Zx81

[John Earland]

Originally Posted by Slothie

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.

Ah-very good point!

[John Earland]

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

[John Earland]

Originally Posted by Slothie

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

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!!

[John Earland]

Originally Posted by Slothie

Inside the RAM pack there should be a 22uF electrolytic across the supply rails. If this has gone bad then it wont be supressing all the noise generated by the 12v inverter circuit used to supply the DRAM chips. This is likely the source of your interference.

Schematic & info:
https://www.myprius.co.za/ZX81.htm

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?

[John Earland]

Originally Posted by SiriusHardware

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.

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

[Slothie]

Originally Posted by John Earland

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?

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!

[SiriusHardware]

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.

[John Earland]

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?

[SiriusHardware]

Originally Posted by John Earland

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?

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

[John Earland]

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.

[John Earland]

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

[SiriusHardware]

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).

[SiriusHardware]

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?

[John Earland]

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.

[John Earland]

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

[SiriusHardware]

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.

[John Earland]

Originally Posted by SiriusHardware

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?

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

[SiriusHardware]

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?

[John Earland]

I think this is a better photo

[John Earland]

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.