ICOM-R7100

[Radiosonde]

Hello
I received (yeah...) a IC-R7100 receiver a few months ago from a ham radio friend for repair.
It was possible to turn the receiver on and hear some noise but otherwise it was totaly deaf, so I started inserting a signal and tracing it back, I stopped at the 2. Mixer Unit which seemed to be broken. I also inserted signals in the 10.7 Mhz and the 455khz IF, everything ok, no problems whatsoever.
My plan was to build a new mixer module around a AD831 and hack it into the receiver, then re adjust it.
Repair isnt possible because the MIX2 IC is the only ASIC in the whole receiver, it is such a PCB-Chip weird thingie which I cant replace or repair.
 
Everything looked nice and I started to build the mixer PCB, I "simulated" the same conditions as in the receiver on my bench with signal generators to be sure it will work, it will, even if I am out of the specified 500 Mhz the mixer works fine and outputs a usable IF.

Sadly now nothing works, the receiver greeted me with various problems when I opened it today to check some values:

1. It doesnt do noise anymore, only a very silent hum in the speaker
2. The demodulator stages dont work, I havent done something with them in the last two months
3. The 1. local oscillator doesnt "track" anymore, it should follow the typed in frequency in a specific way to keep the IF on a constant value, nothing, it is about 100 Mhz of and only chages value slightly, I tried it with the adjustement procedure but here I only can tweek in khz ranges at best.

What could be the reason for getting worse and worse?

I am very frustrated now, it is like one step forward and five back?
Could this be affected by bad electrolytics?
The unit was in storage for 10 years as defective.

Like I said it is annoying because now the design of the new mixer and invested time is wasted, I am sceptic that I can an want to fix all these problems that were not here in the beginning.


Thank you for support