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Hi y'all, need some old school help.

Discussion in 'Power Electronics' started by Nessmuk, Jan 25, 2021.

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  1. Nessmuk


    Jan 25, 2021
    I've got this old school power transformer. 120v to 12v 1.2 amp.
    It doesn't work. How do I test it out?
  2. Martaine2005


    May 12, 2015
    Have you researched it?
    Have you got a multimeter?.
    Have you opened it?.

    First check you have mains voltage to the primary side. (Fuse and mains cable).
    2. Check secondary voltage.
    3. Check bridge diodes.
    4. Check filter caps.
    5. Check LM7812 regulator (if it has one).
    Let us know your findings in a little more detail than your question!

  3. Nessmuk


    Jan 25, 2021
    I was trying to post pictures. No luck.
  4. Nessmuk


    Jan 25, 2021
    I know enough to test a capacitor and the breaker. That's about it.
    How to test:
    Transformer, rectifier, capacitor?
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Resize the pictures to < 300 kB.
    Measure the transformer windings for continuity.
    Measure the rectifier by measuring each diode: about 0.6 V to 0.7 V in forward direction, no current in reverse direction. Many multimeters do have a "diode" option.
    Measure the capacitor with a capacitance meter. Some multimeters do have such an option, but not all.

    This is akin to "my car won't move, what's wrong?"
    Way too general. As Martin asked in post #2, describe the details. Can you measure a primary current into the transformer? Does the transformer output AC but no DC after the rectifier? Is the DC present but noisy or too low?
  6. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir Nessmuk . . . . .

    Post # 5's photo seems to cover all parts that we need to see. . . . . a la . . . . .

    My rare, imported 30 inch Swarovski crystal ball is predicting that the REAL problem will be either the frontal mounted circuit breaker or the internal thermal fuse hidden down within the power transormer. (Its reason for physically being placed there, for establishing good thermal contact / sampling )

    The Yellow Brick Road path . . . as it is now being encountered . . . . .

    The units two AC line power wires come into the unit at the rear crimp grommet and then both wires pass up to the point where one wire goes / solders direct to one transformer primary winding terminal.
    The other incoming AC power wire goes into and then out of a power slide switch and connects to the other extreme end terminal of the power transformer .
    Now the REAL primary connections into the transformer are being that first wire soldered on, and the MIDDLE terminal of those 3 terminals, with this HIDDEN thermal ffuse having its two wires being connected between the switch derived wires . . . end terminal . . . and the center transformer terminal.
    Therefore if you have a dirty corroded switch or that thermal use being blown open, no AC power in will get to the power transformer primary winding.
    But . . .BUT . . .BUT ! . . . if all was being well in that circuit path, then the transformers other sides secondary's three terminal will have your 12V AC outputted across its two opposite terminals. . . . with the center tap being unused..
    The most visible RED wire then passes to a Littlefuse circuit breaker to then pass out via a BLACK wire to one of the YELLOW dot coded AC input wires to a heat sinked FWB rectifier block.
    The transformers secondarys other wire connects to the other YELLOW dot coded wire of the FWB block.
    We also have the units frontal LED indicator, being wired in across those two YELLOW dot coded wires of the FWB.
    Therein, you will find a voltage droppng / current limiting fixed resistor that passes up to one lead of the LED and then out of the LED into a properly polarized series diode, then that spliced on RED goes down to the other YELLOW dot lead of the FWB.

    Now if you have been receiving / comprehending ( apprehending) a proper Edd-i-cation so far, your . . . . EUREKA ! . . . . .light should have gone off in your head and your now realizing that . . . . .
    You have some Built In Test Equipment, as is being present in that units design.
    If you plug the unit into AC power and then turn on the AC power slide switch, the LED indicator should light. ( Considering with a 99 86/100 ths degree of probability that the actual LED proper junction is being good. )
    If so, you then know that the transformers internal thermal fuse is not blown open nor is the secondarys circuit breaker being either tripped or just plain BAD / open.
    If not,there is only the FWB and its connected GREY 470 ? 1000 ? 1500 ? 2200 ? 3300 ? 4700 ufd filter E-capacitor, being suspect.
    A bad / shorted FWB rectifier unit could have the circuit breaker always tripping on initial power up.

    Thaasssssssssssssit . . . . . . liesurely*** . . . .awaiting for your comebacks findings . . . . . . . . . .
    .*** [​IMG]

    73's de Edd . . . . .

    A short line outside a building can sometimes become a long line inside.

    Last edited: Jan 25, 2021
  7. Nessmuk


    Jan 25, 2021
    Thank you!:)!
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