Benq G2255 doesn't power on - amber power light when plugged in

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in

Will, just in case (it might have no bearing) but R802 is a 304 resistor which is 300K ohms, and I get 25K ohms across it?

Ok, for resoldering the 2 pin and removing jumpers. I will also resolder every through hole component from the transformer pin 7 to CN801, including CN801.

Thanks again for your incredible support and knowledge!

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in

Something quick to check... R838, R839 & R837 are used in the current detection circuit on cn801. R842, R843 & R841 are for the cn802 circuit. The pairings below should be very near each other on resistance check:
R842-R838
R843-R839
R841-R837

Also, is D801 marked "A1"?

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in

Pairing of :

R842-R838
R843-R839
R841-R837

read the same value.

Also, is D801 marked "A1"? Yes it is marked A1.

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in

Replying to your question about R802:
C805 and R802 go to pin 7. That is your enable pin. C805 is to remove ripple from the signal, and R802 is pulling the pin low to keep the chip off until it receives a sufficient voltage from your main board. We know the chip is running, so R802 is not a problem.

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in

The attached is a pin-out on that diode. Please check it. If not 100% sure on checking, watch a video on checking a diode.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in

These are the readings I get on D801.

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in

You're going to have to lift the top pin, or take the diode out of circuit. In diode mode, reverse biased (red to either of the bottom pins) meter should read OL.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in

Hi Will,

I'm a little nervous about removing the diode or lifting the top pin, because last time I worked on a diode I guess my iron was too hot, so I killed it. Do you have any advice on how I should heat it (like not more than XX degrees)? I checked my inventory of used parts (motherboards, pcb boards, etc.) and didn't find another A1 diode, so this is the only one I have. If I kill it, I could get some from Digikey, but any advice you can give me on how to work it would be welcome! Tks!

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in

Before attempting the diode lift, check your other boards for same size diodes, and give me the markings on them. In the mean time, I'd like to rule out connector problems. Going to need you to take some resistance readings. With lamp connectors plugged in, back probe each wire and take resistance to the points in the pic. Just give me color and resistance. If you don't have back probe tips, put an alligator clip on a pin or needle. Follow the wire in from the back of the connector until you hit the metal barrel.

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in

For the diode lift/removal, try ~350c. Go a little higher if solder is not melting quickly. Adding flux and fresh solder helps a lot. Practice on your old parts boards first. Watch the video below. We're going to have to get back to that diode pretty soon.

https://www.youtube.com/watch?v=LADgxZgyJ04

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in

ok,

Blue: .3 ohm
Red: .4 ohm
Yellow: .4 ohm
Black: .4 ohm

Some other diodes same size I have on boards are:

A7W (lots)
DB6
5D
AAAA (not sure if this is a diode)
3D
5C
B6

and probably some others.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in

Ok, Will thanks for the advice. I practiced on a few diodes on scrap boards with flux and fresh solder and wick and it's working! I feel much more confident. Had my iron set at 350 degrees with a flat tip and it works pretty well.

Ready when you say go!

Tks

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in

No problem with the connectors. Try lifting that top pin on d801 and recheck
the diode. I'll look into those markings that you listed.

Also, can you give me the resistor values for those 3 pairings on post #143?

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in

Here you go for the resistor values. Off to my surgical task of lifting that leg. Will report in a few minutes.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in

Ok, Will, checked the diode. Lifted the pin, was fairly easy with your guidance.

Measuring in diode mode, black to lifted pin and red to either of the 2 other pins I get 0L. When I reverse the probes, I get .580V on one and .582V on the other.

Does that mean the diode is good?

Will, you mention in an earlier post while we were diagnosing with the jumpers on pin 2 and 3 and 1 and 16 that we have an over current problem? Can we not measure current at different points on the trace of CN801 to see where it might be coming from while we jumper C803? Probably a dumb question, but my Fluke measures milliamps.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in

Another thing to consider. On CN801, coming from the transformer there is a blue cap (C831) on the trace from pin 7 of the transformer going to ground and it is marked
10J
3KV


Whereas on the CN802 connector line coming from the transformer pin 8 I see a blue cap (C834) on this trace to ground marked
8
3KV
SEC

Why different values? Shouldn't they be the same?

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in

Diode checks good, you can put it back down. The chip is measuring amperage by checking the voltage drop across a known resistor, so it's actually looking for the voltage to be in a certain range.

Might be a good thing to check the actual current on each connector so that we can see if there is a significant difference between them, or the chip just thinks there is because of an out of range component in the circuit.

You'd have to lift the end of R841 and R837 (one at a time), and measure current between the lifted end of resistor to the pad it was lifted from. Power on, check lights and measure amps. (jumper on pin5)

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in

Will, also in post #127, did you notice Vmax on pin 2 and 3 with the cfl's in the original position? Quite a difference in voltage?

Or is it my oscilloscope that is not that accurate?

Will try to do the current testing (never have done current testing in the past, I think I need to change where I plug my probes in the Fluke, but I'll check) probably tomorrow night. Having a snow storm here which will last overnight so will have to get on the road early so as to not be late at work.

Have a good evening and talk to you tomorrow. And again, thank you!

[will62]

Re: Benq G2255 doesn't power on - amber power light when plugged in

C831, C833 & C832 form a capacitive voltage divider for the chip to monitor voltage on cn801. The values are critical. You can see that they had to parallel
C833 and C832 in order to get an accurate divider. C834, C835 & C836 serve the same purpose for connector cn802. We don't have a voltage problem on the chip, so I would leave well enough alone with all 6 of those.

What I am curious about is C817. What is it marked, and do you have a possible replacement on another board?

Edit: I'll look back at those voltages. You're welcome and stay safe on those roads.

[rddube]

Re: Benq G2255 doesn't power on - amber power light when plugged in

C831, C833 & C832 form a capacitive voltage divider for the chip to monitor voltage on cn801. The values are critical. You can see that they had to parallel
C833 and C832 in order to get an accurate divider. C834, C835 & C836 serve the same purpose for connector cn802. We don't have a voltage problem on the chip, so I would leave well enough alone with all 6 of those.

Ok, but C831 and C834 don't have the same values. But the other 2 caps on each line looks to be excactly the same (they are SMD so I am going by color and size) on each line.

What I am curious about is C817. What is it marked, and do you have a possible replacement on another board?

In fact C817 is marked the same as C834
8
3KV
SEC

and it is close to C831. Could they have inadvertently been swapped during fabrication? Should I try and swap them around?


No, I don't have another a replacement for 8 3KV SEC, in fact they don't have them on DigiKey either. Momaka mentionned in thread #111 "It's probably an 8.2 pF capacitor (as that's a standard value).", but I don't have any marked exactly like that. The spares I have are marked 5 on another inverter board and all sorts of marks on caps from scrap power supplies (I can see 3KV) at the AC entrance, but not sure what the other values are. I can unsolder some and give you the exact markings if you want me to.