CTX PL9 Blowing HOT

[Kirk S.]

Hi,

Hope someone can help me. Got a CTX PL9 monitor just quit working. Power
LED comes on ok and all voltages are good at SMPS. The HOT (Toshiba
2SC5411) was fused and during my probing, I killed the FS3KM-18A MOSFET on
the flyback. Power supply was not coming up after I shorted it. I replaced
both and still had no Horizontal at the yoke. Found fusible 10 Ohm 1 watt
resistor (R341) open and replaced. Now the HOT is fused again and R341 is
open. Basically, I think I'm at where I was when I started only about $12
poorer and a little wiser.

I supposedly have the manual yet it is for a 1995UE and nothing matches at
all. No schematics for the main board at all.

Any ideas? I don't want to keep blowing HOTs; they aren't cheap. The
deflection is controlled by a TDA4857PS and the voltages are within the
range on the manual. I was also getting 0.4 vdc at the base of the HOT
before it fused.

Kirk S.

 

[Kirk S.]

FWIW:

Removed the fused HOT and replaced the open resistor. Now have voltage at
the collector. Seems a bit high for the resolution I am running. On
multi-scan monitors, is the voltage to the horizontal varied depending on
the scan rate? Probably depends on the make, eh?

The HOT dies instantly so I'm thinking it has something to do with:

a) bad base drive - oscilloscope waveform looks ok however I don't know what
it should look like. It has a nice steady form that is broken up on regular
intervals.

b) bad snubber cap - .0047uF 2kv; how can I test it?

c) damper diode - passes the diode test however what does that tell me?

Of course, this is all just from reading the repairfaq. My assumptions are
likely incorrect.

 

[Cobalt]

| b) bad snubber cap - .0047uF 2kv; how can I test it?

Check the solder on all the timing caps too. Unless the monitor has a LOT
of hours on it , these usually are ok.

| c) damper diode - passes the diode test however what does that tell me?

you can try replacing that or try an equivalent sub HOT with internal
damper. Did you use the correct HOT as original? Are you sure it was indeed
the original and no
no one worked on it before and subbed it? Usually a degraded damper will
allow the monitor to function for a brief time before the HOT overheats
without protection. A higher refresh mode excellerates the time before it
quits. If your HOT instantly pops, check if there is a FET feeding its
collector, it may be shorted, else perhaps a bad FBT. If you post the FBT
number I can tell you if its a common fail part.



| Of course, this is all just from reading the repairfaq. My assumptions
are
| likely incorrect.
| | > Hi,
| >
| > Hope someone can help me. Got a CTX PL9 monitor just quit working.
Power
| > LED comes on ok and all voltages are good at SMPS. The HOT (Toshiba
| > 2SC5411) was fused and during my probing, I killed the FS3KM-18A MOSFET
on
| > the flyback. Power supply was not coming up after I shorted it. I
| replaced
| > both and still had no Horizontal at the yoke. Found fusible 10 Ohm 1
watt
| > resistor (R341) open and replaced. Now the HOT is fused again and R341
is
| > open. Basically, I think I'm at where I was when I started only about
$12
| > poorer and a little wiser.
| >
| > I supposedly have the manual yet it is for a 1995UE and nothing matches
at
| > all. No schematics for the main board at all.
| >
| > Any ideas? I don't want to keep blowing HOTs; they aren't cheap. The
| > deflection is controlled by a TDA4857PS and the voltages are within the
| > range on the manual. I was also getting 0.4 vdc at the base of the HOT
| > before it fused.
| >
| > Kirk S.
| >
| >
|
|

 

[Kirk S.]

| b) bad snubber cap - .0047uF 2kv; how can I test it?

Check the solder on all the timing caps too. Unless the monitor has a LOT
of hours on it , these usually are ok.

| c) damper diode - passes the diode test however what does that tell me?

you can try replacing that or try an equivalent sub HOT with internal
damper. Did you use the correct HOT as original? Are you sure it was indeed
the original and no
no one worked on it before and subbed it? Usually a degraded damper will
allow the monitor to function for a brief time before the HOT overheats
without protection. A higher refresh mode excellerates the time before it
quits. If your HOT instantly pops, check if there is a FET feeding its

Yeah, there is a FET feeding it. Voltage is high on all pins. I traced it
back and it is connected directly to the +200v line from the power supply.
Gate is connection via zener and resistor to same line. Gotta be some sort
of signal from the horizontal oscillator going there, right? There is a
100uf 250v Elect Cap between the source and ground of the FET so I have to
believe that is where the voltage comes from.

