magnavox 37MF321D/37 no power

[rsrace]

Magnavox 37MF321D/37 television. I am referencing the Service Manual posted in another similar thread and reading off the troubleshooting flowchart and 7.4 power supply schematic. I have read every other thread on this forum with similar problems...just haven't hit the "magic" solution yet.

No power light on front LED/manual power switch (or the remote for that matter). No picture, sound, or menu. Pushing power switch will show a slight grey screen for 2-3 minutes (backlight?) and then go dark. I also see a red LED on the backlight/inverter board (Master?) which goes out with the darkening of the screen. Pushing power a second time does nothing (toggles off?) then again repeats the slight backlight for a few seconds.

The front power switch activates two micro switches on the small IR/switch board. The switches seem to work fine – 0 ohms/continuity when pressed, infinity when released.

On my power supply board, I have 24v and 16v. I do NOT have the 1.8v standby signal on pin 4 of connector 1089 as suggested in the troubleshooting guide. As a result, I do not have +24B off the STP45NF3LL MOSFET (apparently it is not being switched on).

The troubleshooting flowchart in the service manual suggested this was a power board issue, although I cannot understand why. But, since I had power problems once before (I replaced the caps a year ago with good quality parts), I put the blame on myself and I bought another (used and listed as working) power supply board...with the same results…absolutely no change anywhere to any voltages or symptoms.

The next most logical cause seemed to me to be the “main” scaler board. With no obvious cap failures, and the troubleshooting manual telling me to test a connector that I don't seem to have (1503), I decided to buy a used “working” main/scaler board. Still absolutely no change in symptoms or voltages.

I have tried unplugging the inverter boards to see if the voltages were better (no change). And I tried unplugging the T-con board to see if voltages came back (nope).

I am at a loss. Either I am really great at finding replacement boards with exactly the same problem (although they came from “cracked screen” televisions) or my problem is not with the power supply or scaler/main boards. I don't know where to look to get the seemingly missing but needed standby power.

[sordids999]

Re: magnavox 37MF321D/37 no power

Posting pictures of the back of your tv showing the boards with clear pictures may help you more with fixing your problem

[ReeceyBurger123]

Re: magnavox 37MF321D/37 no power

Posting pictures of the back of your tv showing the boards with clear pictures may help you more with fixing your problem

I knew as soon as I see you posted it would be that

[rsrace]

Re: magnavox 37MF321D/37 no power

Pictures of both sides of the Power Supply Board, Main Scaler Board, and the IR-Power Switch board plus a schematic of the Power Supply showing where I do not have expected signals.

Many thanks for any insight. Let me know if there is something else I should have photo'd.

Randy

[rsrace]

Re: magnavox 37MF321D/37 no power

...and back panel layout. Power supply and main are in the center "box", with backlight/inverter master? on left (viewed from back) and slave? on right. IR power-switch is lower left in picture.

The only markings on the chassis label are LC370WX1 (SL) (04) and 37065K4635051 F12C.

I have been using service manual TPE1.0U LA.

[CapLeaker]

Re: magnavox 37MF321D/37 no power

Could you post all the pinouts with the voltages on the PSU?

[budm]

Re: magnavox 37MF321D/37 no power

Post 1:
No power light on front LED/manual power switch (or the remote for that matter). No picture, sound, or menu. Pushing power switch will show a slight grey screen for 2-3 minutes (backlight?) and then go dark. I also see a red LED on the backlight/inverter board (Master?) which goes out with the darkening of the screen. Pushing power a second time does nothing (toggles off?) then again repeats the slight backlight for a few seconds.
From the above description you must have have the standby Voltage (16V) and the PS-ON (STANDBY pin), BL-ON (Backlights ON), 24VDC to run the inverter circuit for you to see slight gray screen.
So I am confuse about you not having 24V, etc. It is not making sense to me.
BTW, the service manual does not have the diagram for the 37" power supply.

[rsrace]

Re: magnavox 37MF321D/37 no power

[B][I]Post 1:
From the above description you must have have the standby Voltage (16V) and the PS-ON (STANDBY pin), BL-ON (Backlights ON), 24VDC to run the inverter circuit for you to see slight gray screen.
So I am confuse about you not having 24V, etc. It is not making sense to me.
BTW, the service manual does not have the diagram for the 37" power supply.

Using the schematic BudM attached (which does not show the 37" - but I have yet to find one that does so I am following the schematic and the physical board and have yet to find differences), I have the 16V at D-16 on the print, I have the 24A voltage at C-16, I do not have the 24V off coil 5079 at the same F-16 location.

I do have 16V on 1089 pin 1&2 at F-1. I also have 24V at pin 3 at F-1. Pins 4&5 of 1089 are very low voltages (mv). I do not have standby on pin 9 of the 1089 at F-1.

