Checking before I do anything

[msknight]

I have what looks to be an issue 1 zeddy.

Firstly, for the 16k memory expansion, I need to take out the existing chip and replace it with the module. Link 1 needs to be broken and link 2 needs to be made. I still solder in to D1, D3 and D5 as before. Yes?

Secondly, the ROM. At least it is socketed. I don't know what it is yet, but if it is a 622 or worse still a 550, then what are my options please? I believe I have a programmer to program BBC chips, and I have some of those, but I'm willing to bet that it's going to be different... isn't it always

[Lardo Boffin]

It’s an issue one.

Given the 82 13 date code on the ROM it’s extremely unlikely to be a 550.

There a few people on here with programmers I believe. I don’t have one but do have a bag of ROMs…

If you like variety in your ROM life you may be able to get a ROM-el, a switchable ROM that is a drop in replacement.

[Lardo Boffin]

For 16K of RAM I just buy a chip and a socket:

Same on an issue one:

[Moggy]

I have had several Mostek ROMs over the years and don't recall them being anything other than 649s and, as Lardo says given the build date, I suspect this to be the case. What's more intriguing is an issue one board fitted with issue three chips. ( ULA at least)
As for for burning your own ROM/EPROM then the only thing I entertain is the "Big Bang 2x" ROM code.

Faster maths, printing and it gets rid of the annoying screen garbage when CLS is called after SCROLL plus no more equally annoying flicker when using PAUSE.

The simplest EPROM route is the 2764/2C764 which only requires two pin bends and wire attachments. The older 2764 however does require a 21v programming source which a lot of modern programmers may struggle with unlike the lesser 12v needed for the 2C764.

[1024MAK]

I have what looks to be an issue 1 zeddy.

Definitely an issue one board. It says so just above the processor (Z80A or D780C-1).

Firstly, for the 16k memory expansion, I need to take out the existing chip and replace it with the module. Link 1 needs to be broken and link 2 needs to be made. I still solder in to D1, D3 and D5 as before. Yes?

If using the module you linked to in another topic, yes. Link 2 connects address line A10. The connections on the diodes pick up address lines A11, A12 and A13. 16K bytes of RAM needs 2^14 (16384) addresses (which is address lines A0 through to A13).

Secondly, the ROM. At least it is socketed. I don't know what it is yet, but if it is a 622 or worse still a 550, then what are my options please? I believe I have a programmer to program BBC chips, and I have some of those, but I'm willing to bet that it's going to be different... isn't it always

If the Zeddy is currently working, the machine can tell you which ROM version it has. You just have to ask it the right question...

Hint, read this topic

Your options are:

• Obtain a replacement orginal ROM chip from either a member here, or from someone else who has one for sale.
• Burn a EPROM and then carry out the small amount of rewiring needed,
• Obtain an EPROM from someone and then carry out the small amount of rewiring needed,
• Buy a replacement module (and get the seller to program it for you). That's the ROM-el that Lardo is talking about.

A programmer for programming EPROMs for a BBC B will work okay.

If you are going to use a EPROM, do as Moggy says and use the Big Bang version

Mark

[msknight]

I use the RGBtoHDMI units and I've found out that clipping in a 220 ohm resistor through a jack plug and a couple of crocodile clips, works just as well as Mark's composite veraboard mod.

The trick is this... all that I've been learning from this process, I've been putting on my web site... and I intend using this second zeddy, to record a youtube channel video for other people to follow. I did this with the BBC B, and so I'm doing with the zeddy. ... but in order to ask it which rom it has, I need to sort the keyboard first... which would wreck the sequence for the "repair" video ... so I'll be in a position to ask it what rom it has, somewhere down the line.

I do know that the second zeddy does boot to K prompt, so at least the video output is working... I haven't tested the tape input yet... that could potentially be faulty... time will tell.

Yes, I will be using the same 16k RAM mod that I posted about earlier, so I will be using link 2 to enable one of the address lines... the only question that remains now, after all your kind input, is whether I need to break L1 in the process.

[1024MAK]

L1 and L2 (or LK1 and LK2) are "one or the other", never both.

So remove L1 (LK1) or cut it in half and remove the remains in the central PCB pad. Then fit a wire link in L2 (LK2).

L1 and L2 on a ZX81 issue one board

L1 and L2 on a ZX81 issue one board

L2 fitted on a ZX81 issue one board

L2 fitted on a ZX81 issue one board

Mark

[msknight]

Given the 82 13 date code on the ROM it’s extremely unlikely to be a 550.

If I read the date code, that's 1982, week 13. I can't quite compute an issue 1 1980 zeddy having an original bios chip from 1982. Grateful for insight please.

[Paul]

1980 is the copyright, not he building date

[Moggy]

1980 is the copyright, not he building date

Agreed, and the issue three boards have a 1981 copyright date I would have said that all the socketed chips on this issue one board have been replaced with later ones if not for the same date soldered in RAM chip which leads me to think that is is a very late issue one,built to use up old m/boards, just before the change to issue three.

What's the date code on the modulator? If 1982, like the chips, then again it points towards a very late issue one as whilst the chips may have been swapped it's highly improbable the modulator was swapped.

As Mostek didn't print the ROM code type on their chip, reading the thread Mark links to in his first post will confirm ROM type which being Mostek I estimate it to be a 649.