Re: Zx81
 

I’ve tried the modified ZX81 on a LCD TV on bot SCART and AV1 both produces the picture attached. The lines give a strobe effect. On the crt via SCART I don’t have a problem. I guess it’s not a great lcd? The Spectrum works fine on it lcd though so maybe the mod just doesn’t work well on this tv.

Re: Zx81
 

You can always try inserting the diode and the diode + resistor as per the slightly more refined versions of the ZX81 composite mod. If it makes things worse, just go back to how it is now.

Re: Zx81
 

I would guess the CRT has a lower bandwidth and filters out the noise. So perhaps the LCD is too good rather than bad! Sinclair wouldnt have bothered with noise you couldnt see.... Or maybe you have a bad capacitor somewhere thats not decoupling the noise.

Re: Zx81
 

Thank you for that-I think I might need to replace the caps. I have noticed a couple of things now:

1. The ZX81 performs well at first but after a while the picture distorts (pictures attached)
2. After the distortion the computer switches itself off whenever you press a key. So for instance when I unplug and plug the psu back in I get the ‘K’ display. When I press a key, Sat P, the picture goes blank and I either have to reboot or sometimes it resets itself.
3. I’ve tried 3 different TVs-crt and 2 lcds -same issue
4. I’ve tried two separate psu’s and with difference
5. The picture quality on the lcds is not as sharp as I would expect- the ZX Spectrum by contrast was very sharp. Same modification as on the ZX81.

This all occurs after the computer has been on for about 20 mins or so. I would appreciate any thoughts you might have.

Re: Zx81
 

Annoyingly I can't remember where I saw the version of the composite mod using a diode but I do remember the OP had said he needed to add that specifically to get a signal which was acceptable for his LCD TV. So it may be worth adding just a diode in the video-out path, just a resistor in the video-out path, and a resistor and diode in series in the video-out path to see if your LCD TV finds any of those combinations more soothing.

If you do find a combination which works better with the LCD TV, then go back to the CRT TV and make sure it still works there. What you really want is whichever arrangement works best with them both.

Edit: Just caught your latest. Try this: From cold, power up the '81 without a display connected. Leave it on for longer than it normally takes for the distortion / odd behaviour to happen, maybe half an hour.

Then, connect whichever display usually works best.

Is the picture already distorted / key problem already present? Or does it take ~ 20 minutes from the moment you connect the display for that problem to happen?

Re: Zx81
 

Ah, right. Thank you for that. I will give it a go when I get home.

Re: Zx81
 

I’ve actually ordered a ZX8-CCB video output module which has to be soldered to the main board but it comes from Germany. Not sure because of COVID-19 whether I will get it for some time. I ordered it to solve the issue of the ‘older’ ZX81 I have which has the 1st ULA but I think I will use it on my second one. However, I am intrigued about the additional modification. I too have seen this somewhere - I shall endeavour to find it and share it with you.

Re: Zx81
 

The mod with the diode was on the page i linked in post #69

Re: Zx81
 

Thanks, that article shows variants both with the diode and diode+resistor - I failed to click through on that one.

It will be interesting to see how well that works. I did wonder whether it would be suitable for a 2C210 ULA because it generates the back-porch portion of the signal and the 210 ULA already does that, but on reading through the info about it I see that it does work with all versions of the ULA.

Re: Zx81
 

Ah yes-thank you for that.

Re: Zx81
 

Would you believe the module arrived this afternoon. Now which mod do I do? I think I might like to try the one Slothie posted-any advice would be welcomed. Mind you I don’t have that particular diode so I’d have to order one.

Re: Zx81
 

Pretty much any signal diode will work if it has a forward voltage of 0.6v - check with a multimeter on diode check mode.

Re: Zx81
 

I’m afraid I don’t have any diodes only resistors! I shall order some.

Re: Zx81
 

If your going to experiment much more get a mixed component pack e.g. https://www.ebay.co.uk/itm/1390pcs-E..._trkparms=ispr search on your favorite auction site for "electronic components" . A breadboard, some wite and a £5 multimeter and youve got months of fun!

Re: Zx81
 

In the meantime, try the version of the mod which places a resistor (only) between the emitter and video-out.

Re: Zx81
 

I think I have already done that one I think? #57?

Re: Zx81
 

Okay, I tried your suggestions:
1. The zx81 was switched on for about 30 mins before connecting to the crt tv - I get a better picture at the moment on this one. The picture was very distorted and couldn't make out anything.

2. I let the unit cool down and then I plugged it in and the picture was great (again on the crt) and strobe effect on the LCD

However, after an hour of continuous use the picture became distorted but there is a pattern before it gets to this stage.

The unit began not to load games and when it did it it sometimes reset itself and sometimes it froze. When I rebooted it it then went through the cycle of not loading then resetting and sometimes freezing. After this behaviour a few times, the picture became very distorted.

Re: Zx81
 

And this also happens when you don't have a RAM pack connected?

Sounds like a thermal-related problem. The best way to hunt for this would be using freezer spray - let the unit run until the faults appear and then spray each major component - regulator, ULA, Z80, memory IC, one at a time to see if that makes the fault disappear. Any electroyltic capacitors in the unit are also a possibility (spray those too).

Freezer spray made for the purpose of diagnosing thermal electronic faults is usually quite expensive but if you take a look at this thread, someone has just discovered more or less the same product being sold for an entirely different purpose, and much more cheaply.

https://vintage-radio.net/forum/showthread.php?t=164937

If you can bring yourself to buy some of that, that should work quite well for the purpose.

