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Thread: S Video from A520

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    Default S Video from A520

    Orignal post here http://members.iinet.net.au/~davem2/overclock/a520.html

    S-Video output from the A520

    The A520 video modulator was made by Commodore to allow TV sets to display Amiga graphics. It provided a standard UHF RF output as well as a standard composite video output. Unfortunately, with the exception of a few early versions, the A520 has atrocious video quality - being little better than VHS! Fortunately, by the deletion of a few components, and a little rearranging, it is possible to have full bandwidth S-Video from the A520.

    Why S-Video?

    As a picture is worth a thousand words, one look at the oscillographs below should be enough to convince anyone. The two top images are from a standard A520, the two bottom ones are from an S-Video modified A520. In the top left is a colour bar signal. Note the rounding in the corners of the colour signals as well as the leading edges of the luminance portions. Also obvious is leakage of the colour oscillator signal used in the A520 as shown in the thickened traces in non-coloured areas, especially obvious in the sync tip and the back and front porches. In the top right is a multiburst signal. As you can see the quality is disgusting. The colour leakage is especially noted here, as I have enlarged the trace for easy viewing. Also easily noted is jitter in the colour burst.



    The bottom left image is the same colour bar signal, but now through the modified A520. Note the composite signal is now two separate signals, the luminance (B&W) signal on top, and the chrominance (colour) signal on the bottom. Note the absence of rounding in both signals as well as the absence of colour leakage in the luminance portion. Jitter is much reduced, being barely visible in the colour burst and in the colour bar signal. On the bottom right, the multiburst signal now looks the way it should....almost flat to 14MHz on the right. The standard A520 was lucky to get to 3MHz! Incidentally, another feature not readily noted from the traces above is that the sync signal is now at the correct 0.3V level. The standard A520 had it at about 3.5V. Of course I did not mention the main reasons, not specifically related to the A520, that make composite video so horrible.

    One problem that existed with the earlier version of the S-video modification was weak colour saturation. Investigation revealed a design fault by Commodore - the MC1377 colour encoder chip used in the A520 requires 1 volt RGB signals for full saturation. The Amiga provides 0.7 volt RGB signals, which strictly speaking is correct as the addition of the 0.3 volt synch pulses gives the correct 1 volt video. To cure this I chose to increase the RGB level from the Amiga to 1 volt to give the correct colour level and reduce the luminance level in the A520 to compensate. Unfortunately this involves a (simple) mod to the Amiga itself, which will result in it putting out a non-standard RGB signal, which will be brighter than it should be (adjustments to the A520 will compensate). Of course this mod will be different for each Amiga model. Other changes to the new modification include the ability to adjust some parameters of the colour subcarrier to allow for component variations as well as some minor simplifications. This new version will be posted in the near future, once I have finalised it. It will also be easy to update the existing version to the new one.

    How to do it - step by step

    Firstly you need a standard PAL A520. You will also need an S-video compatible monitor or VCR. Otherwise there is no point to doing this. Apologies for those in NTSC countries - I live in Australia, and I have never seen an NTSC A520. These modifications may not work on NTSC units. I do know however, that pin 20 on the MC1377 IC is grounded for NTSC, there is a solder pad for this on the PCB. On the other hand, from what I understand, the way the NTSC A520 handles the synchronisation of the colour subcarrier to the Amiga is completely different. In any case I tried changing the jumper and replacing the 4.43 MHz crystal with a 3.5MHz crystal with no luck. All I got was B&W. The oscillator circuit was not working.... Be aware that conversion to S-video will render the RF modulator inoperative, as of course, there is no way to broadcast S-Video on a standard RF carrier. The existing composite output connector on the side will now become the Y or luminance output. The existing audio input socket will become the C or chroma output.



    Click on the picture above for a larger view. Items to be changed are highlighted in coloured boxes. Red boxes denote components no longer required, and they are simply removed. Yellow boxes denote items that are either changed or are added. Refer to the designations on the silkscreening on the PCB.

    1:- Remove C1 and L1 (may already not be present).

