68kMLA Classic Interface

This is a version of the 68kMLA forums for viewing on your favorite old mac. Visitors on modern platforms may prefer the main site.

Click here to select a new forum.
Classic II Motherboard issues
Posted by: CompuNurd on 2016-01-16 13:03:03
It's never worked since day 1. I have removed and cleaned and reflowed the RAM and EGRET chip, as well as a few corroded voltage regulators. I think I might have fried a ROM but don't have any spares to test with unfortunately.

Any pointers?

image.jpg

image.jpg
Posted by: techknight on 2016-01-16 14:25:33
That isnt a ROM issue. that is definitely a RAM issue. Keep in mind the video RAM on the Classic II is shared with main memory. 

Looking at that screen tells me the bus to the RAM is all jacked up. Also check the state of the RESET line coming from the EGRET, and make sure its being toggled on first power on, and then driven high to remain high. Thats another point. 
Posted by: CompuNurd on 2016-01-16 14:49:45
I don't know how to check bus to RAM or the EGRET chip, nor can I find schematics. I can do all of this with a DMM right?
Posted by: Elfen on 2016-01-16 20:34:45
The Classic II is more like a LC II than an LC III. You can use a multimeter to check the RAM SIMM Socket to each other and then to the databus. Between the two sockets, a minimum of 8 limes should not connect as they are the 8 lines for the data bits (8 bits high and 8 bits low). But they should connect 4 megs of RAM on the board. Pin outs for SIMM sockets and RAM Chips are on the net.

Did you turn it on without RAM in the SIMM Sockets? There is 4megs on the board; so if it still messes up, you can look the connecting lines to where they go (Egret or 68K).
Posted by: CompuNurd on 2016-01-17 06:48:53
This is without the RAM SIMMs installed.

image.jpg
1