RF output instability
Hello!
Now i need help :)
One of my rubber keys (48K) behaves a bit strangely on behave of RF TV output.
When booted the picture is stable.
Then i load Exolon and the title screen (with menu) is good.
Then, when i press 1 (start game) the picture becomes unstable and fuzzy, especially the lower part.
When i move around the main character the picture stabilizes.
SO, it seems like stability of the picture depends on some statistical data of a shown image (brightness or color, i don't know).
ANy idea how to fix it? Is it a dead cap somewhere?
Regards,
Artem
Now i need help :)
One of my rubber keys (48K) behaves a bit strangely on behave of RF TV output.
When booted the picture is stable.
Then i load Exolon and the title screen (with menu) is good.
Then, when i press 1 (start game) the picture becomes unstable and fuzzy, especially the lower part.
When i move around the main character the picture stabilizes.
SO, it seems like stability of the picture depends on some statistical data of a shown image (brightness or color, i don't know).
ANy idea how to fix it? Is it a dead cap somewhere?
Regards,
Artem
Post edited by ArtemKuchin on
Comments
The mean level of (a part of) the video signal is used to create a determined ('black') level. The synchronising pulses are recognised in relation to this mean level. So a different screen might somewhat change the syncs.
But such minor changes should still be inside the normal catch range of a TV/monitor from the eighties.
Did you try with a different TV.
I checked the RF of the output and it is 594-595 instead of 591 (channel 36 UK). Any way to adjust it? And why is it shifted?
Also the two ZTX313 transistors right under the rf metal kit could solve it if replaced. Done it many times and I have seen good results.
http://www.youtube.com/watch?v=EyID-WNpO-k
You can begin by changing the two electrolytic caps on the far left part of the board which is the easiest thing to do.
If after that the problem is still there then replace the two ztx313 transistors under the rf modulator kit. If that doesn't help either then I would go for the whole modulator kit (actually I've never come across a bad modulator kit but others have).
Also a worn out 7805 is known to affect picture quality.
I cannot test it on another tv because i don't have it, but, i have plenty of spectrums (rubber key and +) and all of then work fine with this tv.
What interesting to me is the frequency shift. What defined the base channel frequency? Is this circuit inside the module or the frequency is defined by external components relative to the rf module?
- Capacitors
- 7805
- 2x ZTX313 or equivalent
- Modulator kit
The frequency shift is not uncommon specialy on aged parts, or maybe someone chenged it. You can change the frequency by twisting the iron core of the coil in the modulator kit. There is a hole for that on the lower right of the modulator kit's cover. Be careful not to damage it.
ps
You can always perform the composite modification and be over with RF.
I had another machine with almost exactly the same fault and did the composite mod on it, but not before hurling the modulator's guts in the bin. I want to keep one machine boxed and as original as possible, but is it even possible for an average electronics enthusiast to repair or replace a modulator - or should I just get over myself and do the composite mod on it?.
When you can't keep something working in original state, all you have is a nice box to look at. At that point, does it make much difference what's inside? :-?
Increasingly difficult to find a replacement modulator. But have you ever looked inside one? Some resistors, capacitors, coils, and HF transistors on a carefully laid out circuit board. With suitable replacement for those transistors, it wouldn't surprise me if a modulator could be repaired indefinitely (like a classic car). Most modulators I own, are basically used as mechanical support for the cinch connector... :-D
Well... it is very easy. The most 'daunting' part is opening up the Spectrum - and the related keyboard connector issues (if that Speccy still has the original keyboard foil, that is). Apart from that, there's little reason not to do it.
Tried to turn it with little force on the speccy i'm tried to fix originally and it does not turn. I don't force it anymore.
So, DOES IT TURN T ALL? I've found docs confirming that turning it shold change the frequency, but it seems to be stuck in all modulators i have (and i have plenty, like 10 48k of different issues).
Tried it on another tv today. This tv a little more stable, but still have the problem.
What exactly is bothering you?
Or maybe you're not using the right tool to turn them. Be careful, if you ruin the coil then the composite mod would be your only solution.
Well, maybe. I did ruin one coil already so that speccy goes composite for sure.. or i'll put a modulator modules from a broken 16k board.
Anyway. What is the right tool for it then?
Personally i've done it using a small flat screw driver that fits the slot but veeery carefully.
SHould be turned counterclockwise?
The coil is actually inside a platic case, a platic tube sticks up and the rotating part is inside it more like at the bottom. The rotating part has a square dent in the middle for the rotating tool and also something wobbly sticks up from this rotating part. It seems to be soft and it's kind of rubber.
Is my description correct?
That coil is part of the tuned circuit which determines the UHF channel. It was quite normal to adjust the Spectrum channel as it might conflict with your favourite TV channel. Even with a broken core the circuit is tuned and the signal most probably still somewhere in the UHF band.