Turning on STacy only 2 times out of 10 does it start.

[exxos]

There is no voltage in that area, not even after L106 and not even on the 3 electrolytic capacitors that are near Q108 (in the capacitors I obviously set the multimeter to direct current).

Do you think it's that Q108's fault?

If there is no voltage at L106 then you need to see what is to the left of it voltage and part wise. Q108 will do nothing without power.

[Roberto]

If there is no voltage at L106 then you need to see what is to the left of it voltage and part wise. Q108 will do nothing without power.

I ran voltage tests exactly at the points indicated by the red arrows, and took note of the results and wrote them into the diagram.
The DC/DC module has 16V input but it seems that after Q107 and Q106 (unfortunately they are read badly in the diagram) there is no more voltage, consequently there is no voltage even on L106 and Q108. Is it possible that one of the two components (or both Q107 and Q106) is giving problems?

Schema DC_DC ATARI STacy_tensioni.jpg

[stephen_usher]

It looks like Q107 is controlled by the output of Q106, so that would be the first transistor to look at.

[exxos]

I'm not very familiar with what that circuit is really doing. Either the transistor pair is blown know what is driving them is malfunctioning for some reason.

My only suggestion, which you do so entirely at your own risk, Will be to lift the end of the resistors highlighted below and tie to 0v to turn the transistor on. Then see if you get voltage on the right side of Q107.

But be ready to turn it off immediately if something starts smoking all goes bang. If the transistors faulty then I would assume it would burn the resistor.

Capture.PNG

[stephen_usher]

Isn’t it trying to be an oscillating circuit, to produce the high frequency transferred through the transformer to produce the 85V needed for the backlight? I guess that if all the capacitors aren’t close to their specification the oscillation feedback won’t happen and the circuit won’t run. It’s interesting that there’s an “EL OFF” line which seems to be possibly clamping one line either low of high, possibly preventing the feedback from happening.

It’s probably time to get an oscilloscope out as it looks to be analogue magic.

[Roberto]

@stephen_usher @exxos
I'm ashamed to write it but I have to do it... That backlight circuit is controlled by the potentiometer that is in the middle of the adjustment card. Well, I thought that there was current there from the moment the computer was switched on, instead I discover that it is controlled by that potentiometer which clicks from the moment the backlight is switched on and by an increasingly higher current (up to 85 volts) towards the end of the run. Such a simple thing and I hadn't noticed it :-( Then I discovered that noise which, as I increased the brightness, a very sharp sound was heard louder and louder: it's simply the fact that, heating the screen with the hairdryer to remove that line, I blew the (already very precarious) contact of the panel.
I put a resistor in place of the panel and the sound is gone.

By the way, @Darklord I didn't listen to you: that is, I decided, given the failures to remove that line with the simple pressure, to disassemble it! Never had I done it....obviously now I find myself, not with one line less, but a myriad of lines missing. But if someone has mounted it, I say, it's not impossible. The point is how! I hope next year starts differently than it ended! :-D

[Darklord]

:cry: . . .

[derkom]

By the way, @Darklord I didn't listen to you: that is, I decided, given the failures to remove that line with the simple pressure, to disassemble it! Never had I done it....obviously now I find myself, not with one line less, but a myriad of lines missing. But if someone has mounted it, I say, it's not impossible. The point is how! I hope next year starts differently than it ended! :-D

Do you want the screen that came out of the Stacy I upgraded to a colour screen? It's got a weak backlight of course, but is otherwise fully functional. (Or at least it was when I pulled it out.) If you want it, it's yours, no charge. :merryxmas:

[Roberto]

By the way, @Darklord I didn't listen to you: that is, I decided, given the failures to remove that line with the simple pressure, to disassemble it! Never had I done it....obviously now I find myself, not with one line less, but a myriad of lines missing. But if someone has mounted it, I say, it's not impossible. The point is how! I hope next year starts differently than it ended! :-D

Do you want the screen that came out of the Stacy I upgraded to a colour screen? It's got a weak backlight of course, but is otherwise fully functional. (Or at least it was when I pulled it out.) If you want it, it's yours, no charge. :merryxmas:

Of course I do! But this time I have to repay my debt somehow. I am writing to you privately.

[Darklord]

@Derkom :bravo: