Cable for Adapt Electronics Speccy 48K RGB Interface

[JohnM73]

Hey Mark. Just wanted to say many thanks for the help you gave me late last year in making up an RGB lead for the Adapt RGB interface. Finally got around to soldering it up a couple days back and it works absolutely fine! Mucho thanks!

Picture is very good, though I do get some dark blue 'snow' on black backgrounds. I thought this might be the result of old electrolytic so replaced all those plus the two trimmer pots (Piher still remarkably make exactly the same type all these years later). No change. Also (before and after cap change) yellow appears to be closer to a honey colour... not unlike a C64 somewhat amusingly!

Was wondering if the Traco Power switching regulator that's fitted in my Spectrum+ might possibly be causing some interference. Otherwise I'm wondering if they were just all like this. The picture is so close to being perfect I can easily live with it. I have a Spectrum Next which can output a perfect picture into the same CRT via RGB so it's all moot really. Could also be one of the ICs on its last legs I guess, though as the manufacturer sandpapered off all the markings from every chip I'll likely just live with it. Was just wondering if there's anything else I can try before putting this project to bed?

John.

[1024MAK]

If the "snow" is always one colour and always random with no visible pattern, I would say that it's less likely to be the Traco Power switching regulator, as that would be more likely to affect all colours. Unless you have an oscilloscope, the only way to know for certain is to remove the Traco and refit a good quality 7805.

It may be something in the circuitry that forms the blue video channel is marginal or is more noisy than it should be.

Maybe there is a faulty capacitor or there is a poor connection on the PCB.

All components create some level of noise, that's why designers have to be careful to try to keep the wanted signal level much higher compared to the noise level (known as signal to noise ratio). But sometimes a fault can cause the noise to increase or the wanted signal level to drop.

Yellow should be an equal mix of just red and green (as far as RGB video is concerned), if it's more orange ("honey") colour then there is more red than green. If there are no adjustments or adjusting does not help, then a component may have drifted out of tolerance. But it's hard to say without more information.

Mark

[JohnM73]

Thanks for your thoughts Mark. Think I’ll likely keep it as-is then. The caps have already been replaced with good quality Vishay types. And have double-checked the cable’s solder joints and all’s good. So it was either always like this, or perhaps one of the transistors or ICs is on its way out. Frustrating as it’s so close to being a perfect picture. I noticed the IC legs look a little blackened actually, but as all IC markings sanded off impossible to buy any replacements, otherwise would have socketed the lot. May try replacing the 4 transistors as they do have the markings intact, and will check the resistors are within tolerance still, but that’s as far as I’ll take this project, though may test on another Spectrum that has still got the 7805 regulator fitted instead of a Traco switcher as a last resort. Thanks for all your help Mark.