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Thread: Replacement transformer recommendation fro Northstar Horizon ?

  1. #11
    Join Date
    Jan 2010
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    Fact of life with lightly loaded S-100 systems using a regular transformer. Later power supplies compensated by using a CVT (constant voltage transformer, uses a separate winding and a capacitor to keep the transformer saturated). Of course the final evolution in S-100 was regulated switchmode supplies.

    You can fill the system up with boards to drop more voltage, increase cooling air flow (especially when you have the cover off!), run a variac on the line input, etc. I use the variac approach if I'm going to be using my IMSAI for a while with very few boards and the top cover off. If you don't mind modifying your boards, there are new drop-in replacement regulators that are little switchers designed in a TO220 footprint. They run cool despite the higher +8V rail.

  2. #12
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    Quote Originally Posted by glitch View Post
    there are new drop-in replacement regulators that are little switchers designed in a TO220 footprint. They run cool despite the higher +8V rail.
    The problem with these retrofit switching regulators is that they have super noisy output. It may not cause problems with the machine running properly, but it can introduce noise in audio circuitry and cause distortions on video output, especially analog like CGA/EGA/VGA or composite.

  3. #13

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    Do note that, for a buck/boots, it is only the secondary current rating that you need to worry about. Do check the ripple voltage on your power supply. If it drops below the 2 volt head room that the regulators need, you'll have issues.
    I recently worked on a unit that looked to have fine voltage with a meter but it had one half of the bridge circuit blown out. The ripple was dropping below 1 volt headroom for the regulators. This cause erratic operation.
    I've seen cases were the design of the boards did not account for all the power needed by the board. Resistors were placed in parallel to remove some of the power being dissipated in the regulators. Like was said, to doesn't change the total heat, but it does move some way from the regulator. A shorted regulator can do a lot of damage!
    Dwight

  4. #14
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    Quote Originally Posted by GiGaBiTe View Post
    The problem with these retrofit switching regulators is that they have super noisy output. It may not cause problems with the machine running properly, but it can introduce noise in audio circuitry and cause distortions on video output, especially analog like CGA/EGA/VGA or composite.
    This is true. Especially if you go with the cheapest available models.

    As I mentioned, I just plug in the variac if I'm going to be running the IMSAI with nearly no load and the cover off. Also, if I've got a board up on an extender, out of the normal chassis airflow, I'll aim a small desk fan at it.

  5. #15

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    Quote Originally Posted by zippysticks View Post
    Can anyone recommend a replacement transformer for the Northstar Horizon please ?

    Mine seems to be producing around 20V and 10V rather than 16V and 8V, regulators on both memory boards are uncomfortable to touch (hot!) and it appears that over time the heat has discoloured the PCB around the heatsink mount (perhaps this is normal?) so I'm looking to try and get things back closer to spec.

    I did look at the article below where another owner has done similar but the spec of that transformer does not look correct.

    http://www.connect.gi/northstar.htm
    A lot of thought and testing went into that, the transformer was custom made as a result. I calculated the
    voltages in excel and built a test power supply before ordering.

    If I made another I might include more voltage taps on the primary to allow for 220, 230 and 240v
    but certainly for my Horizon it works nicely and the regulators on the boards no longer cook.

    The machine is an early Horizon and NorthStar issued a transformer upgrade later to allow for powering
    eight UP-8 slave processor cards fitted and a 5.25 HDD.

    However if you are in the UK I can strongly recommend Eastern Transformers Ltd as being responsive and
    producing a custom transformer at a very sensible price. I asked another company to quote and got no answer
    but the transformer made was exactly the right size mechanically. It looks better than the original.

    AND it works.

  6. #16

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    I'm using Eastern Transformers but have taken the variac route with them - they do quite a nice unit HERE

  7. #17

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    I did not include the transformer spec on my website but have now;

    SE250 1PH 220VA double wound transformer made to order
    Primary: 0/240Vac centre tap
    Secondary 1: 0/12Vac @ 60VA
    Secondary 2: 0/12Vac @ 60VA
    Secondary 3: 0/6 Vac @ 50VA
    Secondary 4: 0/6 Vac @ 50VA

    Remember the voltages quoted are RMS, not peak and there is a voltage drop on the diode.

    With that transformer the voltages on the motherboard with a basic set of cards is
    8.5v 17.5v and -17.5v and everything runs cool.

    The transformer is an exact physical replacement for the NS one.
    Cosmetically better looking with screw connections rather than flying leads.

    Otherwise its ... a transformer.

  8. #18

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    Thanks Jim - I might get one of those too.

  9. #19

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    Incidentally my input voltage is 245v yours may be 230v so you need to consider primary taps;

    I did a centre tap to allow running a 240v fan at 120v for reduced speed as it cuts the noise down dramatically. The airflow is adequate.

    Having got this far and with two decent half height floppies working I need to spend some time finishing the rebuild.

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