The AM Forum

I have this O.B. McCLINTOCK milliamp meter with three scales: 0 to 15, 150 and 300. It would be great for a current (no pun intended) project. I've successfully shunted meters before to change the scale so I'm familiar with the process but this one has me stumped because I don't know which range is "normal" without a shunt or multiplier.  I reads 6.6 ohms on my meter. Suggestions??

Two choices:
- it's the low scale
- there is no shunt, it's just the native movement

                         _-_-

I have the very same meter set up to read plate current on my  813 AM rig using the 399 ma scale;

The shunt that I put directly on the terminals using solder lugs was about 1 1/2 inches of small gage Manganin ( zero temp. coef. wire )  A piece of copper wire would be close enough for amateur work.  You calibrate it by tinning the end and trial and error shifting the solder point while watching with a digital current meter and low voltage source..

The current mentioned in my forst sentence should have been  300 mA ( trying to type in poor light )

If you need to set it up to use it in more than just one current range then the problem is a bit more complicated.  You would put individual shunts in the circuit right at their source and calibrate with series resistors treating the meter as a low voltage voltmeter, after the selector switch.

By the way,  I went to Tech school and lived in Minneapolis in the early 1960s.  A partial AM transmitter was given to me by one of the instructors who was also a Ham.  O B McClintock in Minneapolis was before my time !

Good luck,   Val  W0LPG   

Those meters resemble Simpsons so I wonder if this McClintock bought up a bunch of surplus and made new faces for them.  I have a LOT of meters in the Jonque Boxe so I'll probably use a couple of those rather than give my new pacemaker an instant workout.

Put 1KOhm in series to an adjustable power supply to find out

Good idea, thanks!