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How picky are power transitors ?

J

jmuller

Jan 1, 1970
0
I have a couple of dead monitors (little 15" ones) which seem to have
the same problem. :(

Dead power transitor.

They have both short-circuited.
One is marked:

T
K2038
'41

My question is:

How hard would it be to find a substitute for it? Are they pretty much
generic? Would any high current (5A) transitor do?



- - thanks in adavnce,

-- jmuller
 
J

jmuller

Jan 1, 1970
0
jmuller said:
I have a couple of dead monitors (little 15" ones) which seem to have
the same problem. :(

Dead power transitor.

They have both short-circuited.
One is marked:

T
K2038
'41

My question is:

How hard would it be to find a substitute for it? Are they pretty much
generic? Would any high current (5A) transitor do?



- - thanks in adavnce,

-- jmuller.

Also out of circuit there is a short between where the outer legs used
to be. Is this normal?

The monitor says that these aren't often transistors but "expensive"
mosfets. What's the chances of replacing these and how expensive?
 
J

James Sweet

Jan 1, 1970
0
jmuller said:
Also out of circuit there is a short between where the outer legs used
to be. Is this normal?

The monitor says that these aren't often transistors but "expensive"
mosfets. What's the chances of replacing these and how expensive?


Look up the numbers on the cross reference on www.nteinc.com and compare
specs. I believe you're referring to the horizontal output transistor, these
can usually be subbed, but they're special high voltage (as in 1400v or so)
high current transistors usually. The "short" on the board is often normal,
caused by an inductor which has a low DC resistance. Often a HOT will also
appear shorted because of the internal damper diode or resistor but you
should be able to test two good diode junctions.
 
J

jmuller

Jan 1, 1970
0
Thanks for your reply.

The monitor in question is totally 100% dead. No HV, no LED, not even
degauss.

I have measured (very carefully) voltages on the board. I seem to have a
good B+ line but absolutely NO lower voltages. 12V and 18V points are
marked but are still dead. I feel the problem most likely lies in the
power supply. (However when the monitor died it did loose H-sync for a
second then the mains filter cap exploded, could this mean the HOT is
also dead?)

Does THIS transistor have an inductor parellel to it normally? As far as
I can see the HOT is OK (not blown up, etc).

Thanks for the link to the NTE cross reference software (downloading on
dial-up is painful).

Are you aware of any other good resources I should know about? For
instance a circuit diagram for this particular monitor would be VERY
helpful, do I have to pay the manufacturer for that (compaq)? Or are
they available elsewhere?

Thank you again =--- jmuller
 
S

Sam Goldwasser

Jan 1, 1970
0
2SK2038 is a MOSFET.

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J

James Sweet

Jan 1, 1970
0
The monitor in question is totally 100% dead. No HV, no LED, not even
degauss.

Is there a ticking or chirping noise?
I have measured (very carefully) voltages on the board. I seem to have a
good B+ line but absolutely NO lower voltages. 12V and 18V points are
marked but are still dead. I feel the problem most likely lies in the
power supply. (However when the monitor died it did loose H-sync for a
second then the mains filter cap exploded, could this mean the HOT is
also dead?)

It's entirely possible that the 12v and 18v supplies come from the flyback
transformer which is part of the horizontal circuit so if that's not working
those voltages will be dead. If the HOT is bad I would expect you have a
blown fuse or flameproof resistor that supplies B+ to it.

Does THIS transistor have an inductor parellel to it normally? As far as
I can see the HOT is OK (not blown up, etc).

I have no idea, I don't even know what monitor we're talking about here.
Thanks for the link to the NTE cross reference software (downloading on
dial-up is painful).

Don't download the software, the cross reference is available on their
website so you can just use it there.

Are you aware of any other good resources I should know about? For
instance a circuit diagram for this particular monitor would be VERY
helpful, do I have to pay the manufacturer for that (compaq)? Or are
they available elsewhere?

