Princeton Ultra 90 Monitor

[glasnostJDC]

Hello, I have received a Princeton Ultra 90 Monitor. It blows fuses, no
posister and every thing else before power supply module checks. Found a
shorted Transistor on the module power board, removed it and left it out but
still blows fuses. I'm wondering were to obtain the service manual or just a
schematic. I will repost in a reply more information as I go along (part
numbers and other information). Any information is help full and anything is
appreciated. Thanks for your help.

--
-glasnostJDC

 

[glasnostJDC]

Checked rectifier and it is fine, checked the switcher FET and it is shorted
out (Q401), I removed it and turned on the monitor just to see if it would
not blow the fuse. It blew the fuse anyway. From there I checked for shorts
off of the paired wires off the module power supply and found one but all it
led to a transformer but for thrill of it I removed the transformer (T902
for anyone with a schematic) and it powered on and worked fine. My two
questions are: Do I need to replace the switch FET before powering it on
again? And why when I removed that Transformer (Tests 1 ohm on primary and 1
ohm on secondary, its a small one and seems fine to me) it would turn and
work fine? If any part numbers or anything please ask and any help if
appreciated and thank you John Gill for your help.

-glasnostJDC

"John Gill" <jdgill@juno.com> wrote in message
news:71016e83.0307110634.5ce02ecf@posting.google.com...

Most of the time, when the fuse blows, the problem is on the
primary side of the switching power supply. Shorts on the secondary
side of the switching transformer cause the power supply to shut down
without blowing fuses.
Check the switcher FET (on the heatsink) and the bridge rectifier (or
the 4 primary rectifier diodes) for shorts. These are the most common
cause of a blown fuse. You can check these with the diode-check position
on your DMM. Look for zero or near-zero readings to indicate a short.
Hope this helps....
John

 

[glasnostJDC]

This monitor is made by (this one particular one anyway) By ADI and its ADI
model number is PD-959. I hope this information helps someone in helping me
solve this problem.

-glasnostJDC

 

[John Gill]

I have been repairing monitors for about 8 years and have never
come across one with a shorted power supply switching transformer.
(I have repaired TV's for about 25 years).
The switching FET drives the primary of the PS transformer. If you
remove the FET, then there should be no drive on the primary, although
one side of the primary may be connected to the input voltage.
I do not see how this could blow the fuse, unless there is an
internal short to ground within the transformer.

I guess it is possible that the transformer could be bad, but I
doubt it. I have had the big electrolytic capacitor short out,
but that is very rare. I would still check parts on the primary side
of the transformer. Also check the diodes on the secondary and the
the horizontal output transistor (HOT), although these normally cause
the power supply to shut down, if bad.
John