Color classic power supply issue

[Bwana_fr]

Hello to everyone,

This is my first post on 68kmla

I recently acquired a Classic Color and I have some issues with the power supply.

I recapped the analog board, and changed opto-couplers. After recapping I've 4.85V on the ADB connector.

When I try to power using the keyboard, I hear a click and nothing more happens.

When I remove the logic board, I can hear an "electrostatic" noise from the CRT but the PSU stops just after.

Does the analog board starts every times without logic board?

Is there some known components causing this kind of failure to be replaced?

Many thanks.

 

[bibilit]

Is your Logic Board OK ? here more an issue with the LB than AB.

4.85 is not bad at all, the Analog Board won't power on without the Logic Board in, or not supposed to do so.

The noise is probably the degaussing process.

 

[Bwana_fr]

Hello bibilit, Merci pour la réponse (I'm french too).

I made more investigations on the logic board and I spotted some potential issues and I have several questions:

Can we trust BOMARC schematics for the logic board?

If yes, using the schematics, I found some shortcuts with power supply lines.

In the attached picture, I highlighted three locations:

- Yellow : this is the +12V comming from the PSU

- Red : Should be the +5V standby (+5VSB) (provided when computer is off)

- Pink : Unknown. I expected to be tied to the +5VSB

I tested with a multimeter and I discovered some weird points (if BOMARC schematics are accurate).

The +12V is connected to the +5VSB? It may explain why the PSU is not able to start.

The pink pads (should be +5VSB) is connected to nothing else (except components near this area, like UC7 and UC8, R24...).

Is my feeling ok?

Many thanks!

 

[Bwana_fr]

Hello,

I reply to myself.

Bomarc schematics are not accurate. The +5VSB shown in the picture is the +12V PSU for the floppy drive.

So this is normal to have them connected together.

After a long investigation on the logic board, the problem doesn't seems to come from this part.

More to come....

 

[bibilit]

I see new capacitors on this board, but two are missing (one close to the main connector, and the other one near the battery holder)

The portion on the board responsible for kicking the power on process is the one near the battery holder...

The U7 chip is called the Egret, and from your picture goo contaminated, the close U8 chip is also important in the process.

Both chips are known for being troublesome at some point.

 

[Bwana_fr]

Missing capacitors is normal, I remove them to check some PCB tracks.
I also desolder U7 & U8, to check pads and via under these chips. The area seems to be ok.

I get +5V on the pin 2 of U8 (while in standby) and a ~1 second pulse on pin 26 of U7 when I try to power the computer using an ADB keyboard.

I'm pretty sure the problem is coming from the analog board (already fully recaped).

Because the SMPS doesn't seem to be able to start (it stops when starting, I can hear it).

I also tried to keep the Mac plugged during some hours, one of the transformer on the analog board (the smallest one) became abnormally hot.

Do have some tricks to check the analog board? Powering it without CRT seems to be a bad idea because the HV can pulse...

Many thanks!