undo 25p64 eprom mod ?

[Crazyboss]

Hi guys
have a 4000-05 board, not sure why but it came with 25p64 Eproms. And some yellow wires at the back of the pcb to route the signals from those eproms, i dont see any cut traces, wonder if i can just remove this "mod" and put in roms from another 4000-05 board.

Wonder why they did this ? ran out of rom chips at the factory?

[Dave]

i dont see any cut traces, wonder if i can just remove this "mod" and put in roms from another 4000-05 board.

Why do you want to remove the existing ROMs?

Have you looked under the existing ROMs? - the cut traces are likely to be there, you might be able to see them though the sockets, or you may need to remove the sockets to see them.

The 4000-05 board originally used Mask ROMs, if Memotech ran out of their initial supply, they were probably broke by this time and didn't want to shell out for another run of factory masked ROMs. If the board has not been modified, and you have a good set of 4764 Mask ROMs from another 4000-05, then you should be able to remove the patch wires and use those ROMS, but unless the potential donor board is beyond repair, why do that?

Unless you do have a spare set of known good Mask ROMs. it's pretty certain that you should NOT remove the patch wires, you need to examine the connections closely, if you are lucky, you might be able to use a 2764 EPROM.

This page should help you a lot

http://primrosebank.net/computers/mtx/r ... eplace.htm

And of course, you could always remove the ROMs completely and have MFX supply the contents

[Martin A]

One of my two MTX500s had factory fitted 2564 EPROMS. If I remember correctly there were cut tracks on the top and the bottom of the PCB. I replaced the 2564 parts with the more modern 27C64, and that needed more track cutting, there's definitely cuts on both sides now as Dave's page points out!

That's actually my MTX with the yellow labels in the photo on the page Dave linked.

The write up for that fix is on this page http://primrosebank.net/computers/mtx/r ... air_ma.htm

[1024MAK]

But the yellow bodge wires make it look pretty

Thing is, as said above, unless you have a spare set of working mask ROM chips, you actually need wiring with any EPROM chip type.

Mark

[Crazyboss]

This mainboard came from (another) MTX which had power issue after replacing the power regulators and the TIP, and the diods i get normal voltages at the 4116, +12,+5v,-5v. The eproms TI 25P64 are in very bad condition, so I actually dont want to put them back in to the mainboard. The problem is I have no other MTX using those type of roms, so cant swap them. I can program some W27C512 eeproms i have a ton of those but they are not 100% compatible with the 25P64 ROMS. And there will not be space to use some kind of ataptor.

i dont see any cut traces, wonder if i can just remove this "mod" and put in roms from another 4000-05 board.

Why do you want to remove the existing ROMs?

Have you looked under the existing ROMs? - the cut traces are likely to be there, you might be able to see them though the sockets, or you may need to remove the sockets to see them.

The 4000-05 board originally used Mask ROMs, if Memotech ran out of their initial supply, they were probably broke by this time and didn't want to shell out for another run of factory masked ROMs. If the board has not been modified, and you have a good set of 4764 Mask ROMs from another 4000-05, then you should be able to remove the patch wires and use those ROMS, but unless the potential donor board is beyond repair, why do that?

Unless you do have a spare set of known good Mask ROMs. it's pretty certain that you should NOT remove the patch wires, you need to examine the connections closely, if you are lucky, you might be able to use a 2764 EPROM.

This page should help you a lot

http://primrosebank.net/computers/mtx/r ... eplace.htm

And of course, you could always remove the ROMs completely and have MFX supply the contents

[1024MAK]

So what you want (I assume) is to change the board and the wiring so that it can take the more modern 27C512 EPROMs.

Mark

[Martin A]

Or mod the board for the 28C64 EEPROM, which seems to be still in production and easier to re-program than the UV EPROMS.

https://www.mouser.co.uk/ProductDetail/ ... cLow%3D%3D

[Dave]

There are lots of options.

As I said earlier, Claus needs to examine the existing patch wires. If he’s lucky, he might not need to do any further modification, even his 64k ROMs might work “as is” if the higher address lines are already tied high or low.

At this point, with no detailed information or photos, we’re all guessing !

[1024MAK]

Claus, is your board wired like the one in this topic?

Mark

[Crazyboss]

Hi Mark, yes it looks the same and also the same rom chips used.

I checked with multimeter, and i found out the layout from the rom socket to the z80 (A0-A10) and (D0-D7). looks quite standard compared to some flash chips W27C512 I have in stock.

So I wonder if you actually could flash one W27C512 (a 64k chip) you could plug it in the socket and have some extra wires and use only one chip ?

and put Chip enable from the other sockets at A11 and A12.

Technical it should work right ?

at the rom socket in the mtx:
1:- 28:-
2:- 27:-
3:A7 26:-
4:A6 25:A8
5:A5 24:A9
6:A4 23:-
7:A3 22:-
8:A2 21:A10
9:A1 20:-
10:A0 19:D7
11:D0 18:D6
12:D1 17:D5
13:D2 16:D4
14:- 15:D3

//CLAUS.

Claus, is your board wired like the one in this topic?

Mark