HP 8568B 1.5 GHZ SPECTRUM ANALYZER
HP 8568B Spectrum Analyzer
and HP 8444A Tracker Generator
In 1978, HP came up with an instrument that would represent a giant leap in spectrum analyzers, and with his big brother the HP 8566 introduced in 1980, would remain the gold standard for a long time. Heck, it is still a standard. Worth a whopping $48,000 at introduction, it was the most expensive instrument on the catalog. Read the story of its development here:
For amateurs, it was the unattainable instrument you could only dream of. Today, working ones still fetch a pretty penny, but those that don't can be had at a relative bargain, meaning 3 digits in the price only. If you can repair them, you'll own one of the finest HP pieces of RF equipment ever made.
HP 8568 YTO Unlocked Error Repair
Mine had the dreaded YTO unlocked error, meaning that its widely tunable internal oscillator could not be precisely locked to the 10 Mhz reference oscillator. When that happens, the whole frequency control becomes open loop and the instrument loses all of its precision. A million things can go wrong that can cause this error. In the end the repair was trivial, but it forced me into a deep dive into the heart of the analyzer, which was very rewarding.
This series of 7 videos, which was my first vintage electronics repair series and would start the popularity of my channel, tells the whole story.
HP 8444A Option 59 Tracking Generator
The addition of a tracking generator enables to take frequency response curves of passive devices. The HP 8444A tracking generator from the previous HP 141T series of spectrum analyzers was slightly modified to work with the HP 8568. The modification is Option 059. This adds an internal 500 MHz local oscillator, and also extends the tracking all the way to 1.5 GHz (instead of the former 1.3 GHz). But although the built in 500 MHz LO is nice, the HP 8444A Option 059, or the regular HP 8444A for that matter) can be quite improved by using an external synthesized oscillator such as an HP 8662 as the LO. This setup also allows to use a regular HP 8444A with the HP 8568. These two videos demonstrate the setup.
Garbled Video Repair and Battery Change
Unfortunately the two volume service manual for the HP 8568 RF section (publications 85680-90127 and 85680-90137) is not available on the web, but you can purchase it from Artek Media, which did a good job at scanning it. The RF section for the HP 8566 is available, included below, but it's quite different from the HP 8568. Most of the other documentation is available, including the IF section service manual, I have assembled it below.
My HP 8568 is a "B" model, so it has the newer Motorola 68000 processor board, instead of the earlier HP Hybrid Processor (the same that was used in the HP 9825 Calculator). I have dumped the ROMs from my processor board, which is a 85680-60180 revision.