CapLeaker - I will try to measure and post the connector voltages on the PSU later today

[budm]

Re: magnavox 37MF321D/37 no power

Since STANDBY (PS-ON) is generated by the main board then that will be the next board to test.
How about the INVERTER-ON/OFF signal, is it present?
The so called working used scaler/main board, is that what the seller claim?

[rsrace]

Re: magnavox 37MF321D/37 no power

Could you post all the pinouts with the voltages on the PSU?

So, in recording these voltages on the PSU, a small discovery - they voltages change when the red light on the Master Inverter board goes out (and the screen goes from slight greyish to black). To turn the light back on, one must either unplug the TV and repower it, or toggle the power button TWICE.

So BudM - this means I DO have 24v on coil 5079 but only when the inverter light is lit. Then it bleeds to 0v.

The first set of voltages is red light ON, the second set, red light off (ON/OFF). All are VDC.

connector 1089 (to scaler):
1-17.7/15.8
2-17.7/15.8
3-24.14/24.14
4-.250/.119
5-2.45
6-.003/.000
7 through 10 (same as pin 6)

connector 1088
1-24/falls to 0 (i.e. bleeds to 0)
2 through 5 (same as pin 1)
6-.252/.118
7-2.42/bleeds
8 through 12-0/0

connector 1063...same voltages as 1088

[rsrace]

Re: magnavox 37MF321D/37 no power

Since STANDBY (PS-ON) is generated by the main board then that will be the next board to test.
How about the INVERTER-ON/OFF signal, is it present?
The so called working used scaler/main board, is that what the seller claim?

BudM - just updated... if the print is right, Standby is Pin 9 of 1089 which is never present (.003v). Inverter on/off is pin 5 of the same connector which is a steady 2.45v

The seller claimed the scaler was working. That and $5 will get you a coffee at Starbucks. (I don't have any reason to believe it is working except that it is EXACTLY the same symptom as my original). All measurements above are on my ORIGINAL boards.

[budm]

Re: magnavox 37MF321D/37 no power

If the command STANDBY signal was HIGH then drops down to 0V, then the switched 24V for inverter will drop out, since the signal is generated by the main board, then that is where the problem is. The inverter ON/OFF also drops out?

[budm]

Re: magnavox 37MF321D/37 no power

BudM - just updated... if the print is right, Standby is Pin 9 of 1089 which is never present (.003v). Inverter on/off is pin 5 of the same connector which is a steady 2.45v

What DCV do you have between the two legs of the main filter cap? about 390~400VDC?
I do not believe you have good working main board.

[rsrace]

Re: magnavox 37MF321D/37 no power

If the command STANDBY signal was HIGH then drops down to 0V, then the switched 24V for inverter will drop out, since the signal is generated by the main board, then that is where the problem is. The inverter ON/OFF also drops out?

If I am reading it right, the STANDBY is always "off" (.003v then .000v) - pin 9.

The inverter on/off never drops - it is always rock steady at 2.45v

[rsrace]

Re: magnavox 37MF321D/37 no power

What DCV do you have between the two legs of the main filter cap? about 390~400VDC?
I do not believe you have good working main board.

Capacitor 2062 of the PSU (I think it shows a different number on the schematic for the 32" board; the big 450V cap, lead to lead): When the inverter light is ON, I have 399VDC. When it goes out, I drop to 164VDC

[budm]

Re: magnavox 37MF321D/37 no power

OK, I hope you can follow my description on how the power supply board function:
When TV is OFF:
1) You will have always on +24A, +24B
2) You will have always on +16V
3) You will not have +24V (switched +24V for running Inverter board) because MOSFET Q7071 is turned OFF by Q7075.
4) The OPTO 7074 is OFF because the Q7076 (PS-ON switch) is not ON due to no STANDBY (PS-ON, >2VDC = ON, <1V = OFF) command Voltage from the Main board yet.
5) Since OPTO 7074 is also OFF, same for Q7063 (switch) so there will be no VCC to run the PFC Voltage booster Controller IC 7056, so the Voltage on the main filter cap will be at around 160VDC.

When TV is turned ON:
1) Q7076 receives the STANDBY (PS-ON) signal from the main board, so Q7075 is then turned OFF which will allow MOSFET Q7071 to turn on and supplies the +24V for the Inverter board.
2) At the same time, the OPTO 7074 is also turned ON, which then turns on the Q7063 so now the VCC will be applied to run the PFC Voltage booster IC which will cause the DCV between the two legs of the main filter cap to jump up to around 390~400V range.

