You will see in the photos that I have not retained the original meter shunts. I began my career in a calibration lab. So I check and often remove or trim the values of the original shunts if they are not accurate to better than 5%. The wire shunts EFJ uses do not lend themselves to calibration well, and are nearly impossible to solder to. Copious amounts of flux and heat are required. Generally they are way off calibration. My rig on RF plate was over 20% off. The Grid meter reading was also way off calibration. Due to the common availability of accurate low value resistors these days, I have no reservation about hacking these out and ditching them for better calibration. It is very difficult to directly measure the low ohm resistors. The end result of accurate current readings is easy to achieve without specialized four lead ohm meters. I was unable to convince a friend to do it this way after lengthy discussion. I suspect he got inferior results and wasted much time the other way. A small variable supply or 12 volt supply and variable resistor are all that is needed to get the panel meter to 1% if you wish using a modern digital meter. These tests are done with the Valiant unplugged from the AC power. Simulate the current in the circuit with a power supply with a digital milliameter and current limiting resistor in the lead and observe the panel meter. The Valiant should be in the normal horizontal position to avoid gravity effects on the Valiant panel meter. This is really important to do before you attempt any modifications. Think about it. If your plate meter reads high, and your output power is low, are you really running rated input power? Bad data does not help you tune up your rig properly.

.

Click on the thumbnail picture to see the plate meter shunt highlighted in green, and the grid meter shunt highlighted in blue.

The formulas will get you close, but make the final adjustment with a power supply, appropriate series resistor to limit the current, and an accurate digital current meter. If you have a 500 mA commercial wall wart with a voltage adjustment switch that can be set to 12 Volts and a series resistor that can be set to slightly more than 27 Ohms (at 6 Watts or more), you have what you need to calibrate the 0.202 Ohm Plate meter shunt resistor for 450 mA full scale. Clearly, a variable power supply with an LM317 that runs off your 12 Volt station supply will be easier to adjust to full scale. The same 12 Volt wall wart can be used with a series resistor of slightly more than 500 Ohms at half a watt to calibrate the Grid current meter of 25 mA full scale. To calibrate the Modulator meter 0.404 Ohm shunt, you will need the same 12 Volt wall wart and a slightly more than 50 Ohm series resistor to limit the current to 0.250 mA full scale. During this process, be sure to apply the current to the points in the circuit that generate the actual current being measured, not parts of the shunt resistor. It is also important to realize that the RF plate meter and the modulation meter are NOT referenced directly to chassis ground! Do NOT put your test supply clip leads to the chassis when calibrating the panel meter. Look at the E F Johnson schematic to be sure where to make the connections, or your calibration will not turn out right.

If the panel meter reads lower than the digital meter used to calibrate it, a resistor must be inserted in series with the multiplier resistor in the Valiant. This is pretty straightforward. Increase the value of the Valiant resistor by 1% if the meter reads 1% low.

If the analog panel meter reads higher than the digital calibration meter, a shunt resistor is required across the existing resistor to reduce its value. It is definitely easier to start with a larger resistor and trim it down from the standpoint of circuit connections. You can just touch the intended trim resistor across the existing one to try it out to see if it is about right. If not, try the next closest value.

Here are some helpful rule of thumb approximations if the meter reads high:

If the meter reads 20% high, put a shunt resistor across the multiplier resistor 5 times the value of the multiplier resistor and the panel meter.

If the meter reads 10% high, put a shunt resistor across the multiplier resistor 10 times the value of the multiplier resistor and the panel meter.

If the meter reads 5% high, put a shunt resistor across the multiplier resistor 20 times the value of the multiplier resistor and the panel meter.

If the meter reads 2% high, put a shunt resistor across the multiplier resistor 50 times the value of the multiplier resistor and the panel meter.

If the meter reads 1% high, put a shunt resistor across the multiplier resistor 100 times the value of the multiplier resistor and the panel meter.

This process can be done repetitively to approach the value with higher and higher shunt calibration resistors until you get as close as you wish.

If you need to fix the RF plate or Modulator multiplier resistors, the best way is to hack out the original resistance wire loop and replace them with a real precision resistor which is cheap and commonly available. Get one slightly higher than the 0.404 or 0.202 ohm values specified by EFJ for the RF and Modulator Plate Current shunts and trim them down by the methods shown above.

If you wish to use math, here are the formulas you need. Keep in mind that these will only adjust for the theoretical accuracy of the resistor specified in the Johnson parts list. If the meter movement is off a bit, the method described above is better because it adjusts the actual meter reading, not just the resistor. I argued with someone on the air for half an hour about why this was the best way before I gave up. Measuring the resistor is an INDIRECT method that assumes the panel meter is accurate. Measuring the actual current is a DIRECT method, since it assumes nothing, and corrects for any errors in the meter movement and switch and wiring resistance. The only possible source of error is the digital test meter used to calibrate the transmitter panel meter. Use the calculations to get you in the ballpark and do the final adjustment with a digital meter and power supply.

The resistors are defined as:

Rt = the total resistance shunt desired, such as 0.202 Ohms

R1 = the first standard value resistor, such as 0.27 Ohms

R2 = the parallel resistance to be added across R1 to obtain Rt

General formula for total parallel resistance of R1 and R2:

(Inverse of the sum of the inverses)

1	=	1	+	1
RT		R1		R2

Specific formula for total resistance with only two paralleled resistors R1 and R2:

(Product over sum)

RT	=	R1R2
		R1 + R2

Specific formula to find the necessary parallel resistance R2 to lower a standard value resistor R1 to a desired total meter shunt theoretical value Rt:

(Product over difference)

R2	=	RTR1
		R1 − RT

Specific example of 0.27 Ohm standard value starting resistor to get a final value of 0.202 Ohms using Product over Difference formula (two iterations):

R2	=	RTR1	=	.27 × .202	=	.0545	=	.802+
		R1 − RT		.27 − .202		.068

To obtain a 0.8 ohm resistor, begin with a 1 ohm resistor. Using the rule of thumb, to reduce the resistance by 20%, use a parallel resistor of 5× the value. Five times one ohm is five ohms, or 4.7 ohm nearest standard value.

SOLUTION: Use 0.27 ohms, 1 ohm, and 4.7 ohms in parallel. THis results in less than 1% error in the value of the meter shunt resistance. Again, I caution you that this is an indirect method that does not account for any inaccuracies from the analog panel meter movement or stray resistance in the meter switch and associated wiring. The final calibration should be done as described above with a power supply, current-limiting resistor, and digital meter of sufficient accuracy to ensure results which meet your standards.

RECOMMENDED READING: Metering amplifier or transmitter from W8JI.