Attic 81's - My collection and attempts to repair

Discussions about Sinclair ZX80 and ZX81 Hardware
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PokeMon
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Re: Attic 81's - My collection and attempts to repair

Post by PokeMon »

Nice TV :mrgreen:
sheddyian
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Re: Attic 81's - My collection and attempts to repair

Post by sheddyian »

PokeMon wrote: Wed Dec 02, 2020 8:54 pm Nice TV :mrgreen:
Thank you :D

The previous owner had stuck parcel tape over the two Bratz characters. I imagine them getting sick of looking at them as they got older!

I found it dumped by the roadside - it's this TV that has caused me to be repairing my ZX81s, as I was waiting until I had a CRT TV to plug them into.

I might paint the case or stick a picture of Sir Clive on the TV if it gets to be used regularly for the ZX81 stuff.

Ian
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1024MAK
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Re: Attic 81's - My collection and attempts to repair

Post by 1024MAK »

Gives a different meaning to ‘a colourful’ TV :lol:

Mark
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Re: Attic 81's - My collection and attempts to repair

Post by sheddyian »

I've been tinkering with my 3 Zeddys a few times recently, finally got one working reliably (so far).

I'd previously got #3 working, but found it would crash as it warmed up. When I was later working on a different board and swapping chips in and out, I found that the ROM on #3 had a leg bent over! Making partial contact I assume. I straightened the leg out and put it into ZX81 board #1 which I was working on.

So, board #1 : Had sockets for the ULA, ROM and CPU but the single 4118 RAM chip was soldered in. I cut the 3 sockets and RAM chip out, cleaned it up and fitted 4 new sockets. I also put a jumper header in where LK1 and LK2 are, so I can choose 1K or 2K from the M5117 that I've fitted. I cleaned all the IC pins with a fibreglass cleaning pencil - thanks for the tip 1024MAX - I was very impressed how clean it got some of the very grubby pins.

After a brief flaky start, where it powered up with a white screen and no cursor, and then seemed to respond to the board being tapped with screen flicker, it settled down and seems to work every time now.
IMG_20201219_223510.jpg
I left it running the above program until the screen was PLOTted black, and it didn't crash. #3 would run this for 10 minutes or so then the screen would freeze or distort.

I've fitted it into my complete case, and loaded a few 1K programs from .P files on a laptop : all working well so far :D

The other two cases are without a keyboard. Back in the 1980s I'd put the keyboard from my original ZX81 onto a piece of wood, layed on top of a piece of corrugated card. The 81 was then screwed to the board. It looked more homebrew and kept it cool. But the keyboard connector has got repeatedly damaged. I tried cutting it back to a good part, but struggled to get the connector in, and split it a little further down. I don't think there's enough left to be able to use it now!
IMG_20201219_212627.jpg
I suppose I could buy two keyboard connectors, plug them into the stub of the ribbon and solder them on flying leads to the ZX81 on the board... but the keyboard here has a cut on the R key and I'm not sure if it works at all, even if the ribbon can be salvaged.

I had a brief look online - can anyone recommend a supplier of new keyboards that I might buy? I remember reading of keyboards for sale, but only found some for around £18 on ebay.

Thanks

Ian
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1024MAK
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Re: Attic 81's - My collection and attempts to repair

Post by 1024MAK »

Try SellMyRetro.

I buy new keyboard membranes from RWAP. But he’s sold out at the moment.
Here’s his listing for normal ZX81 types.

This one is for people who use a ZXpand or ZXpand+.

At the moment, there is only a blank ZX81/TS1000 type available. I’m sure at some point more ZX81 keyboard membranes will be manufactured.

RWAP is a member here, so you could always ask him if/when new stock may become available.

He has excellent service, reasonable prices and I’ve bought various items from him over the last ten years. [And no, I don’t get a cut].

Mark
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Re: Attic 81's - My collection and attempts to repair

Post by sheddyian »

Thanks for the links, I'd discovered I couldn't find any on sellmyretro (except blanks) but I've requested notification if they come back in.

What with Covid-19 Christmas and Brexit I can easily imagine supplies may be short so I'll sit back and wait a little to see what reappears.

That said, today I was tinkering about with two of the boards and I've got a 2nd one working now, tho with a very dark screen as per the buggy ULA.

I've now tested all the ROMs, ULAs and CPUs, and found just one failure - an NEC Z80!

It came out of the board that had the socketed RAM chips in upside down, and the CPU itself had a leg missing! I soldered a wire on, but it doesn't work.

Still, not too bad that all the ULAs work, even if two are the buggy variant without backporch.

Thanks to everyone for their help so far!

Ian
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1024MAK
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Re: Attic 81's - My collection and attempts to repair

Post by 1024MAK »

I wouldn’t describe the first two ULA versions as buggy, as the ZX81 was intended to be used with the less expensive monochrome black and white CRT TV sets. Remember, this was 1981. In the U.K., BBC1 and ITV had only started regular colour programming just over eleven years earlier. So black and white TVs were still available in 1981. Back then why would you use a colour TV with a computer that could only produce a black and white picture?

