Question about Samsung DynaFlat CXM2785TP 27 in. TV/VCR/DVD Combo

3 Answers

Samsung CXM2785TP When I got the tv from a friend it had no power.I found the horizontal output transistor shorted and replaced it.Now the tv fires up but the screen is very bright with white horizontal lines.I cannot run it for too long as the horizontal ouyput transistor gets very hot.There is no video,audio,or on screen display. As it warms up the high voltage drops to 25kv.The B+ voltage should be 200v but is 230v.

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  • 2 more comments 
  • Rick53 Feb 02, 2008

    I would like another opinion.

  • Rick53 Feb 02, 2008

    I don't know what you mean by floating the B+ output of the FBT. Could you explain further? thanks Rick53

  • Rick53 Feb 12, 2008

    I have checked the Flyback Transformer and Yoke with a Sencore Ring Tester and they checked OK. Any ideas where to look now?

  • Rick53 Feb 24, 2008

    Well guys, I finally found the problem and thought you would like to know what it was.I checked the flyback and yoke with a ring tester and it chacked OK. I was reinstalling the flyback by soldering some connections and not others but didn't help. You mentioned the CRT board with possible shorted outputs so they were checked and were OK. When I removed the CRT board from the Picture Tube the heavy current load went away.I reinstalled the board and the tube lit up with On Screen Display came on.I checked the HOT for overheating and it was normal.I removed and reinstalled the board again and it acted up again. It seemed like an intermittent connection so I resoldered all the CRT PCB connections as I could not find any One connection that was the problem.I have had another Samsung TV before that was acting erratically and it was intermittent, so I soldered the entire PCB and has worked perfectly for over a year now.Poor solder connections may be a problem with Samsung TV's.Thanks for the help guys as it is now working perfectly.I have also resoldered the main PCB to be on the sefe side.




3 Answers

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With the indulgence again of robotek,

1. 230V instead of the B+200V - normally I use A variac to supply a unit in question. This way I have control of the input voltage which reduces risk of burning a component such as the Hout.

2. Overheating Hout - is indicative of a short in the FBT area or its output including the vertical (as robotek have posted). You can try having the FBT tested and/or run the unit with the low B+ output of the FBT floating and determine which one causes the Hout to overheat.

3. In some Samsung units that I have worked on, the Yoke tends to be the culprit when the Hout burns out or overheats.

Hope this be of additional help/idea. Pls post back how things turned out or should you need additional information.

Good luck and kind regards.

Posted on Feb 02, 2008

  • 2 more comments 
  • Louie  Role
    Louie Role Feb 02, 2008

    Hi again,

    Floating the B+ output of the FBT - refers to desoldering some of the
    FBT pin/terminal connection points going to the diodes. Pls refer to
    the images below:

    normal solder floated/desoldered pins

    Going back to your initial post:

    1. screen is very bright with white horizontal lines - indicates
    excessive screen voltage normally adjustable using the screen control
    located at the FBT or if something in the HV is way off;

    2. Hout transistor gets very hot - as posted initially, indicates a
    short or partial short. If the FBT doesn't heat as well, then it can be
    a HV cap or the yoke. If the FBT heats up likewise, then it can be the FBT
    itself or any of its load;

    3. There is no video, audio or OSD - more often B+ supplying these secondary circuits are from the FBT. These normally 12~18, 36~42, 80~100;

    4. As it warms up the high voltage drops to 25kv - normal tendency if something is still wrong with the HV section;

    5. The B+ voltage should be 200v but is 230v - this appears to be another issue on the same unit. As robotek have
    posted seems to be a power supply problem. Not unless you are measuring
    the B+ at the collector of the Hout which tends to give you erroneous
    DC readings due to the HV & Hfreq component. You might get more
    realistic B+ readings at the FBT pin that leads back to the PSU.

    Things that you can try:

    a. Float as above described;

    b. Remove the 4-wire yoke connection. Even with no display, you can still assess if the Hout will abnormally heat up;

    c. desolder/remove the FBT and have it tested;

    Pls post back any development(s) or lack thereof.


  • Louie  Role
    Louie Role Feb 04, 2008

    Hi robotek,

    Appreciate the pat on the back and likewise the invite to be your tech buddy.

    Regards all.

  • Louie  Role
    Louie Role Feb 12, 2008

    Hi again Rick,

    Copy your last post. To summarize so far:
    a. The HOut, FBT and Yoke has been checked and determined OK;
    b. the unit powers up but HOut heats up;
    c. you have floated the FBT's secondary outputs but still the HOut heats up;
    d. you have determined/verified that there are no shorts in the FBT's secondary circuitry which includes the Vertical (36~42VDC), Color Drive (80~100VDC) and audio (12~18VDC).

    What can still be checked:
    1. IF there is a tripler, that could be a possibility;
    2. the three (3) color drive transistors at the small PCB at the rear of the CRT's neck. If there is a problem with one of them, the screen tends to be very bright with fine slightly diagonal retrace lines;
    3. the power supply section, if the should be 200B+ is still reading 230.

    Since the Samsung DynaFlat CXM2785TP is not readily available in my region, I have no service manual/schematic hence have to resort to winging it based on other comparable models.

  • Louie  Role
    Louie Role Feb 25, 2008

    Hi again Rick53,

    Glad that your TV now works and to have been of some assistance. Thank you too for sharing the information/update.

    Would also appreciate if you can rate the posted solutions as you see it fit pls?

    Thanks in advance.


