128k upgraded to 1mb

[crazyben]

I recently picked up 512k Mac that turns out to be 128k upgraded to 1mb. Serial number says it is 1114th Mac. How lucky did I get?

Analog board is corroded so that will need replacing. Shall I revert the upgrade and make this 128k mac or leave as it from the value/collecting point of view.

 

[Tobias]

I think your Mac was updated twice, one time by replacing the logic board 128k to a logic board 512k (blue 512k label) and then later with this extension cart to 1mb. So I think you cannot "go back" to a 128k Mac logic board (without changing the whole board), because this was never a 128k board.
Nevertheless, for collecting point of view it can be interesting to go back to 512k. But I don't know in which way this update was made, is this extension cart only plugged or is it soldered in any way ?

 

[bibilit]

I agree with Tobias, in my opinion, a lot of work should have been done on this board already, so reverting back to 512k will be a pain by itself.

Maybe finding a 128k board is the best option.

Have you got the 400k drive or the new 800k ?

 

[cheesestraws]

If you are thinking about this as an investment, or in value terms, but asking these kinds of questions, you are going to lose money on this. Canny investors in any area are those who are ahead of the community's opinions, not behind them.

From the point of view of interest, I'd suggest leaving period upgrades in place, especially in a machine that was apparently upgraded twice, is probably a way to make the machine more interesting; it tells you about the history of the machine.

Agree with those upthread that to "revert" this to 128k, you'd need to replace the logic board with a 128k one, and at that point given that you're replacing the entirety of the electronics in the computer, is that really a good use of this machine?

 

[Tobias]

And the other bad news is: I think you have MacPlus Rom's (this match also to run 1MB of RAM), but the MacPlus Rom's don't work on an original 512k logic board (the first 512k logic board is more valuable) but only on 512ke or later.

 

[crazyben]

I asked the seller what happened to the original board, and he mentioned it was still there. I was hoping it could be reverted back, but it turns out the seller was mistaken—the original board had been swapped out for a 512K upgrade. Since the analog board is corroded, I haven’t been able to test it yet.

I do have another 128K machine with its original case and board, so I’m planning to replace the analog board and keep it as is.

 

[Builder68]

And the other bad news is: I think you have MacPlus Rom's (this match also to run 1MB of RAM), but the MacPlus Rom's don't work on an original 512k logic board (the first 512k logic board is more valuable) but only on 512ke or la

I recently picked up 512k Mac that turns out to be 128k upgraded to 1mb. Serial number says it is 1114th Mac. How lucky did I get?

Analog board is corroded so that will need replacing. Shall I revert the upgrade and make this 128k mac or leave as it from the value/collecting point of view.

The RAM upgrade board shown in the picture its a MacSnap 2048, fully populated. Meaning it should bring 1.5 MB more to the system. On a 512K/Ke, total memory with this board should be 2 MB.

 

[Builder68]

And the other bad news is: I think you have MacPlus Rom's (this match also to run 1MB of RAM), but the MacPlus Rom's don't work on an original 512k logic board (the first 512k logic board is more valuable) but only on 512ke or later.

The only difference between a "512K logic board" and a 512Ke LB is precisely the ROMs (128KB MacPlus ROMs); the LBs themselves are the same. Additionally, both the 512Ke and the Apple upgrade kit to 512Ke come with an 800K floppy drive. MacPlus ROMs do work on a 512K LB.

The original Macintosh 128K came with PCB revisions 820-0086-C/F (630-0101).

PCB revisions 820-0086-C/F differ from 820-0141-A in that the latter has an additional 74253 IC to generate the RA8 signal when 256Kb ICs are installed (512KB of RAM). Jumper W1 on the 820-0141-A board should be bridged if the logic board revision is 630-0101 (Mac 128K) and open if it's a 630-0118 (Mac 512K).

Here is a picture showing where the "PCB revision" model's location is on the back.


And here is a picture showing where the "LB revision" model's location is on the front. For a PCB rev 820-8141-A could be either a Macintosh 128K (630-0101) or a 512K/Ke (630-0118)




If you are trying to obtain an original stock 128K logic board, an unmodified 820-0086-C/630-0101 is what you are looking for

 

[Builder68]

According to the serial number shown in the picture, your machine is a Macintosh 128K with the hybrid PCB revision 820-0141-A. Meaning, it can be reversed to its original state without changing the whole LB.

However, you will need to do a few things, with one being a little complex if you are not skilled with desoldering:
1. Unplug the MacSnap Board (Super easy, nothing should be soldered. The board "snap-in" into several ICs and is fully removable)
2. Find the original 64KB ROMs, maybe on eBay or someone on the community willing to sell them to you.
3. Swap the sixteen memory ICs back to 64Kb from 256Kb. That can be only done by someone truly skilled to avoid damage to the LB.
4. Bridge W1 after the swapping.
5. Find a fully working 400K Floppy drive, which become scarce and expensive lately.

 

[jasonfortheworld]

Just in case you end up wanting to revert the 512k to it's stock configuration, I've got some spare 512k roms from machine that I put a rominator into.

 

[Tobias]

Very interesting Builder68!
So what your are saying is that every 512k / 512ke LB can be transformed back to a 128k LB ?
(I already tried to transform a 512ke(not exactly sure which version, but surly 128K size ROM and 800K drives) board into a 512K original LB, just by replacing the ROM and removing the 800K, but that didn't work for me.)

 

[Builder68]

Yes, the only difference between a 512K LB and 512Ke LB is the ROM. An 800K floppy drive will not work with 64KB ROM. Many of us who have been around these machines for a while are familiar with this, but it's always helpful to share the knowledge!

I can't tell you what was wrong with your conversion, but I would check the 64KB ROMs and look for poor connections with their sockets, which is a very common issue with these machines.

 

[Tobias]

So maybe the ROM I bought was not working
Ok I will check also the connections, thanks for your great knowledge sharing.

 

[crazyben]

According to the serial number shown in the picture, your machine is a Macintosh 128K with the hybrid PCB revision 820-0141-A. Meaning, it can be reversed to its original state without changing the whole LB.

However, you will need to do a few things, with one being a little complex if you are not skilled with desoldering:
1. Unplug the MacSnap Board (Super easy, nothing should be soldered. The board "snap-in" into several ICs and is fully removable)
2. Find the original 64KB ROMs, maybe on eBay or someone on the community willing to sell them to you.
3. Swap the sixteen memory ICs back to 64Kb from 256Kb. That can be only done by someone truly skilled to avoid damage to the LB.
4. Bridge W1 after the swapping.
5. Find a fully working 400K Floppy drive, which become scarce and expensive lately.

Thank you for the detailed explanation. I am quite comfortable desoldering. I recap my own stuff. I haven’t decided on if I want to revert back or keep as it is yet.

 

[Byrd]

Personally I'd just check it all over and keep it as it is, because it's an early Mac that's much more useful than a 128K. Maybe you'll find a spare 128K motherboard and keep it close to this for a quick switch over for the prestige, but those upgrades are useful.

 

[Tobias]

I don't think useful is the most accurate term for this old Macs. I think it's more a question of authenticity. To have the configuration of the first Mac.

 

[crazyben]

Personally I'd just check it all over and keep it as it is, because it's an early Mac that's much more useful than a 128K. Maybe you'll find a spare 128K motherboard and keep it close to this for a quick switch over for the prestige, but those upgrades are useful.

I do have another 128k. Not as early as this one. It is 1361. So I am going to keep it as it is for now.