The back porch part of the waveform (that is not produced by the first two ULA versions) is not as important in most black and white television sets, for a video signal derived from a simple home computer like a ZX80 or ZX81, with only three available voltage levels [*]. This part of the signal is present to allow TVs to auto calibrate to set the black level correctly. This is more important in colour television sets.

So I would call it a design choice or design limitation rather than a bug.

[*] white level, black level and sync level.

Mark
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Re: Attic 81's - My collection and attempts to repair

Post by sheddyian »

Good point. I was mostly being lazy, as I couldn't remember the two ULA versions I've got offhand, so typed a generic "buggy" instead.

I remember noticing the backporch issue at the time without knowing what it was or why. When I got my ZX81 in 1982, it was first plugged into the family colour TV, and I had to turn the brightness up high. Once I did this, the picture was fine, not dark at all.

I mostly used it on a portable B&W TV I'd been loaned, that had a great tuner - just turn the dial no pushbuttons to program. Ideal for the ZX81 to get the picture just right, and I wish I still had that set now . You quickly learnt whereabouts on the dial Ch 36 approximately was!

Ian
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Re: Attic 81's - My collection and attempts to repair

Post by sheddyian »

I've been tinkering some more, and now got ZX81 #1 and #3 working reliably. Both now have all chips socketed, a jumper to select 1 or 2K, and both have an M55117P 2k x 8 static RAM chip installed.

Zeddy #1 is in it's original case, original keyboard but with a 2C210E ULA from #2. It gives a very clear picture on the colour TV with pleasing sharpness.
IMG_20210101_160549.jpg
It draws 310mA when running.
IMG_20210101_160508.jpg
Zeddy #3 is going to be mounted on a board so it's easier to tinker with. It has a 2C184E ULA so the picture is very dark on the colour TV set. I shall be getting a ZX8-CCB board for this I think.
IMG_20210101_160301.jpg
It draws 420mA when running, and the ULA gets unpleasantly hot to the touch around 1/3rd of the way down from the notch.
IMG_20210101_160405.jpg
Output from the 7805 is 4.9V according to my meter. Maybe I'll modify it to run off a USB cable and get rid of the 7805.

I'm shortly going to modify it to have 16K internally with a 32K byte SRAM chip I've bought (see earlier post)

Questions / advice please :

Should I worry about the hot ULA? I have a heatsink to fit to it, and it won't be in a case.

I'd like to try Wilf Rigter's "Why Wait" mod http://www.user.dccnet.com/wrigter/inde ... 81WAIT.htm and I see that in this thread https://sinclairzxworld.com/viewtopic.php?t=1742 there is a discussion about which might be the best transistor to use. Can anyone recommend a particular transistor I should get to make this mod work reliably and with no ill-effects?

(And on a related note, is there any benefit in upgrading TR1 or TR2 in regard to better performance, reliability or better screen output? Eg on some of Dr Beep's 1K hires games, the top line of text bends over - is this expected or can I do something to remedy it?)

I'd quite like to try one of the modded / rewritten ROMS I've been reading about. I'd idly assumed I could get some compatible EPROMS and burn some, but I think that's no longer possible in 2021. Can anyone recommend a way that I might easily switch ROM images or flash them into something?

Many thanks

Ian
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1024MAK
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Re: Attic 81's - My collection and attempts to repair

Post by 1024MAK »

If it’s working, definitely don’t change TR2. And unless you think TR1 is actually faulty, again don’t change it. ZTX313 transistors have a fast switch off time and I have still not found an in-production good direct equivalent.

The top line bending over is not uncommon. It’s generally assumed to be caused by the ULA.

If you’ve seen any (all?) of my heatsinking ULA rants, err, discussions, you may want to say unless you really want more fun from me :lol:

Yes, the earlier ULA versions do run hot. It’s normal. If it’s not in a small case, I don’t see a problem with a heatsink being fitted. But do read this post (I’m talking about a rubber key ZX Spectrum in that post, but most of it, apart from the bit about the resistor, which is not applicable to a ZX81 as it only has a single positive supply pin, is the same).

Fitting a heatsink where there is a good airflow (if done correctly) will do no harm and it may help with the lifespan of the ULA. But don’t worry about it, or not fitting a heatsink to it. Use the Zeddy until it fails (which may be a long lime). If the ULA does die, well, there are ULA replacement modules available. And there are plenty of ZX81 and TS1000 machines around if you prefer to keep to original parts.

I did post up how I did a Why Wait modification. I’ll try to find it.

The mod. to fit a 2764 or 27C64 (8 kbyte), or a 27128 or 27C128 (16k byte) EPROM is not very complex. Alternatively RWAP on Sell My Retro does sell this which is a plug in module that needs no wiring changes.

Mark
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