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

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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.

 

[Franc Zabkar]

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.

You may be able to substitute each lamp with a 100K dummy load:
http://www.myslotnotes.com/Dummy Load.htm

Here is a service manual for your monitor:
http://archive.espec.ws/files/SDM-HS74P KRSVM000081483.pdf

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
http://www.mcmelectronics.com/product/6871TPT285C
http://bestbuy.partsearch.com/Part/...ed/LG+Electronics/Zenith/6871TPT285C/New.aspx

6871TPT285A
PWB(PCB) ASSEMBLY,POWER, HS74 AUO 2X4PIN POWER TOTAL POWERNET
http://www.mcmelectronics.com/product/6871TPT285A
http://bestbuy.partsearch.com/Part/...ed/LG+Electronics/Zenith/6871TPT286A/New.aspx

Here are photos of the IP boards:
http://global.gmarket.co.kr/challenge/neo_goods/goods.asp?goodscode=160493545

- Franc Zabkar

 

[PlainBill]

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.

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