As an electronic hobbyist, having multimeters and having lots of them seems like a necessity. Well, no, not really, but I tend to get a bunch of them for the heck of it. I got this full sized Heathkit V7A, originally sold as a kit, from a hamfest. It was probably introduced around 1958 making it more than half-century old technology.
|R x 1||ohms|
|R x 10||ohms|
|R x 100||ohms|
|R x 1K||ohms|
|R x 10K||ohms|
|R x 100K||ohms|
|R x 1M||ohms|
This meter is analog with vacuum tubes (12AU7 dual triode and 6AL5 dual diode), and has just one semiconductor element in it - a diode for rectifying AC power. It still has a battery inside to deal with measuring resistance on its logarithmic (or is it 1/x, or is it something else?) scale. A single C battery is needed for measuring resistance. The power/mode select has OFF, AC, DC-, DC+, and ohms. Note that ohms and AC volts (or dB) use the two banana jacks where black is common, and red is volts or for the other terminal of the resistor. For DC, the phone plug tip is used with the black common banana jack. The DC+ and DC- modes will let you switch the polarity of the input, so two shielded phone probes are needed. This helps against introducing noise into the system as the input impedence is high enough to pick off signals off the air.
Now most digital multimeters these days have input impedences at 10MΩ like my Fluke 77, no longer making the VTVM special.
The other strange thing is because this is powered, all the current draw is through the tubes and not much going through the battery even in ohms mode. I
find the circuit pretty interesting: basically the 1.5V battery is hooked
through the resistance tree straight to the input of the differential triode
pair. So that means, when you have nothing hooked up, you have voltage going
to the grid of the tube. As it's a grid, no current really flows.
But this is the infinite
resistance case - so when the full scale is shown on the meter, it's actually
infinite resistance! This is the opposite of most regular VOMs;
then again, keeping 200µA flowing will eat battery life.
Well, how does this meter actually work for measuring resistance?
With the unknown external resistance, you shunt the resistance
of the divider to ground! So the meter drops toward 0µA (but not really
- but the meter zeroing makes it look that way)
as you reduce resistance to 0Ω.
The meter scale is marked accordingly. However, as in most multimeters of the
era, the internal 1.5V battery is passed through a low value resistor and you
can get over 100mA going through your test probes. This is sufficient to turn
on many diodes, and if it can't support 100mA, you can fry these diodes.
Here's a trick that works on old analog ohmmeters with Rx1 mode: you can use the ohmmeter's internal battery to light up an LED with the "Joule Thief" circuit - but a digital voltmeter won't work! Then again, I'm not sure you want to waste the battery in a VOM.
Luckily it's a Heathkit and one of the most common VTVMs out there. But this just means you don't know who assembled the unit. And sure enough, there's plenty of cold solder joints in the meter. Also it looks like some liquid has met some components. It's in fairly sad shape.
The main cold solder joint that restored lots of functionality was resoldering the common point to the PCB. That fixed all of the meter flakiness that I saw. The next problem was that it was really inaccurate about the voltage, implying there's resistance where it shouldn't. I checked the highest DC measurement resistors in situ in pure laziness but the 7MΩ and 2MΩ resistors seemed a way off. Thus I had to stop being lazy and disconnect them to be sure.
They turned out to measure correct out of circuit. There must be a short elsewhere. After more disconnecting and measurement, I found that the DC ¼ inch phone plug was shorted and causing all the grief - I was shocked because mechanical devices like this usually don't short out! I replaced it with what I had on hand: a TRS stereo ¼ phone plug, and left the side connector open.
A third problem was that though the internal battery has its negative terminal on the chassis, the chassis was not connected properly to the rest of the circuit - it's open in places. I added a jumper wire to connect the two parts of the circuit. Fixing that, and now the ohmmeter works. (It remains to be seen whether I modify the unit to completely isolate the case. As designed, the negative common is hooked up to the case.)
I also replaced the #50 screw in bulb by ripping out the glass part and adding a small 6V incandescent lamp in the screw on base. Unfortunately the new bulb isn't as bright as the old one, but bright enough to indicate the unit is on. Old bulb was probably bright enough to backlight the meter face.
The DC probe itself was well worn out, at least on the phone plug end. It frayed out so badly that it was hanging on by a thread of stranded wire and could short out any time. The probe end had its 1M resistor fried, so I had to replace that too. This is not to mention the other two probes' insulation were dry rotting, cracked along the whole length of the wire. They need to be replaced, alas, I don't have any 1KV rated wire to use as probe wire.
After replacing the resistor, soldering a jumper, resoldering a cold solder joint, fixing the light bulb, replacing the phone jack, and then recalibrating, I now have a fully functionl VTVM. I still think I'd rather use the Fluke for most purposes, no power cord or warm-up time to deal with...