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Weird power transformer problem

Discussion in 'Electronic Repair' started by Ancient_Hacker, Aug 20, 2007.

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  1. This is the weirdest thing I've ever seen.

    I'm repairing a sick Tek 1502 TDR. Several transistors are blown in
    the power supply and the nicad pack looks like it's been overheated.

    I replace the shorted parts and apply power through a variac,
    slowly. WTF? I have the variac up to about 45 volts AC out of 120
    and the power supply output voltages are up to normal! Even more
    bizarre there's 12 volts across the main filter capacitor, rated at 16
    volts! No way I can turn up the variac any more without blowing this

    I first think: Aha, somebody set the jumpers to the wrong line
    voltage!, But no, that's backwards thinking, I'm putting in 120, and
    a quick look shows the jumpers are set properly, with the two
    primaries in parallel, just as it should be for 120 VAC in.

    Aha, I think, somebody replaced the power transformer with one meant
    to put out 24 volts. But no, the tranny looks original, and the part
    number matches the one on the schematic.

    Now I'm totally stumped. It's the right transformer but it puts out
    like 28 VAC no load instead of 12 to 14 volts expected.

    Put the transformer on the LC bridge --

    primary #1: 72.4 ohms 2.18H, Q = 8

    Primary #2: 84.2 ohms, 2.2H, Q = 7

    secondary: 89 mH, Q = 6

    So apparently there are no opens or shorts.

    Anybody have any idea how this transformer is violating the laws of
  2. N Cook

    N Cook Guest

    Loss of isolation between primary and secondary ?
  3. msg

    msg Guest

    Are all of the outputs regulated? If so, the supply is exhibiting a
    wide dynamic range. If your "main filter capacitor" is before
    any regulation, perhaps it was replaced with an incorrect WVDC
    rating? Is it feasible to disconnect all of the supply's outputs
    for further analysis and testing? How about disconnecting the
    transformer secondaries and running it at its rated input voltage
    and monitoring its temperature (shorted turns in the primary)?


  4. hr(bob)

    hr(bob) Guest

    The unit appears to have a rectifier followed by a switch-mode power
    supply. The rectified AC probably goes into the SMPS and is
    supposed to have a constant low voltage output. There might/ust be a
    failure in the feedback circuit that sets the output of the SMPS.

    H. R. Hofmann
  5. Thanks for all the suggestions. I think I've figured out what
    happened, a bit far-fetched but here goes:

    By taking the square-root of the primary to secondary inductance
    ratios, we get a step-down ratio of nearly 5, which implies a step-
    down ratio to 24 volts. Which is what we measure the transformer

    Now that tranny goes into a full-wave bridge, then thru a SCR to the
    12-volt nicad pack and 16V 470F capacitor.

    A completely non-sensical situation.

    Now a couple more observations:

    (a) The transformer doesnt smell cooked, but it does have a few
    droplets of varnish oozing out.
    (b) Somebody replaced the dual AC line fuse holders with a single
    fuse holder.
    (c) The unit arrived with no fuse caps or fuses in the AC line fuse
    (d) The nicads, although overheated and with 1983 date codes, are in
    good electrical condition.
    (e) These 1502's tend to get very beat up, as they're meant for
    portable use in the field. This one looks like-new.
    Not a scratch or scuff anywhere.

    The only explanation I can come up with is:

    (1) The guys at Tek wound the transformer wrong. Twice as many turns
    on the secondary than required. They slapped the "0151-xxx" tek
    sticker on it anyway.

    (2) The transformer had just enough primary resistance to limit the
    primary current to around one amp average.

    (3) The nicad pack (remember, it looks overheated) acted like enough
    of a zener to clamp the voltage to about 14 to 16 volts.

    (4) The rest of the power supply aged very quickly with the
    overvoltage, frying a transistor and a zener.

    (5) ... and the unit ran just long enough for Tek to calibrate and
    ship it.

    (6) Since then it's been blowing AC line fuses every few minutes, bad
    enough so someone rewired it to use just ONE line fuse.

    (7) It's been used very little, probably put on a shelf as a
    unreliable fuse-blower. That explains the good nicads and the
    pristine condition.

    Weird, but I don't see any other logical explanation!

