HP 1955 Monitor Repair Guide

[sermulis]

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?

Multimeter was set to diode test mode, so as i understand these was Ohms. All testings performed in the circuit.

[budm]

Re: HP 1955 Monitor Repair Guide

When test in Diode mode, the display will display forward voltage of the semiconductor junction, typical silicon will show about 600~700 range (which is 0.6~0.7V), so the one with 165 and 1065 are not right. I use Ohm mode to look for low resistance reading since bad semiconductor will no longer act like semiconductor any more when it is bad. I would test them out off he circuit to be accurate.

[nickBcap]

Re: HP 1955 Monitor Repair Guide

The the capacitor I removed was the original and I have seen reports about the original caps, that they do blow often. The original cap from memory is PowerStor and the replacement I am getting is a Panasonic and the price seems to indicate quality lol....I hope so. The corrosion appeared as silvery lumps on the capacitor pins.

[Behemot]

Re: HP 1955 Monitor Repair Guide

There are those grey PVC caps between the transistors, don't forget about that, they create distortions in the measurement.

[budm]

Re: HP 1955 Monitor Repair Guide

The cap is part of the Royer Oscillator tuning circuit in con junction with the Inverter transformer, if it is bad it will cause the Oscillator to run at the wrong frequency as designed and it damage the C5707. So check the value of the cap and for leakage resistance.

[sermulis]

Re: HP 1955 Monitor Repair Guide

The cap is part of the Royer Oscillator tuning circuit in con junction with the Inverter transformer, if it is bad it will cause the Oscillator to run at the wrong frequency as designed and it damage the C5707. So check the value of the cap and for leakage resistance.

You are talking about 0.22 uF 160V caps?

Removed C5707 - all of them shows about 17,26 MOhms (charging or discharging)

[budm]

Re: HP 1955 Monitor Repair Guide

The tuning cap is next to the two c5707, for example:
http://s807.photobucket.com/user/bud...tml?sort=3&o=1
Is the version of your board made by BenQ. the circuits may match your board.

[sermulis]

Re: HP 1955 Monitor Repair Guide

hmm..
is there any other options, before i start replacing (if i will) all components one by one.
from measuring tools i've got only multimeter.
i have no tool to test cap. visualy it looks OK - no leakage, corrosion or smth else.

[budm]

Re: HP 1955 Monitor Repair Guide

By the way, that black FUSE PF701, what resistance do you get, from my experience repairing these board, that bad black fuse will show 3~10 Ohms of resistance instead of less than one Ohm.

[sermulis]

Re: HP 1955 Monitor Repair Guide

By the way, that black FUSE PF701, what resistance do you get, from my experience repairing these board, that bad black fuse will show 3~10 Ohms of resistance instead of less than one Ohm.

0.8 Ohm

[budm]

Re: HP 1955 Monitor Repair Guide

So if you check the voltage on either ends of that black fuse with ref. black lead to the chassis, what do you get?

[budm]

Re: HP 1955 Monitor Repair Guide

You have indicated on your post that the customer can shake the monitor to make it work but you cannot see any bad solder joints, from the pictures I can see small ring around the transformer's leg. When I work on these boards, I will also re-flow all the solder joints. See example of the ring develops around the component'e lead: http://s807.photobucket.com/user/bud...tml?sort=3&o=0

[sermulis]

Re: HP 1955 Monitor Repair Guide

So if you check the voltage on either ends of that black fuse with ref. black lead to the chassis, what do you get?

if understood you correctly, than i recieve 0 Volts dead.

You have indicated on your post that the customer can shake the monitor to make it work but you cannot see any bad solder joints, from the pictures I can see small ring around the transformer's leg. When I work on these boards, I will also re-flow all the solder joints. See example of the ring develops around the component'e lead: http://s807.photobucket.com/user/bud...tml?sort=3&o=0

already resoldered transformers, diodes, and caps in inverter circuit. still no success.

checked connector to logic board - receiving 4.97, 3.20 and 1 Volt

[budm]

Re: HP 1955 Monitor Repair Guide

OK, just to be sure, black probe on the chassis, red probe on either end of the fuse and it shows 0V, then the power supply for the inverter is not working, but since the 5V power supply for the main board is also generated by the same transformer, then I will expect you not to have 5V power supply working either. Please post good clear pictures of YOUR board, pictures of both side of the board.

[sermulis]

Re: HP 1955 Monitor Repair Guide

Yes. Measured as you advised. Red probe on Fuse lead, Black to the chassis. Measured 0V.

It was hard to take clear photos of the back than in post 134

[budm]

Re: HP 1955 Monitor Repair Guide

OK, so more likely you do not have the 5vdc output to feed the main board either. So carefully check the DC voltage of that large main DC filter cap, it should have more than 300VDC on its two legs (the reading should be Line voltage x 1.414= xxxx DCV).
I can see ring (cracked solder) on one of the AC input connector. Similar to this: http://s807.photobucket.com/user/bud...tml?sort=3&o=2

[sermulis]

Re: HP 1955 Monitor Repair Guide

OK, so more likely you do not have the 5vdc output to feed the main board either. So carefully check the DC voltage of that large main DC filter cap, it should have more than 300VDC on its two legs (the reading should be Line voltage x 1.414= xxxx DCV).
I can see ring (cracked solder) on one of the AC input connector. Similar to this: http://s807.photobucket.com/user/bud...tml?sort=3&o=2

Hi!

resoldered bad joints on AC jack and large capacitor. Measured line voltage on large cap legs - got 300V (so 300 X 1.414 = 424V DCV) so it looks ok?

[budm]

Re: HP 1955 Monitor Repair Guide

your line voltage is 220 x 1.414 = about 311V, so the board is getting the Main DC to feed the switching power supply circuits OK. Your board seems to match the diagram I posted on post 147. Did you down load that diagram? So with just power supply board by itself with the board power cord connected, check the DC voltage on the TWO LEGS OF Zener Diode ZD601, DO NOT TOUCH anything on the board with your hands!

[sermulis]

Re: HP 1955 Monitor Repair Guide

Hi,

yes, i spent almost an hour reviewing the schema and came to conclusion that may be IC601 NCP1200 should be checked...

ok. i will check DC on Zener diode D601 between it's two legs

[budm]

Re: HP 1955 Monitor Repair Guide

That the VCC for the IC601, it can easily be the filter cap in that power supply section that supplies the VCC for the IC.