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Sony SDM-HS74P computer monitor 2nd post. can anyone please help me?

The raster comes on for about 2 seconds and then disappears. The power supply and inverters seem to be one unit. I've ESR'd the caps and checked for any other obvious things. There are two high voltage outputs marked 800V 15ma. I measured these with an AC meter and noted one side was about 650v and the other was over 1kv.I then disconnected the cables to these two jacks and noted that the 650v output remained about same while the other one shot up way above 1kv. I'm wondering now if I may have a bad inverter or perhaps is there a bad lamp? I'm not sure how to determine that though. Can anyone please point me in the right direction? Thanks for any help. Thanks, Lenny.

Reply to
captainvideo462002
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On Fri, 1 May 2009 13:46:27 -0700 (PDT), " snipped-for-privacy@yahoo.com" put finger to keyboard and composed:

You may be able to substitute each lamp with a 100K dummy load:

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Here is a service manual for your monitor:

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I don't see the inverter, though. :-(

FWIW, I believe the monitor is a rebadged LG.

According to page 18 of the manual, you could have either of the following two power PCBs:

6871TPT285C (4x 2-pin) PWB(PCB) ASSEMBLY,POWER, HS74 CMO, HS94 LPL/AUO POWER TOTAL POWERNET
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6871TPT285A PWB(PCB) ASSEMBLY,POWER, HS74 AUO 2X4PIN POWER TOTAL POWERNET
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Here are photos of the IP boards:

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- Franc Zabkar

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Please remove one \'i\' from my address when replying by email.
Reply to
Franc Zabkar

A quick explanation of how the inverter works might help. The heart of the inverter is a controller chip. It generates the drive waveform for the transformers, adjusts brightness, and monitors the current through the CCFLs. The controller itself draws very little current, the usual design turns the backlight on and off by switching the power to the controller chip on and off.

When the controller chip is turned on it drives the lamps at full power for about 2 seconds, then monitors the current flow through each CCFL, as well as the output voltage from each transformer. If the output voltage is too high, or the current is too high or too low it will shut down. This is sometimes refered to as the 'two seconds to black' problem. The trick is to determine WHY it is shutting down.

Substituting dummy loads for the CCFLs is one technique. Another is to substitute known good CCFLs. An inverter from a case mod kit can be used to drive the original ccfls and possibly identify a problem. The CCFLs from the kit provide known good CCFLs. With a schematic (either the manufacturer's or one you draw) can help identify which input to the controller is causing it to shut down.

In this case, the fact that one output rises when the load is disconnected while the other stays low is significant. The likely cause is a shorted turn in the transformer. A ring tester - like those used with TV flybacks - would help verify this.

If substitution shows the problem is in the inverter, not the CCFLs I suggest identifying the controller and searching for the datasheet and any application notes for it. These can provide a wealth of information.

PlainBill

Reply to
PlainBill

If one of the lamps isn't working the inverter will shut down after about 2 seconds. It can be very difficult to tell if one of the lamps isn't working, so ether remove the lamps so you can see them, or test it with only one lamp plugged in at a time. Chances are, one half of the inverter is bad (I've never seen a bad lamp that wasn't broken). Your voltage measurements point to the inverter. Andy Cuffe

snipped-for-privacy@gmail.com

Reply to
Andy Cuffe

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