OK...I spent a few more hours working on the thing, checking the blanking signals that control the intensity / Z signal to the CRT, the third and last information that goes to the CRT, last hope of finding something wrong.
To cut the story short... I did not find anything wrong, all signals in this area looked as I would expect. None of them had blatant "blank" portions that would correspond to the 3 missing channels / traces on the screen.
So that made no sense. All the signals that go to the CRT should make it display my 4 channels, period.
So I must be missing something, just not competent enough for sure, hey I am not a professional after all, no shame...
So decided to give up on it, having run out of ideas as to why it would not work. Bought if for 50 euros not working at all... at least I improved on it quite a bit, now it powers up, runs, and works in analog and storage mode, albeit on channel #1 only... that's a lot better than the brick it was when I got it. So hopefully I can still sell it for at least what I paid for it !
Proceeded to reassemble it, but wanted to give it one last test anyway just to see, so reassembled it only as partially as possible.. did not even put the front panel assembly back onto the chassis, I just laid it on top of the scope... ribbon cable is just long enough to allow for that, and works just fine ! Hell I should have done that from the beginning, saves time !
Plugged the PSU connector to the main board.... BIG SPARK, BANG, FLASH !!!! Oh no... just because I resorted to give up on it doesn't mean I wanted to kill it for good ! Damn it ! Went to turn the power off.... Ah, power strip was ALREADY OFF hmmm.... maybe the spark came from some big cap on the PSU that got shorted... at any rate the chance of the scope surviving are slim. So turned the power back on... scope still alive and kicking, powers up and runs !!! That's one tough guy !!!
Then what do I see... some garbage on the screen, fuzzy and all.... let me adjust focus and intensity a bit... OK... WHAT ?!
I have FOUR traces in analog mode ?! They are BACK ?! Fed the calibrator signal to all 4 inputs at once, I get 4 square waves no problems ! Even better : played with the attenuators : NO MORE random big offsets problem ! Traces don't move around any more as I change range !
I proceed to switch to Digital / Storage mode, anxious.... IT WORKS !!! FOUR traces on the screen, four real traces this time, not " place holders " / test patterns like before !!! IT BLOODY JUST WORKS !!!!!!
Even I, can not believe it, so I took some pictures below to prove it.
Oh boy !!!
Can you believe it ?!
I spent countless hours probing around, studying schematics, pulling hair and scratching head.... but did not actually DO anything to it ... yet it WORKS !
So I guess all that time and effort put into it was not in vain after all... I was just rewarded for my perseverance, there is some justice out there in the end !
So no corrosion problem any more in fact, not a blown tantalum cap, no nothing...
Only explanation is some cold solder joint somewhere, which I must have triggered while working on the board.
That's bad because that means it can stop working randomly at any time. Might not even work after I am done putting it all back together properly, and even if it behaves for so long as to let me take pics of it so I can advertise it online... it might break again during transport to its new owner, and he will be mad at me : "You said it was working perfectly !!! "..... yada yada yada.....
Anyway, even it it lasts only for 5 minutes we shall see, I am none the less overjoyed to see it working, and my hard work rewarded !
At least if it fails again, I know there is nothing wrong so to speak, so no need to chase red herrings and waste time anymore... just a bad connection god knows where.
So I am super happy, I managed to fix what the previous owner failed at !
So to summarize the repair : was bought "No Power". Found a 5V rail that was way high at 5,40V. Fixed it, was a buggy remote sense feedback line. Then it would power up but not come to life/run. Found the CPU was not running. First found its 8MHz oscillator was kaput, replaced that, still not running. Found that the Reset line got broken in the corroded area due the leaky battery. Bridged that track. Scope now back to life !
...and wasted time by trying to be too thorough and fixing two broken traces that turned out to be factory mods hence did not need " fixing " !
Thank you Tautech and Shakalnokturn for your support, much appreciated, we won !
Now here is hoping the bloody thing will still work in 10 minutes once I have put it all back together again... cross all your fingers and those of your cats and dogs, I need as many fingers as you can come up with !....