Magnavox 42MF531D/37 - no power

[SurrealMustard]

Further troubleshooting

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.

The measurement does appear to jump up to around 400 on the filter when I press the power button and then it falls back down at the same time as the PS-ON. This is with the inverter boards unplugged. Should I attempt to repeat the test with them plugged in? I ask because it'll take a bit of effort since there isn't much slack on them.

[budm]

Re: Magnavox 42MF531D/37 - no power

"pin 9 jumps to 2.9volt for a few seconds before returning to zero. The remote control has no effect." OK that will explain why you did see the Voltage on the main cap jumps up to >360VDC and then drops down when PS-ON goes away.
So some how the PS-ON goes away.
BTW, does the 16VSTBY stays at 16V during this start up?
I would also try disconnecting the T-CON board LVDS cables just to see what happen.
The power supply does not any error signal to tell the main board to shutdown either.

[SurrealMustard]

Voltage and test conditions update

BTW, does the 16VSTBY stays at 16V during this start up?

Yes, it holds steady at around 15 at all times.

I would also try disconnecting the T-CON board LVDS cables just to see what happen.

I'm sorry, I probably should have mentioned earlier that these tests are being/have been done in that condition already. Because the way I have to flip the cage upside down to get measurements reduces the slack available on the cables, the T Con and inverters have both been unplugged while I'm probing, if that makes any difference.

[SurrealMustard]

Mainboard

Does this mean I'm probably looking at a bad mainboard?

[budm]

Re: Magnavox 42MF531D/37 - no power

Yes.
You can verify if the power supply and the inverter are working by disconnecting the cable between the main and the power supply and use 2K 1/4w resistor between PS-ON and 16VSTBY to turn on the power supply first to see if the >360V boosted Voltage will stay on, then use another 2K resistor between INV ON/OFF and 16VSTBY to see if the backligthts will come on.

[SurrealMustard]

Success! (mostly)

Yes.
You can verify if the power supply and the inverter are working by disconnecting the cable between the main and the power supply and use 2K 1/4w resistor between PS-ON and 16VSTBY to turn on the power supply first to see if the >360V boosted Voltage will stay on, then use another 2K resistor between INV ON/OFF and 16VSTBY to see if the backligthts will come on.

Good call. Mainboard fixed the problem! The set now powers up and behaves how it is supposed to.

There is one other question I have though. After the unit has been on for a short period of time, five bright spots begin to appear at the very top of the screen and eventually become more pronounced after a couple of minutes. They are most obvious when the screen is dark. I know this thing was sitting in the previous owner's garage on the floor (resting with all its weight on its speakers ) for quite some time (possibly over cold winters) before I got it and it may have been improperly handled at some point.
Any ideas as to what could have caused this or if there's anything that can be done about it? Do I risk damaging it further by leaving it on?

[SurrealMustard]

Spoke too soon

Hmmm... Further testing reveals some problems. Is the power light supposed to be illuminated when this thing is off? I'm having issues with it sometimes not powering up. If the light is green while the power is off, it powers on with no problem, but if the light is off while the power is off, the set will rarely turn on (it merely flashes the backlight for a split second and turns the power light off after some time).

Any ideas?

[budm]

Re: Magnavox 42MF531D/37 - no power

Hmm, will have to look up user manual to find out about the Green power light.

[SurrealMustard]

More troubleshooting information

Hmm, will have to look up user manual to find out about the Green power light.

I looked into it a little more closely, and while the user manual is pretty vague on the topic, the service guide is crystal clear:

"Power indicator : LED (On: Green ,
Standby: No light )" it says.

What I have found is that when the set has been unpowered for some time, it's easy to get it to start when it is first plugged in. Attempting to turn it off is partially successful because the screen and backlight shut down, but the green light stays on (which it is apparently not supposed to do; if left long enough, the green light will eventually turn off). If I plug it in a second time, it has a hard time powering up; the green light will come on for a few moments when the button is pressed, but the television never starts. Sometimes when it does start, there is no sound.

Could I have purchased a bad mainboard or might something else be the problem?

[SurrealMustard]

Whack a mole

Well, I contacted the seller of the mainboard and was sent another mainboard. This one works correctly, but (yeah yeah, here it comes) I'm getting a high pitched whining noise out of the speakers while the set is powered on. It goes away when the sound is muted, but the pitch and loudness of it remains the same no matter how high or low the volume is (when the sound isn't muted).

Any ideas as to what could be causing this?