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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 23rd Mar 2020, 10:04 am   #141
John Earland
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Default Re: Zx81

I've just plugged the '81 in to the crt and the picture is immediately distorted. This is from cold. It seemed to be working perfectly last night. I haven't moved the unit but simply plugged it in.

I've now sprayed components. I have to say I couldn't see any discernible difference. I switched off the unit.

After about 10 minutes I plugged it in again and the '81 sprang to life. I loaded a games and set it running continuously. After about 1hr and 10mins of continuous use the '81 rebooted for no reason and froze. I unlugged the unit and it began this rebooting cycle before finally emitting a distorted video.

I sprayed the ULA and instantly the video was back to normal. I'm leaving it on again now to see what happens for another hour.

The strange this is though, it gave a distorted video right from cold this morning. However, it does seem to be a fauly ULA.
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Old 23rd Mar 2020, 10:41 am   #142
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Default Re: Zx81

Maybe, but replacing that would be expensive so I suggest you rule the composite mod in or out by reverting the machine to the way it was (ie, with modulator in circuit). Let the machine run for a long time with the best picture you can get tuned in on a TV and see if the machine stays stable.

if it does, remove the simple mod and fit your new CCB PCB, and see how the machine behaves then.
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Old 23rd Mar 2020, 10:55 am   #143
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That's a good idea. I will try that.

I have been running the '81 all morning and each time the video goes bad I spray the ULA and each time the video is restored so it is a bad ULA. I don't think fitting the CCB PCB will be useful?

If this is the case, and the ULA is faulty, should I still revert to its original modulator setting and remove my mod as you suggested above?
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Old 23rd Mar 2020, 11:02 am   #144
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My hypothesis is that the ULA is not faulty but it is objecting to the composite mod for some reason. Remove the composite mod, see if the ULA still behaves the same way. If you think about it, although you had problems before you fitted the composite mod, you didn't have that particular problem.
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Old 23rd Mar 2020, 11:17 am   #145
John Earland
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That's very true. It is definitely worth an investigation. Actually, this is rather fun! I am getting quite an appetite for investigation - but couldn't do it alone. your guidance and advice is brilliant. Thank you.
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Old 23rd Mar 2020, 2:30 pm   #146
John Earland
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One small issue! I cant see where the 5v wire should go! I wish I had taken a photo before I made the mod! I presume the video feed is going to the video out pin but I can see where the 5v wire is I cut!
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Old 23rd Mar 2020, 2:32 pm   #147
John Earland
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For reference for anyone viewing this thread, this link is really good:
https://www.bytedelight.com/?page_id=3560
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Old 23rd Mar 2020, 2:37 pm   #148
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Default Re: Zx81

Nice clear photos of the composite mod there, although that version is the version with the resistor (no diode) in the path from the emitter to video-out.

Did the photos in the article answer your question re: Where 5V wire should go? If not I will dig out one of my (unmodified) ZX81s tonight and take a photo or two of the inside of the modulator.
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Old 23rd Mar 2020, 3:15 pm   #149
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No-unfortunately not. If you could find how yours is wired that would be great. I did try this mod-picture attached-I think my soldering is improving! I have tried this on the LCD and it worked really well-100R to gnd and 330R to video out. However, after a while the same issues occurred-cooling the ULA again did the trick.

However, I am in the process of removing it now.
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Old 23rd Mar 2020, 3:30 pm   #150
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Default Re: Zx81

Will do, but I'm afraid I will have to get home first. I'll try to post some pics early-evening.

It was worth trying the mod + resistor. Your soldering certainly is improving.
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Old 23rd Mar 2020, 5:46 pm   #151
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Quote:
Originally Posted by SiriusHardware View Post
Will do, but I'm afraid I will have to get home first. I'll try to post some pics early-evening.

It was worth trying the mod + resistor. Your soldering certainly is improving.
I found them! Please don't go to any trouble. I have a picture and it is as was before. Good resolution but wobbly image. I shall load in a game and let it run.

