Low Voltages/Analog Board Issue - Macintosh Color Classic

[EagleTG]

Hi there.  Long time lurker, recently registered...

Been having relatively decent luck restoring old Macintosh machines for my collection.  As luck would have it, the one I want to work the most out of my collection (the Macintosh Color Classic) has also proven the most difficult to repair.

This machine has had all of the classic issues, I've already replaced all of the logic board caps (which cured a lot that ailed it), replaced the battery, cleaned contacts, reflowed the analog board, replaced all bad caps on the analog board (anything that tested bad via ESR or was common brand/model cap of any other ones that tested bad).  I've disassembled the hard drive to cure a stiction problem with the arm, fabricated a new rubber "home position bumper" out of a grommet.

The hard drive now spins up reliably when outside of the machine, but I found that it wouldn't spin when installed inside the machine.  A quick check of the voltages at the HDD connector showed that I've got 4.5v on the 5v and 10.6v on the 12v lines.  Not good.  I proceeded to investigate the power supply circuit in the machine.  I read some things indicating that the power supply requires load to initialize, but this doesn't seem to help mine.

I've searched and searched online, I've poured over the circuit boards and any available information I can find relating to this machine.  I've tested, poked, prodded, and diagnosed but am still scratching my head.

I found a few posts referencing the voltage feedback optoisolators in situations like this.  Surely that has to be the solution!  So I replaced the CNY75GB optocouplers (both of them, they're cheap).  Still no joy.

Can anyone make some recommendations on next steps here?  I feel like I'm soooo close to having this guy work.  I've got hours and hours and bunches of money in parts into it at this point, don't want to give up now.  

I find the machine a bit tough to troubleshoot as the boards have to be installed to get it to boot, makes testing at the board with it powered up very difficult.  Additionally, the combination of monitor and power supply voltages on this board makes me cautious about poking around with it powered-up anyway.  If it were just AC and DC power supply-level voltages, I'd have no issue doing-so.  The HV stuff for the monitor makes me a bit more leery.

Schematics seem to be unavailable, I've been looking through some of the information for the B&W Classics as the circuits seem to be similar-ish.  So far nothing has jumped out at me as the problem here.

I'm assuming the 5 and 12 volt rails are derived from each other in some way, I did read a post that indicated this as well.  I'm assuming once I get one of them straightened out, the other will follow suit.

Thanks in advance for any troubleshooting advice you can provide.

-Todd

 

[techknight]

the feedback resistors have probably drift out of tolerance. 

 

[EagleTG]

Thanks techknight, I appreciate the assistance.  I know it's been a while, but I'm poking at this machine again.

Current status, I've completely recapped the analog board.  I had wanted to complete this, but needed to order some additional parts as there are some bizarre values on this board (and very tight quarters so physical dimensions are of more importance than most electronics I repair).  That's done.  Glad I did, too, as I found some additional caps that had quietly leaked (no visual indication, passed ESR test, stank upon heating/desoldering and had small puddles under them).

I've also spent a lot more time researching the issue, but I'm still in the same boat.  Oddly the machine boots up to a flasing ? floppy icon every time (because the drive is disconnected or doesn't properly spin up).  The voltages being low don't seem to bother the logic board from a base functionality perspective, if in fact the voltages are low there as well.  The hard drive definitely doesn't like it.  Are there good points to test for power at the logic board?  I know the floppy port is a great spot on other "toasters", but not on this one.  

I don't recall yet having checked the feedback resistors as you recommended.  Forgive the potentially stupid question, but would these be resistors that are part of the optocoupler/optoisolator circuit?

I'm also wondering if it's worth getting the schematics from MacCaps?  I know they are reverse engineered, are people generally happy with the purchase?  I don't mind spending the money for a quality product.

Thanks for any additional hand holding anyone here can provide.

Edit: One additional question, is there any way to work on these more easily?  Getting the AB in and out of the frame is a painful and tedious process.  I haven't tried ripping the entire thing apart to see if I can isolate the board connectors for use in a "bench test" style environment.  Any tips from that perspective, or is it just "take board out, work on something, put board back in, test"?

-Todd

 

[techknight]

I don't recall yet having checked the feedback resistors as you recommended.  Forgive the potentially stupid question, but would these be resistors that are part of the optocoupler/optoisolator circuit?

-Todd

Yes, and I would even shotgun the opto-isolator itself. If that doesnt work, Next in line would be the main switching IC. 

 

[EagleTG]

Yes, and I would even shotgun the opto-isolator itself. If that doesnt work, Next in line would be the main switching IC. 

Ok, thanks.  I did replace both of the CNY75GB opto-isolators, so I'll investigate the resistors and switching IC.

Thanks again for the help.  

 

[lfletche]

Hey @EagleTE, did you ever end up solving this one? I have a similar issue with a Color Classic AB at the moment.