Re-creating a Sinclair QL power pack

Discussion in 'The Projects Forum' started by bootlegger17, Feb 10, 2016.

  1. bootlegger17

    Thread Starter New Member

    Feb 10, 2016
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    Hello all - thankyou for taking the time to read my 1st post!
    I am a sinclair fan and recently picked up a cheap Sinclair QL computer from the 80's. Unfortunately it did not come with a power pack, and it appears the power pack is an unusual beastie and very hard to come by. So I wondered how difficult it would be to re-create such a Power pack.
    Looking online it shows that the original power pack carries two loads - a 9v DC 1.8a and a 15.6Vac 02.a, and a common ground.
    http://www.dilwyn.me.uk/docs/hardware/qlpsu.png
    I guess its easy enough to get hold of a 9v dc power supply, but I am stuck on trying to find a 15.6Vac power pack.
    I was thinking about re-creating the 15.6vac line myself, but this is where my knowledge lets me down - i know very little about designing electronics. i have a very basic understanding of components, but have no idea on how to create my own circuits etc.

    From my rudementary understanding I thought I could just use a transformer to generate this power line.
    I see on ebay I can buy a transformer that will convert 230vac to 15vac, but this is not 15.6vac. Do I really need 15.6vac, or is 15vac close enough - is there usually a tolerance allowed (like a cap with +-20% etc)
    http://www.ebay.co.uk/itm/Encapsula...hash=item2a1b163b77:m:mZyvhEy-bcWlksgtjwXKggQ
    Do I need to go for the higher rated 230vac to 18vac maybe, then somehow step down again to 15.6v. If this is so, then i am lost. What do i need to do to achieve this. Also I believe these dont have enough amperage so how do i get around that - these ebay transformers show 60mA but I believe the 0.2a requirement on the ql psu diagram equates to 200mA??

    So sorry for the dumb questions, but I was hoping for a quick easy resolve to this, and the more I look into it, the further out of depth I feel!

    Regards
    Jason
     
  2. #12

    Expert

    Nov 30, 2010
    16,305
    6,814
    How many people here own a Sinclair QL computer? Only by knowing its insides would anybody know how accurate the voltage needs to be.

    However. power supplies are designed to work as the power in the wall outlet varies by +/- 5%
    Is 15 volts within 5% of 15.6 volts?
    Is the power in your house really 230 volts? The alleged 240 volts at my house is really 250 volts.
    Is the transformer rated at 230 volts of input?

    You are getting so picky here that you have to do actual measurements to find the answer.
    and, yes, the transformer has to be rated for at least the current you need.

    http://www.mouser.com/Power/Transfo...uyyrmZ1yzuxsfZ1yzuxtdZ1ysxjzyZ1z0j7osZ1z0wd2x
     
    Last edited: Feb 10, 2016
  3. bootlegger17

    Thread Starter New Member

    Feb 10, 2016
    2
    0
    Hi and thanks for the reply.
    All I can tell you is what I know from what i can find online.
    Assuming in UK i have anywhere between 230 and 250vac, i wouldnt have though it would matter if i want to step down to 15.6vac.
    if at 230vac shouldnt the transformer give me 15.6, and if at 250vac, shouldnt the transformer also still give me 15.6 - or is the output all relative to the input? i thought it would be down to the transformer to create the exact output required somehow.

    The link you provided is great, but they all output 16v? I thought voltage had to be exact, but amperes could be more. is it safe to use 16v or am i likely to blow the circuitery on the motherboard?

    jason
     
  4. #12

    Expert

    Nov 30, 2010
    16,305
    6,814
    I told you that the voltage at the wall outlet matters.
    I told you that you would have to measure the voltage available at the wall outlet. You did not do that.
    I told you that nobody here is likely to know how exact the motherboard wants its voltage supply, but it's probably within 5% of the labeled rating. You still ask us if it is safe to apply an unknown voltage range from an unknown wall outlet to an unknown requirement of the motherboard.
    I told you that the motherboard is probably designed to accept a range of voltages within 5% of the labeled voltage.
    I showed you the math which demonstrates that 15.0 volts is within 4% of 15.6 volts.
    I expect you can do the math to see that 16.0 volts is within 2.6% of 15.6 volts.
    I gave you a source for transformers and so, some choices are available. You chose not to do the measurements or look up the requirements.

    "Exact" is impossible to produce because transformers don't produce exact voltages, they produce a proportion of the input voltage. The power company doesn't produce exact voltages, they guarantee their product within +/- 5%. Transformers always produce more voltage than the label says when they have little or no load on them. Transformers only load down to the labeled voltage when under full load.

    Either do the measurements and look at the specifications for the available transformers, or just guess at which one is best. That's as much as anybody else can do.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
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    Uk voltage tolerance is +10% -6%, thats anywhere between 14.5V to 17.5vac on your pc input, get a transformer for a car battery charger they're about 15vac, the circuit diagram inside is using the AC to produce +/- 12V dc ICs 36,37 using the half wave diodes.


    Top left cct.
    http://www.sinclairql.net/srv/QLSchematicIssue5.gif
     
    Last edited: Feb 21, 2016
    #12 likes this.
  6. bertus

    Administrator

    Apr 5, 2008
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    #12 and ronv like this.
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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  8. ian field

    Distinguished Member

    Oct 27, 2012
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    Just throwing a wild guess - but the 15VAC probably supplies 78/79 regulators to furnish +/- 12V rails for fully compliant RS232 ports.

    The 9VDC probably feeds a (or more likely several) 7805 regulators for the main logic circuitry.

    A quick look inside could make better than a wild guess a possibility.
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    I posted the circuit diagram in post#5
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Finally got the image to open...........................

    A few days ago the power company hit me with an outage that corrupted my network drivers.

    The power came on just long enough to invoke system restore, then they hit me with another outage timed to do some *REAL* damage.

    Got everything going again - but it seems to be running real slow!
     
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