HP 1955 Monitor Repair Guide

[MrMaguire]

Re: HP 1955 Monitor Repair Guide

Hello, I've known of this forum for a while but never browsed or posted on it, but now I have an excuse to.

I also have a HP 1955 monitor, that I got because it had been thrown away, but my problem seems to be a little bit different. Even though the monitor will power on and display an image, the backlight does not come on at all. Which I think would indicate that the fuse is open.

The fuse is a bel brand fuse and is rated for 2.5A 250VAC, I think this is because I'm in Britain where we use a 240VAC 50Hz electrical system.

I looked at the components and found a C5707 that looked as if it had a small hole blown in it. I do not own any test equipment so I couldn't check the resistors, capacitors etc. to see if they were out of tolerence.

I also looked at the solder joints on the underside of the PCB and some of them don't look exactly right, for example the solder joints on the IEC power socket are loose. And the grey capacitors feel loose. I couldn't reflow or touch up any of the joints as I don't own a soldering iron.



I mentioned above that I don't own any of the proper equipment that should be used to repair this power supply board. The point of this post is to get some good feedback on what might be wrong with this and ultimately the goal is to possibly take the board to a professional to have it repaired.

I am new to repairing electronics in this way, so forgive me if I'm not being detailed enough.

Thank you in advance for any feedback.

George

[Behemot]

Re: HP 1955 Monitor Repair Guide

Probably typical problem with blown 2SC5707 transistors, blown fuse.

If you'll go to somebody, have him check all the inverter transistors, also the grey capacitors between them.

[MrMaguire]

Re: HP 1955 Monitor Repair Guide

Thanks for the feedback.

So I'm thinking the inverter transistors are the C5707s and what else?

[Behemot]

Re: HP 1955 Monitor Repair Guide

These 2SC5707, every time two pieces around the gray foil caps in push-pull configuration.

Also there are two 2SJ598, one for every invertor, these turn it on and off. Just posted problem with one of these on page 6. It was also shorted making the invertor stay on all the time (even when display was turned off).

[budm]

Re: HP 1955 Monitor Repair Guide

The inverter circuits for this chassis uses Royer Oscillator setup, the gray cap is part of the tuning circuits with inverter transformer, if the cap changes it value, the Oscillator frequency will shift which can cause the C5707 to run hot. The B+ feeding the Royer is done by the power transistor that is driven with PWM signal to produce variable DC feeding the Royer Osc, less B+ less brightness, if that power transistor shorted out, the Royer will be running full blast or the time since the PWM Power transistor is not turn off when the monitor is turn off because it is shorted out. New monitor no longer use Royer Oscillator type setup any more. Bad solder joints on the inverter transformer legs are another common failure.

[rexmeridiem]

Re: HP 1955 Monitor Repair Guide

Guys. Sometimes not the transistors, fet or such is the problem only the "bolting". Happend to me once that the power input bolting (all three) were "lazily" connected and it made my screen go black after 1 sec. I have rebolted it and wunderbar, All working. TRy that first to see if there is any loose bolting.

[Behemot]

Re: HP 1955 Monitor Repair Guide

You mean grounding screwes or what?

[rexmeridiem]

Re: HP 1955 Monitor Repair Guide

You mean grounding screwes or what?

No, the soldering at the condensator etc. At the AC it's typical it gets loose but it can happen to any part on the motherboard so check them out carefully. If you havent seen any at the electricity part then check out the computer motherboard.

[Behemot]

Re: HP 1955 Monitor Repair Guide

Ah, soldering. Yeah, we are used to check it, at least most people are around I think. BenQ has usually poor board and soldering quality, so do all other BenQ-made displays (like HP).

[budm]

Re: HP 1955 Monitor Repair Guide

Cracked solder joints at AC inlet: Example of Dell and HP
http://s807.photobucket.com/user/bud...tml?sort=3&o=7
http://s807.photobucket.com/user/bud...tml?sort=3&o=2
At transistor C5707:
http://s807.photobucket.com/user/bud...tml?sort=3&o=0

At inverter transformer:
http://s807.photobucket.com/user/bud...tml?sort=3&o=8
BenQ made some monitors for ACER, HP, Dell.

