HP 1955 Monitor Repair Guide

[retiredcaps]

Re: HP 1955 Monitor Repair Guide

The logic board is the board with the the connectors on it? (DVI, VGA, USB etc.?)

Correct.

In addition, can you just restate what the latest activity of the power LED and screen is? With the number of components being removed and tested, I just want to see what the current state is. Please also connect a working computer to the LCD to view your desktop.

[krantz]

Re: HP 1955 Monitor Repair Guide

The logic board is the board with the the connectors on it? (DVI, VGA, USB etc.?)

[retiredcaps]

Re: HP 1955 Monitor Repair Guide

Though Im guessing this is a dead end with this board?

1) Let's get a picture of the logic/main board.

2) What are the DC voltages on the connector from the power to main board? List all the pin number, its color, and the DC voltage. If there is a legend detailing the voltages, please also post a picture of that.

[krantz]

Re: HP 1955 Monitor Repair Guide

Oh, haha. Sorry.

Though Im guessing this is a dead end with this board?

[retiredcaps]

Re: HP 1955 Monitor Repair Guide

Ok. In that case I only get 1 on the left hand side of the multimeter display.

I have allready taken pictures of the board on top and underneath and even given links to the full resolution pictures on my webserver.

My sig file is a standard part of the post. It doesn't apply to you.

If you get "1" or out of range, then those components are likely NOT shorted.

[krantz]

Re: HP 1955 Monitor Repair Guide

There seems to be a lot of people who are mis-interpreting their multimeter readings.

1) A multimeter always gives a reading. "1" on the left hand side of the display means the reading is "out of range". 1.0 on the right side on the display is a unit of measure (i.e. 1 unit of measure). As you as you turn on the multimeter, it gives you a reading. There is no such thing as "nothing".

2) When testing for a short, put your multimeter on 200 (two hundred) ohms. Any reading under 30 ohms suggests a short. If your multimeter reads "1" on the left hand side, it is telling you that the resistance is greater than 200 ohms (out of range for the dial setting). At this point, we don't care what the actual resistance reading is. We know it is NOT less than 30 ohms and thus it is NOT shorted.

Ok. In that case I only get 1 on the left hand side of the multimeter display.

I have allready taken pictures of the board on top and underneath and even given links to the full resolution pictures on my webserver.

Thanks for the info about the multimeter btw!

[retiredcaps]

Re: HP 1955 Monitor Repair Guide

I get nothing on the transistors at that measuring, jetadm123

There seems to be a lot of people who are mis-interpreting their multimeter readings.

1) A multimeter always gives a reading. "1" on the left hand side of the display means the reading is "out of range". 1.0 on the right side on the display is a unit of measure (i.e. 1 unit of measure). As you as you turn on the multimeter, it gives you a reading. There is no such thing as "nothing".

2) When testing for a short, put your multimeter on 200 (two hundred) ohms. Any reading under 30 ohms suggests a short. If your multimeter reads "1" on the left hand side, it is telling you that the resistance is greater than 200 ohms (out of range for the dial setting). At this point, we don't care what the actual resistance reading is. We know it is NOT less than 30 ohms and thus it is NOT shorted.

[krantz]

Re: HP 1955 Monitor Repair Guide

I get nothing on the transistors at that measuring, jetadm123

[jetadm123]

Re: HP 1955 Monitor Repair Guide

The tranformer values seem a bit low, but are pretty consistent, so we'll assume that they're good.

Concerning your values for the signaling transistors. I'm assuming your testing the 3904 and 3906 transistors?

Let's try a different test for these transistors. Again, with your meter set to resistance, touch the black and red probe tips together. You should see something between 0 to 1 ohms. This is what you should expect to see if something were shorted. Now, check the transistors again with your probes across the following:

E-B
B-C
E-C

If you see a zero reading, then you have a short between those two legs of the transistor.

[krantz]

Re: HP 1955 Monitor Repair Guide

There are 4 inverter transformers on your board. They're the yellow rectangular devices with multiple pins coming out each side. They appear to be labeled T801-T804. The secondary pins are numbered 8 and 10. With your meter set to resistance, measure across these two pins and post your results. Difference between the 4 transformer secondaries should be no more than 3%.

I start the measuring from the top transformer...

188.6 ohms
184.3 ohms
187.1 ohms
188.5 ohms

[jetadm123]

Re: HP 1955 Monitor Repair Guide

I measured the SMD 4-pin 4431 Backlight ON/OFF FET's. From drain to source I got 0.535v-0.540v, and from source to drain i got overload.

I measured the inverter fuse and it gave the resault of 0.05 ohms.

But what is the "secondaries of the inverter transformers" and where do I find it on the board?

