Hp Vs17

[Oklahoma Wolf]

Ok, I've about had enough of this one. The patient, an HP VS17 (PK657AA), was given to me by a friend and is determined to send me to the crazy house.

Initial symptoms were the thing would turn on for a couple minutes and then go dead like the power was cut. Power button would not turn it back on. I went through it and touched up every bad solder joint I found, and replaced several capacitors.

That got it to run for a couple hours instead of a couple minutes, but it would always go dead again. Pointing a fan at it made it stay on indefinitely, but then if you tried to turn it off with the button it would play dead again. It would not power up again unless you cut the power to it for a while.

Replaced the 47uF 50V capacitor near the FAN7601 PWM controller. That got the power button to work properly, so I figure I'm getting closer. Measured the voltage at the Vcc pin on the PWM... 61 volts. Seems a bit high - the chip is rated for 20V. That's gotta be part of the problem.

But, I'm now at a dead end - not sure what's causing the Vcc pin to get that much voltage. I have a part on a heatsink I can't ID - picture attached (it's the MOSFET dead center in the pic). 5V output reads 6.31V, 12V output is at 12.01V. 12V happens at the regulator at the very top of the board, next to the 5V and 25V output heatsink. 25V output reads just under 25V.

Anyone have any thoughts on where next to look? Any help is appreciated.

[Oklahoma Wolf]

Re: Hp Vs17

Nobody has any ideas on this one? I can't decide if this is more likely to be issues with the FAN7601 or the highish 5V supply. This one keeps tying my head into knots when I try to trace anything on it. Tried to find a Schottky I can use to replace the 5V part just in case, but no luck there.

I'm tempted to hack a little fan into the 12V supply and just leave it as is. It's not exactly the best LCD monitor out there anymore, and not really needed at the moment.

[Scenic]

Re: Hp Vs17

sounds like some component is getting too hot and goes into thermal protection.
trace down where the VCC voltage for the PWM controller is coming from and why its so high..?

also.. if the voltage on the VCC pin really is 61V, the 47uF 50V cap isn't going to last long i'd say..

edit: just looked it up

I have a part on a heatsink I can't ID - picture attached (it's the MOSFET dead center in the pic).

http://www.alldatasheet.com/datashee...AP2761I-A.html

[Oklahoma Wolf]

Re: Hp Vs17

That's what I thought. The topology of this one makes it hard to see what's running hot and what isn't due to short cables between parts. I need to find a way to run it so I can see what's overheating.

The supply for the Vcc seems to be coming directly from pin 1 of the bridge rectifier, a U4KB80R. I checked that pin four times when I saw the 61V... couldn't believe my eyes. Maybe I was checking the wrong pin, but I was going by the datasheet so I don't know what to think there.

[Oklahoma Wolf]

Re: Hp Vs17

Some more progress made. As it turned out, I remembered wrong about how the inverter board mounts - it was easy to check the running temp of the components.

Observations - the only chip running hot is the GM2621 display chip. It wasn't bad when it first fired up, but after a while my brain made me take my finger off the thing. I could have held it longer, but it was getting uncomfortable. In the below pics, you can see the display board. On it, there are a ton of Capxons. Don't have replacements for any of them.

There's an AIC1084-33PM on the upper left - I got 6.31V at the input and 3.31V at the output. Next to it, there's an LM1117-18PE - got 1.8V at the output. The IC by the VGA connector is a TDA7496SA that was barely warm at all - has lots of heatsink.

I just wonder if those Capxon caps need to be swapped, if the 5V input is just too high, or both. Nothing on the inverter board was running hot, including the PWM controller.

Any more comments?

[pedro]

Re: Hp Vs17

Some more progress made. As it turned out, I remembered wrong about how the inverter board mounts - it was easy to check the running temp of the components.

Observations - the only chip running hot is the GM2621 display chip. It wasn't bad when it first fired up, but after a while my brain made me take my finger off the thing. I could have held it longer, but it was getting uncomfortable. In the below pics, you can see the display board. On it, there are a ton of Capxons. Don't have replacements for any of them.

