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Help needed for Troubleshooting Sony KV-G21B1

Pavi98

New Member
Hello all ☺..,
I got an Sony Crt Tv (model number - KV-G21B1) and i found its damper diode is short. Then, I replaced it with a new diode (with same number as the old one) and after trying to turn on the Tv, the indicator led turns On and Off continuously with all the voltages of the board turning On and Off repeatedly. Then I found that the new damper diode I replaced was also short. I tried with few diodes but all of them became defective. I couldn't find the reason behind damaging the damper diode repeatedly and the following are the things I did with the TV board.
  • Resoldered all the dry joints including the Neck Card.
  • I found the 33v zenar diode is short at tuner and replaced it.
  • All the suspicious mylar capacitors are replaced around the H-OUT section.
  • Replaced the flyback transformer but nothing changed.
  • I checked the HOUT and VOUT transistors and they are fine.
Outputs of the chopper transformer are 115V and 15V. The flyback gets 115V properly.
Please support me to solve this problem and I am clueless in this case to repair this Tv.
Thank You very much for your time.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Hello all ☺..,
I got an Sony Crt Tv (model number - KV-G21B1) and i found its damper diode is short. Then, I replaced it with a new diode (with same number as the old one) and after trying to turn on the Tv, the indicator led turns On and Off continuously with all the voltages of the board turning On and Off repeatedly. Then I found that the new damper diode I replaced was also short. I tried with few diodes but all of them became defective. I couldn't find the reason behind damaging the damper diode repeatedly and the following are the things I did with the TV board.
I used to be chief engineer at a Sony service dealer, I've repaired thousands of Sony CRT sets.

I've no idea what you mean by a 'damper diode'?.

  • Resoldered all the dry joints including the Neck Card.
  • I found the 33v zenar diode is short at tuner and replaced it.
That's EXTREMELY bizarre - they don't usually go S/C, and it wouldn't make the set trip anyway - I'm bemused you even tested it.

  • All the suspicious mylar capacitors are replaced around the H-OUT section.
Never an issue in Sony sets, as they use higher quality components than the cheaper brands - I've never known one fail in a Sony.

  • Replaced the flyback transformer but nothing changed.
LOPTX's DO fail in Sony sets - they also commonly arc internally and take out the LOPT transistor with them - if you change the transistor it blows instantly again. Depending on the chassis, and particularly if it's 16:9 or 4:3 (even if the board is the same) it's very common for them to take out the main micro-controller, which is a large SM device.

  • I checked the HOUT and VOUT transistors and they are fine.
Outputs of the chopper transformer are 115V and 15V. The flyback gets 115V properly.
Please support me to solve this problem and I am clueless in this case to repair this Tv.
Thank You very much for your time.
For a start remove the LOPT transistor (so the line can't run), and fit a 60W bulb across the 115V - this should light the bulb dimly, and the PSU should run with no issues.

I presume you have the schematic?, with the LOPT transistor in place check the various trip circuits to see which one is making it trip.
 

unclejed613

Well-Known Member
Most Helpful Member
did this set use a tripler block? iirc they were more prone to failure than the flyback
 

Pavi98

New Member
I used to be chief engineer at a Sony service dealer, I've repaired thousands of Sony CRT sets.

I've no idea what you mean by a 'damper diode'?.



That's EXTREMELY bizarre - they don't usually go S/C, and it wouldn't make the set trip anyway - I'm bemused you even tested it.



Never an issue in Sony sets, as they use higher quality components than the cheaper brands - I've never known one fail in a Sony.



LOPTX's DO fail in Sony sets - they also commonly arc internally and take out the LOPT transistor with them - if you change the transistor it blows instantly again. Depending on the chassis, and particularly if it's 16:9 or 4:3 (even if the board is the same) it's very common for them to take out the main micro-controller, which is a large SM device.



For a start remove the LOPT transistor (so the line can't run), and fit a 60W bulb across the 115V - this should light the bulb dimly, and the PSU should run with no issues.

I presume you have the schematic?, with the LOPT transistor in place check the various trip circuits to see which one is making it trip.
Hello Nigel ,
Thank you very much for the advice. Actually I am a newbie in electronic repairing. What I meant as a "damper diode"(just found that term on Internet) is a diode which is connected to the Collector of HOUT and other side connects with the ground.
IMG_20210223_121438.jpg
IMG_20210223_121544.jpg
(The diode and the flyback transformer are removed)
This diode (part number C06-15) was found as short and everytime I replaced a new diode (with same part number) all of them became defective (short) and the indicator led keeps turning on and off continuously with all the voltages of the board changing. (HOUT becomes hot during that but It was not defective).

I removed the LOPT transistor and added a 60W bulb across the 115V line as you said and the bulb lighted dimly.

Actually I don't have a schematic right now but I will find it and could you please explain me about using trip circuits to Troubleshoot this tv set?
 

Pavi98

New Member
did this set use a tripler block? iirc they were more prone to failure than the flyback
Hello unclejed613,
Thank you very much.
"did this set use a tripler block?"
Actually I'm not sure about that. The marked diode was found short and everytime I replaced a new diode (with same part number) they became defective (short). I am clueless about the reason behind damaging the diode. All the components around it are fine. IMG_20210223_121438.jpg
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Hello Nigel ,
Thank you very much for the advice. Actually I am a newbie in electronic repairing. What I meant as a "damper diode"(just found that term on Internet) is a diode which is connected to the Collector of HOUT and other side connects with the ground.
Those were called 'efficiency diodes' in TV terms, but that's not what it is anyway - it's the top diode of the pair used in the pincushion modulator circuit, the other is D802. The actual modulator transistor is Q821 - I'd check components round there. If Q821 or D802 are S/C it might kill D801.

Actually I don't have a schematic right now but I will find it and could you please explain me about using trip circuits to Troubleshoot this tv set?
There's a schematic for the chassis here:

 

Pavi98

New Member
Those were called 'efficiency diodes' in TV terms, but that's not what it is anyway - it's the top diode of the pair used in the pincushion modulator circuit, the other is D802. The actual modulator transistor is Q821 - I'd check components round there. If Q821 or D802 are S/C it might kill D801.



There's a schematic for the chassis here:

After replacing new diode, indicator led turns on and off continuously with all the voltages varying. Can this be an overload problem on power supply?
 

Pavi98

New Member
Yes, sounds like excessive current is been taken, so it's shutting down.
In the power supply section what I noticed is there are three 15 KΩ, 2W resistors which are in series directly with the positive lead of the smoothing capacitor to power up the Vcc pin of the STR S6707, are extremely hot (can't even touch) even if the Tv in standby mode for very few seconds. Then I removed the STR and powered up and I noticed that again those resistors have become extremely hot. 25V is feeded to the Vcc pin of STR. Can this be a problem for this case or what can be causing the for the overload problem? I tested these resistors with a multimeter and found that they are fine.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Those are the startup resistors - they should only be in use very briefly as the PSU starts up. If the PSU is tripping, it will repeatedly be restarting, so they will get much hotter. If you remove the chip you will almost certainly cause damage, C624 is going to die if nothing else.

As I said above, remove the LOPT, fit a 60W bulb across the 115V rail (where the LOPT transistor fits is easiest) and make sure the PSU runs without tripping, and it's output voltages are fairly correct - this eliminates the PSU as a problem.
 

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