ZX81 essential maintenance?

Discussions about Sinclair ZX80 and ZX81 Hardware
hjalfi
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Re: ZX81 essential maintenance?

Post by hjalfi » Tue Aug 21, 2018 11:35 pm

Well, it's all arrived --- I haven't powered it up yet, though. One of the keys on the keyboard has a pinhole through it, so I'll have to see if it works. I suspect an order from sellmyretro is in my near future. (It came with an almost untouched manual, Sinclair product catalogue and ZX81 flyer, and three games on tape. Sweet stuff.)

The Memotek 16kB rampack (it has a metal case! Metal!) has some caps in it, although it doesn't look like a power supply. Are they worth replacing? It's all tracks and solder joints as big as my thumb so I don't think even I can cock that one up.

As an aside: the rampack has some dip switches on it. I can't find a manual online; any idea what these are?

McKlaud
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Re: ZX81 essential maintenance?

Post by McKlaud » Wed Aug 22, 2018 12:10 am

Claudius
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1024MAK
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Re: ZX81 essential maintenance?

Post by 1024MAK » Wed Aug 22, 2018 12:30 am

I have multiple Memotech 16K RAM packs. With the exception of the one with a broken edge-connector, they all work fine. Unlike the Sinclair 16K RAM pack that I have...

I use either a ZX81 with a 32K SRAM chip in place of the original 1K SRAM chip (but only 16K bytes is usable in BASIC), or a ZXpand (which has 32K bytes of SRAM on board), or sometimes a Memotech 64K RAM pack.

Hence I have not replaced the capacitors in any Memotech 16K RAM packs. I do have a schematic for them. I would offer to dig it up, but only one part is scanned. As I drew it on paper. And did not scan every page. And now I can't remember which box file the paper version ended up in :oops:.

Mark

hjalfi
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Re: ZX81 essential maintenance?

Post by hjalfi » Wed Aug 22, 2018 11:29 pm

Woohoo, it all works! It'll even load and save, although it won't load the four commercial ZX81 games I have (Psion Flight Simulator, QS Defenda, JD Arcades, and Adventure C: The Ship Of Doom); but this could be anything from cassette tracking problems to the 16kB rampack not working properly on a 9V supply. Yes, I chickened out and didn't try 12V.

I took the lid off, and inside it's a 1981 Issue 3 and looks nice and clean. I hooked up my two composite CRT monitors to the modulator feed and I'm pleased to say that they both more or less work; I have a Sony PVR which produced a beautiful crisp display but only if I set it to low-impedance mode; and I have a cheap and nasty mono portable TV with composite input which displayed a smeared image.
1.jpg
2.jpg
That means that both devices can cope without a back porch; the smearing is consistent with the low-strength signal produced by the ULA not being able to push through a standard 75R load, and that a simple Spectrum-style transistor mod should sharpen it up. Is that correct? (I want to use the TV for the ZX81, so I can use the nice Sony on my Spectrum.)

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1024MAK
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Re: ZX81 essential maintenance?

Post by 1024MAK » Thu Aug 23, 2018 12:48 am

You mean like this:
Image

Alternatively buy this (highly recommended ;) ).

Mark

Moggy
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Re: ZX81 essential maintenance?

Post by Moggy » Thu Aug 23, 2018 3:40 pm

hjalfi wrote:
Wed Aug 22, 2018 11:29 pm
Woohoo, it all works! It'll even load and save, although it won't load the four commercial ZX81 games I have (Psion Flight Simulator, QS Defenda, JD Arcades, and Adventure C: The Ship Of Doom); but this could be anything from cassette tracking problems to the 16kB rampack not working properly on a 9V supply. Yes, I chickened out and didn't try 12V.

I took the lid off, and inside it's a 1981 Issue 3 and looks nice and clean. I hooked up my two composite CRT monitors to the modulator feed and I'm pleased to say that they both more or less work; I have a Sony PVR which produced a beautiful crisp display but only if I set it to low-impedance mode; and I have a cheap and nasty mono portable TV with composite input which displayed a smeared image.

1.jpg2.jpg

That means that both devices can cope without a back porch; the smearing is consistent with the low-strength signal produced by the ULA not being able to push through a standard 75R load, and that a simple Spectrum-style transistor mod should sharpen it up. Is that correct? (I want to use the TV for the ZX81, so I can use the nice Sony on my Spectrum.)
I have the same mono composite TV and far from it being a weak signal causing the smearing the signal is in fact too strong, the transistor mod and the various composite add on boards will make the smearing worse on this TV(been there done it etc).
My cure for this was to put an extra 100ohm resistor from the central core to the earth/ground wire within one of composite leads plug which along side a simple transistor mod works ok for me .

hjalfi
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Re: ZX81 essential maintenance?

