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

[will62]

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

I'm going to add another thing to check, as it may be later in the evening before I can get back on. There's a lot covered in these 2 posts, so just take your time and do everything. No need to post back until all has been checked.

When you had pin 2 lifted, and pin 3's input jumped to it, the lamps came on. I want to make sure that I didn't jump to conclusions that it was pin 2's input causing the problem. It could be the difference between pin 2 & 3's inputs when they hit the internal pull up current source, "OR" gate and comparator.

Lift pin 3, jump pin 2's input to it. Do the lamps come on? If they do, swap lamp connectors.

[will62]

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

The diode is rated in mA, so try that first. Yes, lift one resistor at a time. The measurement you got on the 8pf is over 300pf. Does the new 10pf measure higher? Not sure if you can accurately measure these HV caps with a DMM.

The waves on your scope were all over the place. Wondering if the pic below showing LI1(p2) with the larger negative swing was a fluke, or if you can replicate that consistently. The way the diode is set up, it looks like it should be clipping some of the negative portion of the wave. Maybe also probe at the voltage divider (between R837,R839 and then between R841,R843 for LI2). Both LI1 and LI2 similar there?

Another thing that I was wondering about, is when you first changed the mosfet to the under rated AF4910N that you had on hand, you said the lamps came on for about 10 seconds. Now that you know how to properly check a mosfet, are the original AP9962 and the AF4910N actually blown? Is there something about the new mosfet that is causing spikes or something on the primary side? Maybe probe each drain, then probe the primary side of the transformer (probe to one side, probe ground to other). See 3rd pic below for a spike that looked small, but was blowing mosfets on a DC motor controller that I made (disregard notes).

As to the VI monitoring diagram above, no, I'm not a CD (we'd wrapped this 8 pages ago). I'm learning just like yourself. You can open a board pic in paint.net, add a layer on top, and trace circuit. You delete the pic (background), and you're left with the tracing.

[rddube]

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

I guess next step is to measure current at 837 and 841, one at a time one end lifted? Should I use mA or uA on my Fluke?

[rddube]

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

Power came back at 4:00am this morning. Sorry for the late reply.

Ok, built Frankie and measured it, it gave .30nf, whereas C817 measured .32nf. Tried Fankie in C817, lights go out after 2 seconds. Switched Frankie to C834, same result. Recuperated the new 10pf from Frankie, tried it in C831....unfortunately, same result. C834 original measures .31nf and C831 original measures .30nf (all these readings done with my Fluke).

[rddube]

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

Sorry Will as I was building Frankie, power outage and no estimate from Hydro when it will be back

[rddube]

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

Will you are a science rocket man!! Wow, I can't believe we are going to make a Frankenstein cap!!

Ok, ran the test as you suggested....left the lights on for approximately 15 seconds than pulled the jumper and the lights went out about 2 seconds later, just like when we put them on without the jumper.

Ok, so I'm off creating the monster.

BTW, who drew the schematic? Is it you? Pretty nice, are you a circuit designer in your work?

Tks Will...I'll report in a few.

[will62]

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

I went back over the circuit. The chip is shutting down because of the LI1(pin2). The current monitoring circuit is pretty basic, and you have checked all of the resistors and the diode, so no obvious component problem causing a bad current measurement. As I mentioned before, HV caps C831 and C834 are used in capacitive voltage divider. If they were bad, I think the chip would have shut down when we had LI1 lifted.

I'd like to rule out C817 (8pf), but you don't have an extra 8pf, so we'll have to make one. Take 2 of those 5pf 3kV caps off the other board. We'll use your new 10pf and 2 of the 5pf to get to 8.3pf. You need to solder one of the 10pf legs to a 5pf leg. These will be in series. Then solder the other 5pf to the outside legs of the 10/5 series, this will be parallel. Confused yet? I'll add a sketch. Try this Frankenstein cap in 817. If it doesn't work, move it to 834. If that doesn't work, put the original 8pf's back in place, pull the 10pf off of Frankenstein and try it in 831. That's the only way I can think of to check all 3 HV caps.

Before doing the above, go ahead and run a quick test for me. Jump 5 to ground. Power up and look at lights. After they've ran for a few seconds, un-clip the jumper. Do the lights go off immediately?

[rddube]

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

ok, here are the HV cap values:

near the cfl connectors:
C834: 8 3KV SEC
C831: 10J 3KV
C817: 8 3KV SEC

near the AC input:
C603: SRC 102M SE 250V 21 C
C602: same as C602

near the yellow transformer:
C724: 404
C615: same as C603 and C602

In my spares, I have a brand new 10J 3KV that I ordered from Digikey with the mosfet and electrolytic caps that I changed. I have some 5C 3KV (not new), 102M KX250 X1Y1, 472M KX250 X1Y1 that are on another scrap power board from an HP monitor.

All the other HV caps I have are from ATX power supplies. Do you want those values too?

[will62]

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

I'm pretty sure that if any out of those sets of 3 were out of spec, the chip would shut down due to voltage readings. When we had pin 2 lifted/jumped, the lights stayed on, so I would say that the voltage readings were within spec.
I'll look back at the circuit to see if I didn't notice something.

You can try that current test mention earlier, but please don't change any caps yet. Can you give me all the HV caps and values? Also, possible replacements on other boards.

I'll get back on in the evening after I've had a chance to check over circuit.

[rddube]

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

Hi Will,

Since you said "C831, C833 & C832 form a capacitive voltage divider for the chip to monitor voltage on cn801. The values are critical", is set out to try and measure C835 & 836 / C833 & C832 with my Fluke multimeter, but wasn't getting any reading.

So I took my transistor tester with which has a capacitance function (and an ESR in circuit function, but I didn't use this one, I used the capacitance function only), and with my probes checked them. Here is what I got and not sure if this little machine is accurate, but it was constant:

C835 / C836 : 42.8 uf ESR .15K Vloss 14%
C833 / C832 : 143.2 uf ESR .15K Vloss 12%

I think these are in pairs, because I would get the same reading say from C835 and C836, the same for the other 2.

Not sure if this can have a meaning of some sort?

[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.

[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

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

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

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?

[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

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

[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

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

[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.