Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

[Hazen]

Hello,

I have been troubleshooting this tv for a little while, and now I need some expert help. It was working fine for about 10 minutes one morning and then just went dead, with no standby light. After doing some research, I replaced the main board, which didn't help. I then ordered a replacement ps board, but was shipped a different version than I ordered. I couldn't see any difference between my ps board and the replacement, so I tried it in the tv. The standby light came on, and it would try to turn on, but then the red light would flash a few times. I tried the ps with both main boards to see if I would get the same result, and I did. So now I'm pretty sure its the ps board, and the replacement just wouldn't work with my tv. It has been hard trying to find a replacement ps board, so I am trying to repair the board myself. I've taken many of the components that I've read fail often out of the circuit and tested them and have found all of them good, although I just checked the caps with a dvm and not a ESR.

Some details:

I have 160v at pfc_vin

I have 11v at the p-on+21v

I have 3v at the all+3.3v

I have 3v at the p-on_h2

I have 6v at the amp+13v.


Thanks

[Hazen]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

Any Ideas?

[dick_barton]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

Read through and carry out the instructions given to test the set and post your results. Read through all the pages as it gives more details of what to do as you read through it.


https://www.badcaps.net/forum/showth...light=LF501EM4

[Hazen]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

I've read through the other post and here are my findings so far. I've put a 1k resistor across the 3.3v and p-on-h2, with 2.5v on p-on-h2. I still have only 11v at the p-on+21v, and 6v at the amp+13v. I've jumpered across the 3.3v to the j106/bl-sw with a 1k resistor and the display did not come on. I've checked the voltage at the protect 1 and protect 3 and get <1v on each.

Thanks!

[patrax]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

I have this exact set on the bench at present, showing the exact same symptoms. And low and behold the exact voltages listed in the above post. I have followed the service manual to try to track down the culprit to no avail. Nothing Ive tried will bring it to life. All I get is a red standby light after its unplugged for about a half hour. Ive tried jumping resistors resetting ribbon cbls. Heat, Im stuck on this one. Anyone feeling they might have a clue please post.Im at wits end.Post some pics of all 2 boards may help. Thanks guys.

[dick_barton]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

So with the main board disconnected and P-ON-H2 connected to AL3.3V using a 1K resistor you do not have the 13V or 21V.

What dc voltage do you have across the legs of C605 (33uF, 400V)

[dick_barton]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

It looks as though the PFC operates by driving two mosfets to produce the 395V. Check Q1601 and Q1602. I think only one of them is working which is why you have half the output voltages. If you give the DC voltage across C605 it may provide a clue.

[budm]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

So what DCV do you have between the two legs of the main filter cap 100uF 400VDC when you force on the power supply?

[Hazen]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

With the 1k resistor across P-ON-H2 and AL3.3V I have 160vdc at C605 and C1621. I've checked both mosfets Q1601, and Q1602 out of circuit and both check ok. I get 160vdc at the drain on both also.

[budm]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

So the PFC Voltage booster is not working.
Check the resistance of the two PFC diodes next that filter cap, D1603, 1605.

[Hazen]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

Both D1603 and D1605 have 1.3M with neg on cathode, and 2.2M with pos on cathode. They were tested in the circuit, although I had previously removed them and they checked good. Should I remove them and check the resistance again?

[budm]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

If they shorted out then you will not get the boosted Voltage, but since it is not bad then the PFC circuit will needed to be probed next.
Did you download the diagram from the link to the other post. You will need to find out if the PFC IC is getting the switched VCC to run or not.

[dick_barton]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

Do you have the voltages given especially at VCC (Pin 12)

[Hazen]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

I'm getting 8v at IC1601 vcc. Should I be getting around 15.6v?

[dick_barton]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

Should be getting 15.6V according to the manual which is attached. That is with P-ON-H2 connected to AL+3.3v using your 1K resistor and mainboard disconnected.

Check components highlighted in the flow chart on page 24 of manual

[budm]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

Check the DCV between the two legs of the C616 (220uF 35v) when force on is NOT present and when force on is present.

[Hazen]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

I have 10v across c616 when its not forced on, and 9v when it is forced on.

Thanks for all the help you are giving me!!

[dick_barton]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

Have you checked the components as given on page 24.
I would also be interested to know what DC voltage is on pin 6 of IC601 (FA5640N)

[Hazen]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

Have you checked the components as given on page 24.

I've removed all the larger components and checked them, and they check ok. The smaller components I've checked in the circuit, and didn't find any shorts. I'm not sure how to check the IC's other that checking voltage readings, and the same with the T601, other than checking the resistence on the differant windings.

I would also be interested to know what DC voltage is on pin 6 of IC601 (FA5640N)

I'm getting 11v on IC601 VCC

[dick_barton]

Re: Emerson LF501EM4 s/n ds4 no standby light, and won't power on.

The voltages to the IC's all appear to be much lower than they should be. What are the voltages on the 3 pins of Q602?
The diodes need to be checked using your meter set to diode mode and would normally read 0.4 - 0.7 in one direction with the meter leads across it and OL (over range) with the meter leads reversed of course with all power off.