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The HP 8556A LF Section is used with the HP 141T Display Section and the HP 8552B IF Section to make a Spectrum Analyser. It cost $2,450 in 1980. I have downloaded the Manual and HP Application Note 134 "Audio Frequency Measurements with the 8556A - 8552B Spectrum Analyzer". The 8556A will operate over a bandwidth of 20Hz to 300kHz which covers the audio band and includes its own tracking generator.

Front view
The Input has (unusually) a 1MΩ input impedance and the screen is not earthed to the chassis to avoid ground loops. This allows use of Oscilloscope probes for high impedance in-circuit testing. Feedthrough terminations are required for 50Ω or 600Ω use (11048B 50Ω & 11095A 600Ω). Maximum input is 10Vrms, ±200Vdc. The Input is switchable dBm 50Ω/dBV/dBm 600Ω.

The Tracking Generator Output is variable from 100mVrms to >3Vrms into an open-circuit. Output impedance is 600Ω, for 50Ω applications a Tracking Generator Shunt (11660A) is required. Frequency tracks the Analyser from 20Hz to 300kHz.

Rear view showing P2 connector
Rear Panel connections to the 141T. For details of P2 see HP 141T Internal Connections.

Top view with the screen covers removed
Bottom view with the screen covers removed
A special knob (08556-00013) was supplied to replace the Log Ref Level knob on the 8552B IF Section.

This added an extra legend in red for the LF Section which was marked 20dB below the RF Section. It also adds positions for .2 & 1 V/DIV.  
February 2009

Apart from a good cleanup and replacing the 1A fuse for the -12.6V supply, the 8556A worked first time. Setting up the First Mixer Balance and the Operating Adjustments is straightforward. Most of the Performance Tests completed.

October 2014

A guide to printing a label for the special knob is available from Bob at http://www.boatanchors1.com/knob.html