Performa 640CD Won't Run DOS after 52 Mb Upgrade

[twillkickers]

Upgraded my Performa 640CD from 16 Mb of Mac RAM to 52 Mb of Mac ram. The ram for the Dos portion was 32 Mb before and after the upgrade. After upgrading, the Macintosh System 7.5.1 portion seems to run without issue, but when I try to boot into the DOS portion, I get a blank screen. The DOS portion was working before the upgrade. Does anyone know why this might be happening?

 

[Brett B.]

Try removing the logic board, reseating everything, and cleaning the edge connector.  My LC630 requires that step at least once every time I change a component, has been like that for years.  I think something in the edge connector circuitry is a little flaky but once it works it will continue to work just fine.

 

[Nathan]

If this is still an issue:

Did you thoroughly test the Macintosh sides of things with something that uses a lot of ram all at once?

The basic OS load doesn't really use that much memory. If DOS has problems it might indicate an issue in the region of RAM allocated to it. And since that RAM is borrowed from the Mac side unless you have a dedicated stick on the board, it follows that it might just seem fine under 7.5.1 as long as you aren't trying to use that bit of RAM.

Of course, as the other guy said it could have something to do with a wonky connector that's unreliably passing signal because the connection is poor, possibly due to oxidation on either the card edge or inside the connector.