TS1000 RF output

[1024MAK]

Excellent information as ever Mark.

Thank you

I have noticed with Memotech ram packs that if you drop the input voltage too much below 9volts in an aid to cooling the regulator then they have a tendency to "drop out".

Yeah, the 16K Memotech RAM packs will, as the ‘+9V’ voltage drops below the level the 4116 DRAM chips are happy with. They are one of the types that employ the cheat I described above.

As a side issue I recently bought a cable to use my Atari STe with a modern TV and if I use it on my supa dupa million pixel RGB monitor those "jail bars" are horrendous but when used with my Mickey Mouse LCD TV then they are totally non existent.

That’s the downside of the drive towards ever higher quality displays
The Atari boards may also benefit from extra decoupling capacitors...

Mark

[bwinkel67]

Thank you all. The information here was exactly what I was looking for. I may try the capacitor mechanism and will also look within the RF modulator for an adjustment.

[bwinkel67]

One more question. When ram pack plugged in I had a video glitch where the top 4 lines would shift to the left about 2 inches and then back again, rapidly. I cleaned the contacts a few times to no avail (it would go away and then come back). I transported the machine to a different home and one final time cleaned the contacts and it ran all night printing "HELLO" with scroll without issue. The only difference was I plugged it into an old 13" CRT color TV vs a 15" LCD TV and I can't imagine that would make any differences since it was a memory problem.

Dirty contacts I'm guessing? Or could it be a problem on the main board?

[bwinkel67]

So the video glitch is actually not the TS1000. It is the LCD monitor. When putting it on another LCD TV or even a CRT TV while the glitch is occurring, it goes away. So something with the Toshiba LCD TV makes it glitch (note that the other LCD TV is also Tosihba but an older model and it actually shows the load screen while loading a new program). All TV's are 13" to 14" so very small.

[Lardo Boffin]

I have various TVs I use with retro equipment and not all of them work well with the same computer. One may produce visual artefacts with a particular computer while the others may not.
The effects vary but with a zeddy it can be a flickery screen or varying degrees colouration of the black text.

I guess some TVs need a signal that is closer to specification than others.

[Moggy]

I have various TVs I use with retro equipment and not all of them work well with the same computer. One may produce visual artefacts with a particular computer while the others may not.
The effects vary but with a zeddy it can be a flickery screen or varying degrees colouration of the black text.

I guess some TVs need a signal that is closer to specification than others.

I agree with you Lardo insomuch as I find my so called topline TV's really need an on-spec signal whereas I have no problem with the cheaper usually home brand TV's.

As for the coloured text effect I find just turning the colour off completely cures it.

[bwinkel67]

So after watching someone play with an old ZX81 motherboard on YouTube, as he was trying to get the video going, he just hooked the lead closest to the edge of the board on the RF modulator to the composite in of his TV and got it to work (adding ground of course). So I did that and indeed, I got a composite signal. I didn't need to use a transistor or resistor and the picture is the same quality as the RF without the added interference.

Here are some pics. Question, am I putting the the ULA chip in jeopardy? Is the transistor/resistor pair to protect it? I'm not going to be using it all that often, just to sometimes do speed tests vs the ZXSimulator and I wanted to avoid opening it up as that usually causes the keyboard ribbon to need cutting and fixing. I did the same trick on the QL, by grabbing pin 7 of the MC1377 and that hasn't caused me any grief.

[1024MAK]

Question, am I putting the the ULA chip in jeopardy? Is the transistor/resistor pair to protect it?

Yes you are putting the ULA at risk The ULA was never designed to feed the 75 ohm impedance of a composite video input on a TV or monitor.
The whole point of the transistor is to boost (amplify) the current so that only a minimum load is presented to the ULA. Plus, if a short occurs, you only blow up a very cheap transistor (cost in the U.K. is less than 10 pence). Original ULAs are no longer available as spare parts, and the ULA replacements are not cheap.

Chips like the ULA are only designed to drive other low power logic circuits. By connecting it directly to composite video input on a TV or monitor, you are asking it for 66mA, but a logic output is normally only rated at around 5mA (depending on the type of chip, it can be less, or slightly more).

The transistor and other components does not have to be in the case of the computer. See here.

You wouldn’t expect the line out on audio equipment to directly drive a full size loudspeaker would you? You would use a power amplifier.

Mark

[bwinkel67]

Yes you are putting the ULA at risk The ULA was never designed to feed the 75 ohm impedance of a composite video input on a TV or monitor.
The whole point of the transistor is to boost (amplify) the current so that only a minimum load is presented to the ULA. Plus, if a short occurs, you only blow up a very cheap transistor (cost in the U.K. is less than 10 pence). Original ULAs are no longer available as spare parts, and the ULA replacements are not cheap.

Chips like the ULA are only designed to drive other low power logic circuits. By connecting it directly to composite video input on a TV or monitor, you are asking it for 66mA, but a logic output is normally only rated at around 5mA (depending on the type of chip, it can be less, or slightly more).

Thanks for the quick response. That was my fear. The composite was a convenience but doesn't improve the picture over the RF so not a big loss. I did check the data sheet on the MC1377 on the QL and that does seem to be able to handle the higher impedance of composite directly. I did add a resistor off of pin 9 to help color but no transistor. I used it for a few months without issue though with my current project I've been using the composite monochrome, which was built-in on the QL, since that is such a beautiful crisp picture (the composite color isn't all that much better than the RF). I figured the TS1000 ULA would handle it similarly so that's why I gave it a go :-/ The ULA is a Sinclair specific chip whereas the MC1377 is for more general purposes.

[bwinkel67]

So I implemented this circuit:

2016-01-03-zx81-composite-video-circuit.gif (2.13 KiB) Viewed 2230 times

And tried it out on one of my TS1000's (the one without "backporch) with success. Note the yellow RCA plug going into the RF modulator is just grabbing the outside for ground and not the RF signal (i.e. the inside pole of the RCA plus has been removed). You can kind of see the very dark image on the LCD TV in the background:

I then tried it on my other TS1000, the one that already has a "backporch" signal and which I had earlier grabbed the composite signal directly through a single wire (which Mark warned against as I could blow the ULA chip) -- that one did not yield a composite signal. I did test the voltages and it is getting the 5 volts to the NPN transistor's collector.

What's going on here? The latter one already gave me a composite signal when I grabbed it directly from the wire going into the RF modulator but when I add the second wire (5v) and insert a NPN transistor and 100 ohm resistor it gives me nothing. The RF signal still works and is still strong.