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CRT TV doesn't turn on

Discussion in 'Troubleshooting and Repair' started by wiru, Sep 25, 2014.

  1. wiru

    wiru

    16
    4
    Nov 27, 2010
    Hello,

    I have a CRT TV of a local brand name (TAURAS) that I got for repair. I tried to troubleshoot alone, but my experience is very limited, so I couldn't find the cause of the problem. This is why I'm asking for your help.

    The problem: the owner said that lately the TV would not always turn on, now it doesn't turn on at all. The standby LED is ON all the time.

    Board model: 40-MNTV2KR-L2T. Datasheet: http://www.eserviceinfo.com/downloadsm/57250/_.html

    I suspected the power supply, because I saw a big dark spot on the board under the capacitor C325 ([email protected], appears almost black). I tested it with a multimeter out of the board and it seems to be OK. Is it still possible for it to be bad?
    DSCF3826.JPG

    I checked the voltages coming out of the PSU after rectifier diodes and they all are present, so this leads me to believe that the PSU may actually be fine. Any other checks I can do to it?
    DSCF3821.JPG

    The small board on the neck of the CRT seems to have suffered. Seems like a few transistors have been running very very hot. I found dry joints and have re-soldered them. The transistors themselves seem to be OK tested with my multimeter.
    I checked the voltage between C and E of HOT, it is 125V DC. So I guess it is not shorted.
    DSCF3824.JPG

    The front panel buttons had broken legs which I soldered back, but I don't think it has had anything to do with the TV set not turning on.
    DSCF3825.JPG

    While the TV was OFF and I was checking the board while the HV wire was still connected to the CRT, I heard a spark. I checked what was going on and discovered that the HV wire has a small hole and must have discharged to the coils on the CRT (see the pictures). Could that be an issue?
    DSCF3828u.JPG

    I attached photos of the affected parts. What could I check next or should I replace any parts?
     
    Last edited: Sep 25, 2014
    Jack Aroy likes this.
  2. shrtrnd

    shrtrnd

    3,676
    454
    Jan 15, 2010
    Lots of questions, will take a shot at some of them.
    The broken front panel legs most likely WERE the reason the set wasn't turning on. If the panel isn't secure, when you depress the on-off button, the whole switch can move backwards instead of just depressing the switch for activation.
    That heavy red wire is called the 'ANODE'. It's connected from the flyback, to the anode plug on the TV tube. I carries 23,000 to 30,000 volts when the set is turned on (your spark through the damaged insulation). And the
    charge held on the CRT after the set is turned-off and unplugged is STILL several thousand volts. You'll need to replace that anode plug wire, but you have to be careful when you do that. You'll have to discharge the voltage charge on the CRT to ground (and if you wait any length of time, you'll have to discharge it again, because it builds-up a charge again, even without plugging it in again). I usually carefully slip a long screwdriver under the anode clip at the
    CRT, with a lead from he screwdriver shaft to earth ground. You WILL get zapped, if you touch the wrong thing here, and NEVER mess with this while the set is plugged into a wall socket.
    If you've got a black spot on the board under a capacitor, you need to replace that cap.
    The transistors on the CRT socket board might work, but as long as I was in there, I'd replace them because of the obvious over-heating. They're liable to go at any time.
    If you've got your power supply voltages, I don't know what else you could ask for.
    Any other questions, post 'em here, and somebody will respond.
    Good luck, and be very careful around that anode-plug wire.
     
  3. wiru

    wiru

    16
    4
    Nov 27, 2010
    Thank you for the answers.

    I did not clarify which switches were broken. The "Menu" and Channel +/- buttons were broken. I soldered them back in place and verified their function using a multimeter - now they do work. The TV set still doesn't turn on.

    I have read that the anode wire is deadly dangerous. I will be cautious when I remove it. I have looked on youtube how it's done with a screwdriver and alligator clips attached to it and grounded to exposed metal pieces on the CRT. I just did not expect it to make a hole in the insulation when it was close to these tube windings.

