McMurdo Silver Model 904 Capacitance Resistance Bridge

Thanks to Alan Douglas I acquired a McMurdo Silver Model 904 Capacitance Resistance Bridge. 


A post on the Antique Radio Forum pointed me to Steve Rosenfeld (oceangate at comcast dot net) as a source for a manual. 

Silver Model 904 ad

He supplied nice copies of the manual, a magazine article written by McMurdo Silver himself describing the features of the circuit, and the advertisement above.

I'm very impressed by the quality of construction for a service-grade instrument.


Thumbscrews hold the front panel and chassis to the case, making it easy to remove for restoration. The main bridge potentiometer is a wirewound with a nice feel to it.


The parts are all mounted on the front panel. The power supply filtering and the bridge capacitances are mounted on a terminal board.


The range switch is a high-quality ceramic rotary switch.


The wiring has a pre-war look to it: cloth insulation and straight wire runs.


The carbon composition resistors also have that 1940's look: wide color bands, rough surface on the molded body. The precision 1% resistances are made from selected series pairs of ordinary 20% resistors.


The bridge capacitors need replacement.


Some were trimmed with selected parallel caps. Others seem to be simply selected components. The parts list calls for 2% tolerances on the bridge reference capacitors.


Here's a view from the left side.


Right side view. Some of the components are buried under the power transformer. Re-capping and re-resistoring is going to require dismantling it, and I hesitate to disturb the historical authenticity of the piece.


I replaced the capacitors and resistors and recalibrated the bridge. 


I had to disconnect a number of wires to free the terminal board. I took lots of photographs of the wiring layout before disturbing anything as an aid to getting it all back together correctly.


I was able to pop off the cover of the main bridge potentiometer and clean the wiper and the resistance element with a drop of DeOxit.


I bought a couple of General Tools precision oilers. I keep one filled with Mobil 1 synthetic motor oil and one filled with DeOxit. Since the DeOxit has a low viscosity, you have to be careful not to open the valve too far, lest you deposit too much DeOxit.


I rebuilt the terminal board with new electrolytics and resistors.


An Antique Radio Forum friend with a General Radio precision capacitance bridge hand-selected the replacement bridge reference capacitors from a batch that I sent to him. Many thanks!


I replaced the bridge reference resistors on the range switch with 1% wirewound precision resistors.


The magic eye tube socket got new resistors. I had to replace one of the cloth-covered wires where I broke off the end while removing it. I don't stock cloth covered wire in my shop, so I had to settle for vinyl insulated hookup wire.


The lever switch and pushbutton got the DeOxit treatment and a new set of components. I reused the original fabric spaghetti tubing on the leads of the replacment capacitor.


I also sparingly used DeOxit on the range switch contacts.


The brackets for the leather handle were pretty rusty.


I cleaned them up with a Dremel with a wire brush ...


... and sprayed them with Testors Gloss Cote lacquer to protect the finish.


I had to clean up the hardware for the brackets, too.


The 904 uses pin jacks so I made up a set of short test leads to connect to my capacitor test fixture.


The banana plugs also fit the barrel of Mueller BU-60 alligator clips.


Calibrating the bridge was a bit problematic. The old 0.01 µF reference cap was just a little bit high in value. The set-screw had marred the shaft of a the potentiometer, so when I set the dial correctly for the new reference cap and then tightened the set screw, the dial would shift a little to line up with the old calibration. I ended up locking the dial with a bit of glyptol rather than fully tightening the set screw.

The original 1.0 µF reference cap was quite high - almost 20% high. And, of course, the old paper caps were leaky.

© Steve Byan 2011-2019