Repair and Cleanup of a ESI 250 DE Universal Impedance Bridge.
The Electro Scientific Industries Inc. (ESI) model 250DE Universal Impedance Bridge is an instrument that measures resistance, capacitance, and inductance, and the dissipation factor (D), and the storage factor (Q) of inductors and capacitors.
The unit that I repaired was built in 1971 in Portland, Oregon, USA.
The unit was purchased on Ebay with “Buy it Now” and listed as “For parts / not working missing meter”. The seller had taken good quality pictures from all angles, and included photos of the internal circuitry. The unit operates on 4 D‑cell 1.5 volt batteries, and the battery holder was severely corroded. Luckily that was the only significant damage found. The unit came with a plastic front cover, and a set of test leads with copper alligator clips on one end and gold plated spade lugs on the other end.
After drilling out the existing pop-rivets, I was able to remove the crusty old battery holder and clean up the battery compartment. I was able to get a battery holder from Digikey that matched up with the existing mounting holes in the unit, and used my pop-rivet tool to install the new holder.
The battery pack plugs into the main electronics assembly with a three pin plug with just enough slack to carefully remove the front panel and main assembly. A single screw on the back panel and an inner clip behind the top of the front panel are the only two mechanisms that secure the front panel.
The main circuit board is a single sided board with thru-hole components lined up in neat rows. The was no silkscreen on the board with component placement labeling, so I created an annotated image.
After cleaning up and testing the circuit board, I turned my attention the LCR DEKASTAT and the Function / Range selector switches. I used DeoxIT D5 on a lint-free swab to individually clean all rotating contacts, and then followed up with a light coating of DeoxIT Gold. All resistances were well within tolerances.
I was not able to find an exact replacement for the front panel Meter. What I came up with was a 2.5” A&M panel meter model 265–874 with a 50 uA Zero center movement. The mounting screws matched up exactly with the existing mounting holes with no modifications needed. I am currently using the 20–0‑20 dial that came with the meter, but may customize it for a closer match to the original if I get ambitious.
Inside the metal gold colored box is a 0.1 uF precision reference capacitor with a fixed and variable trimming capacitor attached to its terminals. I currently do not have any test equipment accurate enough to verify or adjust it.
Next to the trimmer capacitor is the three pin socket for the case mounted battery pack.
Inside the cover are the basic instruction for measuring basic resistance, capacitance, and inductance measurements. Very handy to have instead of pulling out a printed copy of the manual or looking up a PDF.
I was very impressed with the quality and level of detail in the design of this instrument. Although designed in the 1960’s it is still useful today.
Hopefully I will have the 1974 version of the manual soon.