Picked up a 128k Mac with what appears to be a DIY RAM upgrade

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bushnrvn's picture
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Picked up a 128k Mac with what appears to be a DIY RAM upgrade

First off, I am experiencing these machines far out of their temporal context. So maybe these were common at the time. For me, it is a first.

Snagged this 128k Mac for a low price. Ended up having 512k RAM. I opened it up expecting to find a 512k motherboard, instead I found this:


The machine had also been upgraded to support an 800k floppy drive. I'm a little disappointed that the machine isn't stock, but it came with an external 400k drive so that helps a little. Smile

Added some photos.

Date stamp and part number



Board SN



ROMs updated sometime during or after 1986



128k board identifier


Note the obvious alteration to the solder joints here - the RAM was removed and replaced with socketed 32k chips.

512k decoder circuit on a different, definitely 512k board.


This is likely (I am not informed enough to know for sure) what the hand built circuit on the 128k board is supposed to be.

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gsmcten's picture
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Re: Picked up a 128k Mac with what appears to be a DIY RAM ...

Bush,

That's a 512k Board, but what is that in the first picture???

Steven Smile

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Re: Picked up a 128k Mac with what appears to be a DIY RAM ...

Are we sure? It looks like the ram was replaced with sockets and new chips at some point. The date on the board is also 1983.

Not saying you're wrong, I'm just surprised.

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Re: Picked up a 128k Mac with what appears to be a DIY RAM ...

Bush,

Your second picture shows two rows of RAM chips. 512k

Steven Smile

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Re: Picked up a 128k Mac with what appears to be a DIY RAM ...

I'll add some pictures when I get a chance so we can verify the serials on the board (it is almost definitely a 128k board.)

Looks like the ROMs were upgraded too. So in essence, yes it's a 512k board, but it is really a 128k board modified to run 512k RAM.

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Re: Picked up a 128k Mac with what appears to be a DIY RAM ...

Bush,

Look at your second picture.

The White Rectangle is around
the two sets of 128k RAM (Upper and Lower).

If this was a 128k board, don't you
think that the Rectangle would be
just around the first 128k?

The RAM on this board has not been modified.

Steven Smile

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Re: Picked up a 128k Mac with what appears to be a DIY RAM ...

If it was originally around two banks of 64k it would make sense

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Re: Picked up a 128k Mac with what appears to be a DIY RAM ...

added new photos to original post.

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Re: Picked up a 128k Mac with what appears to be a DIY RAM ...

Why wouldn't it simply be around all the ram on the motherboard? No need for complex explanations here.

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Re: Picked up a 128k Mac with what appears to be a DIY RAM ...

Keatah

http://forums.macrumors.com/threads/anyone-ever-see-a-mac-128k-upgrade-like-this.1242947/

Also - I'm not looking for a complex explanation - just the correct one.

The serials on the board match the 128k board, the RAM has very obviously been replaced by hand, and the circuit next to the 68K is for a decoder.

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Re: Picked up a 128k Mac with what appears to be a DIY RAM ...

Here is a photo of my stock 128k (original 128, not labelled as 128 to differentiate from 512) Mac128_1984_LogicTop.jpg and (crappy) Mac128_1984_LogicBot.jpg

Both ram banks are outlined. The dead-bugged upside-down IC is likely availing an extra addressing line to access the higher 128K to make 512K. I've seen photos of this before, It's not a new board.

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