Sinclair ZX80 / ZX81 / Z88 Forums

Well bless them for thinking of the environment. Such a forward thinking organisation!

1024MAK wrote: ↑Sat Jun 13, 2020 8:29 pm ZX81 Video Transistor Buffer Amp

There is also a design using a NE555 timer chip to generate a pulse for the back porch. I’m not tried it. Alternatively, there is the ZXVid.

Mark

With another zeddy on its way (yeah, I know!) I will need some sort of composite mod so I will need to be brave and give it a go.

So, I have a few (yeah, I know!!!) questions about the buffered mod:

Are all the capacitors electrolytic?

C1 100uF
C2 100nF
C3 100uF

Are the resistors:

R1 2K2 ohm
R2 100 ohm
R3 10 ohm

In your prototype R3 is 150 not 10. Does it matter which is used?

Q1 - I have few ZTX313s knocking about. Will one of these work?

In the schematic there are three connections with the zeddy (X1 to 3). It doesn’t look like X2 is being used - presumably if this mod was done directly to a zeddy it would be - in your photos the ground comes from the video socket?

C3 isn’t fitted to your prototype - would it go straight from R3 to ground? Sorry - reading schematics is still a skill I don’t have. Sadly there does not seem to be enough time to learn everything I want to learn.

Thanks as ever for your eternal patience!

Lardo Boffin wrote: ↑Mon Jun 15, 2020 9:33 pm So, I have a few (yeah, I know!!!) questions about the buffered mod:

Are all the capacitors electrolytic?

C1 100uF
C2 100nF
C3 100uF

C1 100uF is a 16V electrolytic
C2 100nF is a multilayer ceramic or polyester film type
C3 100uF is a 10V or 16V electrolytic

Lardo Boffin wrote: ↑Mon Jun 15, 2020 9:33 pm Are the resistors:

R1 2K2 ohm
R2 100 ohm
R3 10 ohm

In your prototype R3 is 150 not 10. Does it matter which is used?

R1 is 2.2kΩ, but the value is not critical, anything between 1.5kΩ and 4.7kΩ should be fine.
R2 is 100Ω
R3 is 10Ω. This is the recommended value. But it’s not a critical value and higher values work. I can’t remember why I used 150Ω in the prototype, it may have been to test how much this resistor affected frequency response and the clarity, contrast and brightness of the displayed picture.

All are 5% or better carbon film or metal film and rated at 250mW/0.25W/¼W.

Lardo Boffin wrote: ↑Mon Jun 15, 2020 9:33 pm Q1 - I have few ZTX313s knocking about. Will one of these work?

They should do (I don’t have any on hand to test to see if they are okay), but as these are obsolete I recommend using a type that is still being manufactured.

Lardo Boffin wrote: ↑Mon Jun 15, 2020 9:33 pmIn the schematic there are three connections with the zeddy (X1 to 3). It doesn’t look like X2 is being used - presumably if this mod was done directly to a zeddy it would be - in your photos the ground comes from the video socket?

Err, it depends on the physical arrangements. A 0V/GND connection to the computer is needed, but if you are fitting this inside the case of the modulator, the phono socket already has a 0V/GND connection. But the circuit still needs its own 0V/GND connection. It may be difficult to see in the photos, but the prototype does use a 0V/GND connection. So ‘X2’ is used. Any nearby 0V/GND connection will do, so the inside of the case of the modulator is fine.

Lardo Boffin wrote: ↑Mon Jun 15, 2020 9:33 pmC3 isn’t fitted to your prototype - would it go straight from R3 to ground? Sorry - reading schematics is still a skill I don’t have. Sadly there does not seem to be enough time to learn everything I want to learn.

Yes, the positive lead connects to the junction between R3 and the collector of the transistor, and the negative lead connects to 0V/GND.

Lardo Boffin wrote: ↑Mon Jun 15, 2020 9:33 pmThanks as ever for your eternal patience!

So that’s ‘a few’ multiplied by ‘a few’ questions then

I blame any errors on the dog getting in the way of the screen/keyboard/brain cell...

Mark

I *think* I’ve squashed all the bugs...

1024MAK wrote: ↑Mon Jun 15, 2020 11:24 pm I *think* I’ve squashed all the bugs...

Many thanks.

Something like this?

Yeah (assuming you wanted a ‘top view’).

Remember the track side will be a mirror image...

C3 can also go to the left (as you have drawn it), then use a wire link to connect it to the 0V/GND track on the right.
Then the left hand lead of R1 goes one track further to the left. Why you ask, well radial 100uF 10V/16V capacitors are often on a 2.5mm or 5mm pitch lead spacing.

Mark

Ignore the reversed text for the minute. Will redraw properly at some point, solder side:

1024MAK wrote: ↑Tue Jun 16, 2020 2:13 am C3 can also go to the left (as you have drawn it), then use a wire link to connect it to the 0V/GND track on the right.
Then the left hand lead of R1 goes one track further to the left. Why you ask, well radial 100uF 10V/16V capacitors are often on a 2.5mm or 5mm pitch lead spacing.

Mark

A quick go with Excel (no layout software...).

Underside of board.

Here is lucky zeddy who will be getting the home made comp mod as soon as the bits arrive! A nice (currently) working issue one.


That somebody paid £19.95 for! Nice.