HP 4260A Universal Bridge

© Brooke Clarke, N6GCE

HP 4260A Front
HP 4260A Rear


My first HP 4260A Universal Bridge has a plate on the top cover that reads:
Bridge, Capactance,-Inductance-Resistance, ZM71A/U, Serial No. 12A, Hewlett-Packard co., PIIN:  DAAB07-82-C-H036, Warranty Thru 02/84, U.S.  So this one was built on a government contract.
This is a manually nulled LCR bridge operating form an internal 1 kHz (or external 20 Hz to 20 kHz) oscillator.  It's packaged in one of the HP "sub modular" sized cases and can be rack mounted.

From the serial numbers it appears this instrument ws made from 1970 through at least 1975.  Maybe this was replaced by the HP 4332 LCR Meter, or maybe one of the digital auto balancing LCR meters in the 42xx model series..

C range 1 pF to 1,000 uF in 7 ranges with D from 0.001 to 50
L range 1 uH to 1,000 H in 7 ranges with Q from 0.02 to 1,000
R range 10 milli Ohms to 10 Meg Ohms in 7 ranges
Accuracy 1% in the center ranges and 2% at the extremes.

What Goes Wrong

This instrument has seen some very rough times.  Looks like it was subjected to an extremely high shock.  

The shaft on the "DQ" control that's attached to the CAL pot was broken.   I'm now trying to find a replacement "DQ" pot or maybe just make a replacement shaft.  This can wait to see if the bridge is operational.

One of the 2 wires going to  the main "CRL" pot with the odometer counter attached was disconnected, I think by someone trouble shooting it since it tests open, and not the 5 k Ohms it should be.  Inside the rod that rubs against the resistance wire was disconnected form the wiper and a very fine wire that connected to the end of the resistance wire no longer makes the connection.  May need acid flux or silver epoxy to get a good joint.  The plastic that supports the resistance wire had numerous radial cracks all the way though, these were fixed using acrylic cement.

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