48K Issue 2 - S-Vid mod colour issue

[SnowBum]

Hi all

A bit of background to my ZX Spectrum, it's a 16K issue 2 upgraded to 48K in '83. I've had it since Xmas '82.
I did a straight forward composite mod on it by removing the input wire and the +5v wire for the RF modulator from the circuit board and then taking the composite signal via a wire straight to the centre pin of the RF modulator output socket (dropping the resistor from the modulator board off it first).
This worked fine but the colours were smeary and there was horrible pixel crawl.

I then did a switch mode voltage regulator replacement of the original voltage regulator (which gets horribly hot on the 48K issue 2's).

I did the LM1889 colour adjustment using a multimeter to get the -50mv between pin 2 & 3 and +50mv between pins 4 & 3.
The colours were a lot sharper but i was still getting smearing & pixel crawl.
I then did the composite mod with a transistor for buffering but it remained the same.

At this point i'd read about the s-vid mod board you can fit.
I bought one and fitted it but unfortunately i mixed the wires up and connected the mod board lumina input to the composite (correct), the +5v input to the chroma and the chroma to the +5v input (with C65 removed). When i realised and swapped the incorrect wires over i thought that it wouldn't have caused any issues and i was getting a really stable luma picture with no pixel crawl (the black and white picture) but the chroma wasn't working properly. I was getting patches of colour jumping around. I have the s-vid output lead going into a scart adaptor plugged into my TV. I've tried another TV and it's the same on that.

I assumed i'd damaged the s-vid mod board, so purchased another one. In the meantime i put the RF modulator board back in with the composite transistor mod, C65 back in and it was the same as before, nice colours but smeary and horrible pixel crawl.

When i put the new s-vid mod board in, removed C65 and put the wires in correct ....... i got exactly the same as the other s-vid mod board.

I have a little LCD display DSO138 Oscilloscope so i decided to check the signals on the luma input and the chroma input.
I've attached a photo of the luma signal i've got (and put the probe placement), this seems reasonable and because i get a nice black and white screen i think this signal is ok.
But the signal i get for the chroma, taken from the side of R72 that goes to pin 13 of LM1889, just seems to be a flat 3.26vdc
I know the chroma signal normally goes through to get mixed with the luma signal output of TR1 before getting boosted a bit again by TR2 ....... and the normal composite mod has colour so something is going through to get mixed with the luma signal.

I'm at a bit of a loss now ......... has anyone got any ideas of what i can check?

[Lardo Boffin]

I got similar awful results plugging an s-video modded Atari 7800 into a SCART adaptor. The colour was all over the place. It was worse on a modern LCD TV than an older CRT TV.

I ended up getting a cheap s-video TV from a charity shop and on this the picture was great.

[Lardo Boffin]

I fitted an s-video board to a Spectrum a few years ago to sell and the picture was fantastic - on the right TV. Looks like the same board supplier you used.

The image was rock solid:

The picture on this was terrible when plugged into a cheap SCART converter.

[1024MAK]

Hi all

Hello and welcome

I have a little LCD display DSO138 Oscilloscope so i decided to check the signals on the luma input and the chroma input.
<snip>
But the signal i get for the chroma, taken from the side of R72 that goes to pin 13 of LM1889, just seems to be a flat 3.26vdc

I don't think the DSO138 Oscilloscope has sufficient bandwidth to correctly display a signal in the MHz frequency range.

... the normal composite mod has colour so something is going through to get mixed with the luma signal.

Going on what you have said so far, I think the Speccy is okay.

I have the s-vid output lead going into a scart adaptor plugged into my TV.

Tell me more about this "adaptor" and please post a photo of it.

Also, on the TVs that you have tried, do they actually support S-Video signals on their SCART inputs (not all SCART input sockets do) and have you tried selecting different "AV" inputs for the SCART AV input. Auto selection of S-Video on a SCART input is not always provided, hence the user has to manually select it from a menu or remote control button.

Mark

[SnowBum]

Tell me more about this "adaptor" and please post a photo of it.

Also, on the TVs that you have tried, do they actually support S-Video signals on their SCART inputs (not all SCART input sockets do) and have you tried selecting different "AV" inputs for the SCART AV input. Auto selection of S-Video on a SCART input is not always provided, hence the user has to manually select it from a menu or remote control button.

Mark

You're correct with the DSO138 oscilloscope, the lowest timebase is 10 microseconds, which equates to 0.1MHz!

I think you've hit the nail on the head, my ALBA LCD32880HDF TV supports over SCART - PAL 50/60, NTSC 60, RBG 50 & RGB 60 ......... i don't think any of them are compatible with S-Video ...... although the luminance signal is obviously working ok as i get a nice crisp black & white picture, even when playing games via a DivMMC Future cartridge.

