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Old 1st January 2009, 01:34 PM   #1
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Default SMPS Trouble, wont start - need some advice! Full schematic

Hello!
I have a SMPS wich is normally fitted into a Pioneer plasma screen. The SMPS is on a separate PCB and made by hitachi.

The first thing i replaced was R133 wich was open due to high
current when Q125 shorted. Both Q125 and R133 are replaced
Q125(2sk2651) R133 = 0.1ohm Brown Black Silver Gold.

I also replaced Q126 with a similar device.
Q119 wich is the "driver" did not have any reading when measured out of circuit so i replaced this one also.

The problem is that i only have the 5V Standby Voltage wich is
fed directly before rectification.

It just wont start up, no smoke no nothing. The "ALARM" LED is
lit as soon as i plugg in the power cable. I dont know what that
means but i guess something is wrong. R362 gets quite hot but
it is a high wattage resistor.

One problen that i can se is that Q109 wont turn on, i just get a
over and over from 0v to 0.45v cycle on the base.

I really need some advice, today i took voltage readings on nearly ALL ICs.

Schematic here, www.itxpress.se/433.pdf page 129 and 131.
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Old 1st January 2009, 09:48 PM   #2
winny is offline winny  Sweden
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Sorry but where are Q126 and Q119 located? Since the schematics are pictures, they aren't searchable and they are very crowded.

"I also replaced Q126 with a similar device."
I have a bad feeling about that.
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Old 2nd January 2009, 04:29 AM   #3
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Join Date: Oct 2007
Hello!
Q126 is placed just to the right of Q125. It cuts out the
gate signal in event of a overvoltage i think.

The funny thing is that relay RL101 wont "close" so i have
no voltages. Except the voltages comming from before
the rectification bridge.

Q119 is placed on page 129 on the bottom halv of the
page in the middle. Hope you will be able to find it!
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Old 2nd January 2009, 05:04 AM   #4
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Location: Silicon Valley
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R362 getting hot indicates something is tripping the SCR latch formed by Q113 and Q114, if I see this correctly. It seems this triggers from a zener biased with may series resitors. Verify these resistors are near nominal impedance.

Yuck, I hate this design and schematic. It's silly flyback! There are off the shelf ICs to integrate these 400? passives down to a few dozen components.
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Old 2nd January 2009, 08:17 AM   #5
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Its a big mess trying to troubleshoot the circuit with nearly
no voltages on the pwm or outputs.

You are talking about R103-R105? I relly havent
checked that they are in tolerance because i could not understand
why they would fail.

But in electronics anything could happen i guess! =)

I will check R103-105 first thing today! By the way impedance
of a resistor is equal to resistance? Just a though...
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Old 3rd January 2009, 01:21 AM   #6
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Join Date: Oct 2007
I lost R175 on the floor tonight ! (((
What the **** should i do know?
I dropped it on the floor before i got it
measured :S

the positive thing is that i found R370 1kohm open!
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