    I will probably temporarily put a lot of electrical tape on that small hole and be very careful about it until I figure out what is actually wrong with this TV set. I don't think it's worth investing too much into it if I can't get it working in the first place.

    I thought about replacing these transistors on the CRT board as well. However again, it's probably not worth it if I can't revive the TV at first. Since they test fine with a diode test range on the multimeter I guess it's OK to leave them alone for now.

    I will absolutely replace that burned cap tomorrow. It tests fine but I'm highly suspicious of it. However, it could be that the cap is really fine and it's the nearby resistors that have overheated. The resistance of these resistors seems to be within spec though.

    I'll see tomorrow if the replacement of this cap helps anything. Otherwise I may be be clueless again. :eek:
     
  4. shrtrnd

    shrtrnd

    3,676
    454
    Jan 15, 2010
    Did you check the physical and electrical operation of that on-off switch?
    (An ohmeter with the set unplugged, across the on-off switch to see if you have continuity).
    Did you look for any blown fuses. Most TV's have one fuse, but some have two fuses, one for input power and one for B+ power.
    Did you carefully look at the components on the boards to see if anything burned 'open circuit' physically? You seem to have a lot of heat related issues.
    That's the simple stuff to check first without spending money on parts.
     
  5. wiru

    wiru

    16
    4
    Nov 27, 2010
    No, I did not. It's a hard on switch connected directly to the mains cable. If it was broken, the set wouldn't get into standby mode and there would be no secondary voltages present.

    I was looking for any more fuses, but there is just the one connected to mains and it's intact.

    I checked every browned area I could find and components there - everything seems to be fine. I replaced that dark cap in the PSU - didn't help at all. I thought heat+electrolytic caps don't go well together, so I replaced most of the caps in the power areas (except the largest 400v 100uF one) - nothing.

    There is a "service in" socket with two pins. In the service manual they write that by shorting them I'd get the TV to display the "service mode" menu. When I short these pins, the TV set tries to start up (makes tick-tick-tick noise), the standby led dims during these "ticks", but nothing else happens.
     
  6. shrtrnd

    shrtrnd

    3,676
    454
    Jan 15, 2010
    So, can you tell where the 'tick' sound is coming from? A relay that might be bad?
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    This "ticking" could be "hiccuping". Hiccuping indicates that the power supply is starting up, then detecting a problem and shutting down. Then the soft start kicks in again and it starts up, and shuts down. And the cycle repeats.

    The cause can be any problem that triggers the power supply's safety shutdown, which generally operates in response to overcurrent and overvoltage. So a shorted rectifier diode, a missing load, and even a shorted turn in the transformer can cause it.

    It's not normally a good idea to put your multimeter anywhere near the collector of the horizontal output transistor. When it's operating, there are very steep high-voltage spikes there, that will make the multimeter wish it had never been born!

    The damage on the CRT board is not likely to be the cause of a dead set. There seem to be some errors in the schematic, so I can't tell you the exact component to lift, but if you can unplug the cables from that board, you can probably eliminate that possibility. You may need to replace them once you get the rest of the set working though.

    The discharge from the ultor (CRT final anode) lead could be a problem. You may be able to surround that part of the cable with silicone sealant. Electrician's tape is not adequate in the long term. Clean the whole cable, and dust the CRT and everything on it using brush and vacuum cleaner, first. Discharge the ultor connection before you start, as suggested by someone else.

    You may be able to find a plastic ring-shaped thing that you can clip around the wire to keep it away from everything that it might try to discharge to. The fact that a discharge has punched through the insulation suggests a poor quality cable.

    That 27 nF cap is for high-voltage suppression for the main switching MOSFET in the power supply (VT301). I haven't seen a "black spot" under this type of capacitor before and I don't know whether it means you need to replace it.

    Do you have access to an oscilloscope?
     
    wiru likes this.
  8. wiru

    wiru

    16
    4
    Nov 27, 2010
    Thank you for replies!

    It does indeed sound like the power supply cycling on/off. The ticking sound is quite faint and high-pitched. It happens a few times per second.

