HP 1702 backlight won't stay on

[Maxxarcade]

I have an HP 1702 LCD that the backlight comes on for about 1 second and shuts off. I replaced all electrolytics, several of which were bulging, but the problem persists.

I did not replace the 100uf main filter cap off the bridge rectifier, but it has good ESR still. Also tried resoldering the inductors.

Any ideas? The picture looks good when it first lights up.

[PlainBill]

Re: HP 1702 backlight won't stay on

I have an HP 1702 LCD that the backlight comes on for about 1 second and shuts off. I replaced all electrolytics, several of which were bulging, but the problem persists.

I did not replace the 100uf main filter cap off the bridge rectifier, but it has good ESR still. Also tried resoldering the inductors.

Any ideas? The picture looks good when it first lights up.

Could be a bad transformer, bad CCFL, bad wiring to the CCFLs, bad solder joints, or other problem.

One picture is worth a thousand words. Picture of both top and bottom sides of the inverter will help.

PlainBill

[Toasty]

Re: HP 1702 backlight won't stay on

See thread:
https://www.badcaps.net/forum/showthread.php?t=5164

[Maxxarcade]

Re: HP 1702 backlight won't stay on

Thanks. I added pics of my boards to that thread, which seem to be yet another different version. I wonder if I have bad lamps?

[Toasty]

Re: HP 1702 backlight won't stay on

Get one of the cheapy-do "case mod" CCFL kits and do plug swapping trying your tube with the mod inverter. Then try the mod tube with your inverter.

Just make sure all tubes are connected before you power it up.
Check those output inverter transformers and transistors for good solder. Reflow all of them with some non-ROHS compliant solder...

You do get video if you shine a flashlight on the screen? You should see something...

Toast

[Maxxarcade]

Re: HP 1702 backlight won't stay on

Get one of the cheapy-do "case mod" CCFL kits and do plug swapping trying your tube with the mod inverter. Then try the mod tube with your inverter.

Just make sure all tubes are connected before you power it up.
Check those output inverter transformers and transistors for good solder. Reflow all of them with some non-ROHS compliant solder...

You do get video if you shine a flashlight on the screen? You should see something...

Toast

I did check all solder joints, and reflow the questionable ones.

The lamps have 3 leads each. I think one is the sense wire. Can I find CCFL tubes with the proper connectors easily?

I was wondering why my service manual didn't match up... How many versions of this darn monitor are there?

The image does stay on screen. I can barely see it, but it seems normal.

[PlainBill]

Re: HP 1702 backlight won't stay on

I did check all solder joints, and reflow the questionable ones.

The lamps have 3 leads each. I think one is the sense wire. Can I find CCFL tubes with the proper connectors easily?

I was wondering why my service manual didn't match up... How many versions of this darn monitor are there?

The image does stay on screen. I can barely see it, but it seems normal.

It's not uncommon to find the same monitor with two or three different versions of the internal boards. Or two or three different brands of monitor with identical internal parts.

You are confused about the configuration of the CCFLs. There are two tubes connected to each transformer. The two pins close together on the connector are the high voltage drive lines; the pin by itself is the return line.

Follow Toasty's advice. You're going to have to determine if the problem is a CCFl or with the inverter.

PlainBill

[Toasty]

Re: HP 1702 backlight won't stay on

>>The image does stay on screen. I can barely see it, but it seems normal.<<

Good. Then AFAIK, it -is- a backlighting problem and nothing else.

Finding a "mod light" with that 3 pin setup might be difficult. Most I've seen are either 2 pin (1 tube) or (2) 2 pin (2 tubes).

Looking at your board over in the other thread (I brought 2 of the pics here), there isn't any discoloration under the trafos. The only bit I'm seeing is near the regulation area, and that isn't bad. I've seen (a lot) worse.

I'm fairly convinced it's tube replacement time. Since it does come on and shuts down, it's likely tripping the OC protection. I just would want you to test with one of those kits or perhaps another panel, before I'd go getting tubes.

@PlainBill - Have you determined what value resistor could be substituted for which size tube? Or what the mA draw is per tube? Have you seen any site that does?




Toast

[PlainBill]

Re: HP 1702 backlight won't stay on

>>The image does stay on screen. I can barely see it, but it seems normal.<<

Good. Then AFAIK, it -is- a backlighting problem and nothing else.

Finding a "mod light" with that 3 pin setup might be difficult. Most I've seen are either 2 pin (1 tube) or (2) 2 pin (2 tubes).

Looking at your board over in the other thread (I brought 2 of the pics here), there isn't any discoloration under the trafos. The only bit I'm seeing is near the regulation area, and that isn't bad. I've seen (a lot) worse.

I'm fairly convinced it's tube replacement time. Since it does come on and shuts down, it's likely tripping the OC protection. I just would want you to test with one of those kits or perhaps another panel, before I'd go getting tubes.

@PlainBill - Have you determined what value resistor could be substituted for which size tube? Or what the mA draw is per tube? Have you seen any site that does?

Toast

Toasty, no I haven't. There are a lot of variables, including the length and diameter of the tubes. In general a longer tube will require higher voltage, a smaller diameter tube wil require more current. Looking through the sites that sell CCFLs will give you an idea of the voltage and current requirement.

