Dove MacSnap 5.0 RAM Upgrade Upgrade

[mitchW]

I am restoring a Mac 512K with a 512K Dove MacSnap 5.0 RAM Upgrade card + SCSI.
This upgrade card brings the Mac RAM to 1MB.

But I've seen some other Dove MacSnap RAM Upgrade cards that contain 1MB or 2MB of RAM.
Is it possible to upgrade my upgrade card to 1MB or even 2MB?

Also, one of the snap connectors is bit corroded, any possibility of getting a new one? Or were those made to order parts just for this upgrade cards?

 

[Nixontheknight]

I am restoring a Mac 512K with a 512K Dove MacSnap 5.0 RAM Upgrade card + SCSI.
This upgrade card brings the Mac RAM to 1MB.

But I've seen some other Dove MacSnap RAM Upgrade cards that contain 1MB or 2MB of RAM.
Is it possible to upgrade my upgrade card to 1MB or even 2MB?

Also, one of the snap connectors is bit corroded, any possibility of getting a new one? Or were those made to order parts just for this upgrade cards?

you can fix the corrosion by using white vinegar

 

[mitchW]

Thanks, I did use a special "gentle" rust remover cleaning product that I usually use for corroded stuff and it turned all green corrosion into dull grey metal. I will try white vinegar if it will make it any better

 

[Byrd]

Can you post pics of the card? It might be an easy upgrade to 2MB. Great cards these, really make a 128/512K useful.

 

[mitchW]

Here, I snapped few pics of the card

 

[Builder68]

The card looks in great shape!

To upgrade it, you just need to populate the missing 32 ICs with 41256 DRAMs (256K x 1-bit) and add that second 74253 multiplexer chip on top. You should also check if the two 150-ohm resistors are present; if not, you'll need to install those as well—along with those 0.1 µF yellow capacitors.

Regarding the logic chip, it doesn’t strictly have to be a 74F253; a 74HCT253 or 74LS253 will work just as well in this application. Once upgraded, the card will provide 1.5MB, which, combined with the 512K on your motherboard, gives you a total of 2MB.

The hardest part will be sourcing 32 matching 41256 chips, as they are becoming harder to find in large batches. As for the corroded snap connector, those were, unfortunately, proprietary to Dove.

 

[mitchW]

Thanks!

I checked my parts bin, and I actually have 18 of 41256 ICs (not tested though) and a 74F253! Other missing parts, I can order

But now I found some photos of the fully populated Dove MacSnap 5.0 on ebay, but it seems it has and additional snap connector for one chip on top of the card (for connecting onto LS04 chip, I guess). And if they were proprietary, then even fully populating won't work.

I did find some photos on of that same board, but with 16+16 memory ICs (two thirds populated, so 1024K). Perhaps this configuration would work without connecting to LS04 chip? I am not sure though. Even having 1.5MB of RAM is better than 1MB though.

 

[Builder68]

Actually, the model of your MacSnap board isn't '5.0', that's just the PCB revision.

Your board is a model '548.' Apparently, it seems the model names with an "E" at the end were specifically for the Macintosh 512Ke. Those 'E' versions have an extra snap-in socket connector for LS04 to tap address bit A20, which is used by an additional logic chip (74F259).

I've reverse-engineered the logic of this board; pin 11 of that 74259 connects to pin 15 of the additional 74F253 found on the fully populated 1.5 MB models.

It looks like what it does is disable writes to a specific memory address region: link


From a functional standpoint, it doesn't affect the normal operation of the machine. We never quite figured out what it might be needed for.

So, you don’t actually need that additional snap-in socket connector and the 74259 to get 1.5MB (Just tie pin 15 to GND). But if you want to implement the 74259 anyway, you could just tap A20 from the logic board with a bodge wire.