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Sharp 51DT25H stuck in standby

 
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electronicsuk



Joined: 02 Dec 2005
Posts: 2
Location: United Kingdom
PostPosted: Fri Dec 02, 2005 3:16 am    Post subject: Sharp 51DT25H stuck in standby Reply with quote
Hi everyone. I'm not a TV repair engineer by trade, but have been landed with a Sharp 51DT25H to repair after after someone recommended me as the 'guy to talk to'. I have a considerable amount of experience with electronics but as yet have never repaired a TV.

I did a search for repair tips for this particular model and have also got myself the service manual. My search yielded a couple of suggestions that seem relevant:

Quote:
Set does not come out of standby, PSU buzzes
Change the two HT smoothing caps (100æF, 160V) and uprate them to 250V and 105 degree types if possible. All caps in the PSU seem to suffer with this chassis, so be thorough in your checks.


I have put a multimeter across these capacitors set to resistance measurement. They behave as expected, starting with a low resistance and eventually rising to a few MegOhms. I assume this is an adequate check and suggests the capacitors are working fine? I have also repeated the same check for the mains power smoothing cap.

Quote:
The set will not come on nothing appears to be faulty all the caps check ok. R705 (560Kohm) goes open circuit, this is in the start up section of the power supply


Again, I have checked this component and the resistance is very close to 560k. The set still refuses to come out of standby, although when power is present there is an intermittent high frequency buzzing sound from the area of the board where the PSU and HT transformer are mounted. By intermittent I mean that the buzzing is not a constant tone, I suppose the closest thing I can relate it to is the sound of random high frequency data stream.

Any thoughts? From the information I've found so far my problem sounds most like the HT smoothing caps, but as I've already mentioned, they seem to check out OK. Another tip suggested that if the set is stuck in standby and there is 'motorboating' (?) from the speaker then C712 in the 10v supply is at fault, I have yet to check this out.

Any suggestions? Components I should check?

Cheers,

Matthew
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gary



Joined: 05 Apr 2005
Posts: 584
Location: MANCHESTER, uk
PostPosted: Fri Dec 02, 2005 7:37 pm    Post subject: Reply with quote
A RESISTANCE CHECK ON CAPACITORS IS NOT ADEQUATE AS IT WILL NOT TELL YOU ANYTHING OTHER THAN SHORT OR O/C, CERTAINLY NOT WHETHER THE CAPS VALUE IS EVEN CLOSE TO CORRECT, IF YOU DO NOT HAVE A CAP METER CHECK ITEMS BY SUBSTITUTION THEY ARE RELATIVELY CHEAP AND IN THESE SETS UNRELIABLE. C712 IS A GOOD STARTING POINT AND HAS BEEN THE CAUSE OF MANY FAULTS IN THESE SETS
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PostPosted: Fri Dec 02, 2005 11:56 pm    Post subject: Reply with quote
Thanks Gary. I will get hold of some replacements and give them a try.
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PostPosted: Tue Dec 06, 2005 1:38 am    Post subject: Reply with quote
Just a quick update. I haven't got all the spares yet but happened to have a 16v 220uF in the parts box, I replaced C712 but unfortunately no such luck Sad Will report back and let you know what affect (if any) the other replacement parts have.
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PostPosted: Sat Dec 10, 2005 5:41 am    Post subject: Reply with quote
Good news! Replaced the 400v 100uF SMPSU cap and two 100uF 160v HT smoothing caps and the set is now operational. Many thanks for the tips!
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