Simpson model 59 panel meter, 150V AC

I plan to monitor the AC power line voltage in my shack with a Simpson model 59 AC panel meter that I picked up as surplus from someplace or other. 

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But it's got an oddity - a external 0.0025 µF capacitor hanging off the back of the meter.

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The capacitor looks like it's a molded paper type. The Simpson data-sheet for model 59 panel meters mentions that the higher range AC voltmeters (500 volts and above) have an external multiplier, while the lower-range voltmeters have internal multipliers. I wonder if my meter was modified to use a capacitor in place of an internal multiplier resistor.

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Inspection of the internals reveals that the capacitor is shunting the multiplier resistance coils. The meter is labeled for "60 & 400 CYCLES". I wonder if originally there was an internal capacitor to compensate for the inductance of the multiplier resistance coils, which failed and was replaced by this external capacitor?

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I cut the cap out, leaving a bit of the leads behind in case I need to replace it to make the meter function correctly. I don't want to mess with the solder joints on the precision multiplier resistance coils.

I don't want to have a suicide cord in my shop, so I'll have to rig up a safe way to test out the meter. Some time ago I built a little rig with a line cord, fuse, and power switch feeding a 25.2 volt center-tapped transformer connected to binding posts; I used it for checking the calibration on the low ranges of my VTVMs and VOMs. I think I'll add a couple of insulated pin jacks to access the 117 VAC side.

Update:

This meter came out of a test fixture built into a small Bud cabinet with a couple of two-prong AC outlets and a hole where I guess a Variac used to be. The fixture had a Raytheon sticker on it. I recently noticed one of these test fixtures for sale on Craigslist and the interior photos showed that its meter has the same capacitor as mine. So I guess the capacitor is to compensate for the inductance of the wire-wound precision resistor at 400 cps. I'll have to complete the job of characterizing the meter and the cap some day.


© Steve Byan 2011-2016