And my TOSHIBA 40BV702B TV supports over SCART- PAL, SECAM, NTSC4.43, NTSC3.58 & RGB 50/60 ......... i don't think any of these are compatible with S-Video either. But again it is accepting the luminance signal, i get a sharp black & white picture.

I'm now thinking, should i have tried the composite video signal in via the SCART socket adapter (i've attached a photo of the SCART adapter) instead of putting the composite video signal into the video-in socket of the YPBPR trio of sockets?

Edited for clarity

[1024MAK]

So, that 'adaptor' is just a box with connectors and wiring (wires or a PCB). It doesn't have any active electronics.

All (well as far as I am aware) SCART sockets on TVs (doesn't apply to other devices) support composite video (on pin 20).

S-Video uses pin 20 for the luminance signal, hence why the TV can display a monochrome picture.

S-Video uses pin 15 for the chrominance signal. This is primary allocated as the RGB red video input.

Hence that 'adaptor' is just connects the relevant pins on the S-Video socket to SCART socket pins 20 and 15 (plus the 0V/ground pins).

PAL is a colour encoding standard. May also refer to a 625 line 50Hz (system I) video signal. 50/60 - PAL can be used on 50Hz or 60Hz systems.

NTSC is the USA TV standard.

SECAM is the French TV standard.

None of these are a physical interface, just signal standards.

RGB is a physical interface. For domestic equipment, that's normally a signal line for the red video component, a signal line for the green video component, a signal line for the blue video component, a signal line for the synchronisation signals, a 0V/ground line and optionally control/switching signals. 'RGB' on its own does not specify the video standard in use.

NTSC4.43, NTSC3.58 are referring to two different NTSC colour systems.

Mark

[roganjosh]

As I see from a search that both of your TVs have a rear HDMI socket an alternative would be to use an s-video to HDMI converter. All my Spectrums have the same s-video mod as yours and I've always found that the Tendak converters to be very good, though that is no guarantee. Double check that the advert specifies 's-video' somewhere in the title as Tendak produce other converters. They cost around £40-45 on ebay.

Alan

[Lardo Boffin]

I tried a SCART adaptor very similar to yours (SnowBum) and got terrible results.

I normally use them with composite modded zeddies for which they are fine (I supplied a comp cable and adaptor) but for this application they are useless.

[SnowBum]

So, that 'adaptor' is just a box with connectors and wiring (wires or a PCB). It doesn't have any active electronics.
Mark

Correct i've had the shell apart and it is a bunch of wires, a switch, the SCART plug and a circuit board with the other sockets on.

All (well as far as I am aware) SCART sockets on TVs (doesn't apply to other devices) support composite video (on pin 20).

S-Video uses pin 20 for the luminance signal, hence why the TV can display a monochrome picture.

S-Video uses pin 15 for the chrominance signal. This is primary allocated as the RGB red video input.
Mark

This is correct to how the SCART adaptor 'box' is wired.
S-video pin 1 (Luminance ground) to SCART pin 17 (Video ground)
S-video pin 3 (Luminance signal) to SCART pin 20 (Composite video in)
S-video pin 2 (Chrominance ground) to SCART pin 9 (RGB red ground)
S-video pin 4 (Chrominance signal) to SCART pin 15 (RGB red signal in)

This kind of makes sense as to why i get occasional blocks of colour (all in the red spectrum) splash across the screen.

RGB is a physical interface. For domestic equipment, that's normally a signal line for the red video component, a signal line for the green video component, a signal line for the blue video component, a signal line for the synchronisation signals, a 0V/ground line and optionally control/switching signals. 'RGB' on its own does not specify the video standard in use.
Mark

I think it's all become clear to me now.
I'd seen the ZX Spectrum 48K S-video mod being used with a SCART S-video adapter 'box' and assumed on TVs SCART just supported S-video as standard.
Sadly i can see that it isn't. I'll explore other avenues now to get a solid, sharp, picture.

Thanks for your help everyone!

[SnowBum]

As I see from a search that both of your TVs have a rear HDMI socket an alternative would be to use an s-video to HDMI converter. All my Spectrums have the same s-video mod as yours and I've always found that the Tendak converters to be very good, though that is no guarantee. Double check that the advert specifies 's-video' somewhere in the title as Tendak produce other converters. They cost around £40-45 on ebay.

Alan

I'm thinking of going down this route as the video signal out of the S-video mod is really good with the SCART adapter..... all be it in black and white.
Cheers for the heads up with the Tendak converters!