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Thread: Imsai 8080 power supply question...

  1. #11

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    I had some interesting issues running my SOL-20 from "230V 50Hz" in Australia.

    A lot of the time in supposed 230V countries the voltage can be over 240, even as high as 250V rms because of solar inputs to the grid. If you buy a stepdown transformer , if you have to use the "110v" because the primary of the power transformer in your computer has only the 110V winding, then you can end up feeding it with 120V or more.

    In these old linear supplies, the dissipation of the many 5V regulators on the S-100 pcb's is increased if you feed them with too high a voltage. The absolute minimum for a 7805 regulator is 7.5V input. In my sol, just with the usual 2:1 ratio step-down mains transformer the voltage presented to the regulators was around 10 or 11V when ideally it would be 8v.

    I found with about 98 to 100V rms line voltage, the Sol's DC supplies were producing about 8 to 8.5V to drive the 5V regulators. This reduced the heat dissipation on all the S-100 boards significantly. (my sol has the power transformer that was designed for brown out conditions and there is a mod to lower its output voltage)

    To get the 100V rms I simply use the step down transformer, like this (make sure to get one where the earth is carried through to the computer chassis):

    https://www.jaycar.com.au/120w-240-1...ormer/p/MF1080

    And to reduce the voltage to this transformer with a variac:

    https://www.jaycar.com.au/search?tex...0-b20a6021696c

    Probably a suitable voltage for most of these old computer linear supplies is in the order of 105V to 110V rms to help keep the heat dissipation on the low side as it can add up with a number of boards and regulators. 115 to 125V is on the high side, especially on 50Hz. Applying a lower line voltage also helps with the frequency issue, one thing with American 60 Hz transformers, some (not all) have their cores pushed harder towards saturation on 50Hz magnetization cycle and they have high core magnetization currents and tend to overheat on 50 Hz. Running a lower input voltage helps this issue too. They can also have higher radiated magnetic fields, which can interfere with your CRT monitor, if it is nearby. If the applied line voltage is too low, ripple will start to appear in the regulated voltages, this is immediately obvious in the video on the CRT output as hum bars appear in the video image.

    Al least if you have the Variac you can tailor it to suit your locality. And of course you could just use the variac on its own to dial the voltage back to around 105V, but I still use the stepdown transformer too as there is less chance of an over-voltage accident if the variac is dialed upwards by mistake.

    (just as an interesting technical point, the peak flux density experienced by the core of the power transformer is proportional to the applied voltage an inversely proportional to the frequency. So if the frequency gets altered by 50/60 this can be countered by dropping the voltage to 83% of what it was, in practice this is quite a lot, lowering a 120V supply to about 100V. So inevitably, American power transformers end up running higher peak flux densities on 50Hz systems).
    Last edited by Hugo Holden; May 22nd, 2019 at 05:21 AM.

  2. #12

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    Variac with sufficient current is a good option but remember that it is not an isolation transformer. Shock hazard or transformer ground failure is still a problem. The the full isolation transformer as Hugo suggest is the best option. To tune the voltage you can make a buck/boost box, with a center tapped 12V filament transformer. This would give you 5 possible settings without the added cost of a Variac. You'd have +12,+6,+0,-6,-12, with just some switches.
    Anyway, look at the lowest voltage with the boards loaded on the +8V, with the scope. If it drops below 7.5V you'll have issues. Also look at the regulator outputs of boards. Regulators seem to age and have poor regulation when they do. Replace any that have significant AC noise on the outputs.
    Also, check that you have the full 100Hz ripple on the diodes. Several times, I've seen people complain about there code dying and are replacing good RAMs when the real problem was that one of the diodes in the power supple's bridge was open.
    Saturating the core of the transformer is more related to the frequency and current draw than the voltage. The regulators on the boards keep the current relatively constant. If you have problems with saturating the core, may have to use a sine power inverter ( $$$ ). Another option is to use a small switcher and current limit resistor to boost the +8V current. Many lap top charges may work as some are +12V. You just need the right sized resistor.
    Dwight

  3. #13
    Join Date
    Jun 2012
    Location
    UK - Worcester
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    2,633

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    I have a Mercury step down voltage converter rated at 200W (part number 651.000).

    The specifications state this will convert from 230V to 110V.

    Note that in the U.K. we also went to 230V. We are still in Europe (albeit we may go back to 240V when we finally leave ).

    Yes, Voltage fluctuations must be taken into account.

    Dave

  4. #14

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    @daver2: If Germany would finally leave europe, so it would be much more better for imsai if we would get back the old 220 V 50HZ voltage...

    I decided me to buy an oldschool professional isolation transformer with integrated variac. It is the "Grundig RT5A" with displayed voltage and ampere. With this transformator i can adjust the voltage between 0 and 250 V. I know the issue with the voltage frequency between 50 HZ and 60 HZ. I thought itīs the best way to disconnect the mainboard and check the idle voltage on secondary side while increasing the primary side slowly. So, i can adjust the secondary voltage exactly and find out the needed primary voltage. I think nearly 100 V AC is enough for working properly with the imsai on standard wiring ?!

    If my imsai have the genuine universal supply, i can check with different jumper settings on imsai transformer. Perhaps i can get the perfect goal to use it in range between 230 and 240 V @50HZ with healthy secondary voltage ?!
    Then, i can use the changed wiring and use it without transformer.

    Next step is to remove the input primary filtering with the coils. In german, it is called "Netzfilter", i donīt know the english word. It is not necessary in year 2019 anymore. The primary ripple is in a small range today, i think. Then, iīll swap the capacitators. I think, this is necessary after 40 years ?!

    Jan

  5. #15
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    2,633

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    Ich verstehe netzfilter Jan...

    >>> If Germany would finally leave europe, so it would be much more better for imsai if we would get back the old 220 V 50HZ voltage...

    He He...

    >>> professional isolation transformer with integrated variac.

    Nice choice. I approve !

    Dave

  6. #16

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    As long as the filter capacitor don't have burst seals, they are worth the effort to restore. I just spent a couple weeks bring up the caps on my Poly88 ( 1976 vintage ) for a weekend of use at the Maker Faire here. It ran fine for the entire weekend. Since you have a variac, you can control the voltage. That is what I did.
    A better way is to disconnect one end lead and add a series resistor. A 5K resistor for the +8V caps and 15K for the 18V caps would be enough. Still start with a low voltage. Measure across the resistors to look for the leakage.
    Dwight

  7. #17

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    Quote Originally Posted by Dwight Elvey View Post
    As long as the filter capacitor don't have burst seals, they are worth the effort to restore. I just spent a couple weeks bring up the caps on my Poly88 ( 1976 vintage ) for a weekend of use at the Maker Faire here. It ran fine for the entire weekend. Since you have a variac, you can control the voltage. That is what I did.
    A better way is to disconnect one end lead and add a series resistor. A 5K resistor for the +8V caps and 15K for the 18V caps would be enough. Still start with a low voltage. Measure across the resistors to look for the leakage.
    Dwight
    It is interesting that the gigantic high uF value capacitors seen in these old computers are a technological thing of the past. There is no need for them now with switch-mode psu's which have much higher operating frequencies and much smaller uF values than used in linear supplies for vintage computers. Because of this I'm thinking they might become a rare part one day, as I doubt there would be a huge demand for new ones. So I decided to buy a spare one for my SOL-20, just in case the original one ever gives any trouble, but so far it is still ok.

    https://www.ebay.com/itm/50-000uf-25...53.m2749.l2649

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