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I think the controller IC chip has gone kaput. BUT, it could also be just a software glitch. So, try a hard reset and see if it solves the problem. If it does not you will need service. If still under warranty the factory service center should take care of it.

Do a hard reset by unplugging the power cord, as you have been doing, but a complete reset requires the power to be unplugged for at least 4 hours. Then when you turn it on all the internal soft codes will boot up to factory default settings.

Also, just in case there are loose connections or cold solder joints at the root of the problem, with your hand or fist go around and smack on the cabinet front, back, sides, etc. to see if that will wake up any dead connections.

if the above above method doesn't solve the problem then it is a bad power supply. a repair cost will range around $120-200. samsung tv's are not what everyone thinks they are. they do not last long.I would suggest you to call a repairman..

Don't forget to rate me as fixya..

Posted on Feb 02, 2008

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It will have a short in the vertical stage or power supply to it, and maybe even a power supply regulation failure(high voltage) that set the whole thing in motion. I will do some research for you and get back real soon :)

Posted on Feb 01, 2008

  • Graeme Ross
    Graeme Ross Feb 01, 2008

    the power supply needs to be rebuilt. electrolytic caps and so on, the high b+ voltage will be affecting all stages of the set and you will need to attend to the that first. The set may not even be worth repair if it has multiple failures.. sorry to have say that to you mate. Good luck and BE CAREFUL with SMPS stages of the set. If you want more particulars of the repair... i will pursue for you. Consider a less blown up one to restore i suggest as the best outcome.

  • Graeme Ross
    Graeme Ross Feb 03, 2008

    heheh well done Benimur... I actually read it as the set had a vertcal collapse not retrace lines for some reason... Nice breakdown of the problem in your speil... you da man :)


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this will require you opening the back cover of the television with the use of an ohmmeter to read components.

A blown fuse is a very common type of fault due to poor design very often triggered by power surges due to outages or lightning storms. However, the most likely parts to short are easily tested, usually in-circuit, with an ohmmeter and then easily removed to confirm.

Note that it *may be* useful to replace a fuse the *first* time it blows (though it would be better to do some basic checks for shorted components first as there is a small chance that having a fuse blow the second time could result in additional damage which would further complicate the troubleshooting process). However, if the new one blows, there is a real problem and the only use in feeding the TV fuses will be to keep the fuse manufacturer in business!

Sometimes, a fuse will just die of old age or be zapped by a power surge that caused no damage to the rest of the TV. However, it must be an EXACT replacement (including slo-blow if that is what was there originally). Else, there could be safety issues (e.g., fire hazard or equipment damage from too large a current rating) or you could be chasing a non-existent problem (e.g., if the new fuse is not slo-blow and is blown by the degauss circuit inrush current but nothing is actually wrong).

If the fuse really blows absolutely instantly with no indication that the circuits are functioning (no high pitched horizontal deflection whine (if your dog hides under the couch whenever the TV is turned on, deflection is probably working).) then this points to a short somewhere quite near the AC power input. The most common places would be:

Degauss Posistor - very likely.
Horizontal output transistor.
Power supply regulator if there is one.
Power supply chopper (switchmode) transistor if there is one.
Diode(s) in main bridge
Main filter capacitor(s).

You should be able to eliminate these one by one.

Unplug the degauss coil as this will show up as a low resistance.

First, measure across the input to the main power rectifiers - it should not be that low. A reading of only a few ohms may mean a shorted rectifier or two or a shorted Posistor.

Test the rectifiers individually or remove and retest the resistance.

Some sets use a Posistor for degauss control. This is a little cubical (about 1/2" x 3/4" x 1") component with 3 legs. It includes a line operated heater disk (which often shorts out) and a PTC thermister to control current to the degauss coil. Remove the posistor and try power. If the monitor now works, obtain a replacement but in the meantime you just won't have the automatic degauss.

If these test good, use an ohmmeter with the set unplugged to measure the horizontal output transistor. Even better to remove it and measure it.

C-E should be high in at least one direction.
B-E may be high or around 50 ohms but should not be near 0.

If any readings are under 5 ohms, the transistor is bad. The parts sources listed at the end of this document will have suitable replacements.

If the HOT tests bad, try powering the set first with your light bulb and if it just flashes once when the capacitor is charging, then put a fuse in and try it. The fuse should not blow with the transistor removed.

Of course, not much else will work either.

If it tests good, power the set without the transistor and see what happens. If the fuse does not blow, then with the good transistor (assuming it is not failing under load), it would mean that there is some problem with the driving circuits possibly or with the feedback from the voltages derived from the horizontal not regulating properly.

Look inside the TV and see if you can locate any other large power transistors in metal (TO3) cans or plastic (TOP3) cases. There may be a separate transistor that does the low voltage regulation or a separate regulator IC. Some TVs have a switchmode power supply that runs off a different transistor than the HOT. There is a chance that one of these may be bad. If it is a simple transistor, the same ohmmeter check should be performed.

If none of this proves fruitful, it may be time to try to locate a schematic.

A blown fuse is a very common type of fault due to poor design very often triggered by power surges due to outages or lightning storms. However, the most likely parts to short are easily tested, usually in-circuit, with an ohmmeter and then easily removed to confirm.
Good luck...

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2 Answers

I replace horizontal output transistor with new one but it happens the same short

check yoke coil for open ckt, and and check for 12 v dc supply to vertical circuit, sometimes resistor open that supply dc voltage coming from flyback transformer.

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