KDS LCD monitor, Sampo Inverter..

On Wed, 04 Feb 2009 22:30:57 -0500, Jamie put finger to keyboard and composed:

It could be that the ON signal is active low, in which case shorting it to ground should turn on the inverter. To find out for certain, you may find an application circuit in the datasheet for the main IC (top centre in your photo). It may also be helpful to view the data for the

8-pin ICs below each of the coils.

The usual culprits in inverters appear to be electrolytic capacitors, so I'd change those first. I'd also reflow the solder joints at the transformers and coils.

- Franc Zabkar

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Please remove one \'i\' from my address when replying by email.

Ok, i've already tried pulling that On/off signal to ground. It does nothing, Normally standard markings of on/off is the same order as +/-, since I'm getting + to that pin from the processor board via a resistor from a 5 volt supply, I guess that thought is out the door, but like I said, I did pull it low with out success.

There is 1 electrolytic on that board that seems to be related to both inverters . I will lift the board from the chassis and inspect closer..

thanks.

"

I'm pretty sure that I can see two electros on there - one conventional through-hole, and one s.m. Have you looked carefully for a s.m. fuse ? They usually have one soon after the incoming supply, and it is not at all uncommon for them to fail for no good reason. Have you followed print from the on / off control input to see if you can find a transistor that does the actual switching ?

Arfa

Actually, I made a mistake on the vender of the monitor, it's AOC, not a KDS, I don't know where my head was at on that one.

Any ways, last night I got back into it, yes, the large cap in the view was defective. Spec'ed out to be a 50uf 30v, tested as 1.2 uf.

So, I replaced it with what I had close to it at the time, 47uf, 35V

Still does not fire up how ever, I'm not sure as to what the "On" signal voltage is suppose to be? Currently I have 4.18 volts when unit is switched on at the control pin. Since this unit operates from a 12 volt level, I'm making assumptions that I'll need to insert a video signal and see if this level turns into like 12 volts of so..

At this point, I just assumed the inverter should kick in with no video signal and at least show something for a short time like a "No signal" or Menus before it goes to sleep.

Tonight, I am charging up one of my older laptops to be used as a video signal to give this a try...

I may have it fixed, Just may not have one of those monitors that shows anything at all unless I active it out of sleep mode with a video signal (yellow light)..

P.S. I didn't want to override the on signal with something higher than that in fear of unneeded additional damaged.

It sure would be nice if I had a service manual for this to so me the prints.. To me, this is just a shot in the dark project to see if I can save this monitor and use it for a spare.. It's not worth it to me to sink lots of money into when you can buy them so cheap these days.

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Jamie wrote in news:P25jl.37$ snipped-for-privacy@newsfe19.iad:

I have often found that the small value ceramic capacitor across the inverter coil causes backlight failure, so you might try replacing it.

On Fri, 06 Feb 2009 20:04:54 -0500, Jamie put finger to keyboard and composed:

FWIW, this 12V Sampo inverter requires a signal greater than 3V to switch on:

According to the circuit diagram, there is a digital transistor (DTC144) at the on/off input.

The document shows a test fixture where a CCFL can be simulated with a

100K or 110K load resistor. The test current at max brightness is 6mA, so that equates to a power dissipation of around 4W. The current at minimum brightness is 2mA. A 10 ohm resistor is used for current sensing.

- Franc Zabkar

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Please remove one \'i\' from my address when replying by email.

Thanks for the reply, I have found the problem with the inverter.

The link you provided seems to be a little different in design.

The digital transistor on this unit is a PNP that drives the PNP that turns on the PWM (dual) vcc.

This unit employs a TI1451L dual PWM modulator. etc.

after experimenting with video signals to get a out of sleep mode state, I've deducted this since it pulls the On/Off trigger low in the attempt to fire it up.

The first digital PNP is bad. (opened)for lack of better terms..

Also, both electrolytic caps were almost dead!. I replaced those first thinking it would solve the issue but didn't, it must of been running on weak caps before the digital tranny died.

I pulled the second PNP base to common and it fired up fine.

Thanks for the help.

P.S. I don't do a lot of repairs like I used to how ever, I remember it was a golden rule to always check the PNP transistors first if the suspected area had one :)

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These days, I would say that silicon NPN/PNP types are equally (un?) reliable, but I too remember those days of that 'golden rule'...

Arfa