Magnavox LCD 37mf437/37 chirping

[topfli8979]

I recently aquired a 37 inch magnavox lcd tv, was told tv shut down after about 5 minutes of running and would not come back on. It has a chirping sound coming from the back. So i pulled the back cover to find a 25v cap bulged on the main board, replaced it, still chirping. Im pretty sure the sound is coming from the main board. Anyone had this issue?

[topfli8979]

Re: Magnavox LCD 37mf437/37 chirping

Its not the main board chirping it is the power supply. Here is a picture of power supply, chirping "seems" to be coming from upper right hand corner area.

[tom66]

Re: Magnavox LCD 37mf437/37 chirping

Replace ZD902, ZD904, D925. Common fault.

Part codes in schematic:

[topfli8979]

Re: Magnavox LCD 37mf437/37 chirping

I replaced all the diodes tom66 mentioned, chirping issue is still there. How is a zenor diode supposed to measure on the diode test, I get o.574 one way on all of them on board, even new ones from digikey.

[topfli8979]

Re: Magnavox LCD 37mf437/37 chirping

Also when main board is disconnected from power supply the chirping is gone.

[tom66]

Re: Magnavox LCD 37mf437/37 chirping

Check 12V & 24V outputs, are they stable or bad?

[topfli8979]

Re: Magnavox LCD 37mf437/37 chirping

no 12v or 24v coimg out of pb.

[notallbad]

Re: Magnavox LCD 37mf437/37 chirping

When PS disconnected from main board there will be no PS turn on signal, hence no chirping.

When no 12v or 24v is the main board connected to the PS? Or is that when it is connected?

Which 25v cap did you replace?

Add a photo of the solder side of the board.

[mr2013e]

Re: Magnavox LCD 37mf437/37 chirping

if there is a short of out power supply ,you can hear chirp in power board.so check main board.measure stand by voltage and other voltages in mainboard.

[mr2013e]

Re: Magnavox LCD 37mf437/37 chirping

by the way sorry for all. i have little english.
as notallbad said that:
Which 25v cap did you replace?

Add a photo of the solder side of the board.

[notallbad]

Re: Magnavox LCD 37mf437/37 chirping

Hi Mr2013
Very good English, good to hear from your side of the country!

[topfli8979]

Re: Magnavox LCD 37mf437/37 chirping

I replaced 25v cap C131 on main board, it was bulging. I have no 24/12v comng out of power supply with, main board connected.

[topfli8979]

Re: Magnavox LCD 37mf437/37 chirping

Here is back of ps board, I have been removing and testing components, thats why some of the soldering looks different.

[notallbad]

Re: Magnavox LCD 37mf437/37 chirping

OK, did you power the tv up yourself to see what the fault was and what happens. You cannot always believe the 'customer'!

Backlights won't be switched on unless theres a 3v signal, blon which comes from the main board.

Power on the tv, ok so its in standby only, list the voltages you get from CN902, CN903 and CN905.

Have you seen this? It lists 3 diodes as you have done and 6 caps
http://www.ebay.com/itm/715T2484-2-P...B:STORE:US:101

[topfli8979]

Re: Magnavox LCD 37mf437/37 chirping

Yes I powered on the tv, it only chirps. Buttons do nothing when pressed. I will measure the voltages across those components.

[notallbad]

Re: Magnavox LCD 37mf437/37 chirping

Do you have an led on the front panel of the tv, i.e power on/standby led.
Or does it appear to be completely dead?

[notallbad]

Re: Magnavox LCD 37mf437/37 chirping

This is for V.1, yours is V.2 but should be applicable still, just cross check between the board and schematic to be sure, there may be subtle differences..

[budm]

Re: Magnavox LCD 37mf437/37 chirping

@topfli8979: OK, since you are not getting the 12v/24v, you need to find out if the PFC voltage booster circuits are working or not since the 12v/24v power supply section requires the PFC to be working.
Carefully check the DC voltage RIGHT AT THE TWO LEGS of that large 450vdc filter cap in the primary side, check it when the TV is off (should be about 160~170vdc) and when the TV is on (should be about 370~380vdc). Do not touch anyhting on the board with your hands since you are working in the HOT side of the circuit.

[notallbad]

Re: Magnavox LCD 37mf437/37 chirping

Budm, could you edit your last post to read 300-380vDC
Thanks

[tom66]

Re: Magnavox LCD 37mf437/37 chirping

Why? PFC circuit capacitor will have around 380 to 400V DC on it. In standby 320V or 160V maybe.