ZX Spectrum 128K composite video fixes in 1 document
I just finished my document with video fixes and improvements for all ZX Spectrum 128K models.
Download it here:
ZX Spectrum 128K composite video fixes.pdf
The topics:
- All ZX Spectrum 128K models: detaching audio from the video signal for better picture
- ZX Spectrum +2: no video-out because of faults with the 2N3904 transistors
- ZX Spectrum +2A / +2B / +3: creating a composite video output signal
Download it here:
ZX Spectrum 128K composite video fixes.pdf
The topics:
- All ZX Spectrum 128K models: detaching audio from the video signal for better picture
- ZX Spectrum +2: no video-out because of faults with the 2N3904 transistors
- ZX Spectrum +2A / +2B / +3: creating a composite video output signal
Post edited by bverstee on
Comments
You can just remove the cap, no cutting of tracks or bits of wire needed :-?
Edit: Oh I see, so there is still an FM sound signal on the RF output.
Frankly I can't see the point keeping RF out if you have composite heh. :)
I agree with you, but if I sell a computer with the improvements, I don't want to remove a function, I just want to improve the quality.
So I still want the sound to be on the antenna output.
That fix (put the sound back tight before the modulator) is a tip from Ingo Truppel.
Include the composite video fix for the 48K too (both the straight-wire version and the inline-capacitor version), then WOS can host it as THE authoritative document and anytime someone asks again in future we can all smugly point them to it.
- IONIAN-GAMES.com -
it would be nice to note my name as co-author on your docs (the composite mod as well as the Stereo mod). Since some years I published the schematics here: https://fotoalbum.web.de/gast/ingotruppel/tech1
Regards Ingo.
But if you take the 12v line off the RGB socket then scart cables won't work properly...
12 V you have on 2 pins (1 and 5). So one line could be used for composite video. For the second 12 V line I would prefer to insert a resistor of about 100 Ohm if you are frightened of shorting this line accidentally.
And one should be aware if the RGB output is modified to declare this fact to avoid using SCART cables that would short pins 1 and 5 and so short the composite aoutput to the 12 V line!
Ingo.
The 12v line on the +3/2B is already fed through a 1k resistor so shorting out isn't an issue.
It makes it totally hit and miss whether it's going to work though... how many people know how their scart cable is wired if they bought it off ebay?
Ingo.
fair point :)
Yes, sure Ingo, I will add it.
I must also add some other sources with info I found on the web, but I'm not sure if I can find them again.
Good idea! I will.
I never made a Scart cable which needed 12V.
Or are you referring to an RGB cable?
Maybe I should add some RGB cable schematics in the document too.
I have not 'warned' readers that this document is only meant for people who are experienced with soldering.
I should also add that.
So now I am trying to cut the line from C31 to IC2 as you suggested (to disable sound signal on composite) , but it seems C31 is not connected to anything on this board..
Grey +2 , rev 8639
Thnx :)
Maybe TR4 has low amplification then a new one could help. You can try to short one or two of the diodes in the base line of TR4.
Are you sure your TV set is adjusted the right way? If the oscillator for the colour carrier is not right adjusted it may be that you have TV in colour but a video source with a colour carrier near the right frequency could be not accepted - so did you try to use a different TV set to check?
Ingo.
According to Ben's document this would fix another issue: namely the interference of audio on the composite signal. I am completely new to this machine, so I assumed this could well be right (other computers sometimes have a similar problem)
I will try that.. D7 and/or D8 I assume you mean here?
Quite positive the problem is not with the monitors (2 commodore 1084's here, of which one I use also for analogue videocamera's)
the UHF regulator outputs the image with colour and without too many problems on these monitors. (using an external tuner)
Thanks for the reply :)
Yes, all the 128k machines mix the audio subcarrier into the luma component of the composite video before feeding it into the RF modulator. Obviously this is separated out of the video again by a TV receiver to feed to the speakers. However when using a composite input to a TV/Monitor the sound carrier shouldn't be there and just causes interference.
By removing the capacitor connecting the audio circuit to the luma in pin of the TEA2000 the modulated audio is disconnected.
If there's colour on the RF signal then the problem has to be from the diode chain onwards.
Do you have access to an oscilloscope? That would be the easiest way to determine whether or not output is right.
Isn't it the second right of the TEA2000?
Not sure, so measure if one of the sides from those (axial) capacitors is connected to pin 8 of the TEA2000. That's the one.
It was actually on the left, but i found it in the end.
However, desoldering it did not have the desired result it seems. I made a quick movie showing the results:
www.youtube.com/watch?v=2p5qWFkFa8M
The issue 3B 48K i have does not show this behaviour at all :)
If it really seems to be related to sound then I would suggest you try removing IC38 (the small 8 pin chip, marked MC1376) That's the FM modulator that creates the sound subcarrier and on my +3 I could see interference from that getting into everything via the power rails. If you are removing the sound from the RF output this is redundant anyway.
with high contrast , like rtype:
look al the lines
the problem is not present in all games ,
and happens with my 2 128+ systems
my 2a modded with composite on the same tvs works great though
removing c126 didn't help
Please resize the photo and rotate it.
I have no idea what is causing that blue stripes.
Are you sure it isn't visible on RF?
the artifacts show only on some games,
looks better lowering the color
have tried my 2 toastracks , with several tvs
same problem
could it be an issue with the cable or is it just how de toastrack look on composite?
can't test on rf, i'm sure it won't show, but i have no euro tvs to connect
that's why i need composite
don't you see lines on some games of high contrast
like above on r-type?
did you just connect pin 1 to video , or did something else with the cable>?
Nope, no lines/artifacts on mine, yes pin 1 video and pin 2 to ground and then RCA on the other end.
The only problem is it's a bit dull but that's mentioned in a few articles I've since read and there is a fix for it, doesn't bother me though I just turn up the brightness/contrast and down the color a bit.
I'm using an NTSC LCD TV (2 actually) which must detect PAL too as the color is fine. No lines or anything like that.
You've made sure you didn't accidentally make a solder bridge with another pin?
EDIT: Actually here is a picture of mine using the new cable I made on a Commodore 1084s monitor too, Not much to see other than it's a nice clear image and remains so using games too, the lines are just the refresh lines of course (although in black and white because it's an NTSC monitor).