Why does my HP 8568A spectrum analyzer have such a high noise floor?

[gabor.dani]

I bought a HP 8568A + HP 85662A for 50 bucks. I was able to get it to a working condition but it has very high noise floor. Where should I start looking?

 - blown caps replaced
 - EMS filter lit on fire in about 2 minutes of use, replaced
 - missing interconnect cables, created one, but I do not want to see soldering iron for the next 2 weeks.
 - blown rectifier diodes replaced
 - input limiter works fine, removing it does not change anything.
 - amplitude manual cal done
 - frequency manual cal done
 - error correction auto cal fails with "ADJUST AMPTD CAL" message. It is trying to see the top of the cal signal in very low res bw, and there is a ripple on it.

[Jay_Diddy_B]

Hi,
Here are pictures from my 8566A under the same conditions:




and




I would start by looking for ripple on the power supply rails. Then I would look for high ESR capacitors in the YIG control circuit.

Regards,
Jay_Diddy_B