I found a Viewsonic A95F that I found on the side of the road while
driving home. The monitor is very nice and I decided to see if I can
fix it. I am an electronic technician with 20+ years experience. This
monitor would power on, green LED lit, but has no high voltage and
henceforth, no OSD or video. I found that the B+ power supply for the
horizontal output transformer drive transistor, QH22 (IRF730) was
missing. I then found that power supply capacitor, CH22 (100uF @ 100V)
was open and vented. Replacing the capacitor restored +B for the output
transistor to about 107VDC. There are pulses on the gate of QH22 but
are small and do not seem to be sufficient to drive the output MOSFET.
Unloaded (Taking the transistor out.) resulted in very clean, beautiful
pulses at about 3.5 volts p-p. Loaded, the pulses drop to a very low
rate and are narrow pulses, not evenly spaced and they do not drive the
transistor. What I am afraid of is that the flyback transformer may be
bad and that would kill this job, out to the side of the road it will
go again.

If I put a scope on the output transistor's drain and apply power, I
can see very nice output for only a fraction of a second, do not hear
high voltage, and the pulses immediately stop on the output and the
output just sits there at 107 volts DC. I tested the MOSFET as best I
could with a Fluke DMM and from what I can tell, it is not shorted and
seems to behave as a power enhancement mode MOSFET should. Replacing it
temporarily with an IRF630 yielded the same results, original
transistor is now back in circuit.

Does anyone have any idea of what is going on here and why the
horizontal pulses do not drive the FBT output transistor? What would
really help would be a schematic diagram for this Viewsonic A95f
monitor, the high voltage section and power supply section especially.
Does anyone have this diagram and could provide it as an image file or
pdf? I can provide Acrobat 7 Professional if anyone could scan the
diagram and would like to help.

Thank you.

Sincerely,
Ohmster

You can try Sams Publications if they can supply a copy of the schematic.
Viewsonic will not sell schematics to non authorized servicers for their
products.

In your case, is possible that the HV multiplier is defective, or there is a
problem with the flyback, and the HT supply or H scan is going in to
protection. A defective CRT can also cause this type of effect.

You may find it difficult to find original parts for computer monitors. I am
also finding that especially with CRT monitors and TV sets, support is
getting very scarce.

--

JANA
_____


"Ohmster" wrote in message
oups.com...
I found a Viewsonic A95F that I found on the side of the road while
driving home. The monitor is very nice and I decided to see if I can
fix it. I am an electronic technician with 20+ years experience. This
monitor would power on, green LED lit, but has no high voltage and
henceforth, no OSD or video. I found that the B+ power supply for the
horizontal output transformer drive transistor, QH22 (IRF730) was
missing. I then found that power supply capacitor, CH22 (100uF @ 100V)
was open and vented. Replacing the capacitor restored +B for the output
transistor to about 107VDC. There are pulses on the gate of QH22 but
are small and do not seem to be sufficient to drive the output MOSFET.
Unloaded (Taking the transistor out.) resulted in very clean, beautiful
pulses at about 3.5 volts p-p. Loaded, the pulses drop to a very low
rate and are narrow pulses, not evenly spaced and they do not drive the
transistor. What I am afraid of is that the flyback transformer may be
bad and that would kill this job, out to the side of the road it will
go again.

If I put a scope on the output transistor's drain and apply power, I
can see very nice output for only a fraction of a second, do not hear
high voltage, and the pulses immediately stop on the output and the
output just sits there at 107 volts DC. I tested the MOSFET as best I
could with a Fluke DMM and from what I can tell, it is not shorted and
seems to behave as a power enhancement mode MOSFET should. Replacing it
temporarily with an IRF630 yielded the same results, original
transistor is now back in circuit.

Does anyone have any idea of what is going on here and why the
horizontal pulses do not drive the FBT output transistor? What would
really help would be a schematic diagram for this Viewsonic A95f
monitor, the high voltage section and power supply section especially.
Does anyone have this diagram and could provide it as an image file or
pdf? I can provide Acrobat 7 Professional if anyone could scan the
diagram and would like to help.

Thank you.

Sincerely,
Ohmster