[Johnson] Johnson Viking II Meter Shunt Question

[Robert Jefferis]

Scott,

You may have finished with the Viking II shunts by now. I went through my
Viking II shunts a couple of months ago. The modulator shunt indicated on
the high side by 37% and the PA cathode current indicated high by 20%. There
is no need to guess - the theoretical (ideal) shunt value is 0.202 Ohms for
the 500 ma full scale meter positions. This will provide exactly 100 mV
across a 5 ma meter with coil resistance of 20 Ohms when 500 ma passes
through the circuit. Of course, there are the practical problems like true
meter resistance, wiring resistance and meter error. 

The original 500 ma shunts were indeed nichrome wire. You need messy,
dangerous acid flux to solder it properly. I do not know, but Johnson may
have provided the original shunts tinned so the kit builder could solder
them. I found bad joints as one of the responders indicated. I do not
recommend messing with the nichrome and required flux, mainly because the
flux splatters all over adjacent parts and the chassis. A mess to clean up,
and it is acidic. 

My solution was to use a 0.2 Ohm 3 Watt current sensing resistor from
Ohmite, Mouser part number 588-13FR200E. These are small, stable and they
solder in like a normal resistor. Careful measurements showed that the real
shunt value for my unit wanted to be 0.183 Ohms after interconnect wiring
resistance and meter error was taken into account. The final trim called for
shunting the 0.2 Ohm current sense resistors with 2.2 Ohms(used 1/2 Watt
precision metal film). At the end of the day, at 300 ma, the PA cathode
current reading was high by 4 ma and the modulator current read high by 6 ma
- better than close enough. Be sure to check solder joint integrity at the
meter switch and at the meter before doing a final trim. Good luck with
yours. 

Bob, KF6BC

-----Original Message-----
From: johnson-bounces at mailman.qth.net
[mailto:johnson-bounces at mailman.qth.net] On Behalf Of Freeberg, Scott (STP)
Sent: Friday, July 20, 2007 7:16 AM
To: johnson at mailman.qth.net
Subject: [Johnson] Johnson Viking II Meter Shunt Question

Greetings!  I got the modulator running in the Viking II.  It is definately
the Ranger modulator circuit in there.  The resting modulator cathode
current is reading 200 ma instead of the 80 ma per the manual.  

Gary  K4FMX suggested I check the meter shunts and said they have a
reputation for changing values.  I found the meter shunt for the modulator
cathode current.  I was expecting a low value precision resistor but found a
piece of insulated wire with a funky bend in it.  It matches the one in the
manual photo.  I'm assuming it's some kind of special wire with a high
resistance. 

What is the shunt resistance supposed to be?  I'm guessing it wants to be
0.5 ohms or 1 ohm. 

Thanks.  73, SCott WA9WFA  
Johnson mailing list
Home: http://mailman.qth.net/mailman/listinfo/johnson
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:Johnson at mailman.qth.net