An EICO 249 VTVM, Part 5: Let’s get the rectifier replaced.
Thursday, July 30, 2026 at 06:32:43
Now that we’ve determined what the EICO 249 needs as far as rectifiers, it’s time to actually replace it. Here’s the part that’s leaving us:
And it’s mount. The part itself has a stud on the back that’s being used for a terminal strip.
A 1N4007 and a 5W, 20V zener were used to simulate the voltage drop of the selenium part. There’s also a capacitor there, we’ll get to that in a minute.
B+ and B- are both good, although B- is a little low. It’s still fine and gives plenty of room for AC Zero. The output wire is long because it will get laid in properly later, and I wanted plenty of length.
I was reminded that the measurements in the manual were probably taken with a loaded-down meter, and that may not reflect what we see here. So, I’m happy with the results.
So, why the capacitor in the circuit?
Because, this:
That’s noise measured right off the end of the zener. Diodes can create noise that you didn’t see in tube circuits, and that don’t matter in solid-state because we bypass the noise away. That’s what this capacitor is, it’s a shunt to ground for the high-frequency noise that may be present on the rectifier.
The noise is all over the signal in the unit, and if you zoom in with a better scope it’s about ~2KHz.
We just have that pervasive 60Hz hum that’s everywhere in a device like this after the capacitor is added. A bit more filtering on the power supply, and we get this:
That’s as good as it’s going to get, and is probably 100x better (or more!) than the OEM would have provided.
The meter itself is working fine, so now it’s time to do the fun stuff. Next few posts are just teardown and prep, stay tuned!
Next part of this series: https://wereboar.com … -6a-teardown-part-1/
Previous part of this series: https://wereboar.com … rmission-whats-next/
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.