There is also a FET on the flyback and voltage is the same on it.

collector, it may be shorted, else perhaps a bad FBT. If you post the FBT
number I can tell you if its a common fail part.

FBT is 47F13-0910A

Wish I had a good schematic...

Thanks!

Kirk S.

 

[Cobalt]

| Yeah, there is a FET feeding it. Voltage is high on all pins. I traced
it
| back and it is connected directly to the +200v line from the power supply.
| Gate is connection via zener and resistor to same line. Gotta be some
sort
| of signal from the horizontal oscillator going there, right? There is a
| 100uf 250v Elect Cap between the source and ground of the FET so I have to
| believe that is where the voltage comes from.

Is the FET shorted? Check it with a meter with power removed from the
Monitor. If they go , they short out, so if it reads infinity
on the DMM its likely ok.

| There is also a FET on the flyback and voltage is the same on it.

Probably ok, but check the same way as above.

| FBT is 47F13-0910A

This is a fairly common fail type of FBT and I can supply you a replacement
if need be.
You also need to be sure the original HOT was indeed the same type as was in
there. Do you think it was serviced previously?
If its just a shorted FET then you need replace the HOT anyhow at the same
time.

 

[Kirk S.]

| Yeah, there is a FET feeding it. Voltage is high on all pins. I traced
it
| back and it is connected directly to the +200v line from the power supply.
| Gate is connection via zener and resistor to same line. Gotta be some
sort
| of signal from the horizontal oscillator going there, right? There is a
| 100uf 250v Elect Cap between the source and ground of the FET so I have to
| believe that is where the voltage comes from.

Is the FET shorted? Check it with a meter with power removed from the
Monitor. If they go , they short out, so if it reads infinity
on the DMM its likely ok.

Yeah, dead short on that FET. Too much voltage for horizontal frequency and
fries the HOT immediately. FET is 2SJ449 and HOT is 2SC5411. Wonder if it
did any damage upstream. MCM has both of them relatively cheap. Bought a
couple just in case. Found some info on the net about testing FETs, wish I
had better equipment. Why are P-Channel FETs so much more expensive?
Materials cost?

| There is also a FET on the flyback and voltage is the same on it.

Probably ok, but check the same way as above.

Replacement FET is cheap. FS3KM-18A from MCM. Some similar monitors have a
fixed voltage on the flyback drive. Voltage wasn't the same on all three
terminals like it was on the FET driving the HOT collector.

| FBT is 47F13-0910A

This is a fairly common fail type of FBT and I can supply you a replacement
if need be.

Thanks... What sort of resistance should I be getting between theFBT
terminals? Probably too low to measure with a DMM, eh? It isn't melted nor
did the monitor smoke. Picture just went out.

You also need to be sure the original HOT was indeed the same type as was in
there. Do you think it was serviced previously?

Not likely, I bought it new three years ago. The service literature I have
is for a newer model; none of the part designations are the same however the
parts themselves are the same. Just hunting around other schematics that
use similar circuits and hoping for the best.

If its just a shorted FET then you need replace the HOT anyhow at the same
time.

I replaced the HOT and it shorted again immediately. Have gotten the FET
out (heatsink had to come out with it) and will replace parts once I get
them. Sure don't make them easy to work on. Now I know that when I see one
of these with a HOT shorted, the driving FET is probably suspect as well.
This FET is a plastic one, would a standard TO-220 case be better for heat
dissipation?
Seems the more I learn, the less I find out that I know... Ignorance is
bliss...

Thanks for the info!

Kirk S.

 

[Cobalt]

| Yeah, dead short on that FET. Too much voltage for horizontal frequency
and
| fries the HOT immediately. FET is 2SJ449 and HOT is 2SC5411. Wonder if
it
| did any damage upstream. MCM has both of them relatively cheap. Bought a
| couple just in case. Found some info on the net about testing FETs, wish
I
| had better equipment. Why are P-Channel FETs so much more expensive?
| Materials cost?


Thats your whole problem I bet. Very common and it baffles alot of
servicers.
MCM is not the cheapest source around because they have a $25 Min order.
If in the USA, check out: www.electronica-usa.net or call in with those part
numbers if you have not placed the order.
Another good Monitor Parts / semiconductor Supplier is: www.ljecc.com

| Replacement FET is cheap. FS3KM-18A from MCM. Some similar monitors have
a
| fixed voltage on the flyback drive. Voltage wasn't the same on all three
| terminals like it was on the FET driving the HOT collector.

If its not shorted , probably ok.

| Thanks... What sort of resistance should I be getting between theFBT
| terminals? Probably too low to measure with a DMM, eh? It isn't melted
nor
| did the monitor smoke. Picture just went out.

