Re: Lardo buys another zeddy
Posted: Mon Jun 15, 2020 2:28 pm
Well bless them for thinking of the environment. Such a forward thinking organisation!
Discussion forums for users of the Sinclair 8-bit range of computers - ZX80, ZX81, ZX Spectrum, Z88, clones...
https://www.sinclairzxworld.com/
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.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
C1 100uF is a 16V electrolyticLardo 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
R1 is 2.2kΩ, but the value is not critical, anything between 1.5kΩ and 4.7kΩ should be fine.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?
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 pm Q1 - I have few ZTX313s knocking about. Will one of these work?
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 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?
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 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.
So that’s ‘a few’ multiplied by ‘a few’ questions then
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.