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Mac Classic II RAM issues
Posted by: azidog on 2020-06-13 01:10:46
Hello,

I recently acquired a Mac Classic II in pretty rough shape and after a lot of work (including epoxying all the plastic screw holders back into place), got it booting with a nice bright image on the CRT. So far I had to recap both the analog and the Rev 1 motherboard after cleaning them thoroughly.

Booting it up shows Mac OS 7.0.1 installed.

I then replaced the RAM with 2x 4MB 80ns SIMMs but even though the system boots, it is only recognizing 4MB (the same as previously installed). I then tried to reseat the RAM chips and clean and reseat the ROM chips but that didn't do anything. Resetting the PRAM also had not changed anything.

I also found an FPU card the seller claims was tested working and the Mac also does not recognize that in the system info but still boots fine. 

Has anyone ever faced a similar issue? 
Posted by: azidog on 2020-06-13 02:11:07
I do also see that some Rev 1 motherboard pictures have a blue bodge wire connecting the eagle chip with another on the top right of the motherboard while mine does not have that. Was that a factory fix for something?
Posted by: MrFahrenheit on 2020-06-14 11:00:33
Here is a photo of my Classic II board for you to compare to. 

265B2718-4862-42D5-BE58-8817D9672C79.jpeg
Posted by: MrFahrenheit on 2020-06-14 11:03:28
By the way, you'll note from my picture, there's a jumper at the top-right of mh photo for 2/2MB.  Does your machine have a jumper installed there??
Posted by: rplacd on 2020-06-15 07:44:51
Surely that would be to configure the ROM/FPU expansion card, no?
Posted by: azidog on 2020-06-15 15:52:38
Thanks for the replies all and thanks for all the help on reddit MrFahrenheit! The RAM was definitely 8x1024 chips and the seller is sending me new RAM (stock was mislabelled).

No jumpers installed on my motherboard and also no external ROM on the FPU card. I have the motherboard revision in the pic below.

IMG_2280.JPG

Looking into the blue bodge wire and the schematics from Bomarc, it seems that PIN 23 from the EGRET chip was supposed to be connected to PIN 49 of the Eagle chip. Not going to do that until I get everything working and I know what PIN 23 on the EGRET is.

image.png

View attachment 34298
Posted by: azidog on 2020-06-15 16:04:17
Hmm... unless that missing component on the FPU card is a DIP-14 crystal. No info with the manual that came with it.
Posted by: azidog on 2020-06-17 02:16:31
Looks like from the Developer guide for the Classic II there is a 16MHz clock coming in to the FPU board so should be working. Ordered an external 5V DIP-14 clock to test (one 25MHz and one 50MHz to see if a 2x overclock works) just in case. Hopefully the MC68882 chip isnt faulty.
Posted by: azidog on 2020-06-23 11:09:42
9 fixed traces later (mostly near the DFAC chip) sound is working and the FPU is being recognized by Speedometer. Added the 50MHz crystal and things are looking good. Will probably have to take off that chip and repair the board further but for now i'll enjoy the Mac as is.

The new SIMMs were delivered and I'm at 10MB as well.

Thank you all for your help.
Posted by: cheesestraws on 2020-06-24 06:30:42
That’s great news!
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