Not one of the most preferred things to repair – a rather rare 26.5-40 GHz sweeper plug-in, not producing any output. Despite its rather simple function as a signal source, typically, not easy to fix if any of the microwave parts are faulty. New, close to 18 kUSD, so it is nothing you can easily replace from a hobby budget, and 26.5-40 GHz sources are not easy to come by.

Inside, all full of heatsinks, and a few waveguides.

The modulator.

The YTO. Not many companies around that can manufacture such devices.

First thing to test, with no output present – the YIG oscillator. This has two main items: the bias supply, which is more or less just a variable voltage power supply which is tuned along with the frequency sweep. Secondly, the main tuning coil current, providing the magnetic field for the YTO. Checked both – and found the bias supply at 0 Volts. No wonder there is no output.

Upon inspection of the schematic, I noticed the fuse, which is rather hidden down on the motherboard. And, it was blown. No idea why – maybe just because of its age? Sure enough, HP did not use just any ordinary fuse, but a BUSS GMW model.

USD 9 per piece – that’s a steep price for a fuse.

Cut the fuse open, and connected a 5×20 mm European style fuse. All protected by a piece of shrink tubing.

Well, an about 1 hour later, the YTO is oscillating again, and you can see a nice and strong signal, well over 90 dB useful range, to test attenuators, or whatever 26.5-40 GHz device you want to put to test.