Hello Howard and all,
As Jon mentioned it can very well be that the mixer diodes have been destroyed.
Below are some level problems I encountered in some of these analyzers.
Please check Q350 - 2N3904 (SMD-marking 1A) in the VR Module. I had a broken one. This transistor gets too hot under normal circumstances because of the combination of UCE en IC. The UCE is about 11,5V and the IC is between 20 and 25mA, in my case 23mA. The dissipation then is between 250 - 300mW and that is a bit too high for a SOT-23. In my case I put in a new one and I cooled it via a heat pad and a small 1”x 1” aluminum heat-sink of the right (I forgot) thickness against the inside of the lid of the VR Module.
Another thing can be a too low level LO signal coming from J380 from the 1st LO Buffer Amp. The level should be about +13dBm, but in one of my analyzers this was way lower. Please measure that if you can. The problem in my case turned out to be caused by two leaking SMD electrolytic capacitors inside this hard to reach unit. Residue from the leaking (a bit of black sticky fluid) was hiding under a SMD capacitor C292. Under this resistor there was a high impedance track going to the input of the opamp that stabilizes the LO output level a bit to a certain level. In the end I removed all the following components that I had my doubts on in that unit: SMD components C292, C298, C828, R280, R166, R168, R266, C267, C160 and R264. I cleaned everything and put them back. The problematic SMD electrolytics C180 10μF/50V and C392 10μF/50V were replaced by new ones of course.
Good luck and please keep us updated.
Jaap
