Issue with Macintosh Classic II: vibrating vertical lines / unable to boot

[Spiros]

Dear all,

I have a Macintosh Classic II that -suddenly- died by being unable to boot; vertical (vibrating) stripes appear when powered-on.
I recapped:
- all surface capacitors of the logic board
- all capacitors of the analogue board
(+ cleaned both boards: logic & analogue)

Still no signs of life!

It could be a malfunctioning logic board (or rom chip?) or an analogue board that -still- provides weak voltages.

Regarding the power output of the analogue board, is there a certain mapping / diagram regarding the voltages I should be reading in each of the pins in the highlighted green sockets?
The purple socket (for the drive) has an output of 4.6V & 11.4V that seems OK (correct?).



Thank you in advance for your time & effort.

Spiros

 

[cheesestraws]

The easiest way to check the PSU voltages is at the floppy port: the 12 and 5V lines are broken out there and you can check what they look like while the whole machine is on.

A vibrating screen suggests an analogue board issue to me. If you can look for ripple on the power rails, it's worth looking to see whether that lines up with the vibration or not

 

[Spiros]

The easiest way to check the PSU voltages is at the floppy port: the 12 and 5V lines are broken out there and you can check what they look like while the whole machine is on.

A vibrating screen suggests an analogue board issue to me. If you can look for ripple on the power rails, it's worth looking to see whether that lines up with the vibration or not

Many thanks for your kind input!
Floppy voltages seems to be OK when all in place (4.7V & 11.3-4V in particular).
Will check again for any ripple in power rails (being also aligned with the vibrating tone); however, I don't recall these kind of readings. If there is a specific output (V) that is expected on the power rail of the monitor, it would be good to know (min/max V for each pin). I sense that the board is providing less output than the minimum threshold ensuring a proper boot sequence.

 

[imactheknife]

think those voltages are low? mine are 4.98-5, and 11.68 or higher. removed a lot of ic chips from logic board and cleaned then replaced. Usually fixed every issue, especially sound

 

[Plasticoat]

 

[Plasticoat]

Vibrating screen , reflow any solder on analogue that’s looks bad,

 

[bibilit]

5 volt line is way too low.

4,85 is fine for a Classic.

 

[dochilli]

5V should be 4.85 or higher. Then the classic will boot. You could turn pp1 on the analog board, but you have to look at the 12 V too.

 

[cheesestraws]

Yes, those voltages definitely seem too low. The computer won't start running code until it has stable power lines, so that's why you're seeing the lines on the screen.

 

[bibilit]

Low voltages are frequently related with the Optoisolator (QP1 in the Analog board CNY17G)

 

[chiptripper]

5V should be 4.85 or higher. Then the classic will boot. You could turn pp1 on the analog board, but you have to look at the 12 V too.

Yes PP1 would be where I'd start too. I often have to remove it and clean in IPA, then adjust. Sometimes the whole part needs replacement.

 

[Spiros]

Yes PP1 would be where I'd start too. I often have to remove it and clean in IPA, then adjust. Sometimes the whole part needs replacement.

Many thanks for your kind advice; will do so & revert!

 

[Spiros]

Yes PP1 would be where I'd start too. I often have to remove it and clean in IPA, then adjust. Sometimes the whole part needs replacement.

 

[Spiros]

Yes, those voltages definitely seem too low. The computer won't start running code until it has stable power lines, so that's why you're seeing the lines on the screen.

Many thanks!
Will try to revive all voltage readings & see what happens.

 

[Spiros]

Yes PP1 would be where I'd start too. I often have to remove it and clean in IPA, then adjust. Sometimes the whole part needs replacement.

5V should be 4.85 or higher. Then the classic will boot. You could turn pp1 on the analog board, but you have to look at the 12 V too.

Much appreciated

 

[Spiros]

think those voltages are low? mine are 4.98-5, and 11.68 or higher. removed a lot of ic chips from logic board and cleaned then replaced. Usually fixed every issue, especially sound

I see!
Makes sense...
Many thanks

 

[Spiros]

Made it, thanks to you guys!
PP1 was cleaned & adjusted...
(Sound was also restored after the replacement of all capacitors)

So grateful

 

[joshc]

Woohoo!