I recall that the meter requires 0.1V for full scale. For example, consider the plate current position:
The shunt is 0.51 ohms. This is in parallel with the meter's internal shunt resistance... which I believe is 15 ohms. The parallel combination is 0.49 ohms
When the plate current is 125mA, that current is passing through 0.49 ohms.
125mA x 0.49 ohms = .062V
So... when the meter is reading about 2/3 of full scale (125mA of plate current) the voltage across the meter is 0.062V
To protect the meter, I suggest the following:
1. Remove the 0.02uF capacitor-to-ground on each side of the meter. [I.e. why hold the voltage on either terminal of the meter, even briefly, at 700V, by including these capacitors between each terminal and ground? Why create sparks that damage the meter switch contacts when changing the meter switch position to "plate" (plate current) from any other position? These capacitors were probably part of the original TVI-proofing done on the Ranger]
2. Place one of the 0.02 capacitors directly across the meter (between the two terminals) to slow down the rate at which the voltage across the meter can change.
3. Place a 1N4007 diode across the meter is each direction (2 diodes, in opposite directions) to bypass current around the meter when the voltage across the meter rises above a few hundred millivolts.
The above should protect the meter from brief transients... like briefly shorting one side of the meter to ground when the meter switch is in the plate current position.
Stu
Stu,
Looks good to me I'll give your suggestions a try. I really didn't know which way to go there seemed to be so many different opinions on meter protection some were simple to very involved. You mentioned the internal meter resistance at 15 ohms I seem to remember 20 ohms, but close enough for now. Hopefully I will be checking back with a positive report on the Ranger working again.
73, Dave
