View Full Version : ZX Spectrum Video Problem

March 11th, 2015, 12:45 PM
Hi there,

does anyone knows what can cause a problem like this? It's not the screen refresh rate... this is really happening on the screen.
It's happening with and without the composite video mod. So I guess it's not a problem with the UHF modulator.

Don't know what to do... help?!



March 13th, 2015, 12:20 AM
I'm not an expert at all; but seems some kind of power ripple. Try to:

- Test another TV set.
- Change power supply. You know, anyone with 9VC (centre negative) will do.
- Check power lines in the machine, look for noise.

The DC-DC circuit must have some problem.

Of course drying capacitors can be also a source of problems; so it's always a good idea to do a full recapping.

March 13th, 2015, 11:07 AM
I agree with the previous diagnosis, this is power supply ripple.

If it's happening twice per 50Hz screen refresh I would say it's 100Hz ripple so the prime suspect may be the smoothing capacitor inside the external power supply.

It should be flattening out the 100Hz pulsed output from the full-wave rectifier, but doesn't appear to be doing it very well.

If you have another PSU try it, if you don't, safely dismantle the power supply (ie, unplug it first) and have a look at the capacitor. Even if it does not look distressed, replace it anyway.

If there are more than two bars on the screen this suggests ripple of a higher frequency than 100Hz, in which case one of the capacitors inside the Spectrum is a more likely cause.

March 13th, 2015, 12:25 PM
You're both right!! Amazing!!
Although i solved it by myself, i did just because i didn't know what else to do, not because i knew what i was doing! :) I tested the speccy capacitors with my multimeter they were all within range and everything else looked well. I then changed my attention to the PSU and read 14.5V! I used an universal power supply and the bars "maggically" disapeared!!
You're great! Thanks!
i'm going to open the PSU and let you know!
Do you think thaht I should change the voltage regulator now?

March 16th, 2015, 04:52 AM
Because of the overvoltage? You could, but might not be necessary. The main thing is heat dissipation, so if the 7805 hasn't gone REALLY hot, it should still be good. Anyway they're really cheap and common, so it certainly couldn't do any harm.

March 16th, 2015, 10:00 AM
Believe it or not the 14.5V overvoltage output from your '9V' power supply is COMPLETELY NORMAL.

Power supplies which are not regulated output a much higher voltage than their marked voltage when they are not powering anything, but when they actually have a load running on them the voltage falls to a lower value. However, I think your power supply is faulty in a different way - bad smoothing.

If your power supply is a 'simple' power unregulated supply it will contain:

An iron cored mains transformer
Possibly, but not always, a suppressor of some sort across the mains input
A bridge rectifier consisting of either one component with four legs (+ - ~ ~) or four separate diodes
A capacitor with a fairly large value, typically 2,200uF or 4,700uF.

The last part is the part most likely to be causing your problem and I would change it regardless of how good or bad it looks even on a meter.

The other (less likely) possibility is that one of the diodes in the rectifier has gone leaky.

March 16th, 2015, 10:38 AM
+1 for post #6.

I suspect you have tested the capacitors inside the Specy as opposed to in the PSU (as I read post #4)?

I suspect (as does SiriusHardware) that either the smoothing capacitor within the PSU has gone to electronics heaven - or one of the solder joints is not too happy.


March 26th, 2015, 06:56 AM
1st of all, thank you all to your helpful replies!
So i changed both capacitors inside the unit (one small and one large) although they both were reading fine, to no avail! I guess i will turn my attention to the 4 diodes now!
I'll let the new capacitors in place though, since the original ones did their work with merit. They're there for 33 years! :)