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ZX Spectrum + Revision b Power Issue

owdeukowdeuk
edited February 2010 in Hardware
Hi all,

I'm trying to repair a ZXSpectrum + revision 3b and at the moment the -5v is missing from the board.

I've replaced the TR4 and will be soon replacing the TR5 and maybe the coil. I was wondering if anyone could answer a couple of queriers of mine.

1] Which component directly produces the -5v, I suspect the TR5?

2] Also, does anyone know of any good reliable suppliers for parts as I'm going to order some other spares also?

Thanks in advance
Post edited by owdeuk on

Comments

  • WinstonWinston
    edited February 2010
    No one component directly produces the -5v, a complete circuit (usually called a switch mode power supply) makes the -5v. However, if any one component in that circuit fails, so does your SMPS, and therefore your -5v supply.

    A switch mode power supply requires an inductor and some kind of switch (hence its name). The coil is the inductor and the transistor is the switch. (There's a little bit more to that too, you also need at least a diode and a couple of capacitors to make the most basic SMPS - and of course it has to be made in such a way that it will oscillate, and there has to be some kind of feedback to regulate the output voltage as the demand for power changes).

    The bad news is I'm afriad - if the -5v supply has failed at any time, then it's very likely that all the 4116 memory is fried - the TMS4116 datasheet specifically warns that this memory will be destroyed if the -5v isn't present when any other supply is present. You may get lucky, but the 4116 memory seems the most fragile and failure prone IC in the Spectrum.
  • owdeukowdeuk
    edited February 2010
    Cheers for the quick reply. I've recently changed the memory and they apppear to be ok. I measured the -5 last night on the memory and got about -0.2. So at least there is something there.

    I also tested the coil. On a friends working zx. I got the -5v on his coil. On mine it is producing 11v and 5v, no minus at all.

    The computer does actually boot, and I can write a short program. But there is slight screen correuption on the first line and it fails to see all of the memory. (On the peek poke test it returns 31236 but can produce others, IIRC.)

    Am I right in assuming first things to replace would be the coil and TR4/5?

    Do you know of a reputable supplier? I'm in the UK btw.

    Thanks again.
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