There's an AIC1084-33PM on the upper left - I got 6.31V at the input and 3.31V at the output. Next to it, there's an LM1117-18PE - got 1.8V at the output. The IC by the VGA connector is a TDA7496SA that was barely warm at all - has lots of heatsink.

I just wonder if those Capxon caps need to be swapped, if the 5V input is just too high, or both. Nothing on the inverter board was running hot, including the PWM controller.

Any more comments?

Presuming that the 6.31V is the 5V rail, I'd be keen to get that under control even if the two regs aren't smoking. It *may* be the cause of the Genesis chip warming unduly.

Also any time I go to the trouble of opening a monitor, I automatically replace any CapXon (or other dodgy/bad_rep) caps I find.

The TDA chip is the audio - it's idling now so I wouldn't expect any significant heating.

[Oklahoma Wolf]

Re: Hp Vs17

Hmm... well, the 5V rail has been recapped - it's the only one I even had the replacement caps for. It's still at 6.31V. I've traced it back to D810, an SB1060FCT which in turn connects right to transformer T802. I'm not sure exactly where else to look for the problem. I don't have a replacement for D810 that I know of, and am not sure where to buy one. Lots of larger Schottkys, but not many that will fit this thing.

Edit - I found two STPS2045CT's - any comments on them as a suitable alternate?

[pedro]

Re: Hp Vs17

Hmm... well, the 5V rail has been recapped - it's the only one I even had the replacement caps for. It's still at 6.31V. I've traced it back to D810, an SB1060FCT which in turn connects right to transformer T802. I'm not sure exactly where else to look for the problem. I don't have a replacement for D810 that I know of, and am not sure where to buy one. Lots of larger Schottkys, but not many that will fit this thing.

If the system powers up the 5V line under load then the Schottky is working OK. I'd suspect something is amiss in the feedback part of the loop, upsetting the comparator function that sets the SMPS pulse width.

[Oklahoma Wolf]

Re: Hp Vs17

That's what I was afraid of.

Dug a little deeper into the unit. The feedback circuit uses an AP431 shunt regulator, it looks like. Replaced the 4.7uF 50V Capxon attached to it - no change. Ref pin is attached to R824 - I measured 6.31V at one lead and 1.8V at the other.

I have a strong feeling the AP431 is defective. There's a variable resistor in the circuit - adjusting it does nothing. It's connected via R827 to the ref pin and R824. Other than that there are just a few resistors and capacitors in the circuit... can't find any issues with any of them.

I have no idea where to get one of these either. eBay has nothing.

[Oklahoma Wolf]

Re: Hp Vs17

/smacks forehead

Cross referencing... of course! I now have ten LM431's on the way. Will now wait and see what happens when I replace the regulator.

I'm not actually sure this is the PWM feedback circuit - it connects to it, but seems to only be there to regulate 5V supply. The fault has to be here somewhere since the 25V supply, also directly connected to the transformer, is working properly.

[Oklahoma Wolf]

Re: Hp Vs17

Well, I think I may be giving up on this one. The replacement for the AP431 helped nothing. A nearby resistor (R824) that was supposed to measure 20k ohms read 34k instead, but replacing it had no discernible effect on the 5V output.

I'm now thinking the area I've been working on is in fact overvolt protection for the 5V rail. Don't know what else to try. The 5V Schottky connects directly to the transformer, which in turn connects to inductors on the PWM control side.

[PlainBill]

Re: Hp Vs17

The application note for the SMPS controller is here. It should give you some information on what is happening.

Referring to that, the 431 (IC201) is a programmable voltage regulator. The voltage is set by R204 and R205. When the voltage at the junction rises above the reference point (2.5 volts) IC201 starts to conduct, turning on the source in IC301. This caused the detector in IC301 to conduct, pulling pin 2 high, and reducing the pulse width to Q101, and thus the output voltage.

In your case, the obvious place to check is the output of the opto isolator.

PlainBill