There are 4 inverter transformers on your board. They're the yellow rectangular devices with multiple pins coming out each side. They appear to be labeled T801-T804. The secondary pins are numbered 8 and 10. With your meter set to resistance, measure across these two pins and post your results. Difference between the 4 transformer secondaries should be no more than 3%.

[krantz]

Re: HP 1955 Monitor Repair Guide

I measured the signaling transistors that also is in the repair guide and I got some strange readings.

The left top gave me:
B-C: 0.724v
B-E: 0.730v
C-E: 0.762v
E-C: 0.780v

The left underneath gave me:
B-C: 0v
B-E: 0v
C-E: 1.660v
E-C: Open

The bottom left gave me:
B-C: Open
B-E: 0.733v
C-E: 0.977v
E-C: Open

The bottom right gave me:
B-C: 0.725v
B-E: 0.731v
C-E: 0.763v
E-C: 0.776v

Maybe some need to be changed?

[krantz]

Re: HP 1955 Monitor Repair Guide

Measure the SMD 4 pin 4431 Backlight On / Off FETs. Measure between source and drain do you get a reading in only one direction? Are any of them shorted?

Reference the datasheet for the pinout.

Also measure the resistance on the secondaries of the inverter transformers the resistances should all be similar on all of them.

Measure the resistance of the inverter fuse it should be a low resistance I've had ones that didn't blow but had a higher than normal resistance.

Check the board thoroughly for cracked solder joints especially around the inverter section.

If these check out it's beginning to look like an issue with the main video board that signals the inverter on.

I measured the SMD 4-pin 4431 Backlight ON/OFF FET's. From drain to source I got 0.535v-0.540v, and from source to drain i got overload.

I measured the inverter fuse and it gave the resault of 0.05 ohms.

But what is the "secondaries of the inverter transformers" and where do I find it on the board?

[Krankshaft]

Re: HP 1955 Monitor Repair Guide

Not for a short just check them in circuit.

No need for hot air either on such a small package. Just blob solder between 4 pins on one side. Insert a sewing needle under the pins while melting the blub and gently pull it up until the pins clear the blob.

For the other side blob up the pins grab with tweezers then remove the component.

This method is only for replacement of course.

[krantz]

Re: HP 1955 Monitor Repair Guide

Im guessing I need to solder the SMD's out to measure them? In that case I need to wait for my hot-air-station that I ordered yesterday.

[Krankshaft]

Re: HP 1955 Monitor Repair Guide

Measure the SMD 4 pin 4431 Backlight On / Off FETs. Measure between source and drain do you get a reading in only one direction? Are any of them shorted?

Reference the datasheet for the pinout.

Also measure the resistance on the secondaries of the inverter transformers the resistances should all be similar on all of them.

Measure the resistance of the inverter fuse it should be a low resistance I've had ones that didn't blow but had a higher than normal resistance.

Check the board thoroughly for cracked solder joints especially around the inverter section.

If these check out it's beginning to look like an issue with the main video board that signals the inverter on.

[krantz]

Re: HP 1955 Monitor Repair Guide

I soldered out the C5707's and here are the results of those with my DMM...

All gave these readouts...

B-C: 0.639v
B-E: 0.639v
C-E: Open
E-C: Open

[Krankshaft]

Re: HP 1955 Monitor Repair Guide

Did you check the driver FETs (C5707s) with a DMM in diode check mode for shorts? They clearly label the collector, emitter, and base on the boards silkscreen.

Look up bipolar transistor testing if you don't know how to test them.

You can't see a shorted transistor you have to check the junctions with a DMM. If I had a nickel for every person who said a semiconductor looked fine and dismissed it as an issue I'd be rich .

The only time there will be external damage to the transistor package is when a serious over current condition existed which caused the package to overheat.

The inverter is going into shutdown because one of the CCFLs isn't coming on.

Don't forget to resolder all of the inverter transformer pins, the pins on the two inductors, and the pins on the two gray poly caps in between the C5707 transistors.

[krantz]

Re: HP 1955 Monitor Repair Guide

Lets try that again shall we?

Here are some new full-surface pictures.

If you want there are fullsize pictures here... http://gkdv.se/bilder/Elektronik%20&...1030/Fullsize/

Update: I removed the fuse and the PLED didnt flash as it did before. I didnt get a a picture on the screen though.

[alexanna]

Re: HP 1955 Monitor Repair Guide

Yes better pictures are needed.
The flashing of the PLED has me wondering if we are dealing with something shorting causing the power supply to restart over and over.
Even though the inverter fuse has not gone open, I am going to suggest removing it.
Would you remove the inverter fuse and see if the power button and PLED respond normally.

As a side note is anyone else watching this Ballgame? WOW!