ZX Spectrum 128K composite video fixes.pdf

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Video fixes
for all ZX Spectrum 128K models
1. Introduction
2. All ZX Spectrum 128K models: detaching audio from the video signal for better picture
3. ZX Spectrum +2: no video-out because of faults with the 2N3904 transistors
4. ZX Spectrum +2A / +2B / +3: creating a composite video output signal
5. Finish it: add perfect stereo audio!
Ben Versteeg – (Document version: v1.0, january 2012)
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1. Introduction
This document describes some reasonably easy enhancements for you ZX Spectrum 128K
model, that should result in perfectly clear and crisp video from your ZX Spectrum!
The focus is on the composite out videosignal, that is often used when connecting to a Scart-
input of a monitor, crt tv or lcd tv.
If you are using an RGB cable, the fixes won’t improve the video quality, as you should
already have the best picture.
But not all devices accept RGB signals, and an RGB cable is more complex as it needs
some electronics integrated into the cable.
2. Detaching audio from the composite video signal on all ZX Spectrum 128K models
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.
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.
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.
ZX Spectrum +2A
The capacitor is C24:
Even more perfect
One more improvement (that I haven’t tried myself) is:
“You should solder a series of a inductance (32µH or 16 µH) and a capacitor (51pF or
100pF) from pin8 of TEA2000 (that former pin of the sound capacitor) to ground.
That series resonance circuit shorts signal components of the video signal that would interfer
with the color carrier.
You will note that after soldering that resonance circuit undesired color effects at vertical
lines (especially at letters) will disappear.”
3. Design fault on +2
There is a major design-fault on the Spectrum +2 (grey model): 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.
Or maybe 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.
This picture shows the 2N3904 right next to the modulator.
I already twisted this 2N3904 around, so this is the way it should have been mounted:
On the bottom side it’s 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:
Solution: desolder the 2N3904 and twist it around.
That will solve the problem and give the correct video out signal.
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