Refurbing an issue one zeddy

[Paul]

Why not cut away the plastic? It isn't needed once the top row is soldered
That might even give a nicer look.

[Lardo Boffin]

Why not cut away the plastic? It isn't needed once the top row is soldered
That might even give a nicer look.

Do you mean the excess plastic or all of it?

[Paul]

I mean all the plastic on the pins, not the regulator of course

[Lardo Boffin]

I have soldered it and I think this rates as 'good enough'!

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I may try and scrape off the remains of the solder pads at the top of the board.

Lardo

[Lardo Boffin]

What I use is
this from ebay.
My main problem with manual pumps was always that during the time from taking away the soldering iron and placing the pump the solder was (going) hard again.
With this heated manual pump this doesn't happen anymore.
Works good and is MUCH cheaper.
Kind regards
Paul

I bought a heated desolder pump for about £10 and it is the best £10 I have spent in a long time! It is sooo much easier than the manual approach! Thanks Paul!

[Lardo Boffin]

Hi Mark

A few questions about your 16k mod as I am approaching a point where I will give it a go!

The modern SRAM chips need a 28 pin socket. I showed how I did an issue 1 board here
Note that my mod is designed to allow a further modification to 32k bytes. Hence the cutting of the +5V track, then fitting a wire link across it.

Dave Curran of Tynemouth Software shows how he does an upgrade on an issue 1 board here.

So take your pick. They all do the same. Expand a ZX81 to have 16k bytes of internal RAM.

The mod to allow for the 32k RAM option - given that I am unlikely to do this at this stage do I need to cut the +5V line? I assume this is the left most track cut of the middle part of the image?

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In the section about signals you note the position of M1 but there is no wire to this?

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The link L2 is joined with uninsulated wire - is the lack of insulation significant? Or is it just easier to work with on such short stretch of wire?

Many thanks - trying to make sure I understand!

Lardo

[1024MAK]

Hi Lardo

Ask away...

The mod to allow for the 32k RAM option - given that I am unlikely to do this at this stage do I need to cut the +5V line? I assume this is the left most track cut of the middle part of the image?

On the new 32k byte SRAM chip, pin 2 is one of the address pins (actually A12 according to the datasheets).
The idea behind cutting the existing tracks on both sides of pin 2 is to fully isolate it from the +5V supply. Then having cut the +5V supply track in two places, the bits either side of pin 2 (but not including the bit left connected to pin 2) need linking back up with some wire.

If you never want any more than 16k of RAM, you don't need to cut either of the two track cuts for pin 2. The reason for doing the track cuts all at once, is simple. It's a lot more difficult to cleanly cut tracks after a socket has been fitted!

Having gone to the effort of isolating pin 2 from the +5V supply, a short wire is soldered to connect it up to a fixed logic level (either +5V or 0V, it does not matter, +5V just happens to be easier) until (if) a future modification is carried out.

Note that a SRAM does not care in which order it's address lines are connected up. As long as all of them have valid logic levels. The CPU also does not care. It may appear crazy, but all that matters is that each and every memory location is individually addressed.

In the section about signals you note the position of M1 but there is no wire to this?

Correct. I marked all the signals that were needed for the 16k RAM and what may be needed for a future modification for 32k bytes of RAM. M1 is one of the signals needed if RAM is to be used to store data in high RAM in the ZX81 memory map.

The link L2 is joined with uninsulated wire - is the lack of insulation significant? Or is it just easier to work with on such short stretch of wire?

For such a short bit of wire, insulation is not needed. And indeed, it becomes a pain trying to keep such a short bit of insulation on such a short wire.

Happy to help. If anything above still does not make sense, or you have any other questions, keep asking

Mark

[Tezz]

There is a forthcoming 16K SRAM module without the need to cut tracks on the board. I'll post some info in a new thread shortly.

[Lardo Boffin]

Thanks for the clarification Mark.

Depending on how easy the cut is on the right I may do the two cuts on the left. As you say once the socket is in place it will not be easy to do the cuts!

Are there instructions anywhere on how to do the 32k mod?

[1024MAK]

There are schematics for 32k RAM. But not highly detailed instructions like you have been looking at for the 16k RAM modification.

In fact, there are a number of ways of mapping the 32k RAM into the Zeddy memory map. The easiest is to put the "extra" 16k at the top of memory (48K to 64K). This area cannot be used for executing machine code, but can be used for storing data.

More info and diagram here.

Wilf Rigter describes various RAM upgrades on his web site. Read this one first

Mark