Help with soldering

[sceva]

Not sure if this is the right place to post this, so Mods feel free to move it if you wish.

I need to test a couple of transistors, but they are the SMD style, and I can't figure out how to remove them from the circuit board. In the picture are two of the components, the easiest to view is almost in the bottom right corner, labeled U7. I have been reading about some desoldering techniques, like using something called ChipQuick, or adding more solder to the solder points and heating them with a couple of irons and picking it up... But I wanted to get some advice before I possibly damaged the board. Thanks.

[Solder Boy]

Re: Help with soldering

Depending how new the board is, it could be lead free solder which requires a higher temperature to melt.

I'd use a decent iron with oomph and use a soldering wick, just hold the iron to the board long enough to melt the solder on the pins and use a dental pick or a very tiny precision screwdriver and tap the part off the board.

I read somewhere when you replace the part, use a tiny bit of contact cement to hold it in place while you solder it back in.

Oh ya practice on a board you don't care about before you touch this one

[PlainBill]

Re: Help with soldering

Not sure if this is the right place to post this, so Mods feel free to move it if you wish.

I need to test a couple of transistors, but they are the SMD style, and I can't figure out how to remove them from the circuit board. In the picture are two of the components, the easiest to view is almost in the bottom right corner, labeled U7. I have been reading about some desoldering techniques, like using something called ChipQuick, or adding more solder to the solder points and heating them with a couple of irons and picking it up... But I wanted to get some advice before I possibly damaged the board. Thanks.

That package is very difficult to remove. The two legs are fairly easy, especally using Chip-Quik. It isn't going to be so easy unsoldering the tab. It and the large copper area it's soldered to absorb a lot of heat. Chip-Quik may help, but it will require a high wattage iron.

One point - it should not be necessary to unsolder the transistor to test it. It looks like there may be a piece of solder between the right leg of the transistor on the left and the trace going to the left leg of U7. Remove that, then use a DMM set on the lowet Ohms scale (typically 200 ohms). In the case of U7 and it's complement to the left, measure the resistance from the tab to the leg on the lower right, then from the tab to the leg on the lower left, and between both legs. All should show a resistance of more than 200 ohms. If any show a resistance of less than 1 ohm they are probably shorted.

PlainBill

[sceva]

Re: Help with soldering

Depending how new the board is, it could be lead free solder which requires a higher temperature to melt.

About what temp? or what wattage iron should I use?

it should not be necessary to unsolder the transistor to test it.

Awesome!! I have only used DMM's ohm settings to test for open/closed circuits in my past careers, so I am trying to get my head around testing resistance and the different settings on the DMM. I did as you suggested, and the meter showed what I would call an open circuit - no continuity - when testing all the leads. I hope this means they are fine?

[PlainBill]

Re: Help with soldering

About what temp? or what wattage iron should I use?


Awesome!! I have only used DMM's ohm settings to test for open/closed circuits in my past careers, so I am trying to get my head around testing resistance and the different settings on the DMM. I did as you suggested, and the meter showed what I would call an open circuit - no continuity - when testing all the leads. I hope this means they are fine?

There is a 90%+ chance that the transistors are fine. Certainly they are not shorted, there is a possibility that there is an open junction if you are dealing with bipolar transistors. Try doing the same test, but with your meter on the diode test function.

PlainBill

[sceva]

Re: Help with soldering

With the diode test I get a reading of .19 to .20 on each of the transistors with the black probe on the tab, and the red on the rightmost pin. When I reverse the probes on the same pins it starts at 1.xxx and goes to 1.0.

Going from the tab to the left pin it reads 1.0 both ways.

Going from left to right pin with black probe on left it reads between .62 and .64. Switching probes it starts at 1.xxx and goes to 1.0.

[PlainBill]

Re: Help with soldering

With the diode test I get a reading of .19 to .20 on each of the transistors with the black probe on the tab, and the red on the rightmost pin. When I reverse the probes on the same pins it starts at 1.xxx and goes to 1.0.

