TV DAEWOO 21T1ST chasis CP380 - tweet tweet....

On Fri, 8 May 2009 04:45:12 -0700 (PDT), b put finger to keyboard and composed:

Service Manual for Daewoo CP380 chassis:

Page 3 of the service manual talks about the EEPROM settings but these unfortunately appear to require a Service Remocon. :-(

Could D811 (near T802) be leaky or S/C?

Are there any shorts on the 14V secondaries of the switchmode transformer?

BTW, I'm having trouble understanding how optocoupler I804 inhibits the supply because in doing so it would turn off the +5V to the uP which controls it.

- Franc Zabkar

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Hi Franc,

Thanks for the info - very useful.

I probed a bit more earlier today. The D811 tested Ok, I replaced it and the supply still kept up its rapid tweeting/LED flashing.

Out of curiosity, I desoldered the eeprom and powered the set up - symptoms stayed exactly the same. So maybe thre's just some short I've overlooked.

I had also isloated the vertical/frame circuit by desoldering one leg of a rectifier diode feeding it from the line transformer.

I should point out that I'm using a dummy load of a 60w bulb in place of/across c-e of the line transistor. No power gets there.....

-B.

a few more tests: the 123v (B+ feed to the line stage) output of the psu transformer never reaches more than about 50v, even unloaded. all the others have negligible volts on. could i802 be the cause?

here's a part of the psu schematic:

I'm all ears to any suggestions. B

The switcher is cycling presumably due to an overloaded/short-circuited secondary load. Without being able to see the feedback part of the schematic but assuming it is from the B+ output, I'd lift D812 first and then D813, checking whether the switcher recovers normal operation. If it doesn't - and I expect it won't - then your fault lies in something hanging off the B+ line.

The psu stops its cyling/tweeting when the 123v line is isolated, but still does not reach over 50v on that pin, which led me to suspect some regulation problem. Not only that, but the line stage seems fine: I'm using a bulb in place of the transistor, and I have not found any shorts on the line transformer secondary loads.

I'm beginning to suspect the replacement line transformer may not be correct. the original was an 1142.5057 but I was sold an ELDOR

1142.5077 (which , it says on the box, will sub. a .5057 or 30473 - This may not be the case! Will contact them and check. .

On Wed, 13 May 2009 06:55:09 -0700 (PDT), b put finger to keyboard and composed:

I

I have seen Sony sets where the B+ cap has gone open and caused this type of problem. However, it would seem unlikely that both C816 and C817 would be faulty.

What are the voltages on C819 and C818? It would seem to me that if the B+ voltage has dropped from 123V to 50V, then the 14V supplies would be around 5V.

Here is the datasheet for the TDA8138 (note that there are three versions):

AFAICS, you can detect the current draw of the +5V rail by measuring the voltage across R831 or R832. That's assuming pin 4 is not disabled.

It might also be worth seeing what optocoupler I804 is doing to the Inhibit pin of the SMPS regulator IC.

The HR Diemen equivalent for both the 1142.5057 and the 1142.5077 is an HR 7880:

- Franc Zabkar

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i've since discovered there is no opto fitted on this set!

7880:

yeah, I checked that too, so the transformer seems OK.

I ordered a TDA 8138 but ended up with a TDA8138A, which is a different package, so will have to wait for the right one. Meanwhile, I changed a couple of tired looking 25v caps near it, and desoldered also the regulator KA 7808 (i803 -under normal conditions this produces 8v.) but things didn't improve. will take a look at the 5v cct and resistors you mention.

-B

Quick update:: I have discovered that the psu operates normally (at least, I measured the 123v and it was Ok) if R704 is lifted. Somehow I didn't do this test earlier (would have saved me some time!) Tracing it back, you can see here :

...that it goes via a UPC 574 stabilizer to pin marked VT on i701. My question is: could this mean the stablizer is faulty, or the ic? I wouldn't fancy changing the latter....

-B

On Mon, 25 May 2009 15:12:57 -0700 (PDT), b put finger to keyboard and composed:

R703 and R704 present a resistance of about 11K to a 123V supply. If IC701 was shorted, then the current draw would be about 11mA. I don't see why this would load your supply.

- Franc Zabkar

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Please remove one \'i\' from my address when replying by email.

changed the stabliser and the eeprom, and still the same. Anything else to try before I toss this thing?