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

[ahickman]

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

With black lead on GND and red lead on Pins 7 & 8, I see no DC voltage.

[selldoor]

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

Hi Chaps - I have located a schematic for an AOC L32w831
Which may be a close relation - it has the same Ics that you are looking at.
Hope its some help.

[ahickman]

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

So very good of you. Any tips on where to look for schematics/service manuals?

[budm]

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

Black lead to GND, you mean the negative leg of the big cap, not the chassis gournd right as I suggested? You have to use the negative leg of the 450vdc as a ground point for the meter.
If you look at the SCH page 12 you will see that when the positive voltage feeds PS_ON (S_ON) signal terminal, it will turn on Q934 which in turn, turns on OPTO IC934 LED inside which turn on the transistor inside the OPTO which allow the current to from VCC to flow though the bias resistor R93 to pin 4 of the OPTO and comes out on pin3 of the OPTO which is connected to the BASE (3) of Q932 which will turn on Q932 which will allow the current from VCC to flow to VCC pin of the PFC IC PIN 8 through Emitter/collector of Q932. So if the circuits matches your board, then it should be easy to find out why you do not have VCC at pin 8 of the PFC IC.

[ahickman]

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

I was using the negative lead of the cap as you instructed. I have printed the SCH and taking it home to review along with the board. Below is the diagram I have drawn out. Looks like I have a few things wrong.

[ahickman]

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

Budm, help me out her on reading the schematic. Owing to the segmented sections, I assume the following regarding the "tags" on the peripherals of each circuit:

1. Tags have a directional quality (pointing in/out)
2. The (+) indicator is distinguishing a voltage supply as opposed to those "tags" without the (+). For instance, the output voltages from the dual Schottky (D935) have a 5V and a +5V tag, before and after the inductor. The +5V "tag" is a voltage source?

[budm]

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

The +5V is the first output filtered by C926 from Half-wave rectifier D935 (2 DIODES in parallel) then it fed into the inductor to get rid of the high speed switching frequency then filterd by second filter cap C928. The red Port symbol with one pointed end are connected together with another port with the same name, but I can see a couple mistake on the drawing such as the Emitter and the Collector of Q932, I would have use VCC on the Collector side and VCCout on the Emitter side, you suppose to name the ports and NET name correctly for the circuit board layout. You need to check to see if you have VCC at the Collector (pin2) feeding Q932 first, it should be there since it is generated by the same standby power supply from D933 output as shown in the diagram page 12. the +5V is the out to the connector CN902/903, as you can see, the pin numbers are also messed in the drawing, there are two pin 2 on the drawing and missing pin number on the connector, I do not see pin 1.

[ahickman]

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

When I look at Pin 8 (VCC) on the 7529 and the base (PIN 3) of Q932 with a scope, I see a "shark-fin" (charge/discharge) of about 5 V peak max and 25 ms in duration. I then looked at Vcc at the point between D932 and D933. It waveform followed the peak above in terms of timing. If I am reading the schematic correctly, this VCC is derived from a leg of the AC line dropped down through high resistance (R920-R923) and a rectifying diode? Am I heading in the correct direction for determining why Vcc on the 7529 is null?

[budm]

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

The DC output from the Cathode of the D933 should be nice and clean steady VCC DC. This power supply is generated from a set of winding on transformer 93. as you can see, this transformer is also used for generating the standby +5vdc power supply.
"this VCC is derived from a leg of the AC line dropped down through high resistance (R920-R923)" That must be an error on the SCH, since you do not feed AC into IC 93 VCC pin with an AC, those are kick start resistors and the R922 should have been connected to the Anode side of the Diode. You need to verify the connection in that area, chek to see if R925 is OK or not.
There are so many error in this SCH: missing pin number on the connectors, missing value on the caps for VCC, ETC.

[ahickman]

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

Between R923 and D933, the voltage shows the same "shark-tooth" pattern in terms of voltage. R925 (in circuit) appears to be 5 ohms. SMD markings are 4A7 and schematic say 47 ohms.