Compaq (and many other monitors) aren't made by the company who's name is on
them, they just pay whatever other manufacture gives them the best price to
build a bunch of monitors with their name on them. Schematic diagrams are
rarely available even from the manufacture as monitors are usually
considered to be disposable by them. Once you fix a few monitors it starts
to become fairly easy to do it without a schematic though.
 
J

jmuller

Jan 1, 1970
0
James said:
Is there a ticking or chirping noise?

No nothing. From what I can gather, the filter cap is charging then no
current draw at all. (I need a series light bulb, I know)

It's entirely possible that the 12v and 18v supplies come from the flyback
transformer which is part of the horizontal circuit so if that's not working
those voltages will be dead. If the HOT is bad I would expect you have a
blown fuse or flameproof resistor that supplies B+ to it.
From what I can gather (at least) the 18V rail is derived from the
switchmode-transformer (if that's the right word)

I have no idea, I don't even know what monitor we're talking about here.
It's a "151FS"
but I will get more specific model details tomorrow. (So few hours in a
day).


Don't download the software, the cross reference is available on their
website so you can just use it there.
Thanks for letting me know about this site. I haven't felt like I've had
this sort of power at my fingertips since I discovered google.

Now all I need to know is where I can get myself a 2SK2038 MOSFET in
Australia.
Compaq (and many other monitors) aren't made by the company who's name is on
them, they just pay whatever other manufacture gives them the best price to
build a bunch of monitors with their name on them. Schematic diagrams are
rarely available even from the manufacture as monitors are usually
considered to be disposable by them. Once you fix a few monitors it starts
to become fairly easy to do it without a schematic though.
With regard to finding a substitute for 2SK2038 what are the critical
parameters? Current comes to mind, power dissipation. Rise / Fall times?
Maximum switching frequency ?


-- Thanks in advance



- jmuller
 
J

James Sweet

Jan 1, 1970
0
With regard to finding a substitute for 2SK2038 what are the critical
parameters? Current comes to mind, power dissipation. Rise / Fall times?
Maximum switching frequency ?


Generally current, voltage, and on-resistance are the most important. It
shouldn't be difficult to find the correct part though, that's always the
best approach.
 
J

jmuller

Jan 1, 1970
0
James said:
Generally current, voltage, and on-resistance are the most important. It
shouldn't be difficult to find the correct part though, that's always the
best approach.

It just occurred to me,

If I replace this EXPENSIVE $5 MOSFET and the HOT is short. How do I
save my $5 MOSFET from blowing up again ?


-- jmuller
 
J

James Sweet

Jan 1, 1970
0
jmuller said:
It just occurred to me,

If I replace this EXPENSIVE $5 MOSFET and the HOT is short. How do I
save my $5 MOSFET from blowing up again ?


-- jmuller

That's cheap for a power transistor, I've repaired a few monitors lately
where the replacement part was $25. As for protecting it, wire the monitor
in series with a 100w lightbulb the first time you power it up.
 
J

jmuller

Jan 1, 1970
0
James Sweet wrote:

As it turns out. I found a "known good" power mosfet of the same type. I
installed it happily hoping the monitor would burst into life. However
it didn't. On the upside there was no smoke. Just absolutely no change.

Assuming I haven't blown the "good" MOSFET, where should I start looking
for problems? I really wish I had a circuit diagram (maybe someone could
help if I posted the correct model details ?.. meh tomorrow night.)
 
J

James Sweet

Jan 1, 1970
0
jmuller said:
James Sweet wrote:

As it turns out. I found a "known good" power mosfet of the same type. I
installed it happily hoping the monitor would burst into life. However
it didn't. On the upside there was no smoke. Just absolutely no change.

Assuming I haven't blown the "good" MOSFET, where should I start looking
for problems? I really wish I had a circuit diagram (maybe someone could
help if I posted the correct model details ?.. meh tomorrow night.)

Well start with the obvious, check the fuse, it usually will blow when the
mosfet does. Aside from that, check any fusible resistors, they're generally
a bit bigger around than normal resistors and have a dull sandy appearance.
 
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