Last edited by John Earland; 23rd Mar 2020 at 6:02 pm.
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Old 23rd Mar 2020, 6:38 pm   #152
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OK, it will be interesting to see if your ULA still breaks down when it is no longer driving the buffer transistor. If it keeps on going indefinitely with no change - don't forget, you may need to tweak the tuning on the TV every now and then as the modulator frequency drifts - then I would say have a go at fitting the CCU PCB.
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Old 23rd Mar 2020, 8:54 pm   #153
John Earland
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A little update:
1. I found a spare rf lead and thought I would try that - the result was excellent! I think the cable was faulty to start with! (Picture attached)
2. I kept the computer running for about 2 hours and there were no problems!
3. I will try the PCB as planned.

I will report back.
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Old 23rd Mar 2020, 9:32 pm   #154
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To be honest I thought you had already tried a second RF lead (thought you had two of everything). I guess you'd still rather have a composite output to make it more interchangeable with the other machines.
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Old 23rd Mar 2020, 9:54 pm   #155
John Earland
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Quote:
Originally Posted by SiriusHardware View Post
To be honest I thought you had already tried a second RF lead (thought you had two of everything). I guess you'd still rather have a composite output to make it more interchangeable with the other machines.
No-not two of everything-only psu and ‘81s. Yes, still would like to use it on composite.

Quick question-once in composite mode could this be plugged into a computer monitor via an adapter? I don’t plan on doing it but thought it might be possible.
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Old 23rd Mar 2020, 10:13 pm   #156
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I assume you are talking about a PC monitor, not an RGB / Video monitor?

If a PC monitor with VGA, DVI or HDMI input you need an intelligent electronic converter which takes in the composite signal and converts it in real time to the desired format, it can't simply be done with a passive adaptor (ie, a piece of cable with the right plugs on each end)

There are any number of generic intelligent hardware video format converters available now, probably including composite to VGA and composite to HDMI, but I can't personally vouch for any of them.

If you mean a typical standard definition video monitor of the sort which would be used with the BBC B / Atari ST / Commodore Amiga, those monitors usually have a composite input.
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Old 23rd Mar 2020, 10:29 pm   #157
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Default Re: Zx81

You can get composite to VGA converters for 10-15 but you might end up with sensitivity to video standard compliance as you do with LCD screens. I've also read that some composite to HDMI converters can suffer from noticeable lag which makes them less suitable for gaming. I had a comp-VGA converter to use on my Atari 800xl and had no problems.
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Old 23rd Mar 2020, 10:41 pm   #158
John Earland
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Default Re: Zx81

Thank you for that. I am happy with the LCD to be honest but thought perhaps in the future sometime.

Here’s an interesting thing. I just put the ‘81 together and when I went to put the kid back on the modulator the picture went completely. It seems when you put pressure inwards on the casing it causes picture issues! I’ve managed it but took a little while. I checked everything and can’t see any sort of short etc.
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Old 23rd Mar 2020, 11:35 pm   #159
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Default Re: Zx81

I think all that happened was that you shifted the output frequency when you put the lid on - as mentioned earlier, the 'transmitter' in the modulator is just a simple free-running oscillator and putting the metallic lid on it probably just made the transmitter frequency detune to one side a bit.

All you likely needed to do after putting the lid on (at which point the signal disappeared) was to retune the TV a little.

You can test this theory if you like: Tune it in as best you can with the lid on, and then flip the lid off - does the signal disappear? If so, tune around and find it again. Then, put the lid back on - does the signal disappear again? If so, tune around again until you find it again.

If everything in the previous paragraph is true, then all that's happening is that the modulator output frequency is changing slightly when the lid is present / is not present.
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Old 24th Mar 2020, 7:17 am   #160
John Earland
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Default Re: Zx81

Quote:
Originally Posted by SiriusHardware View Post
I think all that happened was that you shifted the output frequency when you put the lid on - as mentioned earlier, the 'transmitter' in the modulator is just a simple free-running oscillator and putting the metallic lid on it probably just made the transmitter frequency detune to one side a bit.

All you likely needed to do after putting the lid on (at which point the signal disappeared) was to retune the TV a little.

You can test this theory if you like: Tune it in as best you can with the lid on, and then flip the lid off - does the signal disappear? If so, tune around and find it again. Then, put the lid back on - does the signal disappear again? If so, tune around again until you find it again.

If everything in the previous paragraph is true, then all that's happening is that the modulator output frequency is changing slightly when the lid is present / is not present.
Yes, that makes sense. I will try the PCB today. I feel more confident about the soldering now. By the way, I bought a new soldering kit with temperature control so I hope my skills improve further. It also came with a desoldering unit.
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