[budm]

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

"R925 (in circuit) appears to be 5 ohms. SMD markings are 4A7 and schematic say 47 ohms." Boy this really bad schematic drawing! Getting hard to tryst the drawing.
Back to the VCC, you should have clean DC on the C96, if you are getting large 5vac ripple on that filter cap, then you need to check the cap out.

[budm]

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

By the way are you getting CLEAN +5vdc on C928 right? The scope ground probe is on the negative leg of C907 right, and again, you are working on the lien side so be real careful.

[ahickman]

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

With ground probe on neg of cap and probe lead on C928 (positive term), I am getting trash. Before, I had used the ground pin on the pinout where +5V standby is available. Ok, explain why I use the negative leg of the filter cap which is on the other side of the transformer???

[budm]

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

When you work on the hot side, you use the Negative lead of the main dc filter cap C905) of the hot side, this will be the ground ref point when making any measurement on the hot side.
When you work on the DC cold side (isolated from the hot side) C928 will be used for ground ref point for the cold side when you are making any measurement on the cold side.
So to look at the wavform on C928, the probe gorund will be on the negative leg of C928, the input pin of the probe will be on the Positive leag of C928.
When you want to work on the hot side, you will have to move the ground probe to the hot side ground ref.

[momaka]

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

When you want to work on the hot side, you will have to move the ground probe to the hot side ground ref.

Correct, but you're forgetting something - the negative probe on most oscilloscopes is grounded. Connecting the negative oscilloscope lead on hot ground will short it to ground!
- Obviously something you don't want to do.
The only way around this is to use an isolation transformer.

[budm]

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

Not forgetting at all, that is why when you work on the hot side of the circuits you will have to use Isolation transformer on what you are working on, I ALWAYS warn people when work on the hot side of the circuits!. I use Isolation transformer, GFCI outlet when I work on line operated equipment.

[ahickman]

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

OK, a little confused now.

Correct me where I am wrong please:

PS boards have three basic sections when considering signal grounding: a.) AC mains, using chassis as GND, b.) primary or hot side / post rectificed AC, using negative leg of main filtering capacitor as GND, and c.) secondary (cold) side of transformer, using chassis as GND.


Is this close?

[budm]

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

On the AC side, you have HOT, Neutral, and SAFETY GROUND, neutral on your outlet in your home is tied to GROUND BUSS BAR in your breaker panel which is tied to Earth ground.
When the AC is applied to the bridge rectifier in the HOT side of the power supply, the Positive output of the bridge is connected to the plus side of the main DC filter cap, the Negative output is connected to the negative leg of the main DC filter cap, this negative lead of this cap is the reference Ground for the primary side of the circuits, this ground ref of this side IS NOT TIED TO THE CHASSIS. The ground wire (green) of the AC POWER CORD is tied to the chassis of the monitor and its whole purpose is the safety ground which will carry the fault current to ground.
On the other side of the switching power transformer is an isolated out put from the primary side, the negative leg on the DC filter caps of this side is called circuits ground ( if it uses negative ground system, which most of them are), this circuit may or may not be attached to the chassis, it depends on what the products is designed for.
I hope it is clear enough, bottom line is that always plug what you are working on into the isolation transformer.

[ahickman]

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

Just wanted to update this thread a bit. The original power supply was damaged beyond my ability to identify and repair. After finding three defective components, I threw in the rag and purchased a replacement PS from ShopJimmy for $29 US. It was not an exact replacement, but looking over the schematics for the ENVISION L32W961, I took a chance and ordered it. The only difference between the PS for the L32W861 and the L32W961 was the wiring harness. I simply rearranged the order of wires to match the original and tested the set. So far, so good.

[Giff1711]

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

Hope you sorted your problem.
I was looking for a little help seen you have the same p/s has me my fuse keeps blowing, i've narrowed it down to a resistor at R915 the pink one just in front of the large cap. Can yo tell me the markings on it (value,watt) mine has lost its coating so i have no idea. i can't test has its reading open and like you can't find schematic.
Any help would be great..