Viewsonic 17PS - No Display but Neck Glow

[twillkickers]

See attached photos - the display is black but the neck is glowing. It seems to be receiving a signal from the VGA I am putting to it, but the screen still stays black. I believe it is getting a signal because when I connect the VGA cable to a source the yellow status light on the front of the monitor goes to green (a short audible buzz is heard too.) When I unplug the VGA source the light on the front goes back to yellow - See video:

.

Photos of the neck glow:



I've also disassembled the monitor at one point to clean out dust using my ultrasonic cleaner and isopropyl alcohol - I cut the three cables from the flyback for ease of removal and crimped on insulated connectors - wondering if these connectors are sufficient or not. See photo where connectors are circled in yellow:


Are there any diagnostics tools I should use to test various components? I'd like to get better at fixing these or at least determining if they are "worth saving." And before anyone asks yes I do discharge these CRTs before working on them

Any suggestions are appreciated!

 

[daver2]

Have you searched the Internet for the schematics or a service manual for the monitor?

Dave

 

[twillkickers]

Found the attached but mine is a 17PS and not a 17PS2

 

[daver2]

Excellent.

So, the first thing to do is to check the DC voltages coming out of the internal switched mode power supply. We know the heater is working, so at least some of the supplies are healthy.

However, before we do that, we need to check to see how accurate the schematics and PCB layouts are between what is in the service manual and what you physically have. Hopefully, the differences are negligible...

Dave

 

[twillkickers]

Found the correct manual for my monitor and hoping I test the correct terminals - will update once I have metered a few points.

 

[daver2]

Excellent...

Sheet 5 is the main power supply unit of course.

Dave

 

[twillkickers]

Sorry for my noobishness here - I'm measuring the terminals on IC871 and I can't find 12V out as the block diagram shows.


I'm getting the following from pins 1, 3, and 5. I didn't measure 2&4 because it was difficult to reach back there with my multimeter lead and I was afraid of shorting something.:


Stopped checking at this point because I want to make sure that this IC isn't faulty before I spend any more time testing... Thanks for your guidance. I've also attached the datasheet for IC871 (SI-3120FALF)

 

[twillkickers]

Aah wait.. now the middle pin switches to 12.15V instead of 0.668V once the display gets a signal. I suppose this is working then?

 

[twillkickers]

Once again being a noob here - I assume the middle pin is always the output voltage? If so IC873 is putting out 30.61V. I suppose that is within the 33V required by the manual? Is +/-10% ok on these circuits?

 

[daver2]

Pin 5 is Vin (the pin on the right of your picture above).

Pin 3 is the output. But (as you have found out) pin 2 is an ON/OFF switch for the output...

At this stage, something approximating the correct voltage is what we are looking for - so what you are reading looks good. I assume you are also not using a calibrated multimeter either!

Yes, feed a known good signal into the monitor. Otherwise, as you have found, some of the monitor supplies shutdown to conserve power! That stuffs up our readings!

Dave

 

[twillkickers]

Tested IC877 and sometimes the voltage was at 6.154V, but sometimes it started dropping below 1.5V and on to zero. The output of IC871 seemed to always be 0V as well. The manual says it should be 12V. Not sure what might be going on there.

Then I started testing on the high voltage side but I guess my hand wasn't steady enough when I touched one end of Q801 - I think I fried something. The front LED of the monitor went off. I powered down and unplugged - now when I plug it in and turn it back on the light switches on for a short moment and then turns off.

Kind of wondering if IC871 was broken... Is this worth trying to save? What would be a good next step?