Workshop Project #14 - ESI 250DE Impedance Bridge

It was in late 2021 when I was browsing ebay for a decent LRC (or RLC) metering device. Originally, I thought I'd go for a cheap-ish digital one, but then this little gem of a retro device popped up for rather cheap. It was cheap mostly because it was untested and had at least one stuck knob. Sounds like a challenge to me!

The device itself is based on a Wheatstone / Wien bridge circuit and allows very precise measurements of resitsors, capacitors and inductors: R, C and L. I always mix up the sequence of these letters in the product names and I'm sure there are multiple versions out there... LRC? RLC? Doesn't really matter: all three of them!

I did the usual recapping and cleaning, although internally not much cleaning was needed. Wow. It looked brand new inside!

I am also not sure if the device originally came with a cover; the case has attachment points for something along these lines on the side, but the manual doesn't indicate one way or the other. Making one might be a neat build project at a later time maybe. I'll keep it on my list of things to do!

It would be interesting to see how accurate the thing is. If I ever get to some calibration lab, I'll be sure to bring it along. All test measurements were agreeing in magnitude and first significant digits with my other means of checking them. The 3rd and 4th digit is where I can't confirm anthing sadly. But since I didn't change anything on the precision resistors and the reference cap, I'd assume it's at least in the "more than OK for hobby use" range.

And before you ask: yes, the "restored by Atkelar" sticker is ever so slightly crooked again. At some day I might get lucky and manage the proper alignment before the sticky side starts to stick to a freshly painted case.

0:00 Intro / Presentation / Inspection
1:31 No Batteries?
2:05 Open Case / Shiny! / Strategy
3:16 Golden Tin Can? / Binding Posts
4:28 Triple Potentiometer
5:16 "Dekastat(tm)"
5:46 Cleaning / BNC / GAIN potentiometer
6:57 D-Q Potentiomater Assembly / Recapping P2P
7:42 Strange Capacitors
8:21 Delete Charger / Battery Tube
9:51 Case Refurbishing
10:42 Reassembly!
11:40 D-Q deba-q-le...
12:22 D-Q wiper adjustment / Tidbits and Dials
13:17 Chassis Layer 2
13:55 Test #1: Batter Check
14:11 Bridge Circuit, Rough Theory
15:36 Resistor...
16:21 Capacitor...
17:18 Inductor Measurement
17:38 D/Q and ESR math overview
18:57 Case Finish
19:51 Done! Overview / Bye / Credits / Outtakes

Music:
My Train's A Comin'
Unicorn Heads
(YouTube Audio Library)

Useful links that I found via Google that were helpful for this project:

[dinocool]

I accidentally found your channel. Your movies are great, and the talking cat undercoat is cool.
I am surprised that there is such a small turnout of viewers. Best regards :-)

[agranero]

The wires spun around a terminal before soldering is an old technique used to lower resistance in soldered joints and avoid that this resistance gets higher with time. Besides the mechanical strength gets in the wires and terminals and not in the soldering joint. Besides yet makes it easier solder many many many connections. Atkelar, this equipment is an art piece. Very well put together. The Vernier type potentiometer is very cool. Imagine if it is used for impedance as it is a huge inductor too with all those winds and if so if this inductance is used too. I love bridges, they are incredible and have so many uses.

[wickerbaxsupreme]

What a neat project, and it turned out beautiful. Thank you!

[gerardzi]

This Brand it, s not New 1960's, but very clean. You are lucky !

[KeritechElectronics]

Lovely overview, though the Wien bridge sounds more like an oscillator than measurement circuit.
I love vintage gear restorations, not only tube devices. Keep up the great work!
You've gor a super cute voice too, subbing you right now. Boop! :)

[agranero]

Some bridges allow the external meter to be a high impedance phone. They said on those times that they were more sensible than needle meters to detect zero. I imagine you can do that with that connection. That is why they use 1Khz I imagine. where human audition is more sensitive. I don't know about cats though. The impedance of the phone does not affect measurement because when the bridge is balanced no current flows by the center. In fact in some experimental arrangements for demonstrations the center has a switch that when turned on and off should show no variation in the current in a meter in the source powering the bridge.

[agranero]

I could swear I saw a comment from @atkelar showing the external meter connected to an oscilloscope showing the 1kHz signal. But I saw on my phone and can't see where it is now.

[agranero]

"The math is complex, literally" Nice joke.