HOT failure on NEC 5FG

[Sam Goldwasser]

Well, it finally happened. My wonderful, trusty, pampered 5FG finally
went belly up. It appeared to be at the point where Windows switches
to the normal resolution at boot time. There was no warning - the green
light just went off. I found the 2SC3998 shorted CBE. There is something
interesting though. Across C-E is a single LARGE damper diode in a TOP3
case but in addition, there are a pair of smaller diodes across this. Why?
They are in the same direction so it seems they should never conduct at all.
Backup? And, I did find cracked solder joints on these auxiliary diodes so
they connections might be suspect but how could this make any difference?
The monitor powers up fine without the HOT installed but I wonder what else
it might have damaged.

Is the underlying cause (assuming it's not those mystery diodes) related
to the likely bad pile of electrolytic caps or something else? The only
detectable indication of bad electrolytics was an occasional tendencey to
be slightly stretched vertically at power-on which would correct itself over
the period of a minute or so, usually it seemed, when it was very humid.

P.S. Is there any chance a 2SC3995 would sub (at least as a test) for
a 2SC3998 since I have one of those.

Thanks.

--- sam

 

[Sam Goldwasser]

It is not interesting at all to find a diode inside the transistor (In fact
I find it boring)

Sorry, I do know about damper diodes inside HOTs. I wrote the S.E.R FAQs!
But I'm describing a situation where there is an external damper diode
across the HOT, and a second set of 2 additional diodes in series across
that. It would seem that the second set of diodes should never conduct
unless the other one failed open.

Some TV use HOT's with no diode inside it, in which case the diode would be
found across the out side the transistor

2SC3995 is 12Amp, 180Watt
2SC3998 is 25A , 250W

OK, too small but might work at low res.

otherwise it is the same.
my information source says both transistors do NOT have damper diodes
inside, that's for the C-E pins.

You need the damper diode across the Collector-Emitter (what looks like
reverse direction) to make the picture horizontal linearity look better and
the HOT will last longer than a few minutes as well,

More like failure in 1 horizontal scan if there is no damper diode.

To avoid making mistakes have a look at the tutorial on the line output
stage. http://www.alphacron.de/download/hardware/horiz-tv.pdf

Nice article but I also wrote the "TV and Monitor Deflection Systems FAQ"
at http://www.repairfaq.org/sam/deflfaq.htm

Thanks.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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[techforce]

I "Think" the extra diodes are for the Higher Scan Rates. If Thats true,
then I bet the 5FG goes Beyond 1024-768?

On an HP M50 I am working on, there is an Internal Damper in the HOT as Well
as a DMV32 Dual Damper Diode on the Outside. I think the Lower Scan Rates
will have the internal Damper working since its a slower Switch rating, and
the Higher Scan Rates will have the DMV32 Taking over. A lot of People will
use the Lower Scan Rates for years, then when they aell the monitor, the new
owner wants to run in hi res mode all the time, so if the Wrong HOT or
Damper is used, it will likely shorten the Life of the HOT. Mine exhibited
Low Beta (gain) so it had trouble turning on at all.

 

[Eric]

(as I was going to post this and had my reply in the outbox, I read the next
posting and ........... is this NEC 5FG a monitor? I thought it was a TV
never mind it should be the same.)

A 90º tube would only have one damper diode

A 110º tube would have two damper diodes, there is a capacitor across the
diode and the diode between each diode goes off the East-West modulator. to
correct the geometry on the sides of the picture.

With no diode on a 90º tube you do get a bit of scan on the other side of
the picture but it is not good, (I would say roughly from memory 50%, only
seen it once), before the HOT packs in, but you might get about 30 seconds
to view it, if you try it (at your own risk), turn it off as son as you see
the picture.


:
: > It is not interesting at all to find a diode inside the transistor (In
fact
: > I find it boring)
:
: Sorry, I do know about damper diodes inside HOTs. I wrote the S.E.R FAQs!

