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Thread: ZX81 video modes help

  1. #1
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    Default ZX81 video modes help

    When you build a Sinclair ZX81, you get the option to set the tuner to output either an NTSC or PAL standard signal by soldering two/three different wires into one of three sets of three holes on the PCB. For some odd reason I will never know, my uncle set his ZX81 to PAL which requires only two of the wires to be soldered and then he clipped off the third so you can't set it to NTSC.
    I could switch it back to NTSC if I added the wire again but looking inside the tuner box I don't see where the wire specifically goes to.



    Anyone know what to do?
    I finally found a cassette of games that will load in the 1K of onboard ram. All the other cassette software I own requires I have the 16K memory module and I don't have that (but if you want to part with a Sinclair branded 16K cartridge, please PM me )
    = Excellent space heater

  2. #2
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    I don't know about the ZX-81, but the spare hole in the RF modulator is what me and other people use to route a composite video signal on the ZX Spectrum, effectively bypassing the RF circuit. More or less the same RF modulator assembly seems to be present in most home computers and video games, although incoming signal strength may vary.
    Anders Carlsson

  3. #3

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    I'll dig one out and check the wiring inside the modulator, unless someone beats me to it.

    You may also need a few extra components (diode, resistors) to get it running in NTSC (the "USA only" parts labeled here).

  4. #4
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    Need more parts?
    Hmm, I read through the owners manual and it didn't say anything about additional parts being needed.
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  5. #5

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    I'll have to dig through the schematics... I thought for switching between PAL and NTSC there was a wire you hooked to either ground or +5v.

    I'll check sometime tonight if I get a chance.

  6. #6

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    Had a look inside another ZX81 here and it looks almost identical to yours. The only difference is that the middle lead of the modulator pokes back up through the right-most of your two red circles on that second photo.

    However, I also found a copied set of the assembly instructions for the US version. On the very last page was:

    Sinclair UHF KITS Card Enclosure
    for 60Hz U.S.A. video

    This package contains an extra component necessary for the assembly of this kit. It is R30, a 0.5 in. jumper wire. Insert this component at the position marked R30 on the PCB. Be very careful in the placement of this jumper. There are three holes to the left of the box marked R30. Insert the jumper into the middle one of these. This jumper selects 60Hz video as opposed to the normal U.K. 50Hz.

    Insert the UHF modulator following the instructions given in the Assembly Instructions leaflet. Be sure to insert the two wire leads into the holes marked "Fr/UK1" and "Fr/UK2". These are in effect the UHF locations for the modulator. (Disregard the USA openings on the PCB.)

    TO OBTAIN VIDEO DISPLAY
    TUNE TV BETWEEN CHANNELS 30-40
    So, long story short: ground pin 22 of the ULA to enable NTSC video. The other "USA only" components on the schematic are apparently not required; they are marked "Not used" even in the US version of the assembly instructions.

  7. #7
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    So just locate R30 and on whatever side of the resistor I prefer, just ground it?
    Sorry, I'm looking at this PCB and I'm totally confused.
    = Excellent space heater

  8. #8

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    No, R30 is the jumper - you need to fit it. The position is marked with a silkscreen box, between the ULA and the voltage regulator. As the instructions say, make sure you get the correct hole on the left:

    zx81_r30.jpg

  9. #9
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    There's already a ten ohm resistor (brown, black, black gold) right there.
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  10. #10

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    Then it would appear it's already set to NTSC. According to the manual, that jumper is the only difference and 10 Ohms should be close enough to zero to work. The "USA" markings on the PCB are a red herring. I suppose you could measure the actual resistance from pin 22 of the ULA to ground and replace R30 if necessary, but I really doubt that's your problem.

    Going back to square one: what makes you think it's not set to NTSC? Be aware that modern TVs with digital tuners can have a hard time tuning to the ZX81's signal. An old TV with analog fine-tuning is best. Alternatively, you could look up how to get composite video out of it and bypass the modulator entirely.

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