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Thread: Replica Science of Cambridge MK14 available for pre-order.

  1. #51
    Join Date
    Dec 2010
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    Stoke-on-trent. U.K.
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    Tried those tests you mentioned.I had already checked all the resistors so R7-R10 are good.
    I didn't test all the parts of all the digits (very tedious) but the first 4 which are the spaces and 00 to the right of the display worked exactly as expected.I checked a couple of elements of all the digits and worked as expected.

    Then I checked the 7445 with the help of my breadboard to switch the inputs and tried 0 to 6 and lit a part of each digit again as expected.
    I also put all the chips back in and used my scope of measure the outputs of the 7445 and got a mess of signals until a key that that line sensed was pressed then I got a simple pulse repeated until the key was released.Still no clue as to why certain keys stop working when using 7V.

    But in the time I was testing with 7V the regulator started getting hot and the reverse C reappeared but those certain keys still woldn't work.Should the regulator be heat sunk?

  2. #52

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    The regulator definitely needs a heatsink, I added a too small one but manage to keep the temperature within touchable limits by supplying 8 Volt input (from a modified Spectrum supply).

    Your problem with the inverted C could also be caused by a timing problem, have you tried swapping the LS157 IC's to see if the pattern or position changes?

    BTW I found out not all LS157 like to be (mis)used as latches. Two old LS257 worked also.

  3. #53

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    Quote Originally Posted by Lurch666 View Post
    But in the time I was testing with 7V the regulator started getting hot and the reverse C reappeared but those certain keys still woldn't work.Should the regulator be heat sunk?
    Lurch, if you have a variable power supply (as in, one you can sweep the output voltage up and down on by turning a control) try the following:

    Attach your variable power supply to the Vin of the regulator. Connect a voltmeter to the +5V rail of the MK14.

    Start with Vin = 5V or less. You will see that the output from the regulator at that point is much less than 5V, which is no good. The MK14 absolutely requires a 5V regulated supply.

    While watching the voltage on the 5V supply rail, slowly turn up the input voltage to Vin. When the output voltage from the regulator stops rising and sticks at 5V, the input voltage to Vin is the absolute minimum it should be. To be on the safe side increase it by half a volt or a volt more.

    The 7805 can handle considerably higher input voltages and still regulate them down to 5V but the greater the difference between the input and output voltages, the hotter the regulator will run. You should therefore use the lowest input voltage which allows the regulator to regulate properly. In practice this will be about 8V, as Gert has used on his.

    My MK14 has had its regulator mounted offboard on a substantial heatsink for most of its life. It was disingenuous of Science of Cambridge to suggest that it 'Might' need a heatsink - it does need a heatsink.

    A disproportionate amount of the total power is drawn by the two bipolar PROMs, which account for almost half of the total current drawn from 5V. You will find that they get quite hot when the system has been on for a while, or that is certainly the case with the original National Semiconductor PROMs.

  4. #54
    Join Date
    Jun 2012
    Location
    UK - Worcester
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    1,975

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    +1.

    I have mounted my regulator vertically, and will probably manufacture my own heatsink out of aluminium sheet.

    You can buy 1.5A or 2A regulators in the same package. Not sure whether these would be better than the original 1A device...

    My displays arrived from Russia yesterday. I have tested both of them out and all of the digits/segments work on both displays. Good news and I have found what seems to be a good Russian supplier.

    I just need the RAMs and the 80L95 (equivalent) now and I should be able to finish mine off.

    Dave

  5. #55

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    Quote Originally Posted by daver2 View Post
    +1.

    I have mounted my regulator vertically, and will probably manufacture my own heatsink out of aluminium sheet.

    You can buy 1.5A or 2A regulators in the same package. Not sure whether these would be better than the original 1A device...
    It does not matter if you use an 1, 1.5 or 2 Amp regulator, the amount of heat generated will be the same It is always (Vin -Vout) * I (the current drawn by the MK14) A heatsink made of a strip of aluminium will keep the temperature low enough.

    Only way to do a more efficient conversion is with a switchmode regulator but that would be not very 'vintage'


    My displays arrived from Russia yesterday. I have tested both of them out and all of the digits/segments work on both displays. Good news and I have found what seems to be a good Russian supplier.

    I just need the RAMs and the 80L95 (equivalent) now and I should be able to finish mine off.

    Dave
    For the 80L95 you can use a 74LS365 instead (pin compatible) or as I did: get a 80C95 online which will work also.

    Ram chips can still be found online, I got mine from an eBay seller from Israel, he sells the AM9111BDC which work great.

  6. #56
    Join Date
    Jun 2012
    Location
    UK - Worcester
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    1,975

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    Sorry - yes, I didn't mean that the amount of heat produced would be less... Only that it may be a more 'robust' part. Sorry, didn't make that clear.

    Yes, I have gone for the SN74LS365. I am just waiting to hear back from my supplier with a price. I am purchasing everything from the same supplier in the UK (although they tend to be a little slow in responding with the price - but the components are brand new and arrive quickly once you can pay them).

    Dave

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