Viewsonic GS790 19" monitor needs above 121VAC to power on
 

Hi again from Montreal to all fellow techs !!

This is my third posting about this problem. (last one was 6 months
ago).
Most monitors will power on normally with only 110VAC.
This one refuses to power on unless I apply 122VAC. (I use a variac to
boost the input voltage a little).
I checked all power supply caps with an ESR meter.(no problems)Thanks
again to Bob Parker and his famous ESR meter.
I also checked all power supply resistors for correct values. (no
problems)
All diodes and transistors seem OK around the power supply.
I have the schematic and service manual.

Is there something else I should check ??

Many thanks,

Jacques Carrier

Jacques Carrier wrote:
Hi again from Montreal to all fellow techs !!

This is my third posting about this problem. (last one was 6 months
ago).
Most monitors will power on normally with only 110VAC.
This one refuses to power on unless I apply 122VAC. (I use a variac to
boost the input voltage a little).
I checked all power supply caps with an ESR meter.(no problems)Thanks
again to Bob Parker and his famous ESR meter.
I also checked all power supply resistors for correct values. (no
problems)
All diodes and transistors seem OK around the power supply.
I have the schematic and service manual.

Is there something else I should check ??

Many thanks,

Jacques Carrier

What's the rectified voltage at the filter cap? Ripple?
I've seen one diode go and drop the voltage enough to cause problems.
Low capacitance value can be a problem. Not sure if an ESR measurement
always picks this up.

Had a power supply just the other day. Voltage at the output of the
supply seemed close enough to work on my voltmeter. Turned out that
there was 60 Hz ripple on it that I didn't discover until I looked with
a scope. All the diodes checked good with the ohm meter. The scope
told me which one "had to be bad". When I pulled it, the lead was
loose in the package. Checked fine at low current, but wouldn't
rectify the high current required.
mike

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Return address is VALID.
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"Jacques Carrier" bravely wrote to "All" (19 Nov 04 11:46:27)
--- on the heady topic of "Viewsonic GS790 19" monitor needs above 121VAC to =
power on"

JC From: (Jacques Carrier)

JC Hi again from Montreal to all fellow techs !!

JC This is my third posting about this problem. (last one was 6 months
JC ago).
JC Most monitors will power on normally with only 110VAC.
JC This one refuses to power on unless I apply 122VAC. (I use a variac to
JC boost the input voltage a little).
JC I checked all power supply caps with an ESR meter.(no problems)Thanks
JC again to Bob Parker and his famous ESR meter.
JC I also checked all power supply resistors for correct values. (no
JC problems)
JC All diodes and transistors seem OK around the power supply.
JC I have the schematic and service manual.

JC Is there something else I should check ??

JC Many thanks,

JC Jacques Carrier

Try checking the gain of the psu switcher transistor(s). If hfe is too
low there might not be enough loop gain to let the circuit oscillate.
If it is a psu ic then check the startup circuit resistor. These ic's
often get the operating power from their own output but to initially
start they need to jump the bootstrap with a starting current from the
ac line. This usually consists of a resistor connecting to the ac line
with a couple of other parts. Glitches in the powerline can kill this
resistor especially if the usual spiral film type. This resistor will
often still look good. Basically the error is that it was not properly
rated for the transient voltages. You should replace it with at least
2 resistors in series adding up to the original value, and leave its
legs high above the pcb to minimize the electric field when a glitch
hits.

A*s*i*m*o*v

.... You mean 15" sparks are SUPPOSED to come out of this thing?!?

is there wax on the PCB for holding wires?

it can absorb moisture and cause all sorts of weird problems

Mark