Restoring ESI 250DA Impedance Bridge


Recently, my wife and I started attending the swap meet at TRW parking lot in El Segundo California regularly on every last Saturday of the month. This is one of the things that we enjoy doing in the morning before we heading to Hong Long restaurant.

On this particular morning we spotted an ESI-250DA impedance bridge made in the 60’s from one of the vendor. I decided to make inquiry of the price and he was asking $20 for it, but the item has rust from water damage and missing the null indicator tube.  I was not willing to pay so much for it, so I made a final offer of $10 for the thing in part trying to give him some business. He agreed and now I am a proud owner of 1960 old relic.

Normally I don’t buy stuff in such a sad condition. However I hate seeing useful instrument going to the land fill and also this will keep my mind busy for awhile.

Day 1

I brought the unit home and open it up. The top part of the interior are okay, luckily no rust there. The bottom section however has large amount rust deposited at the bottom. Luckily the key components like standard capacitor, resistors and switches are in decent condition.

The first priority is to remove the rust on the transformer and the chassis. I didn’t bother to remove the rust on the case until the very end.

Day 2

Cleaned all the contacts and switches with contact cleaner and oiled the switches.

Day 3

Inspected all the capacitors and they are all bad and need to be replaced.

I went a head and replaced with all electrolytic capacitors with near equivalent, since exact replacement is impossible to find. As long as they are still within the original specified tolerance it should be okay.

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Day 4

There were a huge amount of white stuff deposited on the Selenium Diodes. So, I decided to replace them with Silicon diodes instead since, they are so old will fail anytime now.

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Turned the unit on and the 150 Ohm resistor in the power supply section started smoking. I quickly turned it off but the damage was done.

Day 5

Replaced the 150 Ohms with 390 Ohm 5W and replaced 2.2K with 2.2K 5W resistor as shown below. When replacing selenium diode with a silicon diode, it is commonly recommended to use 300 Ohms or more in series with silicon diode to limit the surge current.

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Turn the unit on and everything seem to be okay and the tubes filament start glowing amber. 🙂

Attempted to measure some resistors, however the needle would not budge.

Day 6

Dissembled the galvanometer to take a look inside. The meter resistant tested fine, however the needle was frozen solid.

Wasted 1/2 hours tapping the tube to loosen the rusted innards. Finally got the pieces separated, the interior were badly rusted and huge amount of  metallic dust stuck on the magnet.

I carefully disassembled the magnet and gently clean up the coil and removed as much rust as I can. As seen below some rust still remain, I don’t want to clean it up too much since it’s very delicate and held together in place by a thin wires.

Finally sprayed some WD-40 on the part to remove any remaining rust.

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Here is the galvanometer cleaned and reassembled.

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I reinstall the galvanometer and did some resistance measurement and the null meter seemed to work now.

Day 7

Did another series of checks to confirm the standard resistors and capacitors. All checked out okay and very close to specified values.

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Once the above check was completed, I wanted to confirm the accuracy of Rx1 and Rx10 measurement. Did another series of resistance measurement, all are very close to what I am getting from a digital multimeter.

At this stage I have to stop working on it, since I can’t test inductor or capacitor with it because the 6U5 null indicator tube was missing. I found some used replacement tube but they are quite expensive on eBay.

More treasure hunting…

Day 8

Must be my lucky day, I found a 6E5 tube and another magic eye tube with no marking on it. The 6E5 tube that I got, it has bulging shape so it won’t fit into ESI-250DA.

After some Googling the unmarked tube could be a 6U5 or 6E5, which will fit it is a straight tube nicely. Since both tubes specs are quite similar, I guess it won’t hurt just to try it out. After some cleaning, I inserted the tube and turn on the unit. The magic eye start producing a nice green glow. Kinda exciting watching it actually.

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Did another series of capacitance and inductance measurement and every thing seem near the ball park.

Day 9

Spend the morning adjusting the rheostats on D-Q dials to what specified in the service manual.

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Day 10

A fully function unit

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Misc photos

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