So what you indicated in post #14 cannot be correct:
‘If I am reading it right, the STANDBY is always "off" (.003v then .000v) - pin 9.
Post #15: Capacitor 2062 of the PSU (I think it shows a different number on the schematic for the 32" board; the big 450V cap, lead to lead): When the inverter light is ON, I have 399VDC. When it goes out, I drop to 164VDC.
You had to have Voltage present at the STANDBY (PS-ON) pin for a while and then it drops out.
You also indicated that the ‘The inverter on/off never drops - it is always rock steady at 2.45v’ It should be <1V when TV is OFF, and >2V when TV is turned ON. It should not be stuck at 2.45V
So my conclusion is that some how the main board sends out the PS-ON signal and then it stops sending it and goes LOW. What is causing that I do not know.
You can disconnect the STANDBY wire going to the power supply board first (must be done), then use 2.7~3.3K resistor between the power supply +16V pin and the power supply STANDBY pin to force on the power supply, you indicated that the Inverter ON/OF pin is stuck at 2.45V so we do not need to do the force on of the inverter circuit.
Then you plug the TV into the outlet, if other functions of the main board are OK then you should see the pictures on the screen and hear the sound from the speakers.

[rsrace]

Re: magnavox 37MF321D/37 no power

BudM,
Thanks so much for your help on this - especially in light of what turns out to be sloppy troubleshooting on my part. (I could not find an emoticon for kicking myself)

OK, I hope you can follow my description on how the power supply board function:

Yes - makes perfect sense - I follow.

So what you indicated in post #14 cannot be correct:
‘If I am reading it right, the STANDBY is always "off" (.003v then .000v) - pin 9.

You had to have Voltage present at the STANDBY (PS-ON) pin for a while and then it drops out.
You also indicated that the ‘The inverter on/off never drops - it is always rock steady at 2.45v’ It should be <1V when TV is OFF, and >2V when TV is turned ON. It should not be stuck at 2.45V
So my conclusion is that some how the main board sends out the PS-ON signal and then it stops sending it and goes LOW. What is causing that I do not know.
You can disconnect the STANDBY wire going to the power supply board first (must be done), then use 2.7~3.3K resistor between the power supply +16V pin and the power supply STANDBY pin to force on the power supply, you indicated that the Inverter ON/OF pin is stuck at 2.45V so we do not need to do the force on of the inverter circuit.
Then you plug the TV into the outlet, if other functions of the main board are OK then you should see the pictures on the screen and hear the sound from the speakers.

With your input, I remeasured. <and am embarrassed>

Inverter on/off: I measured the collector of transistor 7078 at 2.45v when "on" and then it slowly bleeds to 0 when off. I guess I was so used to signals on for 2 secs then off, I did not wait long enough. the base of the same transistor sees 3v then 0v when off and the emitter (Inverter-Control) is getting the same 2.4v then 0...so the switch is working there and I do get inverter on/off

Standby: I went right to transistor 7076 and found 0.654 on the base, then 0 when off. I went to the other side of resistor 3098 and found 1.25v. This ties to pin 9 of connector 1089 where I must not have been making good probe contact - because I see the same 1.25v.

Knowing from your info that I was looking for >2v for "on", I disconnected the wire from the connector (pin9) and measured it to see if something on the power board was dragging it down. I found 1.4vdc coming from the main/scaler board (and nothing on the PSU side), dropping to 0.011v when off. So I am getting a signal but a WEAK signal.

I can try to diagnose the PSU with the resistor "trick" but wanted to make sure whether I now needed the inverter on/off (in light of the discovery that it also shuts off)...I suspect not. And also to confirm that I want 16v - is the resistor to drop it to the needed 2+volts?...just confirming before I burn something out like 7076....so I am putting 16v on resistor 3098 (standby pin) through a 3k resistor, right?

Also, this pretty much confirms a bad main/scaler, doesn't it?

Thanks again for your enormous help!

Randy

[rsrace]

Re: magnavox 37MF321D/37 no power

Also, this pretty much confirms a bad main/scaler, doesn't it?

Helping answer my own question, I plugged in the "replacement" main/scaler board and got the same results.

I then went over both boards using an opti-visor and flashlight and found my original main/scaler had a small "wart" of carbon-like material on IC7003 (see attached picture). The replacement board also had a lot of crud on the same IC and, when I touched it, the corner literally crumbled (see picture with flashlight on). These IC's are obviously heat generators as they are pressed onto a heat-sink (?) foam when the PCB is mounted.

I looked up the part - LD5973D which is a 1-35V 2.5A step down switching regulator.

I'm not one for coincidences - two boards with an apparently burnt-looking IC which happens to be a voltage regulator, and both boards putting out low voltage on the Standby signal.

Unless someone can suggest something else to examine, I am thinking I have two very nice pieces of junk for main/scaler boards and ought to try to get a third...?

Any reason to believe my other circuity can be the cause of the failure? (I never smelled burning IC with either board).

[johnboy1313]

Re: magnavox 37MF321D/37 no power

Those chips are cheap and easy to replace. Buy two chips and a heat gun. Fix both boards, use one, and sell the other.

[budm]

Re: magnavox 37MF321D/37 no power

You should check the resistance between the output of that circuit and GND to see if it shows low resistance reading, you may something that is drawing too much current and cause the regulator to burn up.