Nobody is going to come check and see what resistors you put in your 1950s ham transmitter.

When giving advice to people on repairing and restoring equipment, there are a few rules that I live by. One of these is not to recommend substituting modern parts that are different from the ones originally used, unless I know for a fact that the substitution will work and not degrade the performance of the equipment. I also disavow "shotgunning" all the parts in a piece of equipment as a repair technique, or out of some notion that it will make the equipment more reliable. Instead, I always recommend troubleshooting to component level and replacing only what has to be replaced.

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