Magnavox 42MF531D/37 - no power

[SurrealMustard]

My Magnavox 42F531D/37 LCD television is exhibiting almost no signs of life (does not power on). I purchased the repair kit consisting of four capacitors and some diodes, which I successfully installed, but there is still no improvement. When the power is attached, the power supply makes standard noises indicating that power has been attached, but pushing the power button appears to have no effect. I've attached pictures of both sides of the power supply board and mainboard for reference. The looked okay to me, but I may have missed something.

Any ideas as to where to pick up with troubleshooting? The fuse is intact.

[CapLeaker]

Re: Magnavox 42MF531D/37 - no power

Sounds like the 5V STBY is there. You have a DMM to check voltages?
How it works: 5V STBY is always on at the PSU. That goes to the main board. Once you press the power button it will send the PSon signal back to the PSU, turning on all of the voltage rails. If you can verify that you got 5V STBY and nothing on the PSon pin (both on the PSU), then you can "force" the PSU on without the mainboard and verify all voltage rails. At that point you know if the main board or the PSU is bad.

[SurrealMustard]

Test results

Sounds like the 5V STBY is there. You have a DMM to check voltages?
How it works: 5V STBY is always on at the PSU. That goes to the main board. Once you press the power button it will send the PSon signal back to the PSU, turning on all of the voltage rails. If you can verify that you got 5V STBY and nothing on the PSon pin (both on the PSU), then you can "force" the PSU on without the mainboard and verify all voltage rails. At that point you know if the main board or the PSU is bad.

I went ahead and downloaded a copy of the service guide and was unable to find how to test the PSOn signal, but I did find a little chart with (poorly translated, but workable) suggestions on how to troubleshoot no power situations, highlighted in the picture. I found that while the voltages for pins 1-3 on the 1089 look okay, the voltages for 4 and 5 (which are supposed to be 1.8 and 4.6 volt) are registering little to no voltage. Does this mean I'm stuck with having to buy a whole new power supply or are there some other tests or repairs that can be done?

[budm]

Re: Magnavox 42MF531D/37 - no power

Does the service manual show the schematic diagrams? Can you upload it for us to see?

[SurrealMustard]

Links

Does the service manual show the schematic diagrams? Can you upload it for us to see?

Yes and yes. Here is the direct link to the document, which is a bit on the slow side:


I have also re-uploaded it here to MegaUpload incase that link goes down:
https://mega.co.nz/#!5ogR2Kra!_ujjMf...40XWu8-MOlk-l8

[CapLeaker]

Re: Magnavox 42MF531D/37 - no power

can you see a pin out on that connector?

[SurrealMustard]

Pin

can you see a pin out on that connector?

On the board itself?

[CapLeaker]

Re: Magnavox 42MF531D/37 - no power

yes... I can't see any... I am on my tablet....

[SurrealMustard]

A closer look

yes... I can't see any... I am on my tablet....

I can't either. I don't think there is one printed directly on the board. I only see pin numbers.

[CapLeaker]

Re: Magnavox 42MF531D/37 - no power

That is a bummer. Can't tell what pin does what, in order to force the power supply on. So all we can do is having a WAG at it or trace. Can you check the plug that goes from the PSU to the main board and verify that there is 5V? If there is 5V, and there is no reaction at all from the TV while pressing the on button, I would put my moneys on to the main board.

[budm]

Re: Magnavox 42MF531D/37 - no power

Power supply pin out.

[SurrealMustard]

Supply

Power supply pin out.

So, pin5 would be the PSOn pin I need to trigger to force the power supply on?

[SurrealMustard]

Clarification request

Would I just short that pin 5 to ground momentarily then or would that be going about it the wrong way?

[Alastair E]

Re: Magnavox 42MF531D/37 - no power

Err--No.

Way I do it is this--

Check if the mainboard (M.B.) is turning the PSU on by monitoring the Voltage at the PS-On pin of the PSU while the On button is pressed....
--Normally it should go from 0V (or thereabouts) to around 3-5V. so its PSU Active High. (Some Plasma sets use 'Active Low' for one/some of their functions ie, there's 5V when the PSU is 'Off' and grounding the PS-On/Vs-On brings the supply on, LCD is nearly Always Active High however)

IF there is NO change in the PS-On voltage--Then the M.B. Isnt Turning the PSU on. This could be a faulty M.B. or faulty PSU.

Check that the M.B. is getting 5V supply from the PSU, this should be present whenever the mains is connected and the vacation-switch, (If Fitted) is On. IF there is 5V supply to M.B. and no PS-On when button pressed, then M.B. is Likely faulty.....

[budm]

Re: Magnavox 42MF531D/37 - no power

Pin 9 (STANDBY) is the same as PS-ON.
I do not remember if this pin is ACTIVE HIGH (>2.5v = ON), or ACTIVE LOW (<1V = ON).
So check that pin 9 to see if it changes state or not when TV is turned on and off.
PIN5 is the INVERTER ON/OFF PIN which is the same as BL-ON.

[SurrealMustard]

YouTube coincidence

Pin 9 (STANDBY) is the same as PS-ON.
I do not remember if this pin is ACTIVE HIGH (>2.5v = ON), or ACTIVE LOW (<1V = ON).
So check that pin 9 to see if it changes state or not when TV is turned on and off.
PIN5 is the INVERTER ON/OFF PIN which is the same as BL-ON.

I'll have to give that a look tomorrow when I have access to the television again.

In the meantime, as luck would have it, one of the YouTube authors that I am subscribed to was working on the exact same television (well, the 37" model). While he was talking about the power supply, he had something interesting to say:

"This is one of those power supplies that does not use a standby circuit and a power on pin... this just runs all the time and it actually puts 24volts out to the mainboard, and then the mainboard has several voltage converters on it...". Does this mean that the test would work differently, or am I just getting similar concepts mixed up?

[budm]

Re: Magnavox 42MF531D/37 - no power

If it does not have power on pin then it is not the same power supply as yours, beside what is controlling the 24VDC big power supply (over 150W) for the inverter backlights circuits.
Do you have the link to that video?

[SurrealMustard]

Video

If it does not have power on pin then it is not the same power supply as yours, beside what is controlling the 24VDC big power supply (over 150W) for the inverter backlights circuits.
Do you have the link to that video?

Yes. This link starts it at the part where he begins talking about the power supply, where it is discussed in detail for a couple minutes.

[SurrealMustard]

Measurements for the Magnavox LCD television

Pin 9 (STANDBY) is the same as PS-ON.
I do not remember if this pin is ACTIVE HIGH (>2.5v = ON), or ACTIVE LOW (<1V = ON).
So check that pin 9 to see if it changes state or not when TV is turned on and off.
PIN5 is the INVERTER ON/OFF PIN which is the same as BL-ON.

I found that pin 9 normally has no volts, but if I push the power key on the front of the set, pin 9 jumps to 2.9volt for a few seconds before returning to zero. The remote control has no effect.

UPDATE:

If it helps, I did some of the measurements described in this post where PlainBill was helping another member troubleshoot this set. I got 170 at both PFC Out and DC in, 24 at DC1 out, and 15 at DC2 out.

[budm]

Re: Magnavox 42MF531D/37 - no power

2.9V is more than enough for PS-ON so that means the PFC Voltage on the cap should jump up from 170V to >360VDC on the main filter cap.
So the PFC is not working, you should try removing the cables to the inverter board to see what you get on the main filter cap.