Lardo buys another zeddy

ZX80 / ZX81 hardware and software offered for sale or swapping
Lardo Boffin
Posts: 1234
Joined: Sat Nov 26, 2016 2:42 am

Re: Lardo buys another zeddy

Post by Lardo Boffin »

Well bless them for thinking of the environment. Such a forward thinking organisation!
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +2a

Lardo Boffin
Posts: 1234
Joined: Sat Nov 26, 2016 2:42 am

Re: Lardo buys another zeddy

Post by Lardo Boffin »

1024MAK wrote:
Sat Jun 13, 2020 8:29 pm
:arrow: 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. :o

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!
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +2a

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1024MAK
Posts: 3032
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Lardo buys another zeddy

Post by 1024MAK »

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 pm
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?
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.
0V/GND connection on the ZX81 video buffer amp
0V/GND connection on the ZX81 video buffer amp
105818D3-4DC2-434A-9E68-E117C10A60AD.jpeg (67.55 KiB) Viewed 622 times
Lardo Boffin wrote:
Mon Jun 15, 2020 9:33 pm
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.
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 pm
Thanks as ever for your eternal patience!
So that’s ‘a few’ multiplied by ‘a few’ questions then :lol:

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

Mark

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1024MAK
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Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Lardo buys another zeddy

Post by 1024MAK »

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

Lardo Boffin
Posts: 1234
Joined: Sat Nov 26, 2016 2:42 am

Re: Lardo buys another zeddy

Post by Lardo Boffin »

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

Something like this?

9D8CE01B-4FC9-442E-89D6-32638BA8E6E0.jpeg
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +2a

User avatar
1024MAK
Posts: 3032
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Lardo buys another zeddy

Post by 1024MAK »

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

Lardo Boffin
Posts: 1234
Joined: Sat Nov 26, 2016 2:42 am

Re: Lardo buys another zeddy

Post by Lardo Boffin »

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

884E1EB7-E152-483A-9047-A8AD0E386B52.jpeg
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +2a

User avatar
1024MAK
Posts: 3032
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Lardo buys another zeddy

Post by 1024MAK »

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.
9D8CE01B-4FC9-442E-89D6-32638BA8E6E0edit02.jpg

Mark

Lardo Boffin
Posts: 1234
Joined: Sat Nov 26, 2016 2:42 am

Re: Lardo buys another zeddy

Post by Lardo Boffin »

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

Underside of board.

CompMod.png
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +2a

Lardo Boffin
Posts: 1234
Joined: Sat Nov 26, 2016 2:42 am

Re: Lardo buys another zeddy

Post by Lardo Boffin »

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.

E5B22301-AAB5-4FCB-B1E0-24B29768515E.jpeg

That somebody paid £19.95 for! Nice.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +2a

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