Envision/AOC power supply - no +12/24V

[ahickman]

Seems both junk TV's I have been given have similar issues where the standby +5V is present, but the +12/24V is not. The power supply is from an Envision L32W861 and I can not readily find a replacement. The part number is (T)24180BB1 (PCB # 715T2804-1-2). ShopJimmy lists them, but not in stock. I did find an identical part number, but it was for a VISIO/INSIGNIA. Not comfortable assuming that the parts are the same so I am going to start troubleshooting.

Picking up what I learned from the previous thread, I pulled the PS and powered it up using the isolated transformer and put 2 12V bulbs across the 24V pin and ground and then pulled the ON/OFF pin high with the +5V standby. Nothing. I then looked along the traces of the ON/OFF pins and found that are unrelated to the board and that it only loops from one connector to the other. Can anyone help me determine which pin I should be using to turn on the +12/24V? Images are attached.

[budm]

Re: Envision/AOC power supply - no +12/24V

The PS pin is the PS_ON PIN, the two pins (looking at the bottom of the board in your picture, labeled ON/OFF, DM are the the inverter on/off and dimming control signals) connector CN902. They are just looped through signals from the main board that way they can do cleaner wiring with one bundle of wires from the power supply to the inverter board, and one bundle of wires to the power supply board from the main board, it is very common wiring used in a lot of TV.

[ahickman]

Re: Envision/AOC power supply - no +12/24V

Many thanks for the insight. I assume the standby voltage (+5V) through a 3.3K resistor will be enough to turn on the additional diodes?

[budm]

Re: Envision/AOC power supply - no +12/24V

You can go as low as 1K.

[budm]

Re: Envision/AOC power supply - no +12/24V

By the way, if you trace out the connection from the PS pin, it is connected to the resistor (this common design) and another end of the resistor is connected to the base of the switched transistor from what I can see from your pictures.

[ahickman]

Re: Envision/AOC power supply - no +12/24V

yes, the PS pin runs to a 10K ohm resistor (capacitor to ground) and into a transistor which turns on/off the optoisolator. When I push PS to 5 V, the 5V supply starts pulsing. Just took the optoisolator out of circuit to test.

[budm]

Re: Envision/AOC power supply - no +12/24V

This is one trick I use for locating bad caps (bad cap can look normal with no bulging top, but it can have high ESR, but if you heat it up, the ESR will go down). I se the hair dryer set to low and heat up the P.S. and see if it will come on when you apply the PS ON signal. The 5vdc should be on steady, I use samll 6v bulb (6v 100 ma lamp from Radio Shack) attach to the 5VDCSTBY line to monitor it.

[ahickman]

Re: Envision/AOC power supply - no +12/24V

I have Googled my heart out for a schematic on this PS board ((T)24180BB1 (PCB # 715T2804-1-2)) with no success. Anyone listening have a suggestion on where I can get a decent schematic so I don't have to strain my eyes?

[budm]

Re: Envision/AOC power supply - no +12/24V

Please see the PDF, and take the reading of the votltage on that large cap.

[ahickman]

Re: Envision/AOC power supply - no +12/24V

157 vdc.

[ahickman]

Re: Envision/AOC power supply - no +12/24V

Budm, when I put 5V on the PS pin I notice quite a cycle in voltage. I have attached a screen shot from oscope. Looks like something is turning on and then shuts down? Tracing the PS pin back to the secondary xformer through the schottky diodes, I don't see anything obvious in terms of shorts/opens. I left the PS pin high for a while and felt for anything warm or unusually warm. Nothing stands out.

[momaka]

Re: Envision/AOC power supply - no +12/24V

157 vdc.

See if this voltage changes when you try to turn on the power supply.

This power supply has APFC, so the voltage on the large cap should go to 380V or possibly more (but not more than 450V) when the power supply is turned on.

There's a diode (D902???) mounted on a heatsink (looks like a MOSFET but has 2 legs only) next to the big primary capacitor. Check it to make sure it is not shorted. Do this with the power turned OFF, of course.
Also, be aware that sometimes it can take a while for the primary cap to discharge. After powering OFF the TV, measure the voltage across the primary cap to make sure it is discharged before touching anything in that area.

[ahickman]

Re: Envision/AOC power supply - no +12/24V

Diode D902 shows 18 ohms / infinite across the two pins. No short.

The voltage across the filtering cap does not rise from 157 when I bring PS to 5V. I am stumped and have gone buggy trying to draw out a diagram for this PS. Anyone have a schematic for this board? would be greatly appreciated!

[budm]

Re: Envision/AOC power supply - no +12/24V

At this point, forcing PS_ON and the large cap did not charge up to 400vdc, that means either the PS_ON did not get all the way to the PFC section to turn it on, this PS_ON is also used to turn on the 24VDC switching power supply section. Did you try the hair dryer trick yet?

[ahickman]

Re: Envision/AOC power supply - no +12/24V

Tried the hair dryer on all related caps. No appreciable change in ESR. Might anyone have a schematic for this board? Just too difficult to trace so much detail and not be positive I have overlooked a trace.

I called Envision yesterday and asked for a replacement part number. What they gave me was a part number that pulls up several versions under ShopJimmy for example. There are several board versions for the same number. I am wondering if any of them are interchangeable. For example, the L32W861 and the L32W961 both share the same part number for their PS (24180BB1). From the pics, they look identical. Anyone have experience in this area?

[budm]

Re: Envision/AOC power supply - no +12/24V

So the circuits did not start after heating the caps?

[ahickman]

Re: Envision/AOC power supply - no +12/24V

No sir.

[budm]

Re: Envision/AOC power supply - no +12/24V

Next thing we need will be the P/N for the 3 IC's on the bottom side of the board so we can get the pinout and check the the voltage on the them next.

[ahickman]

Re: Envision/AOC power supply - no +12/24V

Fan7529
ld7522ps
l6599d

[budm]

Re: Envision/AOC power supply - no +12/24V

Wel, lets concentrate on the PFC circuits (FAN7529) first since we know that it is not booting the Voltage up like it supposed to. See the attached PDF and check the voltage on the VCC pin, you must be real careful since we are working on the HOT side with lethal voltage here, isolation transformer and GFCI outlet are recommended. Negative pin of the probe will be connected to the Negative lead of that large 450vdc cap as a reference.
More likely this company will use the same circuits as shown in the application notes.