48k Spec with composite video mod, screen adjustment possible?

I've had a 48k Speccy with a composite video mod for a few years now, and although it works perfectly otherwise, the screen isn't centered correctly - it shows up kinda left off the center. Click here to see the pic.
I was wondering, if it was possible to somehow adjust the centering of the image, or is this just a result of the mod used here, thus unalterable? Also, the output is kinda dark compared to RF, which would be nice to get fixed as well, if possible. Any suggestions?

Comments

  • Have you tried it on a different TV?

    Which version of composite video mod. has been done?
    • The wire link only version,
    • The electrolytic capacitor version,
    • The transistor version,
    • A different version to the above,
    ?

    Mark
  • That's a good question, actually. I have no idea, nor does the guy whom I bought the Speccy from. Guess I'll open up the machine and look for something for clues. Any pointers?
  • edited August 20
    Take the metal lid off the modulator, take a photo and then post a link here to the photo...

    Oh, and be VERY careful with the keyboard membrane tails, they can be VERY fragile.

    Mark
    Post edited by 1024MAK on
  • Realise this is highly subjective and open to both interpretation and the display device (be it TV or perhaps monitor with composite video input) , but I've always done well with the 100uF capacitor erm 'mod version' :)

    I'll just articles seem to suggest radial but axial -might- be better for this as lead length may just be enough given the insulated pass-through ports in the modulator to actually get away without any jump wire.
  • Sorry, I have no idea about how to post pictures here, so here's an outside link for a picture of what's inside the modulator. Hope it's clear enough for you.
  • It’s not possible to upload photos or images to the forum.

    Assuming there is no extra capacitor or transistor on the outside of the modulator (on the side where the wires come out), then that’s the “wire link only” version of the composite video modification. Some TVs don’t get on with this version.

    So if you can, try another TV.

    Mark
  • Nope, no other caps or anything outside the modulator. The problem is not the TV itself, but rather I'm using a DVD recorder in between, which I'm using for making some unemulated gameplay videos for youtube - been using that despite the slight visual defects, because it's "authentic" in a way... but anyway, that's the problem. I'll test the Speccy on another device and see if there's any differences, but I'm considering turning to my unmodified 128k+2 model, because although the picture quality isn't quite as solid, the RF signal gives better colours and the centering is correct. But let's see. Thanks for the info.
  • edited August 21
    Which +2 do you have?
    A +2 grey (uses a 9V PSU) or a +2A / +2B black (uses a +2 PSU with a DIN plug to connect to the computer)?

    Well, on its own, I can’t think of how a composite video modification would be able to shift the picture sideways. The position of a video image on screen is purely down to the time between the horizontal synchronisation signal pulse (part of the composite signal) and the visible part of the video signal. And if anything, it’s more likely to be delayed (image shifted to the right) rather than anything else. The timing is fixed inside the ULA chip. The only external part is the crystal that the ULA uses for its master clock timing.

    Maybe the DVD recorder does not like the non-broadcast video signal generated by the Spectrum. But then, the +2 grey would have the same timing. The +2A/+2B will be very similar in terms of the signal timing.

    Mark
    Post edited by 1024MAK on
  • I have a grey +2. Based on what you're telling me, I can only assume something went a bit awry in the modification, or that something else has happened since the mod. I have actually had to get a new Z80 switched since purchase, could that have anything to do with it?

    I'll work on a series of pictures from a couple of different TV's with both the modded 48k and the unmodded 128k +2 later this weekend, when I have more free time.
  • On the +2 grey (U.K. model), the video/monitor/RGB 8 way DIN socket has composite video available on pin 1. GND/0V is on pin 2.

    Mark
  • edited August 22
    Oh yeah, I have a couple of different RGB cables, which I bought for various other purposes, but neither of them have worked with the +2 for some reason. The other one doesn't have enough pins, but otherwise fits, and that one shows everything in white; and the other one has the right amount of pins, but they're in the wrong order for the +2, because the screen goes a bit yellow (the colour blue seems to be missing) with odd NTSC-like signal effect and there's no sound. Guess I need to buy yet another RGB cable... 8-|

    EDIT: Found a third RGB cable from my cables collection, which gave yet another wrong result. This time, the screen was purple-tinted, and cyan was missing, but it didn't look like it was mixed with NTSC signal, like the yellow-tinted screen was. I still guess I need to buy yet another RGB cable. :))
    Post edited by Skaepoy on
  • edited August 22
    Unfortunately the outputs on the video/monitor/RGB DIN socket varies depending on which country the machine was sold in, or which model you have.

