Yesterday I was digging deeper into the circuitry..
Found out that there is no power supply to the lens connector (it should be there when lens is attached (pin 3, red wire).
N-MOSFET transistor provides this voltage, it is controlled via LV8015 from processor board by signal VB2ON.
And that signal is not there when shutter button is half pressed, and consequently there is no power supply (4.75V) to the lens and some other circuitry, and this the reason for error 99.
Now I have to check ribbon cable, associated connectors and source of VB2ON signal (it is coming from pin14, connector on main board closest to shutter).
Of course, the real reason for N-MOSFET not being ON could well be outside the processor board and could be because some other signals are not as expected by firmware.
That wandering pin from CF connector must have done a lot of damage...
BTW,
ElectroTanya has much higher resolution copy of service manual (35MB).
Stay tuned..