[Behemot]

Re: HP 1955 Monitor Repair Guide

It's very dangerous on the primary side, due to high voltage there may be huge losses on a bad joint. This is one under freewheeling diode in Acer AL2423W:

[rexmeridiem]

Re: HP 1955 Monitor Repair Guide

It's very dangerous on the primary side, due to high voltage there may be huge losses on a bad joint. This is one under freewheeling diode in Acer AL2423W:

Oh god. That's very very bad there. I guess you are loosing a lot of power there. Might be the problem then.

[Behemot]

Re: HP 1955 Monitor Repair Guide

Well actualy problem was that almost all caps were bulged and it stopped powering on, this was just a bonus…

[sermulis]

Re: HP 1955 Monitor Repair Guide

Hi there,

Received same HP 1955 monitor. Turned it on and it worked. Opened it and replaced Elite caps with new 105C ones. Switched it on and tested. It worked for 1 day till I switched it off. Now - I just doesn't turn on. As it has also bad sleeve connector from menu keys to logic board - i can't identify if it's showing some led activity (first time it switched on w/o menu control pad), and i am not able to see any activity on screen using flashlight (which makes me think that smth is shortened)

As I received information from the previous user of this monitor - that it turned on after some kind of shaking it. (This one is leading to bad soldering joints)

Could you please point me what, and more importantly - how should i check components?

What I have already done:

1. Replaced caps in primary circuit (monitor worked with new caps) - OK
2. Checked 5A Pico fuse PF701 - OK
3. Checked Q805, Q812, 4431 mosfets in the circuit. They are not shortened in Source to Drain. So seems to be OK.
4. Was not able to see any bad joints (searching for magnifying glass)

Have no idea how to check FETs and C5707
Any advices what else should i check :?

I am very new to electric theory, so please be understanding.

Thanks in advance.

[Behemot]

Re: HP 1955 Monitor Repair Guide

Just check all three pairs between, between two legs at a time.

Just digged mine from a hill of stuff and found it's display is scratched. Damn it, I just found a buyer for it

[sermulis]

Re: HP 1955 Monitor Repair Guide

hmm..

C5707 all are showing 980 and 620 AND 165 and 1065
FET's are showing around 170 and 990

[Behemot]

Re: HP 1955 Monitor Repair Guide

Should be OK as long as they are not fully opened.

[nickBcap]

Re: HP 1955 Monitor Repair Guide

Switching the monitor on and then have it not turn on again is like a dieing capacitor symptom, well it is a possible symptom in my short monitor and power supply repairs experience and all my repairs to now have been capacitor replacement and a cracked transformer on a 24 inch all in one PC power board. Perhaps one or more capacitors you replaced with are not good. I bought a tester for testing Capacitance of new replacement capacitors before installing.

Also, I pulled an original Microsoft XBox out of a neighbors bin and it worked the first time then stopped. When I inspected the insides I bumped a capacitor and it moved too easily to make me take a closer look at it. I wiggled it gently and one leg on the capacitor broke. Close inspection revealed so much corrosion around the capacitor pins and that it had been falling off the board. The board itself looks fine so have bought a replacement cap for NZ$9 and hoping the high price will pay off.
So perhaps check the pins on all the capacitors on the board.

[budm]

Re: HP 1955 Monitor Repair Guide

"C5707 all are showing 980 and 620 AND 165 and 1065
FET's are showing around 170 and 990" Are those reading in Ohms, K Ohms, M Ohms?

[Behemot]

Re: HP 1955 Monitor Repair Guide

Also, I pulled an original Microsoft XBox out of a neighbors bin and it worked the first time then stopped. When I inspected the insides I bumped a capacitor and it moved too easily to make me take a closer look at it. I wiggled it gently and one leg on the capacitor broke. Close inspection revealed so much corrosion around the capacitor pins and that it had been falling off the board. The board itself looks fine so have bought a replacement cap for NZ$9 and hoping the high price will pay off.
So perhaps check the pins on all the capacitors on the board.

Input Elite caps tend to do that, I've seen two bad already (one here on picture, one in real life). One (another brand) was having some salts around one leg, electrolyte already started leaking.