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Using VAC PSU in DC device

protekprotek
edited April 2014 in Hardware
Having fixed the dead Issue 3B, I began to wonder about the PSU that came with it. It was pretty obvious what had killed the poor Spectrum as the PSU supplied 12 VAC. :o

I really can't imagine what was going in that person's mind, who decided to plug it into the Spectrum. Is there even a theoretical chance that the Spectrum will last longer than a few seconds, when overvoltage and alternating current is supplied to a DC device?

Well, whatever the motive was, it resulted with a dead 7805, dead TR4 and TR5, probably dead lower and upper RAM, partially dead ULA and lifted trace from CPU.
Post edited by protek on

Comments

  • ZupZup
    edited April 2014
    I don't understand.

    Speccy PSU gave almost 12v without any charge, and descended to about 9v when connected to the computer. Anyway, the 7805 will accept any DC input from about 8v to 18v, but more volts means more heat to get ride of, and a hotter heatsink.

    If you supply AC power to a 7805, it will fail. I inverted polarity by accident on my 48k+, but fortunately the 7805 blew without letting any current flow to the rest of components. I don't know if a 7805 can fail in a manner that could supply overvoltage or inverted polarity, but it will surely destroy those transistors.

    In any case, I don't think a lifted trace from CPU is a symptom of a power failure. If it has some black spots, it may have failed because overcurrent... but it seems that could be a failed repair from unexperienced people.
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  • balfordbalford
    edited April 2014
    I'm sure (if I saw the original pictures right), that if a big hole gets blown in the regulator case then all bets are off, and it's entirely possible that the 12VAC made it out the +5v pin.

    Also, this voltage would have been applied to one side of the coil, for which I'll leave the more experienced electronic engineers to speculate on the potential carnage :)

    B
    The Spectrum Resuscitation Thread - bringing dead Spectrums back to life
    zx-diagnostics - Fixing ZX Spectrums in the 21st Century (wiki)
    Sinclair FAQ Wiki
  • 1024MAK1024MAK
    edited April 2014
    There are many failure modes that occur to silicon integrated circuits (chips) when too high a voltage / incorrect polarity / or worse, a large AC voltage is applied.

    It is possible for some of the structures to be converted into a resistance, hence letting the AC voltage through to the +5V line.

    And yes, anything connected to the +9V line may also suffer damage.

    Now you ask why people connect incorrect PSUs to electronic items. Well, most people don't have a clue about electricity when applied to electronics. If it has the correct connector, some people try it to see if it works...

    Other than membrane failure, DC power socket broken/dry jointed and "natural" death of DRAM chips, I strongly suspect that the two main causes of damaged / broken Spectrums is wrong PSU used or edge-connector misuse (plugging or unplugging while the Spectrum is still powered up).

    Mark
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