Heathkit IM-13 VTVM Repair

If you are under a certain age, you might not know the initialism VTVM. It stands for vacuum tube voltmeter. At first glance, you might just think that was shorthand for “old voltmeter” but, in fact, a VTVM filled a vital role in the old days of measuring instruments. [The Radio Mechanic] takes us inside a Heathkit IM-13 that needed some loving, and for its day it was an impressive little instrument.

Today, our meters almost always have a FET front end and probably uses a MOSFET. That means the voltage measurement probes don’t really connect to the meter at all. In a properly working MOSFET, the DC resistance between the gate and the rest of the circuit is practically infinite. It is more likely that a very large resistor (like 10 megaohms) is setting the input impedance because the gate by itself could pick up electrostatic voltage that might destroy the device. A high resistance like that is great when you make measurements because it is very unlikely to disturb the circuit you are trying to measure and it leads to more accurate measurements.

We take that for granted today, but a typical voltmeter in the old days was just a meter with some resistors in front of it. While a good meter would have relatively high resistance, it wasn’t as high as a FET. However, with a tube amplifier, a VTVM could also show a very high resistance and still make good measurements. The Heathkit meter used a dual tube as an amplifier along with some input resistor dividers to provide an 11 megaohm input. There was also a rectifier tube switched in to make AC measurements. In the end, the amplifier drove a conventional analog meter, but that load was isolated from the device under test so its relatively low resistance wasn’t important.

The repair seemed pretty simple, but it was fun to see the inside of one of these. Compare it to a digital meter today and it seems very strange, doesn’t it? If you want to read more about how VTVMs were used, there’s a copy of a 1951 Sylvania book about them online. Some people still prefer meters that move and, we admit, for certain tasks they beat even a digital bargraph.

16 thoughts on “Heathkit IM-13 VTVM Repair

      1. And then in the early seventies, Heathkit had a kit to solid state a VTVM. Plugin solid state equivalents for the dual triode and diode tubes.

        I vaguely remember that they sold a VTVM with those “tubes” in place, but maybe a false memory.

    1. Got one of these kits for Christmas when I was 15 in 1961. Even though I was still in high school i was taking an electronic tech evenibg course one night a week.The instructor was so enthralled with this device that we studied the schematics and the theory of VTVMs for a few weeks, and used these meters throughout the course. Great memories.

  1. A significant bit of information is that meters in the early days were very insensitive. So they really loaded down the circuit under test. Fifty years ago, that still varied, but was way better than it had been.

    Schematics would often include what voltages to expect at a given point, and specify the sort of meter used. If you didn’t use a meter with the same loading characteristic, the readings you got wouldn’t match the sample readings.

    1. In Basic Electronics Lab, one assignment was to measure various voltages with 3 analog VOMs with different ohms per volt ratings. IIRC, one of VOMs was 1 Kohms per volt.
      We were surprised by how much our readings differed between meters.
      Sometime after that, I bought my first VOM, a RadioShack analog with Range Doubler. By switching to the Range Doubler setting, the ohms per volt dropped 50 %.

    2. The circuits were all vacuum tubes; high impedances. Today 11 MEG is huge. Most DVMs are 1 meg, but the impedances in the circuits being measured are at least an order of magnitude lower.

      That Heathkit was an industry standard.

      1. And VTVMs didn’t exist in the early days when meters were very insensitive. They came along as a solution. But fifty years ago, VOMs were still common, and better than they had been, they weren’t as good as VTVMs.

        I went from a used VOM with the back missing in 1972, which I rapidly destroyed by wrong settings so the needle soon wrapped around the stop to an HP-410B a few years later. If I’d not been loaned that, I’d coukdn’t have afforded a VTVM.

        It was a big thing when I got my first DMM in 1984, after my grandmother died and left me some money.

        The distinction doesn’t really exist nowadays, DMMs are the entry level meter.

        Yes, some cheap DMMs have 1M input, but I doubt that applies to “most”

        1. Looked it up you are right. My FLUKES have 10M input.
          My Tektronix scopes both have 1M before probe attenuation. That must be where I got the 1M from.

          I think my original DVM was a no-name with 1M.

          I started with a Simpson 260. The last one I owned actually had a protection circuit that kept the needle from pegging. I was working on an old Tube TV with it once and couldn’t figure out why the “breaker” kept popping before I could touch the test point. I reached up and closed the door on the high voltage cage on the TV and it stopped.

  2. You had to zero a pot every time you switched to a different range or mode. On ohms there was zero and short-the-tips for full range as well. Lots of knob tweaking fun! Otherwise you’d use a 20,000 ohms per volt meter, in the new solid state stuff that was a little less demanding though.

  3. The older models had an aluminum extrusion for the handle. It was connected to the case (and, of course the black lead). I really got a jolt when I had elevated the meter to about 400V on the previous day and then came in the next day, turned on the bench and decided to move the meter.

  4. Early D’Arsonval meters might have jewel bearings, which could have a small amount of friction, hence the recommendation to tap the meter face. Later meters used taut metal bands, which were essentially frictionless, so tapping was not necessary.

  5. When I was a pre-teen, a family friend who knew I was into electronics asked if I wanted his old voltmeter. I said sure, and it turned out to be a Bell & Howell VTVM / oscilloscope combination that he’d built a decade earlier as a part of their “Learn electronics in your spare time” course. Not only did I have the fairly decent voltmeter, but an oscilloscope opened a whole new world for me.

    I used that thing until I couldn’t get tubes for it anymore. Being large enough to be the box a boat anchor comes in didn’t help when it came time to moving, and eventually it went to a bad home during a garage sale. But I loved that old gear.

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