    2:- Remove K2 (the coil), C8, C13, C20, C21, C24, R6, R8, R15, R16, R19.

    3:- Remove K1 (the delay line) and place a 470 ohm resistor between pins 2 & 3.

    4:- Add 0.1 uF capacitor between pin 10 of the IC and ground (underneath the PCB).

    5:- Remove C18 and replace it with a 330 ohm resistor.

    5:- Cut PCB track to base (B) of transistor Q1 (marked with a cross on the picture of the PCB underside).

    6:- Connect with wire pin 13 of the IC to the now isolated base (B) of transistor Q1 (underneath PCB).

    7:- Cut the wire leading to the audio input socket (inside the modulator shield - as denoted by red square in picture).

    8:- Connect the now isolated audio socket to the leftmost pin on the white header (with letter "V" marking - as denoted with yellow square).

    9:- Remove R14 and replace with a link.

    10:- Cut track to pin 8 of the IC (marked with cross on the picture of the PCB underside)

    11:- Connect with wire pin 8 of the IC to pin 3 of the former K1 (now occupied with 470 ohm resistor - refer step 3).

    12:- Change R7 to 180 ohms.

    13:- Remove C4, R20, R13, and replace each with a wire link.

    14:- Change R11 to 1K.

    15:- Add a 220 ohm resistor to the formerly vacant R18 position.

    16:- Add a 1M resistor from pin 17 of the IC to +12 volts (underneath PCB).

    17:- Remove R5 (if present) and replace with a 2.2M resistor.

    18:- Add a 470K resistor from pin 14 to pin 19 of the IC (underneath PCB).

    19:- Add a 220uF capacitor to pins 3 & 4 of the former K2. Positive of capacitor to pin 4.

    20:- Add a 390 ohm resistor from pin 9 of the IC to pin 1 of the former K1 (underneath PCB).

    You will need to make an appropriate S-video lead with 2 RCA plugs to suit the new A520.
    Back to main Amiga page.

    Introduced 7th June 1999. Updated 23rd June 2002. Version 1.2
    http://members.iinet.net.au/~davem2/overclock/a520.html
    Attached Thumbnails Attached Thumbnails a520sm.jpg   scope.jpg  

  2. #2
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    I'll be having a go at this later in the week

    http://members.iinet.net.au/~davem2/amiga.html

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    I'm in the middle of doing this mod at the moment, with removed all mentioned elements

    Where do you get the new resistors and caps from?

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    I buy my stuff from eBay usually

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    This one looked promising when I first looked at it. I'm hoping to get a clear s-video signal out of my modded A520 box

  6. #6

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    I made one of the initial description long ago, worked perfectly, usable also for a monitor with Chroma/Luma in and no RGB as well as Video grabber card with s-video.

    I was hired to mod one for a friend, found the newer description (that I wasn't aware of being new at the time). I recognized the image.
    Built it, seems the PSU overloaded or something cause I got a funny squeeling noises from my switched model of Amiga-PSU when stuffing it into my Amiga 500, image was showing fine but the look of the screen was like someone was running a RC car without noise supression.

    So off it goes, quickly... My old mod still worked, computer didn't break.

    Couldn't find anything wrong, no solder bridges, nothing, very strange - so I decided just to do the old mod as it is explained in the first post, straight copy of my old build.
    Worked right away, perfect PAL image - not sure why the description was changed - what was wrong with the old one?

    Here's the mod I just finished tonight:


    It's going to be used with a projector that doesn't have anything better than s-video for input.

    I mainly used mine with a grabber card to have the Amiga in a window/full screen and at the same time being able to use the PC on the same screen.

    Great build, would be fun to try and figure out if there's anything wrong with the new description or if I had done something (I checked all of it three times, only reason I can think of is a possible non-visible solder bridge somewhere that was removed in the rebuild).

    Thanks for keeping the old version - I found my print-out from back then as well - finally.
    Feel free to try the new description, let us know if it worked.
    http://members.iinet.net.au/~davem2/overclock/A520.html

  7. #7

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    Got a reply from the publisher, the updated mod fixes the color saturation and sync pulse level to something better than the old modification.

