Introducing me

General Chit Chat about Sinclair Computers and their Clones
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Introducing me

Post by Moggy »

angus wrote:Well it arrived today.....wasn't expecting much for £9 including post.



BUT.......it works! Fired it up and got a K cursor immediately. The keyboard works, I've run a couple of simple BASIC programs to display moving text to my work colleagues (I had it delivered to work)...and the venerable computer is working fine.

Only problem is the image is very difficult to tune and the picture is almost inverse video. On the left it starts out with black text on a white background but this tends towards inverse by the middle of the screen. This is on a CRT television, I haven't tried a flat screen yet. I thought 10 year old CRT would give the ZX81 the best chance.

So....composite video mod is in the offing methinks.

Colour TV or B/W?

If colour TV then it looks like it is fitted with one of the two earlier ULA chips
No's 2C158E (earliest) and 2C184E . both of these dont work with colour sets and, If I recall correctly,I don't think the composite mod improves the picture on colour TVs if the Zeddy is fitted with one of the earlier chips, something to do with missing "back porch signal. (google is Your friend here)
the later and last ULA No 2C210E. cured this.

If the mainboard has a green underside then it is likely that it has one of the earlier chips, red coloured board (issue 3) has the later one.

Hope this helps.
Regards Moggy
angus
Posts: 142
Joined: Mon Jun 20, 2011 1:28 pm

Re: Introducing me

Post by angus »

Its a colour TV, the oldest I could find at work was about 10 years old, I don't even have any CRT equipment at all at home.

I'll open up the '81 and have a look at the ULA tomorrow and probably start on the composite mod. Could take a while as I am keeping the ZX81 at work for now (promised the wife it's not coming home) and will work on it in my spare time.

But I was amazded that it worked. I was expecting to have to buy a new keyboard...but every single key works.
angus
Posts: 142
Joined: Mon Jun 20, 2011 1:28 pm

Re: Introducing me

Post by angus »

OK 2nd day with the ZX81 :)

Is there anything odd about the wiring on the ear/mic sockets? I've been trying to load and save to/from tape (and CD) and haven't got far yet. I did manage to get a signal out of the ZX81 onto cassette and I got the impression that the tip of the 3.5mm jack socket carries the earth while the barrel has the signal. Is that correct?
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Introducing me

Post by Moggy »

angus wrote:OK 2nd day with the ZX81 :)

Is there anything odd about the wiring on the ear/mic sockets? I've been trying to load and save to/from tape (and CD) and haven't got far yet. I did manage to get a signal out of the ZX81 onto cassette and I got the impression that the tip of the 3.5mm jack socket carries the earth while the barrel has the signal. Is that correct?
No it follows the normal convention of tip "positive" sleeve earth. Mono of course.
I have never had any luck with Mp3 players, cd, or sticking wires in pc parallel ports etc, I think a lot gets lost in translation, but if working with tape this good old fashioned casstte player/recorder works superbly..

http://www.argos.co.uk/static/Product/p ... PLAYER.htm

Barring that why why not get this the best thing made for the Zeddy in the last thirty years by Our own Sir Morris.

http://www.sellmyretro.com/offer/detail ... rface-1589

Regards Moggy
angus
Posts: 142
Joined: Mon Jun 20, 2011 1:28 pm

Re: Introducing me

Post by angus »

I've got a few cassette recorders, what I am lacking is mono jacks. I've cannibalised some other cables and cobbled one cable together with mono jacks at both ends but I've only managed to save one program to cassette, and not to load anything from cassette or CD. I need to root around for a genuine ZX cable or at least a genuine mono cable.

Part of the issue at the moment is that I am using the RF output into a DVD recorder, thence into a TV. I now have a perfect picture but the DVD recorder thinks it is receiving no signal when the ZX81 is in load or save mode....so I can't get a visual idea of if it's able to 'hear' the recording.

And I am seriously looking into buying a ZXpander interface when funds allow. This is a cheap hobby for me so far, just £9 spent and while the cost of the ZXpander is certainly realistic I can't afford one at the moment.

Anyway, enjoy a little video I made today

http://www.youtube.com/watch?v=3Izmfea1JU8
angus
Posts: 142
Joined: Mon Jun 20, 2011 1:28 pm

Re: Introducing me

Post by angus »

BTW Moggy I took the ZX81 apart today, green "Issue One" motherboard (c) 1980. The ULA seemed to just have 8112 stamped on it. I didn't take any photos, and just carefully put it back as I could almost sense the keyboard ribbon cable cracking. Its still working fine :)
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Introducing me

Post by Moggy »

angus wrote:BTW Moggy I took the ZX81 apart today, green "Issue One" motherboard (c) 1980. The ULA seemed to just have 8112 stamped on it. I didn't take any photos, and just carefully put it back as I could almost sense the keyboard ribbon cable cracking. Its still working fine :)
Hi Angus It would seem You have one of the earliest ULA chips.
Look on the underside (the early boards usually used sockets for the ULA so should prise out) for its type Number; the 8112 is , as far as I know , a year and week build Number ie 19-81 week 12 and is printed on top, any other Number than the ones listed in previous posts means You have discovered a fourth ULA.
If it is an early ULA then it follows You also have the early ROM complete with the floating point maths bug ( And Intel thought They were first!) try typing PRINT SQR .25 any answer other than 0.5 then its a clutzed ROM , makes no difference for playing 3d pacvaders just don't try working out any stress analysis equations for a box girder bridge section!!
In remote cases older programs that made calls to routines within the ROM may fail as in the newer chip some addresses are changed.
If on the other hand the CPU has a small circuit board with two chips attached, then You have a rarity, Sinclairs "bodge" to fix the floating point error until a new rom could be written, see attached pics.

Have fun

Regards Moggy
Attachments
ULA .rar
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angus
Posts: 142
Joined: Mon Jun 20, 2011 1:28 pm

Re: Introducing me

Post by angus »

I might have another look inside the unit tomorrow, and will certainly try the SQR function - I'd completely forgotten about the bugged ROM.

What I did do today was visit my mum's house, where some of my childhood posessions are still stored....and found my JK Greye 1K Gamestape (first ever computer game I bought) and QS Scramble which will come in handy when I get a RAM pack. Also located a couple of issues of Sinclair Programs and issue 1 of Big K magazine - so glad I kept these things!
angus
Posts: 142
Joined: Mon Jun 20, 2011 1:28 pm

Re: Introducing me

Post by angus »

Hey guys,

I didn't want to open the machine up again but I obtained a RAM pack and was getting very inconsistent connection. So I opened up the Zeddy again to clean the edge connector contacts, and took the opportunity to photograph the motherboard. The ULA is attached directly and just has the numbers 8112 on it.
Attachments
zx81board4board.JPG
zx81board4board.JPG (228.7 KiB) Viewed 9972 times
angus
Posts: 142
Joined: Mon Jun 20, 2011 1:28 pm

Re: Introducing me

Post by angus »

And it is the klutzed ROM....PRINT SQR .25 returns 1.7xxxxxxx I thnk. Certainly not 0.5.
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