Macintosh Classic II -12 Voltage Issue

[SchoolBusII]

Hello,

I’ve got a Classic II with a strange voltage issue. I was set to adjust the display recently after a recap and RAM upgrade to 10mb. I double checked the voltages based on Larry Pina’s Mac Classic & SE Repair and Upgrade Secrets (Page 162, Table 8.1).

The +5V reading is spot on, so is +12v. -12v consistently measures at -2.67v. This isn’t the ideal -12v reading. Pina’s book doesn’t have a solution for -12v issues. I’ve used two different multimeters with the same result.

The machine boots, works fine. I’ve tried it with and without the hard drive and upgraded ram. Always holds at -2.67v. Is this a big issue? Is there a way to address this reading if it is a problem?

Thank you for considering.

 

[speakers]

The +5V reading is spot on, so is +12v. -12v consistently measures at -2.67v.

I'm confused. The Classic II doesn't have a -12v rail. It has +5v, +12v and -5v. and I've recently restored one - see https://tinkerdifferent.com/threads/a-mac-classic-ii-restoration.4313/

If you're measuring -2.67v on the -5v line, that's not good though. It won't prevent booting but it will stop your serial ports working.

This may not apply, but related to the LCIII, if the decoupling cap on the -5V line is replaced with a tantalum installed *backwards*, the line may get pulled down to about -2v level.

 

[SchoolBusII]

Hi - thank you for your reply. I didn't realize the Classic and Classic II rails were different. Is testing the voltages off the floppy port still a reliable place to do so? Your post is very helpful- thank you. Where is the -5v decoupling cap? I had someone else recap the board, no tantalums were used. Also, do you think the QP1 Decoupler might be a culprit, like it was for you?

 

[speakers]

Hi - thank you for your reply. I didn't realize the Classic and Classic II rails were different. Is testing the voltages off the floppy port still a reliable place to do so?

Oh, my mistake .. I was looking at the wrong schematic .. the Classic II does have +5v, +12v and -12v on the floppy port. That explains my confusion

Your post is very helpful- thank you. Where is the -5v decoupling cap? I had someone else recap the board, no tantalums were used.

Doesn't apply.

Also, do you think the QP1 Decoupler might be a culprit, like it was for you?

Nope - QP1 affects all rails. Your problem is likely a bad IP3 regulator on the analog board.

 

[SchoolBusII]

Thank you - this is great. Would you mind sharing the schematics you referenced for the Classic II?

 

[speakers]

They're the widely-available Bomarc schematics, e.g. at https://github.com/nickpunt/nubus-se30/tree/master/Schematics/bomarc

Nate that the analog board for the Classic II is the same as what's on the Classic schematic.