HP 1955 Monitor Repair Guide

[jetadm123]

Re: HP 1955 Monitor Repair Guide

Yes, I do have a multimeter but not sure what component to measure.
Here are some pictures. Ive focused on the components as Krankshaft did.

I think what alexanna was suggesting are photos of your entire board top and bottom. It's difficult to point things out for you to test if one can only see portions of the board.

[krantz]

Re: HP 1955 Monitor Repair Guide

Do you have a multimeter?It's time to look at some voltages.
Even though this a long thread with photos,It would help to see photos of your power supply front and back.Try to get them focused so we can read the writing on the board
Please use managed attachments to post the photos.

Yes, I do have a multimeter but not sure what component to measure.
Here are some pictures. Ive focused on the components as Krankshaft did.

[alexanna]

Re: HP 1955 Monitor Repair Guide

Do you have a multimeter?It's time to look at some voltages.
Even though this a long thread with photos,It would help to see photos of your power supply front and back.Try to get them focused so we can read the writing on the board
Please use managed attachments to post the photos.

[krantz]

Re: HP 1955 Monitor Step By Step

First describe the type of problem you are having.
A monitor can fail in various ways.
Does it have what we call 2 sec to black?
Dose the power LED come on and recognize a signal?
Can you turn the monitor on and off with the front power switch?

The LED comes on and recognizes a signal when i push the power button. It then gives a picture for 0.5 seconds, restarts (also the LED blinks at the restarts) and repeats.

I can turn it off if i hammer the power-button during the restarts.

[alexanna]

Re: HP 1955 Monitor Step By Step

I have a 1955 that I am trying to get working. I have changed all the caps except for the 150uF 450v. I cant see any blown FET's or anything else that might be shorted. What could it be?

First describe the type of problem you are having.
A monitor can fail in various ways.
Does it have what we call 2 sec to black?
Dose the power LED come on and recognize a signal?
Can you turn the monitor on and off with the front power switch?

[krantz]

Re: HP 1955 Monitor Step By Step

I have a 1955 that I am trying to get working. I have changed all the caps except for the 150uF 450v. I cant see any blown FET's or anything else that might be shorted. What could it be?

[bomcla]

Re: HP 1955 Monitor Repair Guide

the monitor has many monitors that use the inverter BQ. usually burn a 4-2SC5707 transistors, the 3 amp fuse or a MOSFET IRFU9024.
It is all in net. If the transistors are smd say I do not know what to say.

[calcu]

Re: HP 1955 Monitor Repair Guide

I have burned Q815 & Q815, pleasetell me what are these transitor?
Do someone have the schematic for HP 1955?
Thanks in advanced

[CatalinaWOW]

Re: HP 1955 Monitor Repair Guide

"As others have mentioned, don't forget to resolder all the inverter transformer pins."

I resoldered them as advised, although all did appear fine under 15X magnification. Examined all joints on the board with the following conclusions:

1. Most reflow joints were good. It appears that a couple of parts had tombstoned and the rework on those was terrible, but probably electrically sound. Redid those for insurance.

2. All through hole parts were apparently done manually, by two different operaters. One was good, the other so-so. There were a lot of very good quality joints (including all of the transformers), and a smaller number of joints that ranged from marginally bad to margninally good.

Unfortunately I didn't take before pics, and don't think the after pics will help enough be worth opening the monitor up again to take. The bad solder joint was on the inductor adjacent (above-right) to the top right 2SC5707 in Krankshaft's picture identifying the drivers. The solder in the joint was fractured, fracture resulted from inadequate solder fill and poor wetting of the inductor lead. When I resoldered I found it difficult to get proper fill and wetting, it took lots of heat and added flux. True on both leads of both inductors. The pair of drivers ajacent to the bad solder joint were the two dead ones. There is a similar inductor between (slightly to the left) the other two drivers.

[retiredcaps]

Re: HP 1955 Monitor Repair Guide

Root cause was cracked solder joint on ferrite core coil near center 2SC5707s.