    So only the correct cable will work.

    Mark
    Post edited by 1024MAK on
  • edited August 22
    Well, yeah. Which is why I haven't bothered to order yet another RGB cable yet, because I can't be sure whether it fits or not. Besides, I found one other thread on another forum, in which you had discussed a similar subject with another Speccy user some time ago, and mentioned a possible problem with how the grey +2 might be built, which might lead into more modding, which I can't be bothered with, because I saw the results of that other guy's modding and it didn't look as promising as I hoped it to.

    Anyway, I made some testing on both my comp.modded 48k and RF-connectable 128k on two different TV's and DVD-recorders, with no differences. Unfortunately, I have no CRT TV at the moment, so I can't say if the picture would center correctly on one of those, but on modern flatscreens, the picture is "centered" slighty to the left. Here's a photo comparison.
    Post edited by Skaepoy on
  • edited August 22
    Are you talking a about the text & graphics area not being centred within the border area?
    That’s a design limitation feature of the ULA chips (type 6C001E-1) used in 16K/48K/+ machines with issue 3 PCBs and all subsequent machines and subsequent ULA versions. Sinclair apparently made a slight change to the internal timing to improve the compatibility with some makes of TV back in the 1980s. The side affect being the text & graphics area is no longer centred within the border area, but is shifted slightly to the left.

    Mark
    Post edited by 1024MAK on
  • I should also add that in the ULA for the ZX Spectrum 128 they again worked on the video system circuitry in the ULA. This ULA is the same as used in the +2 (grey) apart from Amstrad had a different part number printed on them.

    Mark
  • Maybe you would like to try the test card program I just posted here (type-in only).

    Mark
  • edited August 22
    1024MAK wrote: »
    Are you talking a about the text & graphics area not being centred within the border area?
    That’s a design limitation feature of the ULA chips (type 6C001E-1) used in 16K/48K/+ machines with issue 3 PCBs and all subsequent machines and subsequent ULA versions. Sinclair apparently made a slight change to the internal timing to improve the compatibility with some makes of TV back in the 1980s. The side affect being the text & graphics area is no longer centred within the border area, but is shifted slightly to the left.

    Well, not initially, although that's a bit annoying as well, but that's something I can deal with, since it's a "feature", as you say. ;) My original problem had to do with a bit of visible blackness to the right edge in the screenshot I linked in the OP, making the entire visible screen (borders and all) slightly shifted to the left, making the entire screen uncentered, which makes the slightly shifted "paper" area even more off the center.

    EDIT: I just realized, though, looking again at the linked picture in my original post, that when I attach the 48k Speccy straight to a TV, the black slab on the right edge you see from the screenshot taken straight from an output video from my DVD-recorder, is changed to the elongated right edge. So it seems like it's basically just a part of the machine's natural quirks, only shown in a different way on different input devices. It still doesn't explain the darker colours, though.

    But anyway, I'll try out your test program as soon as I'm able to, thanks for the tip.
    Post edited by Skaepoy on
  • Skaepoy wrote: »
    Oh yeah, I have a couple of different RGB cables, which I bought for various other purposes, but neither of them have worked with the +2 for some reason. The other one doesn't have enough pins, but otherwise fits, and that one shows everything in white; and the other one has the right amount of pins, but they're in the wrong order for the +2, because the screen goes a bit yellow (the colour blue seems to be missing) with odd NTSC-like signal effect and there's no sound. Guess I need to buy yet another RGB cable... 8-|

    EDIT: Found a third RGB cable from my cables collection, which gave yet another wrong result. This time, the screen was purple-tinted, and cyan was missing, but it didn't look like it was mixed with NTSC signal, like the yellow-tinted screen was. I still guess I need to buy yet another RGB cable. :))

    ouch ... dont just stick a din in with the correct amount of pins ... it needs to have the correct config .. depending on how your +2 is set up ..or you may be sticking 12v into your tv where it doesn't belong ... there are a few different configs ... also there is the issue of the TR4/TR5 being fitted wrong on some boards and the audio carrier interference ..
  • Yeah, that's exactly why I haven't bothered to hunt for a correct cable yet. Luckily, my TV is none the wiser (or dumber) for my random cable experiments...
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