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LG LST-4200A Digital Tv Tuner Rectifier Replacement
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navagator



Joined: 16 Sep 2014
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PostPosted: Tue Sep 16, 2014 4:48 am    Post subject: LG LST-4200A Digital Tv Tuner Rectifier Replacement Reply with quote

Hello,

I don't know very much about electronic repair but from what I can tell my LG LST-4200A ATSC digital tv tuner STB needs a new rectifier. One of the voltages on the power supply board is to low and the service manual says to replace the 0DR104510AA Diode, Rectifier when this condition occurs. Specifications for the rectifier are: B10A45V1 BK KEC TO220 45V 10A .

I can't seem to find the exact part, will any 45 volt 10 amp rectifier work or does it have to be the exact replacement?

Digikey gave me a list of 45 volt 10 amp rectifiers at this link: http://www.digikey.com/product-search/en?pv69=80&FV=fff40015%2Cfff8007f&k=rectifier+45v+10a+to+220&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=100

Could someone point out which rectifier on the list would interchange correctly for me, if there are any?

Thanks.
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jts1957



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PostPosted: Tue Sep 16, 2014 7:00 am    Post subject: Reply with quote

Any of these will work. Most significant difference I see is whether tab is insulated or not, but there are ways of insulating metal tabs.
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PostPosted: Tue Sep 16, 2014 9:18 am    Post subject: Reply with quote

jts1957 wrote:
Any of these will work. Most significant difference I see is whether tab is insulated or not, but there are ways of insulating metal tabs.


Thank you for your reply.

I thought maybe one of them would work for me but I wasn't sure.

The one that I removed from my tuner appears to have an all plastic case, there's no metal tab showing. It closely resembles the MBRF1045-E3/45GI-ND from the list which has an isolated tab.

I suppose the MBRF1045-E3/45GI-ND is probably the best one for me.


Thank you for your help.
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jts1957



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PostPosted: Tue Sep 16, 2014 6:11 pm    Post subject: Reply with quote

IF it's even bad at all.

You DID test the original?
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PostPosted: Wed Sep 17, 2014 1:54 am    Post subject: Reply with quote

jts1957 wrote:
IF it's even bad at all.

You DID test the original?


Well, I don't know for sure.

I did test it but I'm not quite sure what the results mean. When I touch the positive probe of my tester to the cathode leg and the negative probe to the other leg I get a reading of 1, when I switch the probes around I get a reading of 167.

I checked the different voltages coming off of the power supply board and they were all correct except for the 3.3 voltage. It was showing 2.3 volts. So, I checked IC153 pin 1 as the service manual said to do, and if I remember correctly it was also around 2.3 volts. The manual said to check or replace D105 (0DR104510AA Diode, Rectifier) if IC153 pin didn't show around 3.8 volts.


I started having trouble with the tuner about a month or so ago, it got so it wouldn't turn on. Some days it would turn on fine, other days it wouldn't.

On the days it wouldn't turn on, It would say "HELLO" on the front LED display briefly (as it always did when you push the on/off button) but then the display would go blank and that was it, nothing, no standby light either.

When it didn't turn on I would have to unplug it from the electric outlet and plug it back in, sometimes several times in a row before it would come on. It got progressively worse to point where I just left it on. Which worked okay for a while, but then the audio became distorted at times. When this happened I found that if I gave it a wrap with my hand on the top of the case the audio would straighten out, usually for a day or two. The audio problem also became progressively worse to the point where hitting it on top didn't make any difference.

The last time it was on and producing the distorted audio I decide to take the cover off and take a look at the inside. I noticed that there was 2 small green lights on the left side of the main board that were lit up, I went ahead and lifted the bottom of the case up slightly and dropped it down, when I did this I noticed a 3rd green light on the other side of the main board light up for a split second and the audio straightened up during that time.

I don't know what this means for sure, maybe that side of the board isn't getting power or enough power???

Thanks.
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jts1957



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PostPosted: Wed Sep 17, 2014 2:56 am    Post subject: Reply with quote

IF your meter has a 'diode check' feature and 'reading of 1' is also obtained with the meter leads NOT connected to anything, I'd tend to say 'it's good.' Most behave as 'no reading one polarity and a number with reversed polarity. If there is any doubt, disconnect either end of diode and test again.

Flow chart troubleshooting trees and Gold Star schematic diagram symptom-to-cures are meant to be suggestions to point you to the general area that's involved. And once in a while they are actually spot on.
A 30 percent decrease in that 3.3 source voltage would tend to lead me to believe an electrolytic capacitor (C118 &/or C119) is the true cause.
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PostPosted: Wed Sep 17, 2014 9:29 am    Post subject: Reply with quote

jts1957 wrote:
IF your meter has a 'diode check' feature and 'reading of 1' is also obtained with the meter leads NOT connected to anything, I'd tend to say 'it's good.' Most behave as 'no reading one polarity and a number with reversed polarity. If there is any doubt, disconnect either end of diode and test again.

Flow chart troubleshooting trees and Gold Star schematic diagram symptom-to-cures are meant to be suggestions to point you to the general area that's involved. And once in a while they are actually spot on.
A 30 percent decrease in that 3.3 source voltage would tend to lead me to believe an electrolytic capacitor (C118 &/or C119) is the true cause.


I have a Cen-Tech meter, it's a cheaper meter but it does have a diode setting, which does show a reading of 1 when the leads aren't touching anything. When I checked the rectifier I had already removed it from the circuit board, so it's probably good then.

I looked at C118 & C119 as well as all of the other caps on the board and I don't see any puffy tops or leakage. Unfortunately my meter doesn't have a capacitance setting so I can't check them. I'll probably just go ahead and order the rectifier and the caps that you pointed out and change them out one component at a time. Hopefully one of those will get it going.

Thanks.
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jts1957



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PostPosted: Wed Sep 17, 2014 5:36 pm    Post subject: Reply with quote

You will only be able to 'see' that a part is defective a small percentage of the time.
Testing a suspect part by substitution IS the best way to go. Now having said that, bear in mind the following: It IS costly to replace 'good' parts with New 'good' parts. It also will prove expensive over time to 'shotgun' a circuit.
I've found in electronic circuits that the vast majority of the time its electrolytic capacitors and semiconductors that fail the most often.
IF you plan on doing repairs a lot in the future, having a selection of the more common parts 'on hand' isn't a bad idea.
A meter as you described will allow you to test AC/DC voltage and current, measure resistance and continuity, and go-no go test many common semiconductors. A means to successfully test for (at least) a capacitor's value and its ESR will help to pick out the ones that are bad but 'look' good.

It's your money.
It's your decision.
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navagator



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PostPosted: Thu Sep 18, 2014 9:18 am    Post subject: Reply with quote

Thank you so much for your help.

I don't plan on doing a whole lot of future electronic repairs, but I do think I'll invest in a meter that can check capacitance sometime in the future.

There is one last thing I might mention, I don't know if it means anything but I noticed when I was looking over the power supply board tonight that L103 & L104 coil, chokes were different in color. The plastic sleeve on L103 is a bright neon green while L104 is a dark brownish green. I was just curious, could L104 have gotton overheated?

Thanks.
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jts1957



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PostPosted: Thu Sep 18, 2014 5:28 pm    Post subject: Reply with quote

Part list says both are same part number. If they used whatever was available shrink tubing instead of neon green as the standard for that coil...
I'd examine both and see if the one you suspect overheated is shrunk 'tighter' than the other.
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