ESI 250 DE Universal Impedance Bridge

Repair and Cleanup of a ESI 250 DE Universal Impedance Bridge.

ESI 250DE-Universal Impedance Bridge front panel after repairs
ESI 250DE-Universal Impedance Bridge front pan­el after repairs (click to enlarge)

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.

ESI 250DE-Impedance Bridge after initial cleanup without meter
ESI 250DE-Impedance Bridge after ini­tial cleanup with­out meter

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.

Keystone_2162 Battery holder 4 D cells
Keystone_2162 Battery hold­er 4 D cells

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.

ESI250DE-Impedance Bridge back battery compartment
ESI 250DE-Impedance Bridge back bat­tery com­part­ment after cleanup

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.

ESI250DE-Impedance Bridge Assy ss shipped
ESI 250DE-Impedance Bridge Assembly as shipped

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.

ESI 250DE-Impedance Bridge board front component labels (click to enlarge)
ESI 250DE-Impedance Bridge board front com­po­nent labels (click to enlarge)
ESI 250DE-Impedance Bridge board side 1
ESI 250DE-Impedance Bridge board side 1
ESI 250DE-Impedance Bridge board side 2
ESI 250DE-Impedance Bridge board side 2
ESI 250DE-Impedance Bridge board bottom
ESI 250DE-Impedance Bridge board bot­tom

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.

ESI 250DE-Impedance Bridge LCR Dekapot before cleaning
ESI 250DE-Impedance Bridge LCR Dekapot before clean­ing
ESI 250DE-Impedance Bridge Range / Function select
ESI 250DE-Impedance Bridge, Range / Function select switch

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.

ESI 250DE-Impedance Bridge Front A&M meter model 265-874
ESI 250DE-Impedance Bridge Front A&M meter mod­el 265–874

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.

ESI 250DE-Impedance Bridge Isolation transformer and reference capacitor
ESI 250DE-Impedance Bridge Isolation trans­former and ref­er­ence capac­i­tor

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.

ESI 250DE-Impedance Bridge Inside cover instructions
ESI 250DE-Impedance Bridge Inside cov­er instruc­tions
ESI 250DE-Impedance Bridge DQ Potentiometer
ESI 250DE-Impedance Bridge DQ Potentiometer before clean­ing
ESI 250DE-Impedance Bridge Select & gain potentiometer
ESI 250DE-Impedance Bridge Select & gain poten­tiome­ter
ESI 250DE-Impedance Bridge Internals top side view
ESI 250DE-Impedance Bridge Internals top side view

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.

ESI 250DE Impedance Bridge Instruction man­u­al Dec 1974 sec­tions: Introduction, Operation

ESI 250DE Impedance Bridge Instruction man­u­al Dec 1974 sec­tions: Maintenance, Parts, Schematics, Appendix

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