Dell M991 monitor question

[Richard Rasker]

Hi all,

I'm having a bit of trouble repairing a Dell M991 19" computer monitor
(chassis CM2519, hardly 3 years old). It wouldn't start up, and it turned
out that the flyback FET + drain series diode (an IRF640 and a 31DF06,
respectively) were shorted.
After replacing these parts, the monitor seemed to be fine - at least
during the VGA boot screen. As soon as the screen resolution changed, both
components immediately failed again.

I can't find any other broken or even suspect components, and without a
schematic diagram or service manual, it's impossible to determine what
else could be wrong. A search by Google didn't turn up anything either.

Before I thrown this one in the bin, I'd like to know if anyone here might
have an idea where I could look for a solution. Any suggestions are
appreciated.

Regards,

Richard Rasker

 

[John Gill]

Richard:
Have you checked for a bad or weak electrolytic capacitor in
this circuit ? Use an ESR meter if you have one.
I would also follow the gate circuit on the FET to see where the
drive signal is comming from. It should change when the monitor's
resolution changes.
You might also consider using a heavier FET, such as a IRF740 or IRF840.
John

 

[Richard Rasker]

Richard:
Have you checked for a bad or weak electrolytic capacitor in
this circuit ? Use an ESR meter if you have one.
I would also follow the gate circuit on the FET to see where the
drive signal is comming from. It should change when the monitor's
resolution changes.
You might also consider using a heavier FET, such as a IRF740 or IRF840.
John

First a little correction on my initial posting - the FET + diode are
*not* the FBT driver components, but are hooked up to a separate flyback
winding; One end of this windind is connected to ground, the other to the
anode of the 31DF6 diode, the cathode of which is connected to the drain
of the IRF640 MOSFET. The FET source is connected to ground, and there are
no other components whatsoever connected to this little loop. The function
of this circuit is still rather unclear to me. Here's an ASCII drawing of
the relevant circuit parts:



| FBT 31DF6
+78V 3 | .---------->|-------.
o-------. |( 4 #6573 '--| IRF640A
)|( .-| #7573
)|( 5 |-|
6 )| `--. |
|--' | |
|-. === gnd === gnd
|-| <- driver FET, #7571
|
=== gnd


There is a "normal" driver circuit, built around another IRF type FET
(#7571), with some extra components hooked up to the drain (not drawn
here). But as mentioned, there are no other components hooked up to the
31DF6 diode. The gate of #7573 seems to be driven with a signal derived
from the flyback.

With the 31DF6 and the IRF640A shorted out, the monitor of course won't
start up. However, I'm reluctant to just remove these parts and switch the
thing on to see what happens on the driver FET gate. With the +78V
detached, the whole monitor refuses to start properly, and I can't measure
anything.

As for the bad cap theory: I don't have an ESR meter, but the standard VGA
screen looked absolutely perfect, and the monitor had a very modest power
consumption of 50 - 60 watts. Only upon changing to a higher res, #7573
and #6573 immediately failed.


Richard Rasker