    OK, I will not measure the voltage of HOT while the set is running. Thanks for the warning.

    I did unplug the CRT neck board from the main board, but the problem persisted.

    Thanks for the tip with silicone sealant, I may do it if I can get the TV set working again. For now I put many turns of electrical tape and the round plastic piece next to that hole. That should do it temporarily.

    I was wondering what that darkened cap was doing. Do you mean it's there for high-frequency noise suppression? It could be that either that cap or R307 and R310 around it were running very hot, because that cap C325 is right in between these resistors. The whole area around them seems very darkened.

    Here's a closer view of the affected area. BTW, I have replaced the dark cap with 33nF WIMA cap. That's what I could order from the electronics shop. Is the value of it critical?
    DSCF3841.JPG

    While I was endlessly trying to power up this TV, there were a few times when it seems to have started up for a second or so. Afterwards the same rapid startup cycle continued. However, I have noticed a burning smell after that one second power-up. It seems to be from the area around IC401 (STV9302A - a vertical deflection booster). Is there any way to test this IC?

    Unfortunatelly, I no longer have an oscilloscope. I may try get an old one from eBay, for example, if it is necessary. My Fluke 17B multimeter has a frequency counter / duty cycle function. I've tried it on the base of HOT while the TV was in continual starting up cycle, it showed ~6kHZ IIRC. This may be an incorrect reading though, because the HOT isn't operating continually for my multimeter to register a steady signal.
     
    KrisBlueNZ likes this.
  9. wiru

    wiru

    16
    4
    Nov 27, 2010
    I have been investigating the TV circuit further and I think I have found the most likely cause.

    First of all, while investigating the purpose of that darkened C325 cap I have found out that VD308, C325, R307 and R310 form a RCD voltage clamp snubber. I found this excellent article about snubbers where I found the exact configuration of mentioned components defined as a RCD snubber together with a description of it. Here's the link to anyone else who may be interested: http://www.maximintegrated.com/en/app-notes/index.mvp/id/848

    Secondly, I removed the suspected IC401 (STV9302A vertical deflection IC) from the board and checked all of the leads for any shorts. I've found that pins 4 and 5 (ground and output respectively) are shorted. So when I tried to switch on the TV, I got lots of current flowing from the output directly to ground. This would trip the overcurrent protection and the PSU would shut down only to try again and again indefinitely.

    Then I powered the TV with the IC401 removed. Since it drives the vertical deflection yoke, I expected to see a bright horizontal line if the TV turns on. I did the experiment and the TV turned on with a bright horizontal line in the middle of the CRT screen. I quickly turned it off so as not to burn the phosphorus coating of the CRT.

    On Monday I will buy a new IC and see if the TV comes back to life. If it does, I will replace the CRT neck board transistors, then buy some silicone sealant and seal the ultor lead where it has been damaged. If there is anything else I could check or replace while I'm working on this TV set, please let me know.

    Thank you for all of the answers so far. They have helped me troubleshoot and I have learned a lot in the process.
     
    KrisBlueNZ likes this.
  10. Neal

    Neal

    23
    12
    Dec 23, 2009
    You may want to check out the yoke winding where the anode lead burned. That is the vertical yoke winding and no doubt took out the vert. IC when it sparked. There may be some shorted turns in the yoke vertical winding from the arc burning through the formvar insulation.
     
    KrisBlueNZ likes this.
  11. wiru

    wiru

    16
    4
    Nov 27, 2010
    Good idea! I haven't connected the dots before you had mentioned it. That may have been the original cause of the failure of that vertical deflection booster chip. The set may have been moved, the anode lead got too close to the vertical yoke windings and ZAP, the chip is gone.

    Since there is a set of two vertical deflection coils and only the top one got zapped, I have done a simple resistance check. I have desoldered one end of each coil and checked their individual resistance. If the top one had shorted coils, it would probably have lower resistance than the bottom one.

    Luckily, both vertical deflection coils have the same resistance. The top one measures 17,2 ohms and the bottom one measures 17,2 ohms as well. So maybe the discharge has affected only one winding or a few windings next to each other, so the resistance of the whole coil hasn't changed.

