HP 85645A Tracking Source 300 kHz - 26.5 GHz

When the HP 8566 Spectrum Analyzer was introduced, there was no official tracker that could match its prodigious 22 GHz bandwidth. But a much later (1995) instrument, the HP 85645A, provides this capability, and much more, being capable to do offset tracking and power sweeps up to 26GHz with a wide variety of HP Analyzers. Because the 85645A is still quite expensive even today, I originally cobbled together my own tracker, but only on the low band up to 2.5 GHz. But I eventually bought a pseudo affordable 85645A on eBay - which of course wasn't working correctly. It is still by far one of the most expensive instruments in my collection. Thanks goodness the fault was not in the RF or control electronics, but in the power supply, which is still unbelievably complicated. Here it stands proudly above the 8566 after its heroic supply repair.

I was able to mostly reverse engineer the schematics of this complicated power supply, and with the help of the description in the service manual, I was able to figure out how it operated. You'll find the operating manual, the service manual, my notes with the recovered schematics, an Illustrator file with the tracing of the PCB and the datasheets of most of the active components below.

In the end, the supply had 1 blown fuse, 3 blown MOSFETS, a short-circuited MOSFET driver IC, a bad op-amp and a bad comparator (OK the comparator is me during the electroboom). It's not 100% clear what started that chain of failures, but I suspect the op-amp and/or the driver IC. And contrary to internet lore, all the caps were good.