Gateway fpd1520 Fuse Blown

[Rolin]

I have a pretty old Gateway FPD1520 that blows fuses on the inverter board. I think I have it troubleshot down to a shorted power transistor with part number D1864. The board has 2 of them, I took them both out and they ohmed out differently so I suspect one of them is bad.

3 questions:

1) What is the best way to ohm out a D1864 power transistor
2) What is a good cross to something Digikey has? I see too much confusion, but I think it's a 2SD1864TV2Q
3) I noticed a letter on the back of the chip - one of them is stamped "J" and the other is stamped "A". what does this mean?

Thanks!

[PeteS in CA]

Re: Gateway fpd1520 Fuse Blown

The 2SD1864 is a 3A, 50V bipolar transistor. If what you are looking at is the two inverter transistors, then they aren't 2SD1864s (OTOH, they may be part of the base/gate drive circuit for the inverters). I'm the worst person to be suggesting this, but pictures would be helpful.

[Rolin]

Re: Gateway fpd1520 Fuse Blown

Sorry, I didn't think a pic would be necessary - but I understand. Here is the pic of the transistor I need to test and possibly aquire. Both of these have legs bent so the transistor lays flat on the board, then a piece of metal mounted over them used as a heatsink.

I don't have a pic of the actual board right now, but I have another board just like it that I uploaded so you can see what it looks like (this board works after I corrected a very bad solder connection!).

How do I test these buggers to confirm my suspicions. I think one is shorted...

[PlainBill]

Re: Gateway fpd1520 Fuse Blown

Sorry, I didn't think a pic would be necessary - but I understand. Here is the pic of the transistor I need to test and possibly aquire. Both of these have legs bent so the transistor lays flat on the board, then a piece of metal mounted over them used as a heatsink.

I don't have a pic of the actual board right now, but I have another board just like it that I uploaded so you can see what it looks like (this board works after I corrected a very bad solder connection!).

How do I test these buggers to confirm my suspicions. I think one is shorted...

Arbitrarily number the pins 1, 2, and 3. Set the DMM to the 200 ohms scale. Measure the resistance 1-2 1-3 2-3. If any reading is below 200 ohms out of circuit the transistor is bad.

That certainly seems to be the correct part. Package and marking matches the datasheet. I'd like to see a picture of the inverter. If my suspicions are true there may be a fault causing this to fail.

PlainBill

[jetadm123]

Re: Gateway fpd1520 Fuse Blown

And if you do find that they are shorted, they are available from digikey:

http://search.digikey.com/scripts/Dk...1864&x=15&y=12

[Rolin]

Re: Gateway fpd1520 Fuse Blown

One of the two transistors is about 75 ohms on pins 2-3 so it's shorted. Of course I'll just replace both of them.

Jetadm, you found the same item on digikey I was looking at. Is it 2SD1864TV2R or 2SD1864TV2Q. I assume the 2SD1864TV2Q is better w/ "180 @ 500mA, 3V"?

What does the "A" and the "J" stamped on the bottom of these mean. Why aren't they both "A"s or "J"s? These letters are stamped or really molded into the chip.

Plainbill, I uploaded a picture of the exact inverter above in this thread ( I had 2 of these monitors to fix - kind of helpful to compare both while metering them) or do you need to see the actual one I'm working on? I have a few components removed from the one I'm currently working on while I was testing for the short - so it's kind of a mess. If you want to see the exact one, let me know and I'll do it. If you could look at the one I already uploaded, what did you have in mind as to: "If my suspicions are true there may be a fault causing this to fail."

[PlainBill]

Re: Gateway fpd1520 Fuse Blown

One of the two transistors is about 75 ohms on pins 2-3 so it's shorted. Of course I'll just replace both of them.

Jetadm, you found the same item on digikey I was looking at. Is it 2SD1864TV2R or 2SD1864TV2Q. I assume the 2SD1864TV2Q is better w/ "180 @ 500mA, 3V"?

What does the "A" and the "J" stamped on the bottom of these mean. Why aren't they both "A"s or "J"s? These letters are stamped or really molded into the chip.

Plainbill, I uploaded a picture of the exact inverter above in this thread ( I had 2 of these monitors to fix - kind of helpful to compare both while metering them) or do you need to see the actual one I'm working on? I have a few components removed from the one I'm currently working on while I was testing for the short - so it's kind of a mess. If you want to see the exact one, let me know and I'll do it. If you could look at the one I already uploaded, what did you have in mind as to: "If my suspicions are true there may be a fault causing this to fail."

Sorry, I missed that. Just as I suspected, it seems to be our old friend the Royer Oscillator. By any chance is there a poly or ceramic capacitor of 18 to 33 nF on the back side of the board? Replace check or replace that, replace C99, resolder all transformer pins, and I would suggest picking up at least 4 of the transistors.

As to the other question, the Q part number would be preferred. And the A and J are either production lots or date codes.

PlainBill

[Rolin]

Re: Gateway fpd1520 Fuse Blown

Thanks PlainBill. OK I'll order 4 of the Q's. So get 4 so I have 2 spares - I understand

I don't see a C99, did you mean c9? 16V 33uf Cap?

Funny you should say to resolder all the transformer pins. Remember I said I had 2 of these monitors? The other one was fixed by resoldering a very bad solder connection on the transformer - it must have been arcing for a while because it dam well almost burned a hole in the board! Part of the mounting hole was burned off, so I scraped some area on the trace and bent the transformer leg down and soldered for a better hold.

A 18 - 34nF capacitor on the backside of the board? I see several very small surface mount caps, but I have no idea what their rating are. I took a picture of the back of the board and uploaded for your viewing!

