EICO Model 221 VTVM

Here is another piece of Eico test equipment I bought at a local Ham flea market.

I had no real need for it but the asking price was low and I like collecting old test equipment. It can measure AC/DC volts and resistance. It came with an original construction manual but no test leads. The manual is dated 1952. The VTVM (Vacuum Tube Volt Meter) contains 3 tubes, a 6X5 power rectifier, a 6H6 rectifier and a 6SN7 DC amplifier.

When I had a chance to try it out I noticed the meter was way out of balance and unusable. Even the meter mechanical zero adjustment would not zero the meter. I had no previous experience in meter balancing and initially was reluctant to try it but I had nothing to lose. I took the meter apart and repositioned the spring that was located on the opposite side of the needle pivot until the meter would mechanically zero. This was all that was required. I physically rotated the meter in various positions to confirm that the needle would remain at zero. It was not perfect but was very good. I then reassembled the meter and proceeded to do the VTVM calibration. The construction manual did not include the calibration procedure but the instruction manual available on the BAMA website did included the calibration procedure.

The calibration went well with the aid of the more modern test equipment I have. For the AC calibration the manual suggested using the utility line voltage for the source. The Common and the Ohm-AC inputs are to be used for this measurement. The Common input is at case ground so connecting the common test lead to the AC neutral becomes very important. Reversing the leads can be easily done. If reversed, the VTVM case will be hot (live) and also if the VTVM is grounded, sparks will fly. The line cord has a non polarized 2 prong plug.

I did not spend a lot of time checking the accuracy of the instrument. Overall it seemed pretty good considering it's vintage and quality (it's not a Hewlett-Packard). The meter tracked better than I expected. In the past I had a Heathkit VTVM. I can't remember the model number but at the time I wasn't all that impressed with the accuracy. Overall the Eico accuracy seems better.

As mentioned above the Eico 221 did not come with test leads. For AC and resistance measurements I used the test leads from my Triplett 630A VOM (Volt Ohm Meter). For DC measurements I build a test lead using a piece of Teflon coax cable. The schematic showed 15 MegOhms of resistance installed in the probe.

It is likely I will not use this VTVM much, if at all. Any of my modern meters are more accurate and easier to use. One advantage of this VTVM is the high DC input resistance "impedance" of 25 MegOhms. A typical modern DMM (Digital Multi Meter) may be around 10 MegOhms. Occasionally the higher input resistance may be useful especially when working on old tube type equipment. Sometimes there is also an advantage in using an analogue meter for voltage measurements.

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