Sinclair ZX80 / ZX81 / Z88 Forums

Hi,

I made a plugin adapter to tap the composite video signal from the PCB edge connector on my Spectrum. I got video, but there is a herringbone type interference and like a speckle noise. I figured it might be because the modulator was still active so I snipped its power line, but still, the composite video looks noisy.

The picture from the RF output had some noise in it too, but I always figured it was just RF interference. It was less noticeable on RF because the TV filtered some of it out. Now, with the composite signal on a larger broadcast-quality monitor, the noise is still present even when the modulator is powered down.

What can I check? Can dry capacitors cause this noise or am I looking at an iffy ULA? Can I tap the composite signal or RGB signal at the actual source and see if it is any better?

It looks like I have a 3 or 3B board where the heatsink is visible through the expansion port. Could it be power supply noise from a failing 7805? Any ideas from the gurus would be welcome. The machine is very clean and complete and besides the slight video issue is perfectly useable, but my OCD is tingling.

I did clean the PCB edge connector with KONTAKT Chemie contact cleaner.

Thanks

The ZX Spectrum uses 4116 DRAM chips for the ‘lower’ RAM, part of which is the video memory.
These actually run from a +12V DC supply. They also require a -5V bias supply. Hence the ZX Spectrum PCB has a DC-DC converter circuit.

If the electrolytic capacitors have aged or dried out, this may well cause interference on the video. So replace the electrolytic capacitors.
Also, the decoupling capacitors for the +12V supply for these DRAM chips could be better. Replace C5 to C8 with 1uF multilayer ceramic capacitors. This can improve the picture quality.

Third, the original ZX Spectrum design used two different oscillators, one for the ULA (which generates the display) and a second oscillator for the colour encoding chip. Unfortunately the interaction creates distortion between different colour borders. There is nothing that can be done about this.

Finally, the method of generating the composite video signal is not a broadcast quality system, instead it was designed to be low cost. So it’s not the best design. And of course a colour composite video combines monochrome video signals with a colour carrier signal, but the frequency ranges of these signals overlaps. So the TV has to try to work out which is which. And sometimes gets it wrong.

There are NO RGB signals in a original ZX Spectrum. The ULA generates Y, U and V video signals. If you want RGB, either a complex analogue circuit to convert to RGB signals is needed (which does not solve all of the problems) is needed. Or buy a Spectra (which generates it’s own RGB video signals) or buy a HDMI Interface.

Mark

Could you please help and point me to a schematic so I can see which components you refer to? Thank you for all your help.

Any conclusions on this issue?
Thanks