[PlainBill]

Re: Gateway fpd1520 Fuse Blown

You are right, my mistake, it is C9. Hmm, with no timing capacitor this may not be a Royer oscillator. Still, replace the transistors and see what happens.

PlainBill

[Rolin]

Re: Gateway fpd1520 Fuse Blown

Well I received the D1864 transistors and soldered them in, replaced C9 cap, and the fuse. Powered up and it worked! I thought everything was OK, but I heard a buzzing noise, became a little louder, the screen dimmed, then out. I had it on for about a minute total.

Found the fuse blew again. I checked a few things, but eventually replaced the fuse and tried again. It worked again, but the buzzing noise was still there. Tried turning off all lights in the room to see if I could see any arcing - none. Resoldered almost all points on the inverter - nothing helped. After trying different things and failing to isolate, the screen eventually became dim then out. Now the inverter is is out, but the fuse is OK.

I have 2 of these monitors so I've been comparing them to each other. Swapped inverters, the problem follows the inverter - so it's not the backlights on the screen.

I checked voltages at various points - found the D1864 transistors are 12v on every leg, but this compares to the good board when it's in sleep mode. When out of sleep, the bad board is still 12V on every leg, but the good board has various voltages. I assume when the backlight is working, some gate changes the feed to the D1864 which changes the output voltages. So something is preventing the D1864 from kicking in?

I'm at a loss as to what to check next. This is a very old 15" monitor and I really should junk it, but I can't help wanting to fix the darn thing!!

[retiredcaps]

Re: Gateway fpd1520 Fuse Blown

I'm at a loss as to what to check next.

Let's see a picture of the actual inverter board since didn't do it in post #3.

[Rolin]

Re: Gateway fpd1520 Fuse Blown

OK, here's a pic of the actual inverter board. Please pay no attention to my quick, temporary 2nd fuse replacement.

Question: I noticed there was some sort of hot glue, epoxy, or silicone over a few of the solder joints on the coil and high voltage points and maybe a few other areas. I was trying to understand what it was for - what is it for?

[retiredcaps]

Re: Gateway fpd1520 Fuse Blown

Question: I noticed there was some sort of hot glue, epoxy, or silicone over a few of the solder joints on the coil and high voltage points and maybe a few other areas. I was trying to understand what it was for - what is it for?

Just a hunch (or more like shotgunning). I count 8 diodes or zener diodes on the top side of the board. Since there are only 8, it takes only 16 measurements to see if you have a shorted diode or not.

I think it is to protect the user/factory worker from touching them as they are high voltage?

[retiredcaps]

Re: Gateway fpd1520 Fuse Blown

TF1 looks like a thermal fuse?

http://en.wikipedia.org/wiki/Thermal_cutoff

[budm]

Re: Gateway fpd1520 Fuse Blown

Can you please tell me if the Q1 and Q2 are the 2SD1864 that you are referring to? Q1 and Q2 are the transformer driving transistors.

[budm]

Re: Gateway fpd1520 Fuse Blown

@retiredcaps, you are correct, that is the thermal fuse that goes open when the Inverter transformer gets too hot.

[budm]

Re: Gateway fpd1520 Fuse Blown

By the way, if C9 is bad, the DC feeding this board will have high frequency riding on top of the DC and it will beat with the Inverter circuit Oscillator frequency and causes the transistor to run hot and short out.

[Rolin]

Re: Gateway fpd1520 Fuse Blown

Budm: Q1 and Q2 are the D1864's. Both of these have legs bent so the transistor lays flat on the board, then a piece of metal mounted over them used as a heatsink. So you can't see them very well. I replaced the transistors first to see if it would power on which it did, heard the buzzing, then I replaced C9 and also heard the buzzing. I thought the cap could make that buzzing too - not in this case.

RetiredCaps, I checked the diodes with the meter on diode setting (I should have mentioned in my post) - I acutally tested both boards (good one and bad one) side by side and compared readings. TF1 is some sort of fuse - it meters OK, so it is good.

Wondering if the transformer was buzzing? Then eventually went out? How do I check the transformer?

Any more suggestions on this one? BTW: the back of the board is pictured in post #8

Thanks....

[budm]

Re: Gateway fpd1520 Fuse Blown

You will have to use a ringer tester to find out if the transformer has shorted turn or not.
I have SENCORE LC-75.
By the way, C4, C8, C14, and C23 are the tuning caps for this Royer Oscillator circuits.

[Rolin]

Re: Gateway fpd1520 Fuse Blown

Thanks budm. I did compare meter readings from the good board to the bad board on C4, C8, C24, C23 and they look the same on both boards, in ohms the meter charged the caps and stopped at the same reading on both - no shorts, but whether they are truly good or not I guess I don't know. I am a novice and I know this isn't a proper way to check caps, but I really never learned a good way of checking caps with the equipment I have. So I check for shorts and check to see if the cap charges a little with the meter. I assume this will help test them at least a little...

I do have part of the mystery solved though. I removed the transformer from both boards and swapped them. So I now have the transformer from the bad board on the good board and tested - backlights lit, buzzing noise, then the fuse blew! No more testing needed! The transformer was causing the buzzing and overloading the circuit enough to blow fuses. And in the bad boards case, it also eventually blew transistors, and some other unknown component. I swapped back the good transformer to the good board and replaced the fuse and it works without issue. Lucky no further damage to my good board!

Bottom line: It appears the heart of the problem was the buzzing (shorting?)transformer, which blew fuses, and eventually the transistors and some other component. Won't be spending any more time on this one - certainly turned out to be a waste of time on the monitor, but in the end it was also a great learning experience.

On to the next...