Mega ST ROM swap - 2 chip to 6 chip.

Upgrade or replace the ROM.
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rubber_jonnie
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Mega ST ROM swap - 2 chip to 6 chip.

Post by rubber_jonnie »

When I added the Beta IDE interface to my Mega ST 4, the installed UK 6 chip version of TOS 1.04 became spare, since the IDE board also comtains a dual ROM with 1.04 and 2.06 on board.

Since I had also recently acquired a Mega ST 1, but with German TOS 1.04, I thought it would be useful to recycle the spare 6 chip UK set I had.

There was a slight bump in the road however, it turned out the Mega 1 had a 2 chip ROM set. So here's how to change it if you decide you want to do the same, and in fact this will also work for the other way too.

The first thing to understand, is that like the STF/M, there are 3 jumpers that need to be set in order to specify just which ROMs you're going to use, as per this table:

ROM W2 W3 W4
256k 1-2 1-2 jumper
1Mx2 2-3 2-3 open
1Mx6 2-3 2-3 jumper

Unlike the STF/M, these are not solder pads, but a mixture of jumpers which use zero ohm resistors as links like the STE, and a single wire link for W4. Below is a reference picture from my Mega ST 4 showing the positions required for 6 chip TOS with 256k EPROMS:

Mega4_JMP_REF.jpg
Mega4_JMP_REF.jpg (132.87 KiB) Viewed 3425 times

Looking at my Mega ST 1, they were set as below:

Mega1_JMP_REF.jpg
Mega1_JMP_REF.jpg (121.57 KiB) Viewed 3425 times

At this point I'd already removed the zero ohm links and replaced them with proper jumpers. As you can see, W4 is unlinked at the point.

I then added the W4 link and swapped over the W2 & W3 jumpers to position 1-2:

Mega1_JMP_After.jpg
Mega1_JMP_After.jpg (119.05 KiB) Viewed 3425 times

So stage 1 was completed, with the Jumpers set for 6x256k EPROMS.


Next was the extra sockets and capacitors. You'll need to add 4 100nF capacitors as below:

Caps.jpg
Caps.jpg (175.75 KiB) Viewed 3425 times

I used components recovered from a dead ST, but up to you where you source from. They are on large copper planes, so soldering is difficult, I did it from both sides to be sure.

Final thing was to fit the sockets and ROMs:

finished.jpg
finished.jpg (164.06 KiB) Viewed 3425 times

After that a boot and test of everything, and all good to go.

If you want to go from 6 ROM TOS to 2 ROM TOS, then all you'll need to do is set W2, W3 & W4 appropriately.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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