magnavox 42mf438b/27

[Coinhunter]

Re: magnavox 42mf438b/27

There are only 2 main boards. The power supply board and what they call the SSB or small signal board. The other 2 boards are the CONTROL BOARD TACT SW and the IR board.

[Coinhunter]

Re: magnavox 42mf438b/27

I am fairly certain the problem is still in the power supply primary side. I have never been able to measure any voltage at the backlight connectors. Is there a chance you can tell me what voltage I should expect to measure there?

[selldoor]

Re: magnavox 42mf438b/27

DO NOT try and measure voltage at back light connectors could be 4000
volts and will fry your meter.

[selldoor]

Re: magnavox 42mf438b/27

can you do a good in focus straight on picture of the SSB unfortunately there is another board between the metal frame and the panel. Oddly enough you have done picture of the
control board and there have been two threads recently one where the cable was shorted and another where the board seems shorted and it screwed up the whole tv.Might be
worth a try plugging it in with that disconnected and use the remote

[Coinhunter]

Re: magnavox 42mf438b/27

These are the best I can do on pictures for focus. I unplugged the control board and used a generic RC to turn the tv on. It didn't make any difference. All I get is the relay click, the front power light comes on, the 2 green leds on the ssb board come on, but the screen stays black. Thanks for the tip on the high voltage. My meter will handle up to 5000 V ac current. It is an old Simpson I think.

[selldoor]

Re: magnavox 42mf438b/27

Its the other board I was wanting pictures of SSB/ Main/AV

If you can work out the pins on the transformer at the top of the picture,
with the power off , check out the secondary windings for short/open.

[Coinhunter]

Re: magnavox 42mf438b/27

I want to apologize for my delay in replying to my thread. My wife found some problems in her breasts and I have been consumed with Dr. appointments, mammography, biopsy and other medical stuff. Results came back benign thank the Lord.

Now back to my problem. Here are the pictures of the board you requested, as best as I can take. The third board is sandwiched in between the metal support pieces and covered with a metal guard. Unless I drill out a bunch of pop rivets and remove the metal pieces I cannot get to this board for pictures. Any more suggestions/ideas? I am about ready to just order a board from ShopJimmy.com. I am terribly frustrated.

[mike123333]

Re: magnavox 42mf438b/27

I worked on the exact TV several weeks back. This TV consumed hours and headhaches needles to say. My issue was the PS. Then after all of that the audio would not work, so I replaced the amp which was a chore, but still would not work. I endd using the headphone jack going to a receiver which worked out well .Here is the serice manual for it. You can search for my thread in here somewhere.

[mike123333]

Re: magnavox 42mf438b/27

Ok I understand what you want now

CN 6

expected
pin # value/id actual
1 3.3v 3.3v
2 standby .02
3 GND
4 GND
5 GND
6 12V ssb 11.92
7 12V ssb 11.92
8 12V ssb 11.92
9 +12V aud 13.36
10 GND
11 -12V aud -12.98


CN 7

Expected
Pin # Value/id Actual
1 12Vssb 11.92
2 12Vssb 11.92
3 GND
4 GND
5 Inv_On 3.23
6 DIM 1.70
7 Boost 1.47
8 GND

I do recall when i was troubleshooting and trying to fix my bad board against a good board, that the standby voltage should rise to 3.3v which should come from the main board I think this turns on the 24v. are you getting any green LED on the main board?

One of the connectors which is not used has the 24V.. on the PS did you measure and see if your getting the 24 volts?

[budm]

Re: magnavox 42mf438b/27

The board that you have is a combo Power supply/Backlights inverter board, it does not use 24V for the backlights inverter circuits like other board with separated Inverter board. The inverter section of this board runs on 400VDC power supply generated by the PFC Voltage booster circuits.
The CN2 pin 2 STBY pin, please check the DC voltage with the TV just plugged in but not turn, turn it off if it comes on automatically, take the voltage reading, then turn the TV ON and see what voltage you have. I expect it to change state when you turn the TV ON/OFF.
The Voltage on CN7 looks OK.
CN6 is also OK, the Standby control pin for this power supply is opposite than most TV, when it is in standby, pin 2 = 3V, when TV is ON
pin 2 = 0V.
So the problem is in the backlight inverter driving section on this power/inverter board.
The bad news is that the service manual does not provide any diagram for the power supply board!

[Coinhunter]

Re: magnavox 42mf438b/27

Okay the CN2 pin 2 STBY pin has 3.0 v when off and 0 volts when on. So what I think you are saying is the problem is now in the SSN board that I just posted the pictures of. Have I successfully repaired the primary power supply board. Or am I confused and the primary power supply board is still bad and the SSN board is good? You confused me when you used the power/inverter board terminology. I am still a newbie in repairing things, but I am learning fast. I have repaired 2 monitor power supply boards that were headed to the landfill so far. This TV is also a landfill rescue if I can get it repaired.

[budm]

Re: magnavox 42mf438b/27

What you have in your TV is the power supply board and Backlights inverter board all in one. Most TV will have power supply board and one or two backlights inverter boards.
Right now your board (Power supply/inverter board combo) is getting all the correct signals from the Main board OK, but the inverter section of the board is not functioning.

[Coinhunter]

Re: magnavox 42mf438b/27

I am still confused BudM. What are you calling the main board? The power supply board and back-lights inverter board or the SSB board? The SSB board is the one with all of the input/output, HDMI connectors etc. on it. From looking at the service manual the power supply board sends all the signals to the SSB board not the other way around.

[Coinhunter]

Re: magnavox 42mf438b/27

After more reading the main board is the SSB board. I just found a voltage regulator chip on the HOT side of the power board that had a corner broken off. I am going to check all of them to see if that is now the problem.

[Coinhunter]

Re: magnavox 42mf438b/27

After looking up the number on the chip MURF 1060 SSG 07421 it is a SUPER FAST RECOVERY SILICON RECTIFIER Reverse Voltage - 50 to 600 Volts Forward Current - 10.0 Ampere chip. Don't have any idea what all this means.

[Coinhunter]

Re: magnavox 42mf438b/27

Can someone tell me what its function is please?
Okay, according to you guys all my voltages look correct at the terminals, yet I still have no video, sound, etc. The set clicks, you can hear it turn on and the 2 green leds come on on the main/ssb board. I still have 3 caps left to change out. A 450v 4.7uf, a 200v 68uf, and a 450v 1uf. There are (3) 450v 68uf that I was told should not be bad, so I have not planned on changing them. I am going to have to order them because there is no where to purchase them locally that carries the low esr caps.

[Joshster]

Re: magnavox 42mf438b/27

I am having problems with this same TV. Did you get yours fixed? Mine turns on and works ok for a while then will either power off and need to be unplugged or the back lights will go out but sound will remain.

[Coinhunter]

Re: magnavox 42mf438b/27

I am having problems with this same TV. Did you get yours fixed? Mine turns on and works ok for a while then will either power off and need to be unplugged or the back lights will go out but sound will remain.

I never have gotten mine repaired. I am at a stand still with it and frustrated. I have had to take time off from working on it and several others to concentrate on some computers I am repairing. If I ever get it working I will post on here.

[bassn1968]

Re: magnavox 42mf438b/27

did anyone ever repair these tv's? I had 2 given to me with the same problem,after weeks of frustration they are working again. reply here if you want the solution. It is a time consuming repair

[selldoor]

Re: magnavox 42mf438b/27

It would be great if you could post your repair - it may not be needed now but may help others in the future, on this and similar sets Please include pictures of your boards so we know what you are talking about.