ZX81 with strange addon board soldered to it

[mstreurman]

So, I recently received a ZX81 from a friend's father who used to be a electrician, he has had this Issue3(?) ZX81 from its inception. When I opened it up, to check for any issues before turning it on, I found a strange board with a piezo speaker attached to it. So I suspect it is some sort of sound mod the thing is, I've been Googling my ass off for the last couple of days to find out what it exactly is/does and found zilch. So here is where you guys come in...

here is a picture of said PCB:

Soundboard mod?

On the right side of the picture you can see the heatsink for the regulator.

Left chip is a Toshiba TC4068BP

Toshiba TC4068BP

Right chip is a Schmitt Trigger HEF4093BP:

Schmitt Trigger HEF4093BP

chip2.png (139.58 KiB) Viewed 3522 times

If you guys need more information, don't hesitate to ask me, I am more than willing to help. The person I got this ZX81 from doesn't remember exactly what it does or if it's even functional, it's too long ago.

I am a bit hesitant to take out the mainboard itself as the keyboard is still attached to it and I don't want to accidentally break it... I did take a look at it by unscrewing it. The solder mask on the top of the board is green and the bottom is purplish red, it has 2 1KBx4bits RAM chips and it seems everything but the ROM is soldered to the board. The ROM is in a socket that seems to be about 2 pins each side to long (IIRC)

[1024MAK]

I found a strange board with a piezo speaker attached to it. So I suspect it is some sort of sound mod

If it has a speaker, it must be a sound system of some kind, given that it uses only two inexpensive simple logic chips, at most, it’s only going to be the equivalent of the beeper on the ZX Spectrum.

The board looks like a DIY electronics project.

I found
Left chip is a Toshiba TC4068BP

This chip is an eight input gate with AND and NAND function outputs, so is most likely being used as address decoding and control. - edited. See the later post below.

The solder mask on the top of the board is green and the bottom is purplish red, it has 2 1KBx4bits RAM chips and it seems everything but the ROM is soldered to the board. The ROM is in a socket that seems to be about 2 pins each side to long (IIRC)

Various coloured solder masks were used. The angular PCB tracks do mean it is likely to be an issue three board. The issue one board was hand drawn and there are no known issue two ZX81 boards.

It’s common for issue three boards to have most chips soldered. And yes, the ROM socket being 28 pins but with a 24 pin chip fitted is normal.

Can you please tell me where the red and blue wires go on the ZX81 board, or post up photo(s) showing this. A photo of the underside of the sound board would be useful as well.

Mark

[jdfan1000]

The 4093 is often used to make an oscillator in the synth community. My suspicion is that it's a "beeper": when any key is pressed, it beeps. There were more than a few designs for this in the US.

[mstreurman]

Can you please tell me where the red and blue wires go on the ZX81 board, or post up photo(s) showing this. A photo of the underside of the sound board would be useful as well.

Here a picture of the whole underside of the board with the connection points for the red and blue wires circled:

Underside ZX81 Board

Blue goes to ground, and Red goes to a live wire on the voltage regulator (I don't have the schematics here so I don't know how much Volt)

Underside Mod:

Underside Mod Board

Can't make a better picture as the speaker is still glued to the chassis.

Top of ZX81 with the mod attached:

Top ZX81 Board

I did find out that my ULA is of the 210E kind, thus the one that generates a backporch, so I should be able to use it with any TV, right? I should be able to connect a 75Ohm cable from the modulator to the back of my LCD TV? What channel is it supposed to be on?

[1024MAK]

The 4093 is often used to make an oscillator in the synth community. My suspicion is that it's a "beeper": when any key is pressed, it beeps.

So having now had a chance to look at the photos without the inconvenience of the train movement (I was travelling earlier), I agree with jdfan1000.

The red wire is picking up +5V from the 7805 voltage regulator. The blue wire is as said, the 0V/GND connection. The five short wires that are soldered to the five way keyboard connector are picking up the (five) keyboard signals that go to the ULA. The 4068B AND/NAND gate is detecting when ANY key is pressed. And it looks like the 4093B is being used as an oscillator/ astable multivibrator to generate a frequency (tone) that gets sent to the piezoelectric ’buzzer’.

Mark

[1024MAK]

What channel is it supposed to be on?

Channel 36 UHF (591.25MHz) is the official channel / frequency. But as the modulator uses just coils and ceramic capacitors to set the frequency, the actual output could be anywhere between channels 35 and 37.

Mark

[rcade]

So just a keypress beep like the Atari computers do?

[mstreurman]

Before I power up the system, is there anything that I need to doublecheck, like caps that need replacing and the likes? I wouldn't want to break my lovely ZX81!

I have the original power brick, which I will connect through a Surge protector that is properly grounded.

[1024MAK]

There are no critical capacitors that need changing on a ZX81 board. Or indeed any other components that need changing.

The ROM chip must occupy the part of the socket nearest the side of the board with the keyboard connectors.

And do test the power supply unit to check that it’s output (off-load) is between +12V and +16V (the output will be higher than the nominal 9V because it’s an unregulated type).

When it’s powering the ZX81, it’s output should be between 10V and 12V.

Mark

[mstreurman]

I haven't powered it up yet as I have some small issues.
Sadly I don't have a multimeter yet so I am unable to do a reading of the voltages on the power supply. Second issue I have is that none of the outlets I have will fit the existing plug on the power supply (it doesn't have ground nodes and all my outlets do, so it is impossible to plug it into the outlet)

Powerplug

Finally it seems that I am missing the cable that fits the modulator, I was hoping I could just plug in an spare 75 Ohm antenna cable I had but it doesn't grip enough on the modulator thus it falls out and doesn't make any connection will have to figure out what to do about that, I will probably just strip one side of the cable and solder on a RCA plug. That should work, right?