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.


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.


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.


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.


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.


Day 10

A fully function unit

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Misc photos

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8 thoughts on “Restoring ESI 250DA Impedance Bridge”

  1. Hi. A very nice job and a very interesting piece of equipment! Isn’t seeing a green (or blue) glow fun? As I say watching vintage oscilloscopes is way more interesting that TV (I don’t have TV since leaving England). Again well done and thanks for following my ramblings.
    Richard Sears

    1. Hi Richard,

      I agree with you wholeheartedly, with these old oscilloscopes and various other old test instruments they are much more fun to play with and look at. I also appreciate their elegant design using these old technologies.

      I also enjoy reading your blog and the way you document your process.


  2. Kevin – We are a museum in Washington County Oregon that is just about to open an exhibit on the early days of ESI. Would you be willing to let us use a photo of the front plate of the ESI 250DA? It needs to be suitable to blow up to about six feet, and so we need something shot straight on with a relatively good digital camera. Could/would you help us?

  3. Hi, I am also restoring one of these however it’s a ZM-30/U. It’s very similar to your unit, also made by ESI but for the military I believe. I was wondering if you could measure the DC voltage across the two mounting screws of the meter? Mine shows now more than 2V no matter the settings which I think is not enough to move the needle, but I’m not sure what the voltage requirements are for the galvo. I placed 9Vdc across the screws and it wiggled but didn’t move… inspected it and it’s not rusted. Thanks!

    1. Hi Tom,

      I no longer have the unit.

      There is a slider metal button in the front that lock the needle for transportation. When locked, the needle won’t move. Try unlock it first by sliding it up or down to see it it can be unlocked.

      The galvo is very sensitive and it only require a few mA to move it. You might want to use a resistor to limit the current before you destroy it with 9V battery. Try with 1mA then 100mA.

      Measure the resistance of the coil and see if it is open.

      The galvo is used as a DC null detector. Make sure the device under test is inserted so the bridge can be balanced.

      Since it is measuring DC. A balanced bridge should have near 0 volt or 0 mA.

      If you can’t achieve null reading and getting a constant 2V, then there must be short/open some where in the front section.

  4. I just bought an ESI 250DA. I opened it up and it’s clean as new inside 🙂 The chassis is dated Apr 1960 and the SN is 12070. I haven’t tried to power it up yet so I don’t know if it works but the seller claimed that it does. But all of the caps inside are original so I’m sure that they’re due to be replaced. Can you give me a list of the values of the caps used in the 250DA and what brand/type caps you replaced your’s with?

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