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What is this clicking in the color tv?

Discussion in 'Electronic Repair' started by mm, Feb 17, 2007.

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  1. mm

    mm Guest

    Hi.

    I fix things as a hobby and not just when it makes financial sense.
    I'm not very good at tv's but I still like to try.

    I have a 15 inch Emerson color TV in beautiful condition made in 1985
    that has no picture, no raster, and no sound.

    When I first turn it on it clicks quietly about 4 times a second for a
    second or two, then about 2 times a second as long as it is on. When
    it is unplugged it stops clicking quickly, but not immediately. Maybe
    after 1 or 2 seconds.

    It seems like this might be related to the other tv problems.

    There are afaict no relays in this tv, and the on-off switch is a
    full-current switch mounted to the back of the volume control, not a
    push-button that activates a relay.

    I've tried touching each part with a thick chopstick to feel for the
    clicking or to hear it at the end of the stick, but haven't found it.

    Not positive even the region where the noise is coming from, but seems
    a little louder on the right side near the flyback and the low-voltage
    power supply parts.

    Thanks.

    If you are inclined to email me
    for some reason, remove NOPSAM :)
     
  2. James Sweet

    James Sweet Guest



    Sounds like you have a shorted semiconductor somewhere causing the power
    supply to shut down. Check the horizontal output transistor first.
    Diodes in the power supply are another suspect.
     
  3. mm

    mm Guest

    Wow. I didn't know they could click!
    That has obviously been replaced once already, so I thought the odds
    were higher that it was good. Maybe I should have thought the odds
    were lower that it was good??

    So I tested it, a D1398, and all three points on the output transistor
    had the same voltage, 128.

    With an ohmmeter, it was almost zero between the base and emitter, so
    I removed the transistor. With it out, it was 46 ohms in both
    driections between the base. It turns out there is a transformer
    across the base and emitter, so that's why it was 0.3 ohm, but 46 ohms
    is too low also, right. I need a new one.

    It is 220 ohms in both directions between the base and collector,
    and 175 ohms in both directions between the emitter and collector.

    I plugged it in again with the output transistor out, and it still
    clicks!

    The powersupply diodes are all in one box, and the chopstick on the
    box doesn't make the clicking any louder.

    I used a digital multimeter to measure the B+. Is it worth digging out
    an analog meter...would I see pulsing in the voltage because of
    clicking in the diodes?
    BTW this thing even has MTS sound, but only one RCA output jack for
    it. I probably don't have a device to separate the left from right,
    do I? Or to extract SAP, which I never listen to on the tv's I have
    that have it.

    If you are inclined to email me
    for some reason, remove NOPSAM :)
     
  4. Jamie

    Jamie Guest

    That does not always mean it's shorted, i assume you're talking about
    the HOT (Horizontal output Transistor), many of those have an internal
    resistor from the base to emitter.
    THat sounds like it's bad. bare in mind that many of those transistors
    also have the Damper Diode in the case between the collector and
    emitter. so you should get the common diode reading on one direction.
    Most likely because the flyback isn't starting up in the required time
    and the power supply is reseting for another attempt.
     
  5. mm

    mm Guest

    Is there any chance that replacing the bad horizontal output
    transistor will get me the voltages I need, and cure the clicking,
    even though it clicks with the H.O.T removed?

    I don't like just replacing parts without being sure I have found the
    root problem, but I'm lucky with your help to have found this, let
    alone other problems. I don't know how to trouble shootwhen the set
    is not running at all, or without replacing bad parts when I have
    found them.

    If so, it's going to be days or weeks before I have time to get the
    part and put it in.

    I realize I'm just not very good at this, for this sort of reason.
    It's confusing. I did learn that at least in tube tvs the horizontal
    output circuit provided the high voltage that was used all over the
    tv. I never learned if transistor sets are the same.


