I think that I damaged my ZX81

Discussions about Sinclair ZX80 and ZX81 Hardware
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PokeMon
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Re: I think that I damaged my ZX81

Post by PokeMon »

RetroTechie wrote: Another reason why not is electrostatic discharge, especially when (dis)connecting big equipment with the Zeddy in between. Say you have a PC soundcard connected to ear input, and you plug in RCA connector to TV. The tip of the connector is normally the first to touch, so any voltage equalization between TV-ZX81-PC then goes through the ULA pin. By the time the outside of the RCA connector (ground) makes contact, the damage is done.
I do not agree in this point. A capacitor wouldn't help to avoid this discharge. As you probably know how a capacitor works, a potential difference between both ends (one at RCA connector and one at ULA) would result in a current peak which goes through the capacitor in the ULA until capacitor has been loaded with the voltage difference. Under normal circumstances your items you should not have these problems. If you have a metal case, it has to be connected to earth or if you have a plastic case it should have a galvanic isolation due to some industrial standards. So if you have problems with a discharge caused from these components you should verify your items. Something must be wrong.

RetroTechie wrote: Wrong! The other end of the capacitor will go to voltage level it's loaded to. For example schematic of my old TV shows a plain 75 Ohm resistor to ground at its composite video input (SCART). So if ULA pin output averages 3V, capacitor would charge to 3V, other end would average near 0V (and thus little point in adding a diode if your goal was a 0V average), and the ULA pin would behave as if it were loaded with 75 Ohm to a 3V DC source. Supplying current when output >3V, sinking current when output <3V, but on average certainly less than with 75 Ohm to ground. Agree with the value though - 100 uF or more is good, with smaller values you might see some distortions in screens with big black (horizontal) bars & such.
Well my description was not exactly, you are more exactly. A normal Video signal has 1 Volt pp - so the average current is less with a capacitor in comparison to a directly connection to the ULA. But (!) it is anyway too much. So 1 Volt would give peak currents of about 14 mA. Depending on screen content could be effective less average but with a empty screen (white background) it's more near to 14 mA. A R/C combination of 75R and 100uF has a T of 7.5 milliseconds.
This means, for a horizontal sync period it does not go to saturation and it's resistance is nearly zero in this calculation. As you know, the output of the ULA is much more (about 2.5 Vpp) - so could calculate up to 35 mA. Even if voltage breakdown with load the chip has more internal power dissipation. So I would be careful connection ULA pin only with a capacitor to a 75 Ohm load. ;)

RetroTechie wrote: You'd think so, but when I tried this once with different capacitor types (like tantalum, which usually has much better high-frequency behavior than aluminium electrolytic), I couldn't find any visible differences. And as long as the pixel frequency gets through, damping higher frequencies may actually improve image quality. :shock: So I guess for this application cheapo electrolytic is good enough, and other factors (ULA, output circuit, power supply, video cable, TV) are more important for image quality.
I didn't exactly try to measure it out. As I developed a video signal reconstruction circuit I found that it is important to have fast transitions form lo/hi and hi/lo for a crisp picture. It does make the transitions new and better than ULA and kills most of internal noise from the ULA. To have fast transitions you need high slew rates which contain a significant amount of 3rd, 5th even 7th harmonic of maximum pixel frequency (could be 3.25 MHz, exactly the cpu frequency).
So for a good signal/picture we are talking about 22 MHz or more which should pass without significant damping. Picture would get weak and you have a kind of antialiasing effect which does not look very well on modern TVs.

If you look at this picture, you can guess why big electrolytic capacitors in combination with 75 Ohm load are not best choice.
http://de.wikipedia.org/w/index.php?tit ... 0220081224
gozzo
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Re: I think that I damaged my ZX81

Post by gozzo »

As for 'static discharge' the answer is easy - connect all equipment BEFORE switching on! As for the diode making no difference - very possibly, but the video waveform is not 'symmetrical' ,the average DC level will vary depending on whether displaying a mostly-white or mostly-black picture, the diode just prevents it going too negative at the TV input - this MIGHT cause a problem with some TV's, I don't know. In all cases an emitter follower would be advisable whatever! (I have got hold of an issue 1 'kit built' ZX81 which has an emitter follower video output stage fitted instead of the modulator, using various 'unused' pads and tracks on the PCB originally for the French/USA versions!!) Unfortunately not much good at the moment as it has the 2C184E 'back-porch-missing' ULA! As for 'big electrolytic capacitors feeding 75 ohm inputs' not a good idea, possibly, depends on the ESR / 'inductance' of the capacitor ...
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Re: I think that I damaged my ZX81

Post by gozzo »

As for finding out what the problem is - the only sure way is to try a known-good ULA.. and use "1024MAK"'s video buffer!!
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RetroTechie
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Re: I think that I damaged my ZX81

Post by RetroTechie »

