4. Low level problem
As I mentioned, detected signals were exactly at the place in terms of frequency, but they were too low in amplitude.
Working on this machine without documentation is little bit hassle, so I had to sent some papers to printer. Omitting computer boards and merging A3s ... still few hundreds of pages.
Attenuator - Isn't it good practice to start here ? Every step appeared correct. I presumed ATT is OK.
Follow the service manual - there is a beautiful troubleshooting flow chart.
So, is signal from 3rd CONV 3.58 MHz going into IF -25dBm or more ? No, it isn't. (IF OK)
Is signal 226.42 MHz from 2nd CONV going into 3rd CONV -27dBm or more ? No, it isn't. (3rd CONV OK)
Here comes trouble. They tell me to look at the connection between 1st CONV and 2nd CONV, that's at 4 GHz. That's a lot. We have R&S FSV in work, however 3.6 GHz only.
This means to me I have to travel a lot, I know some places where they have more GHz's, but I don't want to shake this Jap in vehicle if I don't have to.
So, is it really necessary to visit places where they have more than 3.6 GHz ? Maybe there is some little problem which can be fixed quickly...
There are a lot of stories about bad mixer diodes and bad solder joints, one successful example here:
https://groups.yahoo.com/neo/groups/advantestinstrument/conversations/topics/16Let's uncover these fragile ceramics. Using magnifier, everything looks perfect. No signs of previous repair attempts.
Turn on 34401A, Ohms. Chassis touch and then track the resistance from input and further. Everythings normal until ... BINGO !
Soldier joint between BP filter ceramic and coupler ceramic has a few more ohms, I remember about 500 ohms. That's not supposed to be, so let my Weller warm up.
Gently touch the bridge until it melt and then let it be.
Quickly attach the cover, flip the machine and turn it on ... PERFECT, CAL signal exactly as expected. Calibration successful, testing procedures OK.
I am happy on this successful repair of the radio section. Have a look on pictures.