Going from the tab to the left pin it reads 1.0 both ways.

Going from left to right pin with black probe on left it reads between .62 and .64. Switching probes it starts at 1.xxx and goes to 1.0.

Those results indicate you have FETs (you can check the part number to make sure) and they appear to be good.

Now the question is: What problem are you having?

PlainBill

[sceva]

Re: Help with soldering

I was successful in removing my first SMD today, although not the above one, as Bill helped me see it was ok. It was a small chip with 4 legs on each side. I used the glob a lot of solder on each side and use two irons to pick it up, and then clean it up with solder wick.

Now the question is: What problem are you having?

LED comes on, but no display. I will open a new thread for it when I have a chance to work on it again.

Thanks

[PlainBill]

Re: Help with soldering

I was successful in removing my first SMD today, although not the above one, as Bill helped me see it was ok. It was a small chip with 4 legs on each side. I used the glob a lot of solder on each side and use two irons to pick it up, and then clean it up with solder wick.


LED comes on, but no display. I will open a new thread for it when I have a chance to work on it again.

Thanks

Attach pictures of the top and bottom of the board. I'll point out a few places to check the voltages.

PlainBill

[sceva]

Re: Help with soldering

Attach pictures of the top and bottom of the board. I'll point out a few places to check the voltages.
PlainBill

This is a Viewsonic VX2025. The backlights never come. The connection from the power board to the inverter board has these readings:

20.1v, 20.1v, 0.0, 0.0, 0.0, .3, 2.6v

I replaced 3 caps on the board at C1-C3 with LXY 180uf, 35V, which is what the originals were. An SMD fuse labeled F1 with TP printed on it was blown. I could not figure out what it was, so I soldered a wire to bypass it... Probably not the best idea, but I took the chance.

There are 6 transformers, and each of them have .3v at 4 of the 6 leads on the primary side, nothing on the secondary side.

At the Inverter IC, which is a bd9883f, reads:
#1 - .5v - Duty - control pwm and burst mode.
11 - 2.6v - STB - stand by switch
18 - 1.5v - BST - bootstrap input for HN output
20 - 1.9v - VCC

all other leads are 0.0

btw, I want to say thanks not only to the guys who respond to this, but to everyone who contributes to this forum. I have learned a lot!! Thank you!

[Toasty]

Re: Help with soldering

TP = Slo-Blo 3 amp

https://cdn.badcaps-static.com/pdfs/...5edad6270b.pdf

[sceva]

Re: Help with soldering

Thanks Toasty. Any ideas on what might be wrong?

[Toasty]

Re: Help with soldering

Check the FET's U2, U3, U6, U7. Shorted? Right inline with the fuse...

Diode D23 or ?D28 (in shadow) checked?

[sceva]

Re: Help with soldering

Diodes are ok.

The two outermost fets read the same, and the innermost read the same. Not much difference between the two pairs...

[Toasty]

Re: Help with soldering

hmmmm...

Problem with power supply not sending "on" voltage? Or not enough?

You didn't indicate wire color to voltage. Is this correct?:

Green - 20.1v (vin)
Yellow - 20.1v (vin)
n/c - 0v
Orange - 0v (ground)
Red - 0v (ground)
Brown - .3v (adj)
Black - 2.6v (on/off)

Does it read the same with the connector off? Use red/orange for neg probe.

[sceva]

Re: Help with soldering

You are correct with the colors. With the connector off the only change is the on/off reads 3.4v.

I want to make sure the transistors are ok. I tested them on the board and with the dmm on 200k I get:
gate/source - 30.4 on the two outermost and 32.3 on the two closest to the center.
gate/drain - it starts in the 30's or 40's and ramps up
source/drain - starts around 0 and ramps up

when I switch the leads I get the same results.

I think they are FET transistors.