    Even though I found a few google hits for a D1398, including a spec
    sheet and a company that sold them, I also found an article in google
    groups from this very newsgroup back in 1996 where JohnSon replied to
    Michael Portmann, and Michaal asked about a 2SD1398, which was also a
    horiz. output transistor, and JohnSon referred to it two ways, as a
    2SD1398 and as aD1398. So I guess they are the same? He also
    suggested as a substitute an ECG2302. That should be easy to get.
    Well, Mouser has an NTE2302. The local electonics store in Baltimore,
    Baynesville Electronics, is usually pretty expensive, and there is no
    rush.



    If you are inclined to email me
    for some reason, remove NOPSAM :)
     
  6. mm

    mm Guest

    On Sat, 17 Feb 2007 20:37:30 -0500, Jamie
    Yes, the HOT. And yes, I found the specs for an NTE2302, which Mouser
    also says is a substitute for the D1398, and it does have a damper
    diode in it. (I never heard of that before, but makes sense.)
    So this might get it working again. I'll try it. It's been replaced
    once already, but maybe it lasted for years after that. Since I
    didn't find out outside an electronics shop, I guess it worked long
    enough to return it to the owners... Unless some guy like me replaced
    it, it burned out immediately, he threw it away and I found it. ;)

    It's a little strange. He cut the leads to the transistor and
    soldered the new one to the stubs. I tried that but for me, it was
    much easier just to unsolder at the pcb. Is his way a common
    techniqe?

    If you are inclined to email me
    for some reason, remove NOPSAM :)
     
  7. James Sweet

    James Sweet Guest



    If it clicks with the HOT removed, you probably have another fault. Can
    you isolate outputs of the power supply to see which bus has a short?
    Sometimes you can lift one side of a component or wire from the power
    supply section to the rest of the set.
     
  8. James Sweet

    James Sweet Guest


    It's not uncommon, particularly in sets where it's difficult to get to
    the bottom of the PCB without disassembling a lot of stuff. I don't like
    to do it that way though, just doesn't seem tidy.
     
  9. Jamie

    Jamie Guest

    if the HOT that bad off and it's been replaced before, i think you
    should also look at replacing the Flyback, it most likely was the cause
    of the HOT short out.
    I use to always replace the Flyback if the HOT was damaged that way
    other wise, it normally came back later with the same problem again.
    finding a HOT opened or just maybe the Base Emitter shorted, i would
    then just replace the HOT>
     
  10. mm

    mm Guest

    Maybe. I'll try.
    Aha. OK.

    Thanks to you and eveyrone for your help.

    If you are inclined to email me
    for some reason, remove NOPSAM :)
     
  11. mm

    mm Guest

    Hmmm. OK.
    If non-high-def tvs weren't going to be almost obsolete in 2009, I
    might replace the flyback, but I guess it would be too much money now.
    At least I won't get too confident that the HOT only will work. I
    wish they weren't going to make all these tvs depend on set top boxes
    and satellites. It's such a pretty tv, although it has no remote
    capability.

    Thanks.

    If you are inclined to email me
    for some reason, remove NOPSAM :)
     
  12. James Sweet

    James Sweet Guest


    I've actually found flyback failures to be quite rare, aside from
    certain specific sets that had a tendency to fail in that way. HOT
    failures I find are often caused by cold solder joints or bad
    electrolytic capacitors.
     
  13. b

    b Guest

    mm ha escrito:
    first things first. remove the suspect transistor and solder a 100 w
    bulb acros c-e terminals on the board. switch on and measure the b+
    across them. if the bulb doesnt light chances are you have a short
    elsewhere, the b+ cap is bad, or the power supply is faulty. post
    results and we can take this a step further.

    B-.
     
  14. mm

    mm Guest

    So I should look at the nearby cold solder joints and bad
    electrolytics. I'll do it when this other one is done, maybe
    tomorrow.


    If you are inclined to email me
    for some reason, remove NOPSAM :)
     
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