As others have mentioned, don't forget to resolder all the inverter transformer pins.

[CatalinaWOW]

Re: HP 1955 Monitor Repair Guide

Just a follow up. The monitor is fine now. Recapped for insurance, although there were no signs of cap failure. Root cause was cracked solder joint on ferrite core coil near center 2SC5707s. Took out both 2SC5707s.

Touched up all four similar joints. All were hard to get right, although only the cracked one was originally bad. Also found poor joints on AC connector.

Thanks again for those who posted this information. It shortened my path a lot.

[CatalinaWOW]

Re: HP 1955 Monitor Repair Guide

Thanks Krankshaft.

This info saved me a lot of time on a monitor with exactly the problems described.

[EGuevarae]

Re: HP 1955 Monitor Repair Guide

Dear Helper
Yes it is a different Monitor and a different board then the Post #1
Is this usually the case with the second set of numbers and sometimes even a third
set.
Thanks for the help greatly apprecaited.
Ron

If it is another board and another monitor,what is it doing here (the post)?

[Ron K]

Re: HP 1955 Monitor Repair Guide

Dear Helper
Yes it is a different Monitor and a different board then the Post #1
Is this usually the case with the second set of numbers and sometimes even a third
set.
Thanks for the help greatly apprecaited.
Ron

[jetadm123]

Re: HP 1955 Monitor Repair Guide

Dear Help
I have a burnt 40T03GP Power Fet and I found some but the second number is Different the ones that blew are 40T03GP and right under that number is 701085. But the ones I can get heve the numbers 40T03GP but the number under that one is 602425 are those fets ok to use. is the socond number of any value I googled the socond number and it never came up.
Thank for your time
Ron

Yes, it's ok to use. The second line is usually manufacturing info like the month/year of manufacture, country of origin, hfe values, etc. Just remember to replace these in pairs, since different dates of manufacture usually indicate different hfe values.

Also, are you sure the 40T03GP transistors were original to your board? If you look at post #1 of this thread, that board used 2SC5707 transistors. Of course, you might have a different board.....

[Ron K]

Re: HP 1955 Monitor Repair Guide

Dear Help
I have a burnt 40T03GP Power Fet and I found some but the second number is Different the ones that blew are 40T03GP and right under that number is 701085. But the ones I can get heve the numbers 40T03GP but the number under that one is 602425 are those fets ok to use. is the socond number of any value I googled the socond number and it never came up.
Thank for your time
Ron

[alexanna]

Re: HP 1955 Monitor Repair Guide

I have one of these monitors and I am finding there is a buzzing coming from the inverter board. Near the entry of the 110v power supply. It could be coming from the transformer for the mains power, or the caps that are near the entry of the 110v. Does anyone have a part number for a replacement transformer? The one for the mains power, not the ones for each backlight supply.

These inverter boards are noted for poor solder joints, if you haven't already I would suggest
Re- soldering that area.
Al.

[fiero79]

Re: HP 1955 Monitor Repair Guide

I have one of these monitors and I am finding there is a buzzing coming from the inverter board. Near the entry of the 110v power supply. It could be coming from the transformer for the mains power, or the caps that are near the entry of the 110v. Does anyone have a part number for a replacement transformer? The one for the mains power, not the ones for each backlight supply.

[garrysoh]

Re: HP 1955 Monitor Repair Guide

I'm sorry, but I guess my question wasn't clear. I know that the 3904 is the npn and the 3906 is the pnp, but I was looking for confirmation in circuit. I know that there is a pair (npn and pnp) is used for each FET. I also know the location on the board of each pair thanks to the picture that circles each pair. What I am not 100% sure on is which one, as shown in the picture, is the 3904 npn and which one is the 3906 pnp...
Thanks in advance.

[pedro]

Re: HP 1955 Monitor Repair Guide

Hi all,
I would just like to double check with someone on this forum regarding the signaling transistors (3904 and 3906) exactly which ones are the npn and pnp.

2N3904 is NPN and 2N3906 is PNP.