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Repair a MFJ switching power supply

Discussion in 'General Technical Questions and Answers' started by W3PNM, Aug 9, 2018.

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  1. W3PNM

    W3PNM XML Subscriber QRZ Page

    Has anyone tried to repair one these sub $100 switchers from MFJ? I have one that started dropping out of regulation under load. MFJ told me the cost of repair is about the same price as a new one. Just curious if it's worth it to try a repair. I hate to pitch it if the fix is simple.

  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Do you have a schematic?

    I've found a lot of SMPS's don't even have published schematics; without a schematic and a parts list, they can be "difficult" to work on.:p
  3. G4COE

    G4COE Ham Member QRZ Page

    Four things that usually go wrong with a switcher I reckon.

    1) Faulty electrolytic capacitors, usually going high in ESR (Equivalent Series Resistance), causing them to 'gas' and then swell, these either leak or their cases become bulged, usually their top is 'domed' rather than being solid flat, this is usually where the vent is - a weak spot in the casing so that they don't explode, like roasted chestnuts.

    2) Output transistor or usually FET these day's 'kaput' sometimes because of faulty electrolytic capacitors in the feedback regulation loop.

    3) For the 'switcher to start operating it needs a 'start up' voltage on the control chip to get things going, this could be anywhere from about 12V up to say 18V or so, this voltage is taken from the rectified line voltage via a resistor and a regulator circuit, usually a high value resistor and a zener diode, this could be at fault, usually the resistor going high in value.

    4) Bad soldered connections.

    Should you attempt repair-:
    Be careful discharge the high voltage capacitors first, a 470 Ohm 1Watt resistor usually does the trick, to be safe check with a voltmeter after.

    K7MH likes this.
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    Looks simple.

    KB0MNM likes this.
  5. K6BRN

    K6BRN XML Subscriber QRZ Page

    Not a new problem...

    N6YFM posted elsewhere that his MFJ-2330MV switcher went down just a few days out of warranty. When MFJ refused to fix it without a hefty charge, he opened is up and found bad solder joints, debris and a solder bridge. So he took out his soldering iron and cleaned it up, whereupon it came back to life.

    His comments are HERE:

    So - if you're lucky, it may be that simple. And a close visual inspection and clean-up is a first step anyway, before pulling components or adjusting pots.

    Brian - K6BRN
  6. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Visual inspection, often with magnification, can quickly find many problems. Bad solder joints, broken traces/wires and loose hardware are common in consumer grade items.

    (I’ve even fixed two large household appliances this way. The typical failure was a cracked solder joint or delaminated trace because of high current and resulting thermal cycling which overstressed the connection).
    KD2ACO likes this.
  7. K6BRN

    K6BRN XML Subscriber QRZ Page

    Interesting schematic... almost no design margin on many components - the flter caps are only 16 volts, barely above operating voltage and some of the noise spikes they see may exceed this, reducing life. Note that there is some OVP protection via the 16V zener feedback - but this is NOT a crowbar circuit and will not trip Q6 until about 17V, whereupon the supply will rapidly "Motorboat"under and over voltage in many failure modes. And in some, it will not work at all. No wonder a few users have reported damage when using these units.

    And... the ammeter portion appears drawn incorrectly. It shows two circuit ground references, one at each end of the ammeter. So, by definition, no current flow. Unless the author is just trying to indicate that the local PWB near the output has its ground return through the ammeter - if so, this is the wrong way to do it.

    Even if I fixed this supply, I'd want to use it with some sort of external OVP circuit.

    Brian - K6BRN
  8. G4COE

    G4COE Ham Member QRZ Page

    If all else fails, I'd be looking at C26 I think it is near the snubber network near the two smoothing blocks, a 2.2u 250V electrolytic looks like, from what I see this is the 'kick start' capacitor that gets thing ticking.

    Whenever changing electrolytic capacitors always use high temperature 105 deg.C low ESR types, avoid using standard 85 deg C types because of the high frequency ripple current.

    The two capacitors on the base of the switching transistors I'd be changing as well

  9. W3PNM

    W3PNM XML Subscriber QRZ Page

    You guys are awesome and given me the info I needed. As a side note I bought an Astron linear supply at 35 amps to replace the MFJ. What a difference $200 makes!
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Yeah, it does.

    Astron's American-made and linear supplies are not only reliable but extremely easy to service, with off-the-shelf, inexpensive parts.

    Drawback: WEIGHT.

    I use only linear Astron supplies here at home, where I don't care about weight (or really, even efficiency). But for "portable" operation, if I have to pack a station in a suitcase or something, switchmode supplies are the obvious choice (smaller, lighter, more efficient).

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