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The article will discuss a cheap and easy way to test panel meters. The meter in your tube tester is an example of a panel meter. Occasionally you see flaky meter operation and suspect that the panel meter may be defective. Most techs know that it may not be safe to test a panel meter by placing an ohmmeter across its terminals — this attempt can ruin the panel meter (depending on the ohmmeter used) and will not reveal all of the panel meter specs anyway.
Most panel meters in vintage equipment (such as tube testers or capacitor testers) will be milliammeters or microammeters. The good news is that these panel meters can be tested, although extreme caution must be exercised. One mistake and you can easily ruin the meter, so “do not try this at home” until you have practiced with a large variety of junk meters and have sufficient experience and confidence in your skills. Even then, “triple check” everything before you proceed.
Many people think that you need to have expensive equipment to test panel meters. It would be nice to own a Simpson 2600 Laboratory-grade Calibrator (which cost $1,620 in 1965 and weighs 132-lbs). But you can make do with the following basic equipment: a variable power supply, a quality resistor decade box, a quality multimeter with mA and µA capability, and another quality digital multimeter that will accurately test millivolts. Likewise, you need an understanding of what you are trying to accomplish. Keep in mind that Ohm’s Law teaches that voltage, resistance, and current are all interrelated, therefore a panel meter that has its faceplate scale in “volts” CAN be tested as an ammeter.
In the following discussion, I will demonstrate testing a panel meter. You will (1) determine the full-scale spec of the meter (using the printed scale as your reference for full scale), (2) evaluate whether the meter action is smooth and accurate across its entire range, and (3) determine the internal resistance of the meter.