New and improved guide for old and obsolete electronics!
Hello, I'm writing this guide because I've seen alot of nice old electrical meters for sale on ebay but most of the people selling them dont know how to test them, so have no idea if the meter they're selling is working or not.
This guide will tell you how to test a few different kinds of old voltmeters (which measure voltage) and ammeters (which measure amps).
If you test your meter and the needle moves; it works! This means it will probably get more bids and sell for more! I have often been put off buying vintage electrical meters because the seller didnt know how to test them so had no way of knowing if they worked or not.
Simply connect a battery to the terminals and see if the needle moves, if it does, great! The meter works! If not you may have connected your battery backwards, if it still doesn't work it may still be alright but there could be any number of things wrong with it from loose connections to a seized up movement.If the meter says anything like Millivolts or Microamperes on it only connect a small button cell battery like that from a watch or hearing aid. Connecting any more powerful battery risks burning out the meter. I would advise using no battery bigger than an AA battery for just testing for needle movement.
Never use a car battery or mains electricity.
More detailed testing ...
First check the needle moves, shake the meter gently and the needle should wobble then go back to its orriginal position, which is hopefully zero but many of these old meters are slightly "off". Some have zeroing knobs to fine tune where the needle is before an electric current is connected to the meter. If the needle doesnt move, it might be broken but dont give up yet we've only just begun! Also some meter's needles only move properly when they are held vertically or at an angle.
Next check your terminals are clean, terminals are where you connect the wires to the meter. Sometimes it is impossible to clean the terminals completely because the metal has corroded too much. Having unclean terminals will decrease conductivity, almost to zero if they are very dirty. You can clean them with fine sand paper, but try not to damage the look of the meter if they are external terminals.
Another note about terminals; if you connect the - (negative) to the + (positive) and vice versa, the needle will move backwards, this shouldn't cause any problems, just switch round your wires and it should move correctly!
What you need to test a meter:
A battery or power supply - Important: Never use a car battery or mains electricity. Just as important: Take note of the range of your meter and choose something suitable, if it says anything like milli or micro before the units then only use a small button cell battery. Even though a meter appears to have a high range it may have been modified internally for a low range so it's best to start with a small battery or a low voltage and work your way up. For the electronics wizards among us, you can use a variable power supply, just start at a low voltage / amperage.
Two wires - With some exposed wire at each end to connect to the meter and battery terminals
Crocodile clips - are optional but helpful!
Heres my testing kit:
Simply some batteries and crocodile clips on wire.
There are many different kinds of electrical meter and each one is tested slightly differently. If you have two brain cells to rub together you should be able to figure out which part of the meter are the terminals and which is positive and negative and get it all connected up okay, but I'll give you a few tips on that anyway.
Most meters designed for pannel mounting have two terminals on the back, one should be marked positive or negative unless the meter is for alternating current (AC)
Back of a pannel meter.
Some have the terminals on the side like the meter photographed below.
Sometimes these kind of meters have two or more ranges, they will have one common terminal and two or more other terminals for the different ranges. The common terminal is usually marked with a 0 or a - or a +, you just have to look and see it's different for all these old meters. To test simply connect the correct polarity to the common terminal and the other one to one of the other terminals.
Another common kind of meter is the pocket watch meter which has it's own arrangement of terminals.
These are some very common voltmeters, the one on the left is the most common pocket voltmeter of all. The terminals in this case are the "plug" on the end of the wire coming from the top of the meter and the "feet" or spike on the bottom of the meter. One of these will be the common terminal.
Ye old Multimeter.
These types of meter are usually in a wooden box or shaped like a box and have many terminals, again, look for that common terminal and connect the correct polarity to it then interchange your other wire between the other terminals to read different voltage ranges or amperages (which are usually marked on the meter's face)
Some notes on meter mechanisms regarding AC and DC.
Two main types of mechanism are used, moving coil and moving iron. As the names imply: One uses a moving coil to move the needle and the other, a peice of iron. Moving iron meters do not wobble when connected to alternating current whereas a moving coil meter would display alot of needle movement and vibration at the frequency of the AC. Moving coil mechanisms require a diode bridge to show AC. Both can handle DC (direct current) just fine.
Results of your tests
If your needle goes all the way to one side, disconnect your battery. You have used either a very low range sensitive meter or a very high voltage/amperage. It is best to start at a low voltage or amperage and work your way up
If your needle doesn't move, your meter may be broken. It could be anything as simple as a loose connection inside or something as bad as a damaged movement. Sometimes the needle just needs a nudge and other times it is physically jammed against the glass of the meter.
Accuracy, if your battery is new or you are using a bench power supply then you will be able to guage the accuracy of the voltmeter you are testing, old batteries will give a lower voltage so don't use them. Some old meters have been tampered with by thier previous owners and so have a widely different range to what they state on thier face, but with old technology like this you should not expect high accuracy.
Is this voltmeter broken?
Look at the voltmeter below; It is a pocket watch voltmeter, its broken. The lead and ring have been broken off the top and the pointer from the needle has been broken off, prising open the back (which you can do with a small screwdriver) revealed alot of rust and corrosion and before I took the lense out of the front and polished it, it was black inside probably from water damage. Definately broken. But don't fret for it's loss, this kind are as common as pennies and not worth much more.
Well I think I covered everything in my new and improved, more direct and less rambling voltmeter and ammeter testing guide!Hope your meter works!