    I'll try again on my own modulator.

    I also added one of these to my friends modulator, removed the RF connector and drilled a 12mm in diameter hole in the metal case to fit it - as well as the opening in the plastic case.

    http://ecx.images-amazon.com/images/...8L._SY300_.jpg

    Looked really nice, makes it easier to find an over the counter cable to use.


    I thought I'd write some more efficient instructions, here goes:

    Needed parts:

    Resistors: 180, 270, 330, 470k, 1M, 2M2, also reuse 1kOhm from R1,R2 or R3 and finally 220 Ohm from R19.
    Capactors: 100nF, 220F, 100F for +12V, also reuse 56pF from C20
    Some hook up wire.

    Desoldering:

    1:- Remove C1, L1 (may already not be present) and K2 (coil)

    2:- Remove R1, R2, R3, R7, R11, R14, R18, R20, C21, C24, R5, R16, C20, R6, R8, C8, R13, R19 and C18.

    3:- Remove K1 (the delay line) and place the 270 ohm resistor between pins 2 & 3.

    Add resistors:

    4:- Solder the 180 Ohm resistor to position R7.
    5:- Solder 1kOhm resistor (from R1, R2 or R3) to position R11.
    6:- Put 220 Ohm resistor (from R19) to the formerly vacant R18 position (if there are no holes for this, you can drill or place the resistor on the back of the PCB)
    7:- Solder 470kOhm resistor from pin 19 of IC1
    (MC1377) pin14, which is +12 volts. (underneath PCB).
    8:- Add a 1M resistor from pin 17 of IC1 to +12 volts (underneath PCB).
    9:- Solder 2.2M resistor on position R5.
    10:- Solder the 330 Ohm resistor on position C18.

    Add wires:
    11:- Put wire links on positions R20, R13 and.
    12:- Cut track to pin 8 of IC1 (marked with cross on the picture of the PCB underside)
    13:- Connect with wire pin 8 of IC1 to pin 3 of the former K1 (now occupied with 270 ohm resistor - refer step 3)

    Solder capacitors:
    14:- Add 100nF capacitor between pin 10 of IC1 and GND (underneath the PCB).
    15:- Add a 220F capacitor to pins 3 & 4 of the former K2. Positive end to pin 4.

    Modify:
    16:- Cut PCB track to base (B) of transistor Q1 (marked with a cross on the picture of the PCB underside).
    17:- Connect with wire the position C20 end of R15 to the now isolated base (B) of transistor Q1 (underneath PCB).

    18:- Add a 56pF capacitor (desoldered from C20) from the base of Q1 to GND.(underneath PCB).

    19:- Cut the wire leading to the audio input socket (inside the modulator shield - as denoted by red square in picture).
    20:- Connect the now isolated audio socket to the leftmost pin on the white header (with letter "V" marking - as denoted with yellow square).

    Optional:
    21:- Add 100F to pin14 (+12V) of IC1 and GND.

    22:- Remove, "RF out" connector, clear space for s-video connector.
    23:- Drill 12mm hole in metal case and plastic shell.
    24:- Solder bottom pins as well as ground pins together with wire, solder to soldered angle of metal shield.
    25:- Solder wires from s-video Y and C to respective RCA-connector.

    Last edited by e5frog; 18th October 2015 at 12:07.

  8. #8

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    Changed my RF modulator modded according to the old description with the new one instead.

    It worked perfectly fine this time, must have done something wrong before - although I couldn't find anything.

    I can't say I see any difference, but I'm sure it can be important on other TV:s, perhaps those that didn't get any color - unless they had the NTSC/PAL mixed up.
    Good luck modding, either version of the mod finally worked out for me.

  9. #9
    Amibayer! Bryce's Avatar
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    For those who don't have an A520 to mod, I still sell these: http://www.cpcwiki.eu/forum/amstrad-...0857/#msg60857 which give a perfect S-video on any Amiga with the levels correct and it's quite a bit smaller too. I only make PAL versions at the moment, but an NTSC version would be possible too.

    Bryce.

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