TS 1000 Composite Mod

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Bill H
Posts: 163
Joined: Sat Nov 27, 2010 6:05 pm

TS 1000 Composite Mod

Post by Bill H »

HI All - I'm stumped on what could be causing my issue. I am trying to add composite out to the TS 1000 basically following this

https://www.classic-computers.org.nz/bl ... r-zx81.htm

And this

https://www.bytedelight.com/?page_id=3560

I am using the correct transistor (2N3904). I do not have a 100 ohm resistor have used 180 & 330 instead but with or without them I get the same scree. the 33 ohm resistor between the transistor and the video out is suggested for brightness - it didn't help. Is it the fact I do not have a 100 ohm resistor on hand or? It works but I get this screen no matter if I add resistors, remove them etc. a washed out "K" that is leaning. Always this screen. Is it the ULA? It's a 2C184E. Also its not the wires going into the can - I added new ones so they aren't the originals hooked up incorrectly.
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dessony
Posts: 342
Joined: Tue Oct 29, 2013 2:26 pm
Location: Indiana, USA

Re: TS 1000 Composite Mod

Post by dessony »

Bill H.,

Try this circuit,
viewtopic.php?f=7&t=2405&p=28600&hilit=veroboard#p25270

I used 1024MAK's circuit on my TS1000. It works beautifully! :lol:

DesSony
bola_dor
Posts: 398
Joined: Wed Oct 02, 2019 5:32 am

Re: TS 1000 Composite Mod

Post by bola_dor »

Yes!! The MAK1024 buffer mod is much more better..
I had several troubles with the simple transistor/ resistor mod.
In the meanwhile if the problem is the intensity try using a signal diode (the smaller glass ones 4148) on the transistor base, the lead coming from the ULA, with anode pointing to the transistor..
Ernesto
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1024MAK
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Re: TS 1000 Composite Mod

Post by 1024MAK »

Okay, four things:

The 100Ω resistor is needed. Otherwise the video levels will be even worse. The transistor needs to have a path back to 0V/GND. When you say that you have tried a 180Ω and a 330Ω resistors, do you mean individually, or do you mean you have wired them in parallel? When wired in parallel, this will result in a combined resistance of about 116Ω which should be close enough.

This simple composite video circuit does rather rely on the TV or monitor accepting the non-standard video signal that does not conform to the standard video levels, so you may have to adjust the brightness and contrast controls. Even then, some TVs and monitors will not produce a very good picture. As bola_dor says, if the image is too bright or has too little contrast, you could try adding one or two signal diodes (1N4148 are the easiest to get types) in series with the transistor base terminal. That is, between the ZX81/TS1000 video output and the transistor base terminal.

The 2C184E ULA does not produce the ‘back porch’ part of the video signal, so some colour TVs and some colour monitors will be confused by this, as they try to use this (missing) part of the signal to auto-calibrate to set the black signal level. Because the video signal produced by the first two ULA versions does not have the ‘back porch’ part, at the time they sample the video level, it’s at the wrong value, hence they get the black level wrong. Even with adjustment of the brightness and contrast controls, this results in a dark grey ‘colour’ instead of a white ‘colour’, and poor contrast. To deal with this problem, you need a more complex ‘composite video’ circuit, one that generates the ‘back porch’ part of the video signal. Alternatively use a 2C210E ULA or a ULA replacement module.

The text (K) leaning is a timing problem, the horizontal video lines that make up the video image are not being produced at exactly the same time. This causes each video line to start in a slightly different place, hence the vertical pixels don’t line up perfectly. Unfortunately some ZX81s and some TS1000 machines do this. It could be the ULA or it could be the resonator.

Mark
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