I found a defective HP8566b spectrum analyzer for sale.

It was working but showed unlock an the screen and it was not showing any positive response on frequencies at input.

Special on this instrument for me personal was that I had been working with this particular instrument in 1981 when I was on internship at the Technical University in Eindhoven. During a period of half a year I worked almost daily with this instrument and I was very fond off it. So I could not let this opportunity go by and had to buy it and see if I could repair it.

I searched for quite some time before I was sure what the problem was.
During internet search I found several others being repaired by replacement of electrolyte capacitors and I started in checking the capacitors in the circuit of steering circuit of the YIG oscillator because I was almost convinced that the YIG oscillator and its steering circuit was causing the problem.

After a long search I tried to measure the YIG itself to see if it was working and found it had no output power at all.
I came the the conclusion that the YIG oscillator itself was broken and needed repair or replacement.
Repair of the YIG oscillator was out of reach for me so I asked Louis CT1DMK if he could repair the YIG and what he asked for that. At the same time I found on Ebay a YIG for sale which was suitable for this analyzer.
After concluding that repair at CT1DMK was more expensive that buying the unit on Ebay I ordered the one on Ebay

After arrival I mounted the replacement YIG into the analyzer and powered the analyzer up.

The YIG oscillator proved to be the problem, the analyzer was working fine

I was very happy with this for me special instrument.

A few years later I decided to open up the defective YIG and take a look inside.

Opening the YIG was not very easy but by cutting the exterior shielding I succeeded in getting to the YIG.

The YIG itself can be opened quite easily so there is almost no risk for damaging the YIG oscillator itself.

This is a picture of the YIG

When zooming more closely I found the problem.
The coil for fine adjustment is burned and most probably the reason why the YIG is no longer working