Sinclair ZX80 / ZX81 / Z88 Forums

r00t wrote:I don't get the point why is so difficult to convert VHF/UHF to composite, I remember back in the VCR era most has VHF/UHF input and composite output, I am right?

RF / VHF / UHF signals are very tricky to work with, because they are radio frequencies. This range of frequencies (and higher frequencies) were chosen because they could be transmitted long distances through the air.

If you do it the traditional way, using analogue circuitry, when trying to design and construct electronics to operate at these frequencies, you have to use expensive, (and increasingly hard to find) transistors and diodes that have specifications that are suitable (low self capacitance and for the transistors, a respectable gain at high frequencies). Then you have to find suitable transformer cores, then wind your own windings (of wire) on the transformer former (core). You have to custom design a PCB. This PCB has to be very carefully designed. If any of the amplified signal gets back to the input, it will amplify that instead of the input signal and goes out of control (producing garbage). It's not easy designing and building good audio amplifiers using transistors. Let alone RF amplifiers and demodulators (where you have a RF amplifier stage, a mixer stage, a detector stage and a video amplifier stage).

If this all sounds like a black art, that's because it is. As digital television has replaced, or is replacing analogue television around the world, so there is less and less demand for products that do what you want. That's why its so hard to find anything suitable.

Add to that, the demand for digital / HDTV, and and as chip based digital processing has fallen in price, so these out compete analogue designs. But in general, the digital / HDTV products are designed for the last versions of the analogue standards using composite signals.

Also, as TV continues to be driven in a quest for better picture quality, keep in mind that analogue RF/VHF/UHF signals are very low quality compared with "standard definition" digital TV picture quality.

The best that a ZX81 / TS1000 can do (after modification), is monochrome composite video (a cleaned up monochrome composite signal is much better than RF/VHF/UHF). Or with a more expensive external interface (the Chroma interface) it can do TV standard RGB (great for TVs used in Europe where RGB SCART sockets are available on most TVs, not so great for elsewhere in the world).

Mark

@1024MAK
Thanks for the explanation on UHF/VHF signals.

Actually I am not sure any of those ZX81's are indeed alive.

When I connect them to the wall I hear no noise at all. I was expecting to hear some noise at least from the ac adaptor.

There is any way to test if the ZX81 is alive without having a monitor?

I was trying to do something like

SAVE "aa"

To try to hear some noise coming from the EAR connector but nothing.

Any advise?

thanks.

btw: I don't have for the moment any electronic equipment, so I need a poor man's solution.

r00t wrote:I was trying to do something like

SAVE "aa"

To try to hear some noise coming from the EAR connector but nothing.

Looking at this thread viewtopic.php?f=9&t=1942&hilit=save they have the same problem.

The sound coming from a ZX81 is so low, you can't hear it in earphones.

@mrtinb

thanks!, they come up with the same idea.

I have just leave the 81 connected for some time and it is hot now, so the AC adaptor looks it is working.

Yes, most Sinclair power supply units / AC adaptors don't make much noise. And of course, the ZX81 has no sound output. So until/unless you get a TV or other video system hooked up, it's not easy to work out if the computer is working or not.

Mark

Instead of looking for shielded video RCA m-RCA m, use a video coax RG-5 (it is more common and is shielded by default) with the items I mentioned previously and the following:

http://www.ebay.com/itm/10-QTY-Pack-F-F ... Swx-9WzgJ-

You're making things rather difficult by trying to use a laptop as display device. That 'TV box' linked above outputs VGA, can you even use that to control your laptop's screen? What inputs does that laptop have to put a video signal into?

r00t wrote:I don't get the point why is so difficult to convert VHF/UHF to composite, I remember back in the VCR era most has VHF/UHF input and composite output, I am right?

The ZX81 produces a b/w composite video signal internally. It's much better to pull this signal from the ZX81 before it ever goes into a VHF/UHF modulator. See for example here (better to add a ~100 uF capacitor in series with the output, but that's the basic idea).

1024MAK wrote:And of course, the ZX81 has no sound output. So until/unless you get a TV or other video system hooked up, it's not easy to work out if the computer is working or not.

Well as suggested, you can try SAVEing a program and see if the MIC output produces anything. But that's a very weak signal, and of course this only works if the keyboard works too... (another for old ZX81's).

Imho the best course of action is to do that composite mod, and find a small/cheap TV (LCD or CRT, your choice) for use as dedicated ZX81 display. Or a composite -> VGA converter board, and use with a cheapo (separate) VGA monitor. Such monitors can be found practically for free these days.

r00t wrote:
I was trying to do something like

SAVE "aa"

To try to hear some noise coming from the EAR connector but nothing.

SAVEing will output to MIC port (not EAR) which is such a small voltage (no where near line level) it needs a preamp to get it to linelevel to send it to an amp to drive a speaker.

You're making things rather difficult by trying to use a laptop as display device. That 'TV box' linked above outputs VGA, can you even use that to control your laptop's screen? What inputs does that laptop have to put a video signal into?

The USB device I referenced would allow him to view on the screen.

The ZX81 produces a b/w composite video signal internally. It's much better to pull this signal from the ZX81 before it ever goes into a VHF/UHF modulator. See for example here (better to add a ~100 uF capacitor in series with the output, but that's the basic idea).

He did not want to open case and solder anything.

@gammaray

I have decided to follow your advise and I got the USB/analog TV hardware.

I'll keep updating.

thanks.