Arrrrggggghhhh!!!! Was cleaning the M9994 when some excessive amount of ethylalcohol (spiritus) hit the tuner dail window...  It immediately turned white and at the same time a crack came...

Got the white stain of by using metal polish... but that crack really bothers me... Is this normal???

I would say the stress crack would be normal if there was a hairline fracture to start with. The plastic is decades old so will be brittle. It makes sense the crack started where it did (existing pre cut screw hole). It's possible the alcohol weakened it but I would think there would be damage to start with, however small. I use soft anti bacterial cloths from the supermarket on my new found Teac system without issues (no alcohol) for cleaning.

You must be right. There isn't an other explanation... This was the last time I used spiritus to clean! Luckily it's not a complicated window, so fairly easy to remake. But I hate to see it brake down after sitting there since 1979...

Nice teac set btw! It looks like it has an all metal front. I like that... Be carefull with it!