ZX Spectrum 128K video fixes and video cables.pdf

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Video fixes and video cables
for all ZX Spectrum 128K models
Contents
Contents ................................................................................................................................ 1
1
2
Introduction ................................................................................................................ 2
Detaching audio from the video signal ....................................................................... 3
2.1
2.2
2.3
3
4
5
Detaching audio from video on ZX Spectrum 128K+ ........................................ 3
Detaching audio from video on ZX Spectrum +2 .............................................. 4
Detaching audio from video on ZX Spectrum +2A ............................................ 4
Design fault on +2 (issue 3) ....................................................................................... 5
Creating composite video-out on a +2A / +2B / +3 ..................................................... 6
RGB cables for all 128K models ................................................................................ 7
5.1
5.2
5.3
5.4
Source of schematics: Paul Farrow .................................................................. 7
Composite video cable for 128K toastrack ....................................................... 7
RGB video cable for 128K toastrack and +2 (grey) ........................................... 8
RGB video cable for +2A / +2B / +3 ................................................................. 9
6
Stereo and crisp audio upgrade ................................................................................10
Ben Versteeg – (Document version: v2.1, februari 2014)
1 Introduction
This document describes some reasonably easy enhancements for your ZX Spectrum 128K
model, that should result in perfectly clear and crisp video from your ZX Spectrum!
The first part describes how to improve composite video output, that is often used when
connecting a ZX Spectrum to a monitor, crt, lcd or plasma tv.
The second part shows how to make an RGB cable with video quality that is superior to
composite video, allthough not all tv’s and monitors support RGB video signals.
2 Detaching audio from the video signal
In every ZX Spectrum 128K model the audio is mixed together with the videosignal, but this
causes distortion on the composite video-out signal and it causes many monitors not to
detect the color signal.
To fix this we need to revise the way the audio signal is added to the video signal.
But it’s very simple!
The solution for this is to reconnect one capacitor in another way.
This capacitor ‘carries’ the sound and normally adds it to the TEA2000 video generator chip,
thus causing disturbance.
By disconnecting one side of this capacitor, and reconnecting it more directly to the
modulator, the picture will be perfect and you will still have sound on the antenna output.
2.1 Detaching audio from video on ZX Spectrum 128K+
The capacitor is C126:
Lift the capacitor on the side where the cross is.
The blue line shows where to reconnect the capacitor to: you need a wire for that.
2.2 Detaching audio from video on ZX Spectrum +2
The capacitor is C31:
As the text at the blue arrow sais, this capacitor is on some models on the component side in
the position that shows C31, but with other models it’s on the bottom – if I remember
correctly it’s under the TEA2000 then.
2.3 Detaching audio from video on ZX Spectrum +2A
The capacitor is C24:
3 Design fault on +2 (issue 3)
There is a major design-fault on the Spectrum +2 (grey model) issue 3 (issue 1’s doesn’t
seem to have this problem): all 2N3904 transistors (3 pieces) are mounted incorrectly.
It’s not clear why this mistake is made, maybe earlier designs used another transistor that
have another pin-order than the 2N3904, and the 2N3904 was chosen later in the process.
One other cause of this mistake may be that they used a wrong datasheet: I found some
wrong datasheets of the 2N3904 on the web.
Whatever the reason is, it causes a unusable videosignal.
The easy solution is to desolder the 2N3904 next to the modulator and twist it around. That
will solve the problem and give the correct video out signal.
The other two transistors do not seem to give problems the way they are mounted, so just
leave them.
This picture shows the 2N3904 right next to the modulator where it is already twisted around,
so this is the way it should be mounted:
This picture shows the bottom side where it is more easy to see that this is true:
The right pin on this picture is the 12V, but in the original way the 2N3904 is mounted, the
12V is connected to the emitter of the 2N3904: that is definately wrong.
The emitter of the 2N3904 should be connected to the output to the 75 ohm resistor:

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