Issue 3 16k internal RAM mod

[Lardo Boffin]

Hi

Has anyone done an internal 16k RAM mod on an issue three zeddy? Is it basically the same as the issue one process (using 1024MAK's process)?

Many thanks as ever!

Lardo

[1024MAK]

Opps, you have asked me about this somewhere before

Yes, RetroTechie's site shows how to do the mod on an issue 3 PCB. See the archived web page here. All the same principles apply. The only real difference is the different layout of the issue 3 board.

Mark

[Lardo Boffin]

Opps, you have asked me about this somewhere before

Yes, RetroTechie's site shows how to do the mod on an issue 3 PCB. See the archived web page here. All the same principles apply. The only real difference is the different layout of the issue 3 board.

Mark

Thanks Mark. Does your method also apply to the issue three? It was somewhat easier for me to follow!

[1024MAK]

Yes, as I said, the principles are exactly the same: cut and remove LK1 and replace with LK2, cut the tracks to isolate the relevant pins (1, 2 & 23) of the 28 pin socket, add wires to restore the path of signals that go elsewhere on the board, add wires to feed the extra address lines to pins 1, 2 & 23.

If you take photo of an issue 3 board, then print it out, you can draw on it. This may make it easier to understand. Or just print out RetroTechie's pictures and draw on them.

Then when doing the work, copy your picture

If you are not happy soldering wires to the thin tracks, just follow the relevant track until it gets to a via or a component pad, and solder your wire there instead.

Mark

[Lardo Boffin]

Yes, as I said, the principles are exactly the same: cut and remove LK1 and replace with LK2, cut the tracks to isolate the relevant pins (1, 2 & 23) of the 28 pin socket, add wires to restore the path of signals that go elsewhere on the board, add wires to feed the extra address lines to pins 1, 2 & 23.

If you take photo of an issue 3 board, then print it out, you can draw on it. This may make it easier to understand. Or just print out RetroTechie's pictures and draw on them.

Then when doing the work, copy your picture

If you are not happy soldering wires to the thin tracks, just follow the relevant track until it gets to a via or a component pad, and solder your wire there instead.

Mark

I had a go at applying your process of working on pins 1, 23 and 26 and have produced the images below (note that I am not doing this ready for a future 32K mod): -

Hopefully the majority of this is correct but I could not find the place to cut pin 26. Is it on the component side?

IMG_6417.JPG

(4.85 MiB) Downloaded 433 times

The component side looks fairly straight forward (famous last words...), cut pin 26 and set LK 2. Unless of course there is a second cut on this side (maybe to the immediate right of pin 26?) that isn't done solder side as above?

IMG_6418.JPG

(4.26 MiB) Downloaded 433 times

Thanks as ever!

Lardo

[1024MAK]

You're getting there Lardo

But some more to do...

In total there are six track cuts to do, and six wires required (not including the link wire for LK2).

Pins to isolate: 1, 23 and 26.

On the bottom (solder) side:
Cut the track to pin 1. As shown in your picture.
Cut the track to pin 23. As shown in your picture.

On the component (top) side:
Cut the track to pin 1. The track going towards the Z80.
Cut the track to pin 23. The track going towards the Z80.
Cut the track to pin 26 either side of the pin (two track cuts). One cut is shown in your picture. The track going to the edge of the board via LK1 from pin 26 also needs cutting.

Because we need to maintain the rest of the circuitry, we now need to link up the isolated tracks beyond the cuts, but keeping the relevant SRAM chip pins isolated.

The former pin 1 cut tracks need linking to maintain /RFSH to edge-connector. See the green dotted line on RetroTechie's picture of the component (top) side.

The former pin 23 cut tracks need linking to maintain /WR to pin 27 and to the edge-connector. See the green dotted line on RetroTechie's picture of the component (top) side.



The former pin 26 cut tracks need linking to maintain the +5V rail to the remainder of the board. Connect a wire on the bottom (solder) side from pin 18 of the now unused pad for IC4A (2114 / 21L14 SRAM) and connect it to the +5V pad for LK1 (edge-connector side).

Address lines can go to pins 1, 23 and 26 in any order. The SRAM does not care which address line connects to which address pin. And the Z80 CPU does not care either.

So I suggest wiring as follows:
A13 to SRAM chip pin 1.
A12 to pin 26.
A11 to pin 23.

IC4 SRAM pin out now:

Code: Select all

Old signal    24p  28p  new signal

/RFSH          -    1 * A13
+5V            -    2   +5V
              (1)   3   A1
              (2)   4   A9
              (3)   5   A8
              (4)   6   A7
              (5)   7   A6
              (6)   8   A4
              (7)   9   A2
              (8)  10   A0
              (9)  11   D0'
             (10)  12   D1'
             (11)  13   D5'
             (12)  14   GND (0V)

             (13)  15   D6'
             (14)  16   D2'
             (15)  17   D7'
             (16)  18   D3'
             (17)  19   D4'
             (18)  20   /RAMCS (via R2)
+5V via LK1  (19)  21 * A10 via LK2
             (20)  22   /RAMCS (via R2)
/WR          (21)  23 * A11
             (22)  24   A3
             (23)  25   A5
+5V          (24)  26 * A12
               -   27   /WR
               -   28   +5V
               
* indicates changes. Pin numbers in brackets refer to a 24-pin IC in that position, as shown in ZX81 schematic.

Mark

[Lardo Boffin]

You're getting there Lardo

Mark

Thanks Mark!

I may get there eventually - certainly it is a good job I decided not to follow a career in bomb disposal... Cut which wire?

New solder side image

image2.JPG

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New component side image (RAM removed)

image1.JPG

(3.65 MiB) Downloaded 389 times

I think I have it all mapped out correct except for the following part -

The former pin 26 cut tracks need linking to maintain the +5V rail to the remainder of the board. Connect a wire on the bottom (solder) side from pin 18 of the now unused pad for IC4A (2114 / 21L14 SRAM) and connect it to the +5V pad for LK1 (edge-connector side).

I have shown pin 18 on the first image but am not sure where it goes! Is it the pad to the immediate left of pin 24?

So six wires for an issue three mod but only three wires for an issue one - that's the price of progress I guess.

Lardo

[1024MAK]

I think I have it all mapped out correct except for the following part -

The former pin 26 cut tracks need linking to maintain the +5V rail to the remainder of the board. Connect a wire on the bottom (solder) side from pin 18 of the now unused pad for IC4A (2114 / 21L14 SRAM) and connect it to the +5V pad for LK1 (edge-connector side).

I have shown pin 18 on the first image but am not sure where it goes! Is it the pad to the immediate left of pin 24?

Yes, on the bottom (solder) side (which on the component / top side is to the immediate right of pin 24).

So six wires for an issue three mod but only three wires for an issue one - that's the price of progress I guess.

Hence why I did my guide on an issue 1 board

Mark

[Lardo Boffin]

I think I have it all mapped out correct except for the following part -

The former pin 26 cut tracks need linking to maintain the +5V rail to the remainder of the board. Connect a wire on the bottom (solder) side from pin 18 of the now unused pad for IC4A (2114 / 21L14 SRAM) and connect it to the +5V pad for LK1 (edge-connector side).

I have shown pin 18 on the first image but am not sure where it goes! Is it the pad to the immediate left of pin 24?

Yes, on the bottom (solder) side (which on the component / top side is to the immediate right of pin 24).

So six wires for an issue three mod but only three wires for an issue one - that's the price of progress I guess.

Hence why I did my guide on an issue 1 board

Mark

That makes sense

Solder side image updated

image2.JPG

(3.83 MiB) Downloaded 371 times

[1024MAK]