ZX81 no picture

Discussions about Sinclair ZX80 and ZX81 Hardware
yukin
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Location: Québec, Canada

ZX81 no picture

Post by yukin »

Hi, im new to the forum :)

And i just got myself a ZX81 (american version) im talking about a real zx81 american version, not the timex 1000.

I receive it today but the computer don't work.

i connect the computer to the tv. on VHF channel 3. when i power the computer, the screen do some flickering but nothing else. no image.

Somebody know what could be the problem?

Oh and when i open the case of the computer i notice the keyboard cable was half cut already lol.
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XavSnap
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Re: ZX81 no picture

Post by XavSnap »

Hi,
We had to know more informations about your TV screen...
The picture is Dark, the screen is white but without "k" cursor or there's nothing (it's swnowing!)
Take a photo !

You had to start the ZX81 with a RAM module. Plug it in, while ZX81 is off, and start the computer.
Xavier
yukin
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Re: ZX81 no picture

Post by yukin »

it is snowing But i think i found the solution this morning while reading the manual. Because my TV use a VHF 75ohm and need a 300ohm signal, the manual said i need to go to my electronic store and buy a VHF UHF Transformer. It supose to convert the signal 75ohm to 300ohm. Will try that tonight(i hope it work)
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XavSnap
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Re: ZX81 no picture

Post by XavSnap »

...Or try to scan others frequencys...
Xavier
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TMAOne
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Re: ZX81 no picture

Post by TMAOne »

I'm in Canada and have a ZX81 (again not Timex) from early on. Unlike the Timex iterations, it has a UHF modulator.

Try reconnecting your antenna leads and looking for your signal on UHF. (Channel 38 rings a bell but I'm not certain of that.)
XorA
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Re: ZX81 no picture

Post by XorA »

TMAOne wrote:(Channel 38 rings a bell but I'm not certain of that.)
Channel 35/36 (ish) if they are tuned the same as UK ones.
yukin
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Re: ZX81 no picture

Post by yukin »

TMAOne wrote:I'm in Canada and have a ZX81 (again not Timex) from early on. Unlike the Timex iterations, it has a UHF modulator.

Try reconnecting your antenna leads and looking for your signal on UHF. (Channel 38 rings a bell but I'm not certain of that.)
Hello brother :) me too im from Canada lol

But for your ZX81, is it a british version or american one? because my zx81 have the switch channel 2 and 3 under the case.

Maybe someone can explain it to me, here exactly what it said in the manual:

If your television has two antenna connections markes UHF and VHF, use the one marked VHF. (if you have cable TV, or a 75-ohm lead from your antenna - the round wire with the screw-on coupling instead of the flat wire - you will need to buy a "UHF/VHF matching transformer" for around three dollars at a local computer or electronics store. This simply converts your cable/75-ohm input to a two-wire, 300-ohm standard that will hook up to the switch box provided with your ZX81). Plug in the ZX81 power supply and switch on the television. You now need to tune in the television. The ZX81 operates on channel 2 or 3 VHF in the united states.

so i have a old TV from 1985 with the VHF and UHF input. So i got today a UHF/VHF matching transformer (1$). connect the video cable to a coax adapter and then connect it to the transformer. now i have the two-wire UHF on the other end. in the instruction they saying i have to screw them to a switch box that came with the machine. i don't have it but i have some already. from there all is good. But oh oh, on the switch box i have it only have the two-wire connector to connect to the tv. behind my tv i see the VHF connector that is a Coax one and the UHF connector who is the two-wire one. I have to plug it to the VHF but how???????????????????????????? do i have the wrong switch box? something wrong i made? on my TV i have the UHF connector but i don't have any option on the tv or remote to go to a UHF signal. :cry:

Here are some pictures

Image

Image

Image
yukin
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Re: ZX81 no picture

Post by yukin »

Or just screw it. i will put the composite mod.
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TMAOne
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Re: ZX81 no picture

Post by TMAOne »

Here in Canada I've seen (and have examples of) three flavors.
  • ZX81 with UHF modulator
    ZX81 with VHF modulator (later version)
    Timex-Sinclair 1000 with VHF modulator
They were all intended for the market on this side of the pond, since the European folks use a different TV system. I don't think a unit from one side would work on the other without a new modulator, or perhaps its a simple wiring change. (Can someone answer that?)

Your unit, if it has a switch for channel 2 vs 3, is most definitely a VHF unit.

Since you already have an RCA to TV coax adapter, try the following deceptively simple solution: Ignore the transformer, and the switch, and plug the RCA coax/adapter combination directly into your TV's VHF input. I'm not kidding. I only discovered recently that it seems to work better without the impedance matching transformer that I thought for 30-odd years was necessary!

I've found it to be a good way to hook up to a modern television too. Not as good as composite mod, but better than any of the plumbers' nightmares I'd wrestled with previously.
RWAP
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Re: ZX81 no picture

Post by RWAP »

Just some comments from the guys on Facebook who watch this feed:

"The 75 ohm vs 300 ohm situation shouldnt be a problem, this is just referring to the 'impedance' of the feed line and system, worst scenario is that you will have a mismatch and would really only affect weaker signals. As the signal from the modulator is pretty strong, and generally stronger then the average TV signal you shouldnt have issues. Im reading between the lines here and wondering if you are trying to view the picture on the wrong frequency? Hence the snowy picture. If you have a UHF modulator in the Sinclair, that would have an output impedance of 75 ohms generally, and if you were trying to view the picture on one of the American VHF channels, American systems are generally 300 ohms impedance. Now the modulators will generate harmonics and yes you could receive a very weak signal on a VHF tv from a UHF modulator, and you could also receive a signal from a VHF modulator on a UHF frequency, but being a harmonic, it would be weak and give the results you are getting. You need to ascertain what the modulator is and its number, and what band the tv is tuned to VHF or UHF.....Hope that helps

.... little bit of a search later...The Timex version was Ch2 VHF, the Sinclair version was Ch36 UHF...I think youve tuned to a weak harmonic and not the main signal."


Personally my bet is that the ZX81 is missing the back porch latch and so the TV cannot find a strong enough signal (this is what normally results on the black text on a black screen) - older black & white TVs did not have a problem with this, but modern TVs do. If so, the composite video mod will not help and you need a ZXVid Module, or a ZXCCB. :D
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