Samsung Chassis KS7b dead.


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Samsung Chassis KS7b dead.

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hpeston
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Sep 8, 2013, 10:56 AM

Post #1 of 7 (2444 views)
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Samsung Chassis KS7b dead.	Can't Post

Hello,

My Samsung crt TV (model CS25M21MA, Chassis KS7b, 220V AC) is dead. The set makes a "ticking sound" every second. With each tick, the main 125V drops to approx. 95V, then goes back up to 125V; the 12V and 3.3V outputs (at the SMPS) also fluctuate with each tick.

B+ is absent on the FBT and the heater.

Also, a small arc is visible in the neck of the crt with each tick.

When I disconnected the collector on the H.O.T, the ticking stopped, as did the voltage fluctuation. I suspect a faulty H.O.T be the cause.

I removed the transistor, but it seems OK (No C-E shorts.)

There are no visible bulging condensors; a polyprop 1uF 630V (shown in the schematic, connected from the collector to GND) is not present on this board.

What else should I check? The 7.4nF 1.6kV condensor (between the HOT and the FBT looks fine...

The schematic can be downloaded here: http://elektrotanya.com/samsung_cs29k30mgdxxsa_cs25m21maexxtl_ch_ks7b_sm.zip/download.html

Thanks for your help, in advance.

Cheers,

H. Peston

Ron.M
Veteran

Sep 8, 2013, 5:26 PM

Post #2 of 7 (2431 views)
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Re: [hpeston] Samsung Chassis KS7b dead. [In reply to]	Can't Post

hpeston;....

Quote

B+ is absent on the FBT and the heater.

IF you are getting a "small" arc in the neck of the CRT , you apparently are getting high voltage even if for a nanosecond...

While I download the manual ,
Try this;...

Remove the small panel at the rear of the CRT...
Hang it in space well away from the CRT & other wiring etc...
Keep your hands & other body parts away from the rear of the CRT & the panel...
Turn on the TV & see if the "ticking & arcing" stops & B+ returns to the FBT...
Please post results....THANKS... Smile

..

Later...Ron.M... Cool

...
Veteran & Retired TV technician

hpeston
New User

Sep 9, 2013, 4:30 AM

Post #3 of 7 (2417 views)
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Re: [Ron.M] Samsung Chassis KS7b dead. [In reply to]	Can't Post

Thank you, Ron.

I re-connected the line output transistor for this test and I carried it out with one hand on the mains switch!

A disturbingly large sound sparked from somewhere in the c.r.t. and I switched off the set immediately.

When I reconnected the p.c.b. to the tube, and switched on again, the ticking resumed.

I did the test again without the transistor attached. This time, nothing happened.

Right now, I suppose I'll just totter down to the local parts shop to purchase a new line output transistor and a 7.6nF condensor.

Cheers,

Peston

Ron.M
Veteran

Sep 9, 2013, 5:24 PM

Post #4 of 7 (2405 views)
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Re: [hpeston] Samsung Chassis KS7b dead. [In reply to]	Can't Post

hpeston;.....

Quote

Right now, I suppose I'll just totter down to the local parts shop to purchase a new line output transistor and a 7.6nF condensor.

HOLD OFF !!!!
Did you check the "line output transistor" to see IF it went bad ???...
Why are you getting a 7.6nF condensor???...

Methinks you have a bad CRT....
With the CRT panel off the CRT , check & see if the 130 vdc to/at the flyback is there...
Do it with the "line output transistor" out of the TV...

Later...Ron.M... Cool

...
Veteran & Retired TV technician

mel
Veteran

Sep 9, 2013, 10:10 PM

Post #5 of 7 (2401 views)
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Re: [Ron.M] Samsung Chassis KS7b dead. [In reply to]	Can't Post

Ron

I think you are right about it being the CRT. I have seen the same symptoms in the past and that was the problem. Unsure

hpeston

Sounds like a good excuse to upgrade to a new flat screen Blush

hpeston
New User

Sep 10, 2013, 4:33 AM

Post #6 of 7 (2395 views)
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Re: [mel] Samsung Chassis KS7b dead. [In reply to]	Can't Post

Thanks for the reply Ron and Mel,

My apologies. By line output transistor, I meant the H.O.T.

As I said earlier, I had tested this off the board (it is a Fairchild J6810A) and it does not exhibit a C-E short; regarding the 7.4nF 1.6kV condensor, I happened to read another post on this forum that mentioned that this part has been known to "dry" and fail. The part on my set is not shorted. However, since it is relatively inexpensive, I thought I would change it as well.

I too am hoping that it is not the tube itself! I'll run the test that you suggested, Ron, and reply later -- my grandchildren are visiting and the house is pleasantly chaotic.

Mel, we do own a flat screen -- kids insisted. The set in question is in my study. In any case, I find the distractions of repairing the set to be far more entertaining than the programmes I would watch if it were in good order!

Thank you for your time. I will post the results of the test.

Cheers,

Peston.

hpeston
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Sep 17, 2013, 9:34 AM

Post #7 of 7 (2354 views)
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Re: [hpeston] Samsung Chassis KS7b dead. [In reply to]	Can't Post

Thank you for you suggestions Ron and Mel.

I replaced the H.O.T and the condensors in the circuit path. The set continues to tick -- a countdown to infinity.

As you suspected, it is most probably the c.r.t that has developed a malfunction of some sort. Perhaps others facing the same problem might be helped by this series of posts...

Once again, thank you all for your replies.

Peston

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