Re: Zx81
 

Just be careful not to buy freeze cracker spray for removing bolts - it contains release oil that will remain on your circuit and possibly lift off solder.mask or screen printing! Its aboult half the price of "proper" freeze spray.

Re: Zx81
 

Thank you for this. I shall take a look.

Re: Zx81
 

Ah-very good point!

Re: Zx81
 

The thread says that it is available in B&M - have you tried this yourself?

Re: Zx81
 

I found a diode among a lot of bits and pieces I've acquired! I set the multimeter to 2000k diode setting (it's the only diode setting this one had) and the result was 734.

Would you be able to explain that result to me? Apologies for my ignorance - I'm still learning!!

Re: Zx81
 

Apologies if this is a daft question - but the piece refers to putting the diode in the video cable? It has an inner and an outer casing doesn't it? How would you go about that?

Re: Zx81
 

I've just purchased the 'Poop' freeze spray, so wil have a go tonight and report the results.

Re: Zx81
 

Well if your feeling brave you could slit the coax lengthways for about 30mm, tease out the inner cable, cut it and solder the diode in (end with band towards tv end) insulating with shrink sleeving or electrical tape, then tuck it all back inside. More tape to cover the slit.
I would think it would be easier to just reopen the modulator and put the diode in there!

Re: Zx81
 

Agree, you could only practically insert the diode into the video lead if it was one you had made yourself, i.e, by unscrewing the cover of the phono plug at the '81 end of the lead and putting the diode inside there.

Much simpler just to place it between the emitter of the transistor and the video-out terminal inside the modulator. Your 734 reading probably meant .734, the forward voltage drop across the diode, as such that would be an OK reading for a silicon diode. If you measure it on the same range with the probes the other way around you should get a completely different reading, usually '1'. This is also normal.

Assuming this stuff is like the normal diagnostic stuff, does it come with a thin tube (like WD40) so you can aim it more accurately?

If so, get the '81 warmed up, let your fault appear and then with the end of the thin tube about 1cm away from the centre of the chip, fire a series of short bursts until you see a layer of frost form on the top of the chip.

Re: Zx81
 

Thank you. Yes the diode did give a reading of 1.

The spray does have a thin tube attached and so I will have a go.

As for the diode am I right in thinking that the grey band end should be at the video out and the other attached to the emitter of the diode?

Re: Zx81
 

Banded end to video out, blank end to the emitter of the transistor (I know that's what you meant).

Re: Zx81
 

Yes - that's what I meant-it's late!

I've just tried it and it hasn't improved the LCD output and there's no change on the crt in terms of quality of image. Maybe I need to wait until the new diode arrives.

I am now hooked up to the crt waiting for the '81 to go into distortion status.

Re: Zx81
 

Quick question - should the diode in the modulator be getting hot? It's very hot right now.

Re: Zx81
 

OK, sorry, maybe I wasn't clear - I would not expect the spray to have any effect until the '81 has warmed up enough for the fault to appear. Only then is it worth starting to spray things.

I would start with the ULA (chief suspect as that is what generates the video signal and reads the keyboard) and also the regulator IC

For the difference in the way the CRT and LCD screens work, the diode and resistor mods may prove a useful line of attack. The aim of the freezer technique is to find out what it is that is going so badly wrong after half an hour regardless of the display being used (in fact, you have already proved that the machine breaks down without a display even plugged into it, thereby ruling out the displays as a possible cause).

Re: Zx81
 

I wouldn't really expect the diode to be getting super hot - what sort / size is it? Can you give us a good close in-focus photo of the composite mod only so we can look around to see if there's anything which jumps out? And what about the transistor? Is that getting hot as well?

Re: Zx81
 

Hi-the transistor is the one mentioned in the blog-2N3904, the diode? I'm not sure, it's too small for me to make out.

No, I haven’t used the spray yet until the distortion occurs. I have a small BASIC program running at the moment. What’s the betting it doesn’t now happen! Picture of the mod attached.

Re: Zx81
 

Sorry Sirius-forgive me - it’s the transistor that’s getting hot not the diode!

Re: Zx81
 

I think maybe we are getting diode and transistor confused again, did you mean that it is the transistor which is getting hot? (Not the diode, which you don't seem to have fitted yet).

Edit: Just got your update. Well, the transistor is between +5V and 0V with quite a low resistance (100R) in series, and that 100R is in parallel with the likely 75R input resistance of the composite-in input on your display, so yes, it would not surprise me if it ran warm. But if so, there is another target for your freezer spray once the fault appears - spray the transistor as well.

Re: Zx81
 

Sorry - it's the transistor getting hot not the diode!

Re: Zx81
 

Thanks for the new photo but it is still difficult to make out one detail - can you confirm whether the transistor is mounted flat face upwards, facing the camera, or flat side downwards, facing down into the modulator?

Re: Zx81
 

I think this is a better photo

Re: Zx81
 

Well, it's 01.08 my time! I think I'm off to bed. The '81 is running fine after just over an hour. I think I will leave it until tomorrow now and try again. If it's running okay after that I will concentrate on getting this working on the LCD. It would be great if I could because I could just keep the LCD in my workshop along with the Spectrum and the C64! All using the one TV.

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.

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.

Re: Zx81
 

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?

Re: Zx81
 

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.

Re: Zx81
 

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.

Re: Zx81
 

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!

Re: Zx81
 

For reference for anyone viewing this thread, this link is really good:
https://www.bytedelight.com/?page_id=3560

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.

Re: Zx81
 

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.

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.