An EICO 249 VTVM, Part 10: Let’s test some parts.
Thursday, August 13, 2026 at 06:28:50
Most of the parts in this device had been replaced at some point - except for the voltage divider on the range switch. For the most part, with the exception of a few in the chassis, that’s all I’m going to test.
Some of these parts came from the donor unit that I had as well, and we’re not worried about them.
Here’s the two things we’ll be testing.
This is the original range switch, as well as a couple of resistors that the previous owner made. In particular, these are R28, an 18MΩ, and R29, an 82MΩ. Or, they’re supposed to be, that is. The 82MΩ was made up of parts that totaled 57MΩ.
Here’s the measurements:
You can see that most of the parts are…ok. A decent amount of them are out of tolerance, but not terrible for the age. However, the 82MΩ part is open. I tried multiple meters, and both of the resistors in the pair are open. I’m not sure if this is something that happened during operation, or if the guy the picked them and put them in without checking. In any event, one of the resistors in this pair is obviously of the correct vintage, so who knows where it came from.
So…why did these get replaced? Some of them are good! Don’t touch them! etc, etc.
I would have loved to do that, but the problem is they are old carbon film resistors, and when you start messing with old carbon film resistors they start changing values. Especially when you start hitting them with heat. Had I taken the out of tolerance devices off, their neighbors would have immediately changed values. This is due to a lot of factors - outgassing of moisture absorbed in the part over the years, potential cracking of the film, and/or other defects that show up after 70+ years and some heat. If you’re taking these off, take them all off. Sure, it means building new parts out of multiple components, but you then have everything back under 1% where it belongs.
How does it work? That’s next.
Next part of this series: Coming soon.
Previous part of this series: https://wereboar.com … inish-this-thing-up/
Wrapup and final thoughts: Coming soon.
References
Manual set: https://wereboar.com … with%20schematic.zip
Youtube Playlist: https://www.youtube. … k&list=PLbV6LYdZISZI
The playlist has more videos scheduled for much later, there are several months difference between here and YouTube.