You rarely see a shorted or open winding on a FBT. Usually what happens is
the built in diodes on the secondary short or open,
or the small built in capacitor off them shorts or opens up. Flyback failure
causing a HOT to short in a monitor is hardly common, usually when flyback
is bad, it will cause a shutdown or a clicking noise on power up. Your
flyback is likely ok.

| I replaced the HOT and it shorted again immediately. Have gotten the FET
| out (heatsink had to come out with it) and will replace parts once I get
| them. Sure don't make them easy to work on. Now I know that when I see
one
| of these with a HOT shorted, the driving FET is probably suspect as well.
| This FET is a plastic one, would a standard TO-220 case be better for heat
| dissipation?

Replace the FET and HOT at the same time, use heat sink compound on the HOT
or a thermopad, and I bet its fixed.
If original parts has plastic cases, then replace with same, metal tabs are
connected to the center lead of the device usually and
if the heat sink is live, it will cause more problems in addition to whats
already being worked on. If these parts fail again in a relatively short
time, then you have a bad / leaky damper. As I said before, the higher the
refresh rate the monitor is used at, the sooner a degraded damper will cause
the HOT and FET to blow again. Im figuring the 2 Parts are probably all it
needs though. Good Luck.

 

[Kirk S.]

| Yeah, dead short on that FET. Too much voltage for horizontal frequency
and
| fries the HOT immediately. FET is 2SJ449 and HOT is 2SC5411. Wonder if
it
| did any damage upstream. MCM has both of them relatively cheap. Bought a
| couple just in case. Found some info on the net about testing FETs, wish
I
| had better equipment. Why are P-Channel FETs so much more expensive?
| Materials cost?


Thats your whole problem I bet. Very common and it baffles alot of
servicers.
MCM is not the cheapest source around because they have a $25 Min order.
If in the USA, check out: www.electronica-usa.net or call in with those part
numbers if you have not placed the order.
Another good Monitor Parts / semiconductor Supplier is: www.ljecc.com

Ok, I'll put them in my favorites list...

| Replacement FET is cheap. FS3KM-18A from MCM. Some similar monitors have
a
| fixed voltage on the flyback drive. Voltage wasn't the same on all three
| terminals like it was on the FET driving the HOT collector.

If its not shorted , probably ok.

| Thanks... What sort of resistance should I be getting between theFBT
| terminals? Probably too low to measure with a DMM, eh? It isn't melted
nor
| did the monitor smoke. Picture just went out.

You rarely see a shorted or open winding on a FBT. Usually what happens is
the built in diodes on the secondary short or open,
or the small built in capacitor off them shorts or opens up. Flyback failure
causing a HOT to short in a monitor is hardly common, usually when flyback
is bad, it will cause a shutdown or a clicking noise on power up. Your
flyback is likely ok.

Got a Hitachi CM715 with shorted damper diode by the flyback. Causes power
cycling. Traced from SMPS, removed FBT driver FET and HOT, and finally
found a shorted diode on 2nd terminal of FBT. At first it was really
confusing and then I remembered on my pinball machines, all the coils have
diodes to handle the feedback surge. Same thing after the FBT field
collapses, large surge has to go somewhere. If diode shorts, B+ goes to
ground. SMPS is overloaded and won't come up.

| I replaced the HOT and it shorted again immediately. Have gotten the FET
| out (heatsink had to come out with it) and will replace parts once I get
| them. Sure don't make them easy to work on. Now I know that when I see
one
| of these with a HOT shorted, the driving FET is probably suspect as well.
| This FET is a plastic one, would a standard TO-220 case be better for heat
| dissipation?

Replace the FET and HOT at the same time, use heat sink compound on the HOT
or a thermopad, and I bet its fixed.
If original parts has plastic cases, then replace with same, metal tabs are
connected to the center lead of the device usually and
if the heat sink is live, it will cause more problems in addition to whats
already being worked on. If these parts fail again in a relatively short
time, then you have a bad / leaky damper. As I said before, the higher the
refresh rate the monitor is used at, the sooner a degraded damper will cause
the HOT and FET to blow again. Im figuring the 2 Parts are probably all it
needs though. Good Luck.

Yeah, the plastic cases aren't as good for heat transfer however it prevent
shorts to ground on the heat sink. Perhaps replacing the HOT damper at the
same time as the HOT would be a good idea.

With help from you, luck doesn't have anything to do with it. I don't have
any electrical engineering training however some of this is starting to make
sense. What I am finding is that typically a $1 (or under) part brings down
a $300 monitor. Just gotta keep my brain thinking and limit live circuit
testing.

Thanks from a hobbyist...

Kirk S.