RCA rlded3950a-b sound but no video

[MrBowtie]

Re: RCA rlded3950a-b sound but no video

I'm going to check that now.

[MrBowtie]

Re: RCA rlded3950a-b sound but no video

The diode is fine.

[philliesfan30m]

Re: RCA rlded3950a-b sound but no video

can you get a better pic of ic2 on the bottom of the board? i was looking at ic3 and that "looks" ok but then i saw ic2 either the pics is distorted or it looks bad

[MrBowtie]

Re: RCA rlded3950a-b sound but no video

It has like a glue all over that spot.

[philliesfan30m]

Re: RCA rlded3950a-b sound but no video

does it look burnt at all by the legs or does it have any holes in the casing that you can see?

[MrBowtie]

Re: RCA rlded3950a-b sound but no video

It looks good from what i can tell.

[budm]

Re: RCA rlded3950a-b sound but no video

OK, so the power MOSFET has Source (3) to Drain (2) shorted. The high voltage cap that blew up is for the RC Snubber circuit (the snubber RC network is connected in parallel with Source and Drain of the MOSFET), the cap is connected in series with R44/R45 (R44, R45 are connected in parallel), You need to check R44 and R45. If the snubber circuit failed, the the MOSFET will be damage.
There is also RCD snubber network (D5, C16, R39 that should be checked) which is connected in parallel with primary side of the transformer.
You also need to check the MOSFET Gate drive network (R23, R26, R26, R30, D25) for damage. The Gate drive network is connected between the Gate of the MOSFET and the GD (Gate Drive) pin 5 of that SMPS IC. You should check the resistance reading between pin 5 and the one end of the jumper JP9, report the reading. We also need the P/N of the SMPS IC.
You also need to check the Source resistor (R13?) that has heat shrink tubing on it sitting between the heat sink and the Main 450VDC filter cap, it could have blown open when the MOSFET shorted out.
So you have lots of components to check, if you don't, you can easily blow up your new parts.

[MrBowtie]

Re: RCA rlded3950a-b sound but no video

Thanks so much Budm. I'm going check all that now.

[MrBowtie]

Re: RCA rlded3950a-b sound but no video

Not sure if my numbers will sound right for resistors but here are my findings. R17=2.5, R44=0, R45=0, R23=480, R26=37.5, R30=9.23, R39=80.9. The ic chip looks good and has a reading of 91.5 if that sounds right. D5 is 0 one way and 558 ohms the other way. D25 is showing 48 ohms both ways. As for c16 not sure how to check but i tried different settings and got nothing. Was there another resistor you wanted me to check? I noticed you had R39 or something twice in your post.

[MrBowtie]

Re: RCA rlded3950a-b sound but no video

Forgot to mention R44 and R45 look toasted.

[vinceroger69]

Re: RCA rlded3950a-b sound but no video

c16 will be a capacitor so you need a capacitor meter really to test the r45 r44 look the bad ones so far as for r13 have you tested that yet? in black heat shrink tubing.

[budm]

Re: RCA rlded3950a-b sound but no video

I am not surprised since they are in series with the shorted out high voltage snubber cap (blue cap), high voltage got dump into those two resistors.

[budm]

Re: RCA rlded3950a-b sound but no video

"R17=2.5" R17? that resistor is not in the area of the circuit (standby power supply) we are working on.
We also need to know the numbers as printed on top of the resistors and compare what you are reading on the meter, you must also tell us if the meter is showing for example:2.5 Ohms, 2.5K OHms, 2.5 M Ohms?

[MrBowtie]

Re: RCA rlded3950a-b sound but no video

R17 is the one in the heat shrink near the big cap. R13 is marked 1001 and is reading 997. R23 is marked 470 and is reading 480. R26 is marked 360 and reading 375. R30 is marked 1002 and reading 9.23 meter set at 20k. R39 is color coded gold, orange, red, silver and reading 80.9 meter set at 200k. R44 is marked 470 and reading 0. R45 is marked but can't tell what it was but reading 0. The IC chip is marked GR8876a C5038 and is reading 92 on pin5 to jp9 with meter set at 200k. C16 looks fine but can't test it. Hope this helps.

[budm]

Re: RCA rlded3950a-b sound but no video

OK, I thought it is R13 looking at the pictures.

"R44 is marked 470 and reading 0" that should be 47 Ohms resistor. You should replace the R44/R45, I believe they are the same value, 47 Ohms.
The readings on other resistors are OK. You may see the color wrong on R39, but it is within the correct range for the RCD snubber.

"The IC chip is marked GR8876a C5038 and is reading 92 on pin5 to jp9 with meter set at 200k" I will need to look up the spec sheet of this IC. I would replace it instead of taking a chance of blowing up the new MOSFET.

[MrBowtie]

Re: RCA rlded3950a-b sound but no video

Any idea where i can order these parts? I tried looking up the ic gr8876a at digi key but no results.

[MrBowtie]

Re: RCA rlded3950a-b sound but no video

I'm having a hard time trying to find the GR8876a ic chip without having to order from China. Was wondering if there are any other chips that i could replace it with? Would a ld7575 work? Or could someone tell me what kind of chip to look for at digikey. Thank you for all your help and sorry to be a pain.

[MrBowtie]

Re: RCA rlded3950a-b sound but no video

Just wanted to update everyone who helped me and to thank you for doing so. All in all I had to change the blue ceramic cap at c22, changed mosfet that shorted "p7nk80zfp" at q3 i think and changed two resistors at R44 and R45. My tv is up and running a half an hour so far. Thanks for all your help everyone.

[philliesfan30m]

Re: RCA rlded3950a-b sound but no video

glad you got it fix!

[wiseowl9000]

Re: RCA rlded3950a-b sound but no video

I had the same problem and fixed my TV following the instructions in these posts. It's the same power supply in an Hitachi LE42EC04AUD.

Using a loupe I saw that the solder joint for one of the transformer pins looked cracked (near the 470ohm resistors) so I re-soldered that for good measure.

For the 470 Ohm Surface Mount Resistors (burnt out like the OP experienced) I used Panasonic ERJP08F4700V SMD Chip Resistor, Thick Film, 470 ohm, 500 V, 1206 [3216 Metric], 660 mW, ± 1%, ERJP08 Series packaged in Cut Tape .

I'd have been lost without this thread so thank you very much.

Cheers from Australia!