HP 8568B Spectrum Analyzer: an amazing find, a few repairs, and a restored marvel of RF engineering

It appears that the US is a land of plenty when it comes to somewhat dated test equipment, otherwise, it would be hard to explain why someone would sell a 8568B analyzer, including a display unit, for just a few dollars. A great find!
In the as-received state, after removing the 8568B and the 85662A display unit from two huge boxes, it was starting up, but did not show any signal, and no annotations on the screen. The latter turned out to be a rather easy fix, a little defect in the intensity control circuit.

First step – adjusted all the CRT circuits, focus/intensity control circuits, and the analog/digital display scaling and stroke generator.
The CRT is of a quite amazing quality, not sure if it is the original CRT – it has a hand-written label sticking to it, which could indicate that it has been replaced at one point in time.

The major item, no signal (but a typical background noise trace) – this can be anything, but unless in cases of several neglect, it is hard to destroy the mixer or other hard to fix ciruits of the 8568B (typically, the attenuator, and the build-in limiter are absorbing any overload power).

Switching the input attenuator, some signal found at -70 dB attenuation! Strange, so there is something wrong with the attenuator.

Similar to the CRT, also the attenuator seems to have been replaced before:
8568b atten 5086-7815 70 db 24 v 4 ghz

8568b atten assy

Opening it up, with the necessary care, what a strange thing – the contacts are not making any contract… the screws indicate that someone has tried to fix it before, or maybe damaged these contact fingers, while trying to fix it.
8568b atten defective 2

This explains why only at the highest attenuation setting, there is a signal: the contacts work when pushed agains the 10-20-40 dB attenuator pads, but they don’t make contact with the pad bypass (“0 dB”).
8568b atten defective

Using some fine-tip tools, re-adjusted the contacts so that they close the by-pass of the attenuators.
8568b atten adjusted

Before re-assembly, make sure that there are no dust particles, and that the mating surfaces are perfectly clean. Best use a small, soft brush.
There is no need to over-tighten the screws. This attenator is the 4 GHz version, and not particularly critical. For the 22 GHz version, of same design, best check for SWR and insertion loss, and carefully tighten all screw with just enough torque to hold the assembly together.

Some checks, some adjustments – and the instrument passed the self-calibration with no issues. The coeffcients are not zero, but close enough, and cross check with a well-calibrated 8642B shows that the amplitude accuracy is perfectly fine, no issues with flatness, any of the attenuator settings, or when switching through the various bandwidths.
8568b corr coeff

Some of the other parts, the 1st LO – a YIG oscillator.
8568b yig

The reference, and OSC 49-61C, unfortunately, I can’t find any spec data for it, but appears to be a rather low phase noise oscillator, with more than adequate stability.
8568b ovenaire osc 49-61C

As a further note, should you be in the market for a 8568B or 8566B analyzer, make sure that it comes with the 85662-60093, 85662-60094 bus and interface cables.
8568b 85662-60094 85662-60093 cables
These cables don’t look like anything special, but are commonly sold for over USD 100 a piece, even in used condition. Often, the cables are lost when the instruments are put in storage, and auctioned later. Fortunatly, the current unit came with all the cables, even with a set of power cables!

A short glance on the main board, it is a marvel of engineering and a pleasure for the eye, all traces layed out by hand, fully gold plated, amazing quality and attention to detail. Might last another 100 years of 24/7 use.
8568b main board

One thought on “HP 8568B Spectrum Analyzer: an amazing find, a few repairs, and a restored marvel of RF engineering”

  1. Well,I am a lucky owner of this masterpiece……have already retired and its the only item I refuse to ‘separate’of it….during my career,had the chance to work and cooperate with various labs arround the globe,still have not met any other INSTRUMENT built like this and lasting/performing like this!!!! HP has published in one of its journals how this magnificent instrument was built…took them 4 years to develope !!!if you find one ,just grab it…
    Those days,had paid a fortune to purchase….

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