Princeton VL1812 Monitor

[drewcam888]

So the power supply is bad. Both 5 and 12 vary greatly under load. Without load the 5 is steady and 12v varies some and is near 11V. I can't see any evidence of bad capacitors. Has anyone worked on a monitor that is similiar? Or anyone have a manual or schematic?

[drewcam888]

Re: Princeton VL1812 Monitor

I have determined it is the same monitor as a Sylvania L181 if that helps anyone.

[drewcam888]

Re: Princeton VL1812 Monitor

Also if it helps it may be made by the JEAN Co., Ltd in Korea.

[drewcam888]

Re: Princeton VL1812 Monitor

I dont get a constant voltage from the rectifiers so I am guessing the switching regulator is bad.

[PlainBill]

Re: Princeton VL1812 Monitor

(Sigh),

Saying the problem is the switching regulator is trite. The whole board is a switching regulator!!! It MIGHT be the SMPS controller, but before changing that, look at the feedback circuit. In the first picture just below the paper label at the top of the board is an opto-isolator. The opto-isolator and components to the left of it make up the feedback circuit. I803 is a critical part of the circuit - check that.

Above all, check the mains filter capacitor. Excessive ripple could be causing the problem. Don't rely on visible inspection.

PlainBill

[drewcam888]

Re: Princeton VL1812 Monitor

well what I meant was the I801 or PWM IC. I will look into the opto. I really need an esr meter to checks the caps. And by main cap do you mean the cap on the ac input line?

[golemmaster]

Re: Princeton VL1812 Monitor

Those guts look exactly like a "NEC AccuSync LCD71V" I opened up yesterday.
The monitor was working fine but I wanted to check parts and cap brands. UCCs Everything nice and clean. Nice display.

[golemmaster]

Re: Princeton VL1812 Monitor

Those guts look exactly like a "NEC AccuSync LCD71V" I opened up yesterday.
The monitor was working fine but I wanted to check parts and cap brands. UCCs Everything nice and clean. Nice display.

Darn it. I can't remember if that was the NEC or the Viewsonic. I opened both yesterday. The Viewsonic needed caps replaced.
I would think that would be the Viewsonic since OP has a Princeton...same OEM I think. crap crap crap brain

[drewcam888]

Re: Princeton VL1812 Monitor

What was the problem with the viewsonic? My power light just blinks and the power supply is flaky. Did you have visually bad capacitors? Do you happen to have a schematic for either? The picture of the NEC look very similiar to my monitor. So your NEC was working correctly?

[golemmaster]

Re: Princeton VL1812 Monitor

The Viewsonic would power on for a second and the the screen would go black.
It had 1 bulging 1000uF@10 cap on the LCD driver board (not the PS or inverter boards). I removed the other 4 or so and tested ESR. They were bad too.
3x 470uF@25, 1x 330uF@16 all ELITE
All replaced. Monitor works fine now.

[drewcam888]

Re: Princeton VL1812 Monitor

Ok well the power supply is my problem so i guess I can't use your info. Thanks anyway. I guess my next step is to verify that I see the same signal on both sides of the opto. Time to find a scope.

[drewcam888]

Re: Princeton VL1812 Monitor

Also another note. I can here a tick tick tick tick from the power supply with 1 second delays between them. Any guesses? That part 1803 is a 2.5v regulator. The circuitry to interesting around it. I will have to look into its function.

[PlainBill]

Re: Princeton VL1812 Monitor

Also another note. I can here a tick tick tick tick from the power supply with 1 second delays between them. Any guesses? That part 1803 is a 2.5v regulator. The circuitry to interesting around it. I will have to look into its function.

I803 and the circuitry around it is a standard system used on most SMPS supplies. It is used to feed back the output voltage status to the SMPS controller A typical schematic is here: https://cdn.badcaps-static.com/pdfs/...adccdf47b8.pdf

A quick analysis of the circuit: The output voltage is applied to the top of R10. R6 and R8 form a voltage divider. If the voltage at the junction of R6 and R8 exceeds 2.5 volts the 'anode' of U3 will draw current through R2 and the LED in U2. This causes current flow through the phototransistor in U2, which pulls pin 8 of down, reducing the drive pulses to T1, which reduces the output voltage.

NOW, that is of secondary importance. The real clue is the clicking sound at one second intervals. This is caused by the SMPS controller restarting. Possible causes are an excessive load, or a bad capacitor on the SMPs controller itself. Because the 12V output is low even without load, we can assume the supply isn't overloaded. That brings us back to the main filter cap (typically 100 uF, 450 Volt). A second possible culprit is a small capacitor near the SMPS controller, typically 10 - 47 uF, 50 V.

PlainBill

[golemmaster]

Re: Princeton VL1812 Monitor

Also another note. I can here a tick tick tick tick from the power supply with 1 second delays between them. Any guesses? That part 1803 is a 2.5v regulator. The circuitry to interesting around it. I will have to look into its function.

I had a DELL 17 inch (with Benq parts) that had the ticking symptom a while back. Failed/shorted mosfets and transistors in the backlight power section. It also had a odd issue. The pico fuse seemed good (DMM diode test) but would partially open/resist when given the working current. Replaced the fuse too.

[drewcam888]

Re: Princeton VL1812 Monitor

I will look into the large cap. It isnt the inverter because it does it without an inverter hooked up. I also dont see any voltage at the because the voltage never reaches 2.5V. So on to your other suggestions.

[drewcam888]

Re: Princeton VL1812 Monitor

Well I soldered in a cap that I think is good. Still ticks maybe I need to find a new one.
So there was a 100uf cap from VCC to gnd on the four pin switch that drivers the transformer. I didnt have a 100uf so I put in a 470uf and the 12 supply now varies more than before. Boy am I confusing myself today.

[drewcam888]

Re: Princeton VL1812 Monitor

Th voltage goes from 8.6ish to 11.7ish then down to 8.6 in 2 seconds then back to 11.7 immediately and repeats over and over.

[drewcam888]

Re: Princeton VL1812 Monitor

With that voltage dropping an repeating I see a small voltage appear on the isolated side of the opto. I can't tell for sure if the voltage at the opto is caused my the low voltage 8.6ish or the high voltage 11.7ish. I guess i need a scope so I can see where the edges line up. It is kinda working it see a low voltage then tries to go higher but then quites again. I dont see the cause of this.

[PlainBill]

Re: Princeton VL1812 Monitor

With that voltage dropping an repeating I see a small voltage appear on the isolated side of the opto. I can't tell for sure if the voltage at the opto is caused my the low voltage 8.6ish or the high voltage 11.7ish. I guess i need a scope so I can see where the edges line up. It is kinda working it see a low voltage then tries to go higher but then quites again. I dont see the cause of this.

Something to try is to look up the data sheet or application note for the SMPS controller. It's probably on the back side of the board.

You are correct, a scope would be a big help. WARNING!!! The SMPS controller and associated circuitry are LIVE!!! You must use an isolation transformer!!!!

PlainBill

[drewcam888]

Re: Princeton VL1812 Monitor

When you say smps controller. I dont have a big ic like the schematic. The opto just drivers a 4 pin TO220 power switch that drives the transformer. So you want me to look at my TO220 IC?