Sylvania LC420SS8 Buzzing When In Standby

[Mr Bill]

I picked this tv up yesterday. It was completly dead. Turned out to be a shorted schottky diode. Replaced it and the tv came to life. It physically works but the only issue (complaint) i have is the transformer emmits a audible buzzing noise when the tv is in standby. I used a cardboard roll and used it as a makeshift stethescope. There is no noise when the tv is on. I did notice on ebay there is a "repair service" for my issue. So this makes me think that it's a cheap and easy fix. I've tested all the diodes on the power supply but thats as far as i have gone.

So what could cause this issue?

Here is the link to the ebay auction for giggles even though i won't spend the $80.
http://www.ebay.com/itm/Sylvania-LC4...item19d218a44b

Thanks for any suggestions

[retiredcaps]

Re: Sylvania LC420SS8 Buzzing When In Standby

Send them an email to ask what the repair service does.

[Mr Bill]

Re: Sylvania LC420SS8 Buzzing When In Standby

I emailed them and asked if they would offer a repair kit. I'm sure you can guess what there answer was.

BUT
In the end i found out what was causing the issue. There was a service bulletin on this issue. D655 needed to be removed from the power suppply. So the repair service on ebay was to remove a little zener diode and charge you $80.

[budm]

Re: Sylvania LC420SS8 Buzzing When In Standby

Thanks for finding this, how did you locate the bulletin?

[retiredcaps]

Re: Sylvania LC420SS8 Buzzing When In Standby

So the repair service on ebay was to remove a little zener diode and charge you $80.

They probably don't even bother to desolder it. Just take a flush cutter and snip off the diode and send it back to the customer.

Despite my number of posts, I still lack a lot of knowledge. For example, I thought a zener diode or any diode was there to protect some part of the circuit. By *cough* hack *cough* taking this diode out, doesn't the customer lose something?

Isn't it like a mechanic rebuilding an engine and finding one left over part and when no one is looking, chucks it into the garbage?

[budm]

Re: Sylvania LC420SS8 Buzzing When In Standby

The bulletin did not indicate that it is Zener Diode, it just indicates it as D655. Be good to see the diagram and what its function in the circuits is.

[Mr Bill]

Re: Sylvania LC420SS8 Buzzing When In Standby

Thanks for finding this, how did you locate the bulletin?

I actually found this on encompass parts website. The service manual parts lists calls it a "switching diode", so thats my mistake on calling it a zener.
http://www.encompassparts.com/webwiz...model=LC420SS8

[rass96]

Re: Sylvania LC420SS8 Buzzing When In Standby

Mr Bill which Diode was your original problem? I have one doing the same thing.

[Mr Bill]

Re: Sylvania LC420SS8 Buzzing When In Standby

Mr Bill which Diode was your original problem? I have one doing the same thing.

D655 was removed and the buzzing in stanby went away? Does your board look like mine in post #1 since i see your from the UK?

[rass96]

Re: Sylvania LC420SS8 Buzzing When In Standby

Yes its the same board, its a US spec TV ( I am USAF stationed here). No I mean for the original power problem. You said it was a shorted Schotkky diode? Just trying to narrow the search. But it did have the buzzing too, so that will get removed thanks to you.

[Mr Bill]

Re: Sylvania LC420SS8 Buzzing When In Standby

I can't remember exactly which diode was shorted making it dead but it was one of the diodes i put squares around.

[rass96]

Re: Sylvania LC420SS8 Buzzing When In Standby

Thanks, I will look at those more closely. When I was randomly testing diodes today I noticed like 4 of those had continuity but I think they had common connections. I will let you know how it goes.

[rass96]

Re: Sylvania LC420SS8 Buzzing When In Standby

I can't remember exactly which diode was shorted making it dead but it was one of the diodes i put squares around.

Ok I found a shorted diode, its the one on the right next to the tall cap in the bottom square. They are parallel so they both shot bad till I desoldered one to isolate. Its an SB 390 which is a 3A forward continuous current, 90V peak reverse voltage. I have a SB 360 on hand which is 3A 60V reverse. Do you think it would be close enough? Or do you think the difference is too much?

[tom66]

Re: Sylvania LC420SS8 Buzzing When In Standby

There's probably a reason they used 90V there, but the 60V will probably work for a quick test, I'd expect it to fail quickly though.

[rass96]

Re: Sylvania LC420SS8 Buzzing When In Standby

Mr Bill

Just wanted to say thanks, the diodes came in the mail today (I decided just to order the right replacement rather than chance it) and I replaced the pair and the set came right up. I instantly heard the buzzing (very obnoxious) and clipped the D655 and it stopped. Thanks for posting that service bulletin.

[Jvercelli]

Re: Sylvania LC420SS8 Buzzing When In Standby

Where did you buy the diodes from?