Meter shunt resistors

[WA2ROC]

A few months ago, someone posted a topic about meter shunts in a Viking Valiant.  My meter readings on my Viking II seem a bit low, so here's how I proceeded.

Looks like the Viking II uses the same value meter shunts.  They call out a 100mv 500ma shunt resistor, which works out to be a 200 milliohm resistor.  I got several from Digikey and they're 1%.  I believe they cost me about 20 cents each and should be a lot closer in value than the 60 year old resistors under the hood of my V II.

Since most other resistors drift in value over time, I suppose the shunts in my TX should be replaced too.

When I get the old ones out, I'll measure them using a power supply and a couple of meters, since a hand held ohmmeter would not measure it particularly well.

[WQ9E]

I also use similar modern shunts for replacement in my Johnson gear.

In general, most shunts are precision wire types that don't drift in value like carbon resistors.  An exception is carbon comp resistors that are often used as multipliers for HV metering and these certainly drift.  But most current shunts will be a nichrome loop like those used in Johnson gear or a small wire wound precision resistor.

The big problem with Johnson gear was the need for a kit builder to properly cut the shunt to end up with a very small precise length of resistance wire after tinning.  At least most were cut long so the meter readings were wrong on the safe (high) side.

Another common Johnson problem with earlier Valiant transmitters was the use of a too small plate coupling capacitor.  Original was 500 pf which resulted in reduced output and unusual tuning on 160 (and 80 depending upon the antenna).  Later production used a pair of 1000 or 1200 pf units in parallel.

[Rob K2CU]

Before running out and buying new meter shunts for your V2 or any other rig, why not try and check them for calibration? IF you have a separate power supply, preferably adjustable, and a handful of resistors, even if you have to buy them, you can pass a test current through each of the current sense resistors and note the meter reading on the meter scale.  You do this test with the V2 unplugged and all power supplies discharged for safety. The full scale readings for a V2  are:

Osc    50 mA
Buf     50 mA
Grid    25 mA
Plate   500 mA
Mod    250 mA

If we believe that the meter is full scale at 100mV, then from the shunt values in the V2, it is a 5mA movement, with a 20 Ohm coil resistance. The coil resistance is in parallel with the shunt to give the actual sense resistance.

Meter movements are usually good to 2% linearity so you can just check each shunt at somewhere between half and full scale. Start with your available external low voltage supply. You will also need an accurate external milli-Ameter. Find, or procure a resistor that will give you something short of full scale current for the meter switch position/shunt you are testing. Then run this current through your sense resistor. compare external meter reading with value you read on the V2 meter.

So, step one unplug V2, open up unit and discharge power supply caps.
Start with the 2.2 Ohms (R55) used for oscillator.
Find combination of external low voltage power supply (such as 12VDC) and resistor (such as 620 Ohms) that will give you about 50 mA. Set V2 meter switch to Oscillator position and connect external supply, external resistor, and external meter in series with the V2 sense resistor R55. IF V2 meter deflects to the left, reverse temporary connections to R55. Compare external meter value with value on V2 meter.  IF they are within 5 mA, you are probably close enough for that position since it is used for checking only. The same combination can be used to check the driver position R56. It is used in adjusting the drive tuning but is also a relative value and not too critical.
You will want to check the grid shunt R52 a bit more closely. Full scale here is 25 mA and you may actually want to check it at 10mA and not near to full scale since you set the drive to about 8 mA. It would seem that 1/2 mA accuracy might be desired goal here. That is 2% of full scale anyway.

For best results, be sure to connect to the same points as the meter switch does, especially for the two high current shunts. And, don't just try and replace the shunts if the V2 meter reads high. it might be easier to just find a resistor to put in parallel with the shunt to bring it in line. If you do replace a shunt, use this simple check to verify the results. It is the safest way to do this since there are no high voltages present at all.

[WA2ROC]

I found the 2 high current meter shunts in the V II.  Not resistors, but pieces of resistance wire.  However when I brought them to the lab here at work and set them up on a power supply, volt meter and ammeter, each measured very close to 200 milliohms. 
 
I did replace them with the 2 watt 1% wirewound resistors I had gotten from Digikey and then re-set the modulator screen supply tap on the big bleeder resistor.  This brought down the resting modulator current and also the current on voice peaks doesn't go up as high when modulating.

The modulator resting current also stays at about 60 ma at rest during transmissions, where it used to climb to about 100 ma.

There's the old shunts.  BTW, the meter shunts for the other 3 meter positions are regular resistors

[KM1H]

I still wind my own shunts from a couple of different reels of resistance wire I picked up ages ago.

By the time I find my Mouser catalog, find something suitable and call I can usually roll by own and there is no waiting

BTW, why would you need to buy replacements for a VII? Do they drift with age or use?
The ones in my VI and VII seem to be fine but I havent done a cal run.

Carl

[WA2ROC]

The first time I actually saw the 500 ma shunts was when I went to replace them.  Up until then, I thought they were just standard resistors. 

The modulator resting current was drifting high during transmission and the PS cathode/screen current seemed to be on the high side for the amount of power I was getting out.

Since I had dragged the V II off the desk and onto the work table, removed the 20 or so screws and located the shunts, I figured that the old ones were coming out and the new ones were going in.

The modulator screen voltage needed adjustment anyway, so while I was there, that was done too.  Everything else appeared OK, so it was buttoned up, put back into service and got about the same reports as usual, Solar Flares not withstanding.