PokeMon wrote:
RetroTechie wrote: Another reason why not is electrostatic discharge, especially when (dis)connecting big equipment with the Zeddy in between. Say you have a PC soundcard connected to ear input, and you plug in RCA connector to TV. The tip of the connector is normally the first to touch, so any voltage equalization between TV-ZX81-PC then goes through the ULA pin. By the time the outside of the RCA connector (ground) makes contact, the damage is done.
I do not agree in this point. A capacitor wouldn't help to avoid this discharge. As you probably know how a capacitor works, a potential difference between both ends (one at RCA connector and one at ULA) would result in a current peak which goes through the capacitor in the ULA until capacitor has been loaded with the voltage difference.
Agree with you, PokeMon - I was talking about transistor buffer in between (which is what I'd do, that circuit posted by Mark looks good). For protection against electrostatic discharge, ULA pin -> capacitor -> TV might as well be ULA pin -> wire -> TV like you say. But IF you go that route, a series capacitor would help to reduce the current load for the ULA output, as compared to ULA pin -> 75 Ohm load. Even with the large peak-peak voltage swing that ZX81 ULA's may produce.
Under normal circumstances your items you should not have these problems. If you have a metal case, it has to be connected to earth or if you have a plastic case it should have a galvanic isolation due to some industrial standards. So if you have problems with a discharge caused from these components you should verify your items. Something must be wrong.
You wish... Take equipment A that has metal case & properly grounded (which isn't always so, btw). And take equipment B which is double isolated. Now lets say equipment B is a big TV which has relatively large capacitors in its AC mains filter (between equipment ground & AC mains lines). This puts equipment B's ground at roughly 1/2 the AC mains voltage - a weak coupling, but you can feel it sometimes. Now connect equipment A & B, and you have continuous equalizing current in between @ AC mains frequency.

These problems are common & well known, unfortunately with no easy fixes. For example: in above situation you could remove equipment B's capacitors from AC mains filter. Problem solved? Nope - equalizing current might be much less, but lots of interference might be radiated into AC mains wires of the home. So that would be trading one problem for another (those capacitors are there for a reason).
gozzo wrote:As for 'static discharge' the answer is easy - connect all equipment BEFORE switching on!
That's usually a good idea, but not a fix for all issues like above.
PokeMon wrote:If you look at this picture, you can guess why big electrolytic capacitors in combination with 75 Ohm load are not best choice.
http://de.wikipedia.org/w/index.php?tit ... 0220081224
Nice picture. Btw it shows all electrolytic caps in the order of 1 Ohm impedance @ 10 MHz - which would make any of those capacitors suitable fine for this purpose. Of course the specimen you have in hand is different... :? (either better, or worse)

So in order of preference:
  • Circuit shown above
  • Simpler transistor buffer, like shown here, optionally with series capacitor added between transistor and TV (that's what I have in most of my ZX81's)
  • ULA pin -> series capacitor -> TV
  • ULA pin -> TV
3rd option is better than nothing, last 2 options are worthless for ESD protection (and 2nd option not optimal for that either).
gozzo
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Re: I think that I damaged my ZX81

Post by gozzo »

that simple transistor buffer would be ok, but I would do it the opposite way, a PNP transistor, connected collector to ground, emitter to the resistor,fed from the 5v line... in the NPN version if an output capacitor isn't used and the transistor should go collector to emitter short (rare,but possible) it would feed the full 5v direct to the composite output ! I wonder, is there any small (8 pin?) video op-amp buffer chips, preferably needing no other components, suitable for this?
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PokeMon
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Re: I think that I damaged my ZX81

Post by PokeMon »

gozzo wrote:I wonder, is there any small (8 pin?) video op-amp buffer chips, preferably needing no other components, suitable for this?
This might be what you are looking for. ;)
http://datasheets.maxim-ic.com/en/ds/MA ... AX7452.pdf

Costs about 3 US$ on 25 piece basis and doesn't need any external components, just some pin on GND or VCC (5V) as desired (8 pin SOIC, quite small).

Of course have many more video products.
http://www.maxim-ic.com/products/video/
luigi
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Re: I think that I damaged my ZX81

Post by luigi »

Hi my friends :D

I have been very busy. My parents visited to me last week and the weekend I was in Gdansk to view Spanish Team to have the EURO2012 match against Italy.

I have tested the voltage at ULA pin 16 and i can read 3.88 Vdc. Does it mean that my Z81 is alive?

Could be that the UHF is damaged? If the problem is this I don't mind, I will made the mod using protection as you are explained me.

Other option is... buy another zeddy but I want my zeddy.
gozzo
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Re: I think that I damaged my ZX81

Post by gozzo »

you really need to try changing the ULA to be sure.(if it's soldered in that can be a pain!) As for the UHF modulator being damaged - extremely unlikely, it doesn't hurt them even if you short the video input to ground or +5v ( +9v may be a different matter,though!) A duff ULA or even CPU is most likely.
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1024MAK
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Re: I think that I damaged my ZX81

Post by 1024MAK »

luigi wrote:I have tested the voltage at ULA pin 16 and i can read 3.88 Vdc. Does it mean that my Z81 is alive?
This is a good sign. Getting this voltage indicates that the output "driver" circuit in the ULA chip appears to be okay. It does not however prove that the whole of the ULA is good.
luigi wrote:Could be that the UHF is damaged?
As gozzo says, these are hard to kill.
When you reconnected the wire or connection, are you sure that your soldering was good?

If possible, some good close up pictures would allow us to see your Zeddy. It is hard to carry out remote fault finding :( Pictures help :mrgreen:

Mark
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luigi
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Re: I think that I damaged my ZX81

Post by luigi »

My Z80 works fine !!!!

I have revised all connections into the RF tunner and there was a component which was touching other. I moved it and I tried to connecto to my TV.

Thank you, you have been very kind.
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