16k Sinclair issue 1 pack

[msknight]

I have an issue 1 Sinclair 16k RAM pack. I can find a few pictures, but only external ... https://www.zx81stuff.org.uk/zx81/hardware/Sinclair16K

Is there a need for internal pictures?

Also, high res graphics is sort of working, but sort of not working... so I'm not sure what set of instructions to follow, to enable high res graphics... or whether to leave this pack as is and give high res a miss.

[1024MAK]

Sinclair (and most other brands/makes) 16K RAM packs that use DRAM chips normally need a modification for full Hi-Res graphics to work correctly.

I thought we had some photos of an issue 1 RAM pack, but either I was mistaken, or I just can't find them... (we do have photos of issue 2 and issue 3 though).

Mark

[msknight]

OK - I'll take some. Where'd you want 'em?

Which mod instructions should I be looking at?

[1024MAK]

Photos in this topic please. Also send copies to me via email (see my PM)

Hi-res mod for a 16K RAM pack, see this topic

Mark

[msknight]

OK - so this is my meagre diode collection. I have some 1N4148 and I presume I'm going to need a slightly higher resistor than the 4.7K specified?

[msknight]

So I'm doing the following...

1) Identify RD on pin 16 top side, cutting the line and inserting a diode in its place.

2) Identify RFSH on pin 23 top side. Cut the line.

3) Take 5v and feed it in to the chip where RFSH went - can normally be done between pins on the same chip, using it's own 5v pin.

4) Take 5v and feed it through a resistor, then through a diode, and out where RFSH originally came in, on the edge connector.

Do I have that right please?

[msknight]

OK - so, RAM pack still works, but high res doesn't. What did I do wrong?

*) Diode on RD which has the track cut underneath.
*) RFSH is cut and destination pin is wired to +5 of the same chip.
*) +5V through 4.7K resistor and another diode to RFSH

System works, but high res is not happening. JBRacing results in a completely black area and also 1K 3D Maze results in blocked maze walls. This is the same behaviour as the pack did before I modded it.

[msknight]

Does the 5v, after the resistor, also have to be connected north of the diode to RD? Is that what I'm missing?

[msknight]

I tried the 5v to the side of the RD as well as the RFSH... it didn't like that at all.

Do I have to raise the value of the resistor because of the diodes I'm using?

[msknight]

Well, if no one has any ideas to help on this, then I reckon I've got a few more days before I'm going to have to abandon this and the vintage RAM, so I can clear my workbench and move on. I don't really know where to go forward from here as I'm just guessing now.