    You'd think Tek would have caught this, especially on such a pricey
  6. Radiosrfun

    Radiosrfun Guest

    Like most everything else - Quality Control has gone to hell - too. Its all
    about the "buck" - never mind how you get it.
    Years ago - I sent in a piece of test equipment to the manufacturere - to be
    repaired and calibrated. I paid like $15 to ship and $75 for repairs. They
    "supposedly" fixed it and shipped it back. I had it a week and it did the
    same damned thing. So much for that. I wasn't about to shell out another $15
    to ship it again. I called the company to complain and they told me how to
    fix it myself - to which I asked - then what did I pay you for? I finally
    acquired a schematic and repaired and aligned it myself.

    These companies are becoming a real trip to deal with. They want top dollar
    for their stuff, make it cheap and expect top dollar to repair it. The
    consumer gets screwed. Lately it is more so the case - than not.

  7. Ivan

    Ivan Guest

    I agree, and the way the whole world is now geared to buy and throw away IMO
    almost borders on the obscene.

    A classic example, about 4 years ago my wife bought me quite an expensive
    electric razor. A year ago it stopped working and from memory the last one I
    looked at (a very long time ago!) consisted of an extremely simple kind of
    (virtually foolproof) vibrating AC powered motor which drove the cutting
    head to and fro.

    However on dismantling this one I was surprised to find a small electric
    motor and a SOPS (self oscillating power supply) and I found out pretty
    quickly that the problem was due to an open circuit quarter watt 2.2 meg
    resistor, total cost to replace about two pence and suffice to say that the
    razor has been functioning perfectly ever since.

    Now I understand perfectly well about labour costs, which would probably
    have ruled taking it to a service depot and having it repaired, but at same
    time I couldn't help wondering how many electrical items are dumped into
    landfills every single day, just for the sake of a couple of pennies worth
    of components.. probably millions on a worldwide basis, which to me begs the
    question, just how much longer can our profligate lifestyles be sustained?
  8. Radiosrfun

    Radiosrfun Guest

    When I was in Elementary School - they showed us "Film Clips" of "Recycling
    Plants" - to do paper, glass, cans, etc.... Said a few existed in the world
    but more to come - it was the way of the future - to recycle and save our
    environment. Yet - here we are - producing JUNK - and filling the landfills
    with it all - and as for "Recycling" - it is NOT used in all areas for the
    "Intended" items and certainly not for electronics. Yeah, I agree - sooner
    or later - we're going to have some real problems.

    One of the bigger towns near me "started" to do Recycling, but like all
    "phases" - it passed. People are using the allocated color coded bins - for
    other things now - much like those milk crates you buy in a store. "Our"
    community has never offered recycling.

  9. Ivan

    Ivan Guest

    I don't know if it's the same where you are, but in one of the side effects
    now beginning to manifest itself here in the UK is the increasingly growing
    numbers of ancient churches having the lead stripped off their roofs and
    even railways and electricity substations are also being targeted and
    literally put out of action because of copper cable and fittings being
    stolen. Apparently there is now such a famine of raw materials in China that
    scrap metal is becoming a very big and lucrative business.
  10. N Cook

    N Cook Guest

    This used to be a third world phenomenon
    in the last month in the UK
    80 drain covers have gone missing from the Wokingham Borough Council
    Hatfield and Brookmans Park and more than 200 drain covers have been taken.
    More than 60 have been stolen in the past two weeks.
  11. Ivan

    Ivan Guest

    Without wishing to stray too far off topic on a non political newsgroup, it
    certainly depresses me to think just how rapidly our fragile materialistic
    societies would descend into anarchy if the global economy abruptly went to
    tits up... here is a link to a local news report from just a few days ago
  12. Radiosrfun

    Radiosrfun Guest

    Some areas a few miles from me have had copper stolen from homes (plumbing,
    etc) being built, siding stolen off the homes being built "and" lived in.
    Theives went to places where stockpiles were kept - too some of it. Some
    were even stealing the bronze flower vases off of Cemetary markers. Was it
    for "China" - I don't know - but doubt it. Probably more like "drugs". Some
    idiot tried his luck here ripping out plumbing from a home of a
    "vacationer" - and let the water run - doing about $10,000 worth of damage.
    He was busted in a week. Our cops caught him as he was making last minute
    reservations to go to Florida. That was his first and last stint - here in

  13. Ivan

    Ivan Guest

    It sounds like your local police are more on on the ball than ours, although
    I love the Simpsons, simply because IMHO it appears to reflect just how
    alike our two societies are.. For instance I was watching an old episode the
    other day when Homer rang the local police station, an answerphone cut in
    and said something to the effect of "The Springfield police station is now
    closed., if you are reporting a felony press one, If you're reporting a"..
    As far as our local police station is concerned it sounded about right to
  14. Arfa Daily

    Arfa Daily Guest

    My 85 year old mother had lead flashing stolen off her house during the day
    whilst she was in, a few weeks ago. The 1000 year old Saxon church in my
    village also had a large quantity of lead stolen from one of the roofs which
    form a more modern part of it. Apparently, road signs are also being stolen
    all over the country, and are now being replaced with glass fibre composite
    ones, which actually have a sticker on the back which says "NO SCRAP VALUE".
    Sad what the world is coming to ...