: But I'm describing a situation where there is an external damper diode
: across the HOT, and a second set of 2 additional diodes in series across
: that. It would seem that the second set of diodes should never conduct
: unless the other one failed open.
:
: > Some TV use HOT's with no diode inside it, in which case the diode would
be
: > found across the out side the transistor
: >
: > 2SC3995 is 12Amp, 180Watt
: > 2SC3998 is 25A , 250W
:
: OK, too small but might work at low res.
:
: > otherwise it is the same.
: > my information source says both transistors do NOT have damper diodes
: > inside, that's for the C-E pins.
:
: > You need the damper diode across the Collector-Emitter (what looks like
: > reverse direction) to make the picture horizontal linearity look better
and
: > the HOT will last longer than a few minutes as well,
:
: More like failure in 1 horizontal scan if there is no damper diode.
:
: > To avoid making mistakes have a look at the tutorial on the line output
: > stage. http://www.alphacron.de/download/hardware/horiz-tv.pdf
:
: Nice article but I also wrote the "TV and Monitor Deflection Systems FAQ"
: at http://www.repairfaq.org/sam/deflfaq.htm
:
: Thanks.
:
: --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
: Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
: +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
: | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html
:
: Important: The email address in this message header may no longer work.
To
: contact me, please use the Feedback Form at repairfaq.org. Thanks.
:

 

[Sam Goldwasser]

I "Think" the extra diodes are for the Higher Scan Rates. If Thats true,
then I bet the 5FG goes Beyond 1024-768?

On an HP M50 I am working on, there is an Internal Damper in the HOT as Well
as a DMV32 Dual Damper Diode on the Outside. I think the Lower Scan Rates
will have the internal Damper working since its a slower Switch rating, and
the Higher Scan Rates will have the DMV32 Taking over. A lot of People will
use the Lower Scan Rates for years, then when they aell the monitor, the new
owner wants to run in hi res mode all the time, so if the Wrong HOT or
Damper is used, it will likely shorten the Life of the HOT. Mine exhibited
Low Beta (gain) so it had trouble turning on at all.

That might make sense though in the 5FG, both sets of diodes are
external. The single diode is in an impressive TOP3 case of its
own so I would be surprised if it weren't spec'd to be acceptable
for all scan rates but this is the first explanation that might also
explain why the HOT blew. I did find cracked solder joints on all
4 terminals of the pair of diodes though I couldn't detect any
intermittent connections with an ohmmeter when wiggling them.

Thanks.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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[Eric]

Some one is trying to pull your tit Sam

As I said if a TV or a monitor uses two damper diodes in series it is for
the East-West modulation, it has got nothing to do with resolution,

You should have a bank (3) of big capacitors & MOSFETS that switch in and
out depending on the monitor resolution.



:
: > I "Think" the extra diodes are for the Higher Scan Rates. If Thats true,
: > then I bet the 5FG goes Beyond 1024-768?
: >
: > On an HP M50 I am working on, there is an Internal Damper in the HOT as
Well
: > as a DMV32 Dual Damper Diode on the Outside. I think the Lower Scan
Rates
: > will have the internal Damper working since its a slower Switch rating,
and
: > the Higher Scan Rates will have the DMV32 Taking over. A lot of People
will
: > use the Lower Scan Rates for years, then when they aell the monitor, the
new
: > owner wants to run in hi res mode all the time, so if the Wrong HOT or
: > Damper is used, it will likely shorten the Life of the HOT. Mine
exhibited
: > Low Beta (gain) so it had trouble turning on at all.
:
: That might make sense though in the 5FG, both sets of diodes are
: external. The single diode is in an impressive TOP3 case of its
: own so I would be surprised if it weren't spec'd to be acceptable
: for all scan rates but this is the first explanation that might also
: explain why the HOT blew. I did find cracked solder joints on all
: 4 terminals of the pair of diodes though I couldn't detect any
: intermittent connections with an ohmmeter when wiggling them.
:
: Thanks.
:
: --- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
: Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
: +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
: | Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html
:
: Important: The email address in this message header may no longer work.
To
: contact me, please use the Feedback Form at repairfaq.org. Thanks.
:
:

 

[Sam Goldwasser]

Some one is trying to pull your tit Sam

As I said if a TV or a monitor uses two damper diodes in series it is for
the East-West modulation, it has got nothing to do with resolution,

You should have a bank (3) of big capacitors & MOSFETS that switch in and
out depending on the monitor resolution.

I'm just stating what I see in the monitor and what's in the schematics:

o----+----+--------+-------+-------+
| | _|_ | |
|/ | /_\ | |
o--| HOT _|_ | _|_ _|_
|\ /_\ _|_ --- ---
| | /_\ | |
| | | | |
o----+----+--------+-------+-------+

or

http://repairfaq.cis.upenn.edu/Misc/5fghot.gif

Sorry, that's the best resolution I have on the schematic so can't read
the part numbers and the monitor is buttoned up at the moment.

That's it. No components attached between the diodes.

The E/W MOSFETs, etc. are separate.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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Important: The email address in this message header may no longer work. To
contact me, please use the Feedback Form at repairfaq.org. Thanks.