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REPAIR HP 8555A RF SECTION
November 2014

The HP 8555A RF Section, although not used a lot, developed a fault which stopped the vertical display on the Spectrum Analyser. Substituting a 8553B and everything was ok, so time to fault find. This is quite a complicated assembly but helpfully the HP Operating and Service Manual is very good.

Front view
The Manual 'trouble-shooting tree' identified a power supply fault which turned out to be a short circuit across the +20V supply. Disconnecting all the Assemblies in turn identified A14A2 PCB which is a bit tricky to remove.

A14A2 Assembly
One of the Sprague tantalum electrolytic capacitors had failed, not surprising after 36 years. It was only rated at 20V for a 20V supply. Replaced with a miniature electrolytic rated at 35V. Failure of these type of capacitors is a common problem but still very time consuming to repair.

Underneath location of A14A2 Assembly
Checking voltage levels
Test Points are provided for the +20V & +10V supplies, these are both regulated from a +26V supply which in turn comes from a +100V switching power supply.

Initial performance tests have been done. Setting up the vertical position, vertical gain, and amplitude calibration, in log and linear modes is a bit tricky and takes some patience to get right.