  15. Radiosrfun

    Radiosrfun Guest

    Not to mince words with you Arfa - but I think DISGUSTING is a better term.

  16. Radiosrfun

    Radiosrfun Guest

    Well, there is a sort of humorous side to this and almost in respect with
    your experience. See - we have a 911 system in place here. IF you have an
    emergency - whatever it is, you call 911, they dispatch the Police, Fire,
    EMS, whatever. Yet, our local police have had a couple idiots call the
    "Station" - to report an accident OR breakin - whatever - and with the way
    our police are set up no one is in the station when they're on patrol. So -
    these idiots wonder why the police don't show up lickity split when they
    call. DUH - they weren't there to get the message. If they had called 911 as
    they should have, the police would have got the call from 911 immediately.
    This 911 system has been in play since 1994. You would think they'd get it
    in their head by now - 2007.

    As to our police being on the ball - they are - I have to give credit where
    credit is due. The funny thing is, many underestimate them. They look at
    them as being the Keystone cops. I can honestly say - not many of their
    crimes have went unsolved. The only one they had issues with - was a guy who
    "allegedly" murdered someone here - and skipped country to one who doesn't
    reciprocate with sending them back here. We get crime waves about once a
    year - usually some ass hole from out of town. I work hand in association
    with a couple of those guys almost daily - and they're pretty good Joe's.

  17. Arfa Daily

    Arfa Daily Guest

    Well yes, you're probably right. As far as cops go, from what I've seen of
    U.S. police (and actually *most* officials over there), they leave ours for
    dead. The system of police officer / sheriff / state trooper is far superior
    to ours. Not so many years back, I would have said that our police force in
    the UK could stand with the world's finest, but as an organisation now, they
    are rapidly becoming a joke. This is being further added to by them
    replacing genuine officers with "PCSOs" - that's Police Community Support
    Officers. These guys (and girls) look for all the world *like* a police
    officer, but only have the powers to observe and report. U.S. police look
    the part. Their uniforms are smart, and from what I've seen around the
    states, always worn with a sense of pride and duty, as are those by customs
    and immigration officials at airports. Ours, on the other hand, now have a
    sort of 'scruffy' and dishevelled look about them. If you call 999 (our
    911), you will get put straight through to the service you need, but with
    the police, you may not get an immediate response. Whereas all calls used to
    be taken and assessed by a police officer, if you were calling the police,
    you are likely now to get a civilian operator, who makes the decision how
    quickly to respond. Many even large police stations are now closed at night,
    so if you are reporting a proper crime, you may not get someone round before
    the next morning, or even afternoon.

    On the other hand, if someone reported you making a racist or homophobic
    comment in public, you are likely to find yourself dragged off to a police
    cell within 5 minutes. It is now also very easy to end up with a criminal
    record for motoring offences such as speeding, and even doing your garbage
    recycling incorrectly, in some towns. I kid you not. Policing here now is
    all about meeting clear-up targets, and it's easier to do this with 'soft'
    offences committed by otherwise law-abiding good citizens, who still have a
    fear and respect for the police, and will roll over and allow themselves to
    be swept along by the system. Junkies, feral gangs, and career criminals
    are, on the other hand, laughing their heads off, and doing degrees in human
    rights law. I hope that the U.S. doesn't finish up going the same way, but
    sadly, I think it is inevitable.

    All of which is a jolly long way from odd transformer problems !

  18. Ivan

    Ivan Guest

    Wasn't it Mahatma Gandhi who said "There's enough in the world for every
    man's need, but not enough for every man's greed," how prophetic that's now
    turning out to be!
  19. Ron(UK)

    Ron(UK) Guest

    Ivan wrote:
    Homer rang the local police station, an
    Erm.. that`s Morecambe police station that is! Closes at 9pm

  20. That's ridiculous, stealing lead can't be in any way profitable.

    Lead is going for about $7 a ton.

    Now manhole covers are quite a bit better- Iron is going for $120-
    $200 a ton.

    Still all it takes is a phone call from the police to scrap dealers to
    make them worhless.
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