"orange" wrote in message
oups.com...
OK, I understand.
its a broken computer, Amiga 1200, it displays for a second blue screen
(which means one of custom chips is broken) and then black screen with
faint parallel stripes.


The faint stripes could indicate HF ripple from the PSU, maybe crashing one
of the chips. In this case overheating capacitors in the PSU might be a
better clue!

In the past when servicing PC monitors, I have used my sense of touch to
test for overheating electrolytics but BEWARE! the can of the capacitors is
not isolated and so the mains reservoir cap on the primary side will have a
large voltage on it! So make sure you can distinguish between primary &
secondary parts of the PSU before poking your fingers in!!!

In any event while inspecting/testing live equipment correct safety practice
is absolutely vital - wear rubber soled shoes, make sure you're not touching
anything earthed (chair next to a radiator?!!!) and keep one hand in your
pocket so if you do touch a live part - you don't give it a path to earth
via your chest!

Some engineers deal with this type of PSU by simply replacing all the
electrolytics and making sure all the new replacements are good quality 105
DEG-C types to ensure future reliability. IIRC - the Amiga PSU has a small
1uF 50V electrolytic that dries up and causes the PSU to radiate RF noise!