Post by hjalfi » Mon Sep 10, 2018 5:41 pm

So... the long delay in response is because I was waiting for hardware to arrive.

I built Mark's circuit; it worked fine on the Sony monitor, with a nice crisp image. No luck on the TV, and absolutely nothing on the composite video capture card or the HDMI converter. So I ordered Pokemon's board, which has just arrived today, and I've fitted it (and fiddled with all the pots).

The status is:

- the picture through the Sony PVM is beautiful and crisp, almost always.

- the picture through the mono TV is horribly smeared and unreadable. In fact, it's actually worse with Pokemon's board than it is with Mark's transistor buffer. I did try building a synthetic load as per Moggy's suggestion (a 200R trimmer across signal and ground) and it had a marginal effect only. I would suspect that the TV's broken, but it displays my Spectrum's video output perfectly fine. I have noticed that the ZX81 picture is very dim, which suggests black level problems.

- the four digital displays I have all show glitchy images --- my big LG TV is the best of all (ZX81 pixels are so big on it!); the video capture card and the mini reversing monitor show images like this:
my_photo-4.jpg
...but the most interesting is what I get through both the HDMI upconverter. The blatant sync failure makes it really obvious to spot where it's failing to line up the scanlines.
lcd.jpg
Pokemon's board looks like it's producing a textbook trace. In the following image, the bottom trace is the input to the board, the upper the output:
osc.jpg
If there were timing issues I'd expect blurred lines as scanlines shift with respect to each other.

My main goal for this were to be able to do video capture; it would have been nice to use the little TV as a monitor for the ZX81. That means I'm 50% there, which is good enough. But it's behaving quirkily enough that I still think there's something wrong. Any suggestions?

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1024MAK
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Re: ZX81 essential maintenance?

Post by 1024MAK » Mon Sep 10, 2018 11:50 pm

Do you know which ULA is in this ZX81?

The three versions are briefly described here. Some ZX81's do have trouble lining up all the vertical pixels. Line tearing (especially the top line) is not uncommon. As Moggy says, signal strength can be part of the issue. Keep in mind also, that the video signal from a ZX81 does not fully comply with the defined video standards anyway, so some modern displays or video capture devices may have problems.

Pokemon's ZX8-CCB module can add the back porch (if missing) if set-up correctly.

Mark

hjalfi
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Re: ZX81 essential maintenance?

Post by hjalfi » Tue Sep 11, 2018 11:05 am

I believe it's the later model of ULA, but like an idiot I forgot to photograph the board the last time I had it open, and I'm a bit hesitant to do so again without good cause (it's a miracle the keyboard tails still work as it is). Judging by the oscilloscope trace a back porch of about 2.5V is being generated by the ULA, which confirms that. (There were only two ULAs, right? The 184E and the 201E?)

In fact, looking at the trace again, I do see that the front porch is missing. Judging by what I've read this is mostly used for calibrating the black level, which explains some issues I've seen. This might also be causing the shunted-left image if the sync pulse is being misread as the front porch.

It's good to know that a little image tearing is normal --- I know that reconstructing crisp pixels from a 37-year old fuzzy analogue signal is a mug's game anyway; besides, it doesn't want to be too perfect or else people won't know it's retro! But I was concerned it might be symptomatic of a further problem.

It's a shame that I get a corrupt image with the HDMI upscaler (the shunted-left image); I wanted to use this for demoing on a big-screen HDMI monitor. It could well be a problem with the upscaler, which was very, very cheap...

Moggy
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Re: ZX81 essential maintenance?

Post by Moggy » Tue Sep 11, 2018 2:27 pm

There are three not two ULA types as Mark has already pointed out ULA 2C158E being the earliest.

As for what's in your 81 if the board is red when viewed through the vents underneath then most probably it has the later back porch ULA 201E, if the board is green then possibly one or other of the two earlier chips are installed.

The earliest ULA was nearly always accompanied by the first issue of ROM chip which had a division rounding error which manifested itself with certain mathematics functions. So if you don't want to take it apart again a quick test is to as ask for the SQR of 0.25, if the answer is anything other than 0.50 then you have the early ROM installed and by extrapolation the early ULA, although these parts could have changed by the owner it has to be said.

EDIT.

Another test for ROM version.

10 FAST
20 LET A = 0
30 FOR B = 0 TO 8191
40 LET A = A + PEEK B
50 NEXT B
60 PRINT A

Will show 854885 for the bugged ROM and 855106 for the 'improved' ROM.

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