New find zx81 and updates

[craftsman1234]

Well a week ago I was able to pick up a zx81 to go with my TS1000 (original owner).

I ordered parts for it to upgrade it to composite video and 16k internally. All the parts came by Friday. So I was up really late hacking it all together.

Some interesting things found inside.

Attached to the bottom of the keyboard. Appears to be some type of buzzer. It was not connected. Anyone know what program used it? The prior owner mainly had business software.



Made up the Composite video based on the 555 and added turn based pots like on the zxvid board.



Then after getting the rf box reattached to the board an wired up and found it was working I adjusted the pots for a nice view. I then followed up with the 16k internal expansion. It was a bit more of a pain as there was a trace on the tip of the board I missed cutting. 4 total were needed. 1 on top and 3 on the bottom. Required removing the socket I had just put in.



Before the clean up.



and all up and running today.



I did the upgrade on the video to allow me to use the zx81 with my Dell monitors but found an interesting problem. They won't sync correcty for any hires screen or PAL game. Bummer. Lucky as seen in the above image it does work my little TV.

[sirmorris]

Good job!

[Tezz]

Good stuff, I've made a few of the 555 based Composite video boards too, do you have a details of adding the turn based pots? That was something that I wanted to get around to asking advice on here

[craftsman1234]

Good stuff, I've made a few of the 555 based Composite video boards too, do you have a details of adding the turn based pots? That was something that I wanted to get around to asking advice on here

The 555 design came from JoulesperCoulomb YouTube videos. On his design there are 4 resisters. I replaced the 68 and 56 ohm resisters with 100 ohm pots. For me R4 the 56 ohm did the more adjustment to the output. The POTs do cause some issues with it fitting in the RF shield. I don't really know what adjustments are at now.

I set the pots to the same values to start but have no idea what they are at now. I wish I could test it out more but the zx81 still has the original keyboard and close to failing. In the future I may relocate the pots to just under the keyboard and drill to small holes to allow adjustment when it is all closed up.

[1024MAK]

His PDF to go with his video is here

Mark

[zx81jens]

I think the Buzzer could be some sort of Keysound to get a response when typing...

Greetings
Jens

[Tezz]

The 555 design came from JoulesperCoulomb YouTube videos. On his design there are 4 resisters. I replaced the 68 and 56 ohm resisters with 100 ohm pots. For me R4 the 56 ohm did the more adjustment to the output. The POTs do cause some issues with it fitting in the RF shield. I don't really know what adjustments are at now.

I set the pots to the same values to start but have no idea what they are at now. I wish I could test it out more but the zx81 still has the original keyboard and close to failing. In the future I may relocate the pots to just under the keyboard and drill to small holes to allow adjustment when it is all closed up.

Thanks for the details. Did you have issues with the output on your TV/s with the documented design? I was concerned that POTs were included with other solutions perhaps as a necessity and Alastairs (JoulesperCoulomb) documented design would not always provide a suitable output on some TVs.

His PDF to go with his video is here
Mark

Thanks Mark, that's the guide I followed to build them, if you recall a few months back with the first one I put together I didn't cut through the tracks properly in a couple of places

[craftsman1234]

Thanks for the details. Did you have issues with the output on your TV/s with the documented design? I was concerned that POTs were included with other solutions perhaps as a necessity and Alastairs (JoulesperCoulomb) documented design would not always provide a suitable output on some TVs.

I am out putting to an LED display. It did work with my old TV but a bit dark even adjusting it. I could not really play too much with it due to the keyboard. Just had to look at the "K". I could see it t least. Before with the RF it was almost black.

My TS1000 has the transistor/resister simple circuit in it and it works pretty well. It seems to be hit and miss with different TVs.

I'll probably build another one for the TS-1000.

[Tezz]

Thanks, I've only tried mine on a 1999 CRT TV that I have in the back room which gave a very good output. I should take a look on the modern LED TVs that I have in the house too to see how it fairs.

[1024MAK]

Ahh LED TVs. That actually use LCD technology for the display. The LED name only refers to white LEDs being used in place of cold cathode tubes used for the back lighting for the LCD.

Well done the TV marketing departments for misleading customers...

From Wikipedia

An LED-backlit LCD is a flat panel display which uses LED backlighting instead of the cold cathode fluorescent (CCFL) backlighting used by most other LCDs. LED-backlit LCD TVs use the same TFT LCD (thin-film-transistor liquid-crystal display) technologies as CCFL-backlit LCD TVs. Picture quality is primarily based on TFT LCD technology, independent of backlight type. While not an LED display, a television using this display is called an LED TV by some manufacturers and suppliers. In the UK, the Advertising Standards Authority has made it clear in correspondence that it does not object to the use of the term LED TV, but requires it to be explained in advertising.

LCD TVs sample the incoming analogue signal and convert it to a digital signal. Unlike CRT TVs which use the analogue signal to drive electron gun(s).

How well (or not) a modern TV does with the output of a 1980s computer depends more on the design of the analogue to digital conversion and subsequent digital processing than anything else.

Mark