One thing that has been mentioned - two panels with identical model numbers may have CCFLs with different diameters.

PlainBill

[Maxxarcade]

Re: HP 1702 backlight won't stay on

I tried connecting the CCFL's from an old 8 or 10 inch Laptop LCD to the inverter, and it did the same thing. Though I was not able to use both outputs on each side, because the laptop only had 2 lamps.

[Toasty]

Re: HP 1702 backlight won't stay on

You have to load at least each plug with tubes. So either both on the "left" or "right" outlets on the board with the grounds together to one pin.

Each trafo drives 2 tubes together off the same winding. If one is bad or weak it could be and imbalance that the circuit is seeing and shutting down. Don't run this without the tubes connected.

Can you confirm the two transformers match in resistance readings across the pins?

Toast

[Maxxarcade]

Re: HP 1702 backlight won't stay on

You have to load at least each plug with tubes. So either both on the "left" or "right" outlets on the board with the grounds together to one pin.

Each trafo drives 2 tubes together off the same winding. If one is bad or weak it could be and imbalance that the circuit is seeing and shutting down. Don't run this without the tubes connected.

Can you confirm the two transformers match in resistance readings across the pins?

Toast

I had one tube on each trans, and it still shut down. I'll ohm out the trans tomorrow, but I highly doubt they are bad. I'm starting to suspect a couple of the SMT transistors or the control IC.

[PlainBill]

Re: HP 1702 backlight won't stay on

I had one tube on each trans, and it still shut down.

%#%@#Q%^@#$!!! That isn't going to work!!! Look at the return lines for the CCFLs. This design monitors the current through the CCFLs. Remove one CCFL from a transformer output and the current will drop. The protection circuit then shuts down the inverter.

I'll ohm out the trans tomorrow, but I highly doubt they are bad. I'm starting to suspect a couple of the SMT transistors or the control IC.

Ohm out the transformers. I'd place the odds of one being bad at 50 /50, but it's a quick test. You don't even have to pull the transformers - note the caps. Then trace the current sense lines back from the CCFLs. I've identified the pins by circling them in red!! Compare the voltages on the return lines while the CCFLs are operating. While the chances of the controller IC being bad is remote, looking at the sense inputs will tell you what is triggering the shutdown.

PlainBill

[Maxxarcade]

Re: HP 1702 backlight won't stay on

I'm aware of the pinout, just wasn't sure how to test it without having 4 CCFL's handy.

So should I measure the voltage between the return lines and the frame ground to see if they are the same? If not, I'm assuming I could swap the lamp connections to see if the voltage change follows a lamp?

I can also get the datasheet for the control IC, once I get a magnifying glass to read the number. That would probably tell me what voltage range should be at the feedback inputs.

I'm going to need my analog multimeter to test the voltages, because my digital one refreshes too slow.

%#%@#Q%^@#$!!! That isn't going to work!!! Look at the return lines for the CCFLs. This design monitors the current through the CCFLs. Remove one CCFL from a transformer output and the current will drop. The protection circuit then shuts down the inverter.


Ohm out the transformers. I'd place the odds of one being bad at 50 /50, but it's a quick test. You don't even have to pull the transformers - note the caps. Then trace the current sense lines back from the CCFLs. I've identified the pins by circling them in red!! Compare the voltages on the return lines while the CCFLs are operating. While the chances of the controller IC being bad is remote, looking at the sense inputs will tell you what is triggering the shutdown.

PlainBill

[Maxxarcade]

Re: HP 1702 backlight won't stay on





Had about 4 volts on one return line, nothing on the other. Bent pin on the return line

So either the bad caps were causing the problem, and I fixed that and bent the pin putting it back together, or the pin was always bent.

Either way, I did learn a lot about troubleshooting CCFL circuits like this. All I had to do was check the voltage on those return lines. Thanks for the help guys!

[Toasty]

Re: HP 1702 backlight won't stay on

>>%#%@#Q%^@#$!!! That isn't going to work!!!<<

Eeeeeeeasy Bill.

BREATHE.

In through the nose, out through the mouth...

Again!


.
.
.
WOW! you really got him wound up.

Glad it was as simple as that.
The damnedest things cause whack-o problems.

Toast

[PlainBill]

Re: HP 1702 backlight won't stay on

Yes. And sometimes simple tests can give a lot of information.

PlainBill

[groovy22]

Re: HP 1702 backlight won't stay on

Hello,
i have same problem with my HP 1702.
When you switch on the monitor the backlight comes on for about 1 second and shuts off. Sometimes when you switch on - the monitor work for about 10 minutes and shuts off again.
One dude told me that could be bad caps, but they looks pretty good.

What you think ?
Thanks

[Th3_uN1Qu3]

Re: HP 1702 backlight won't stay on

Don't think it's the caps. Bad transformers, broken wiring, or bad CCFL.

With the monitor off, unplug all CCFLs and try measuring the resistance of the transformer secondaries (the side with fine wire). Use the 2kOhm scale on your multimeter. Both should read within 3% of each other if they are good.