    I will give it a try the way it is then. Maybe I'll put some silicone sealant onto the affected yoke coils as well.
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    Nice work Wiru!
    Right!
    That will be because of the resistor(s) getting hot. Check under the board for dry joints. But you could check the diodes, especially VD308, for leakage. Desolder and lift one end, and measure the diode on the highest resistance range with the red lead to the cathode. It should indicate open circuit. This is not a conclusive test but it's worth trying.

    Eventually if the board keeps getting that hot, it will start to conduct and fail. It may already be starting to conduct, and this makes the board itself get hot. So its days may be numbered. Make sure your fire insurance is up-to-date ;-)
    There was no need to replace the capacitor, but you can leave the new one in there. No, the value isn't critical.
    Right.
    Good!
    Good! But I wouldn't bother replacing the neck board transistors if the picture is OK.
    Yes, Neal's advice is very good. If the arc-over did damage the insulation on the yoke winding, you might be able to clean it up and move the wires apart slightly, to stop the exposed parts from touching. Then after checking, put some glue to hold them in place. You won't get any arcing there; the voltages are low.

    Some sets have a plastic thing clipped around the ultor lead to prevent it from getting too close to anything. There's one on this page: http://www.indiamart.com/tantia-electronic/other-electronic-parts.html (it's called an "EHT Wire Clamp (Insulator Ring) Spacer"). As well as the silicone, you could put several of those on the cable. If you can get them in small quantities.

    You might also want to check VD401 and VD502 for leakage or shorts, in case they were damaged by the arc-over. Though I don't think it's likely.

    Neal, I hope we see more of you here :)
     
    afrida76 likes this.
  13. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    Me neither.
    Not necessarily. A short between adjacent turns won't affect the DC resistance measurably. But it will affect the picture noticeably, and will stress the output IC.
    I would use strong, hard glue. Like that epoxy stuff that comes in two tubes with a resin and a hardener. But clean and check the yoke windings first!
     
    afrida76 likes this.
  14. wiru

    wiru

    16
    4
    Nov 27, 2010
    Okay, here's the progress update.

    I have changed the vertical deflection booster IC STV9302A to IC STV9302B. I've ordered STV9302A but they gave me STV9302B. I checked the datasheets and there seems to be almost no difference. The STV9302B datasheet lacks a few lines where standby mode is mentioned, that's all.

    I powered up the TV. It works! However, there is one small issue. I get thin RGB lines on top of the usual picture. Here's a photo of these lines:
    DSCF3845.JPG

    I'm moving out of my place today, so I called the owner and showed these lines. He said it's OK to leave it like that. So I gave the TV back already. However, I'm still interested in what (theoretically) could be the problem.
     
    afrida76 likes this.
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    Thin faint not-quite-horizontal lines at the top of the picture are called retrace lines. They indicate that the electron guns are being enabled before they have returned past the top of the screen ready for the start of a new field. It's normally caused by a problem in the frame output stage, such as low supply voltage or faulty electrolytics.
     
    afrida76 likes this.
  16. 73cat

    73cat

    7
    3
    Dec 30, 2014
    If you need to enter to factory mode and make any change to your tv factory data, Try browsing the tv service menu codes at All TV Service Menu Code
    Here is the sitemap link http://tvservicemenu.blogspot.com/p/content.html
    I hope the source is usefull for you..

    TV Service menu-mode Brand list

    TV Service Menu-mode Collection
     
    Last edited: Dec 30, 2014
    Jack Aroy, afrida76 and Allen Bong like this.
  17. afrida76

    afrida76

    1
    0
    Jan 4, 2015
    :) Nice
    Board model: 40-MNTV2KR-L2T can be download at elektrotanya.com
     
  18. Jack Aroy

    Jack Aroy

    1
    1
    Jan 4, 2015
    The problem is looklike hard to repair. I hope someday I will come a good technicians.
     
    Rick79 likes this.
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