LA7838 Vertical IC and Mitsubishi CS-3535R

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I'm at the end of a long repair of this Mitsubishi CS-3535R.
I've replaced the switching regulator and related caps to
get the primary side working, the 30 volt regulator
transistor and related zener with bias resistors that burned
up, a burned zener and bad cap near the "black level
corrector IC," a couple of fuses  and finally the vertical
output IC - a Sanyo LA-7838 and all the caps in the vertical
section.

The initial failure involved smoke and the repair was a
pain, as I smoked several components on the main board and
saw smoke once in the vicinity of the CRT neck.  The last
bit of repair was the replacement of the LA-7838, which I
replaced with an NTE sub.

The picture currently has a slightly brighter horizontal bar
about 20% down from the top.  The scan lines are slightly
closer together there, and I suspect that it is the closer
scan lines that make it appear brighter there, but it may
actually be brighter.  The picture is slightly compressed at
the line, but looks OK above and below.  The caps were all
replaced in the vertical section hoping to get rid of the
vertical problem (which was not there prior to the power
supply problems.)

I've gotten advice from some here whose advice I really
respect, and the consensus is that it's probably not the
vertical IC, despite the fact that I used a sub.  However,
having replaced all the electrolytic caps in the vertical
section, I'm not sure where to go now.  

I'm going to put a scope on the output, and I expect I'll
see a kink in the ramp at the 20% point.  I thought I'd
scope the power to the vertical IC too, just to see if it's
fluctuating, but it goes through a pair of zeners and a
large cap, and the voltages look OK on the DMM.

Anyone want to give some more advice?
Thanks.

The natural function of the wing is to soar upwards and carry that which is
heavy up to the place where dwells the race of gods. More than any other thing
that pertains to the body it partakes of the nature of the divine.

Plato, 'Phaedrus.'

Re: LA7838 Vertical IC and Mitsubishi CS-3535R

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Sounds like you either missed the vertical boost cap, usually about 100uF at
35V, near the vertical output IC, or else a B+ filter near the flyback,
something like 4.7uF at 160V or 10uF at 160V, etc. If it's the boost cap,
the vertical output IC will be running hot.

Mark Z.



Re: LA7838 Vertical IC and Mitsubishi CS-3535R

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There were two of these - both replaced.  One on pin 8
(second B+) and one on pin 9 (Pump Up Output).

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If you suspect this, where would you put the scope to verify
(as long as I've got it out and hooked up?)

Here's a photo of the current LA7838 output.

http://delpet.com/images/LA7838Output.jpg

Re: LA7838 Vertical IC and Mitsubishi CS-3535R
On Wed, 12 Oct 2005 09:38:54 -0400, T o d d P a t t i s t

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What about the corresponding diode? Could it be leaky?

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Re: LA7838 Vertical IC and Mitsubishi CS-3535R
On Wed, 12 Oct 2005 09:38:54 -0400, T o d d P a t t i s t

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What voltage are you measuring on pin 13? Is it significantly higher
than pin 8? If not, then you will be running out of headroom. I'd
replace D4E2 just in case. A 1N4007 should be OK. I'm also curious as
to where the LH trace at pin 9 goes.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Re: LA7838 Vertical IC and Mitsubishi CS-3535R

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I had no chance to work on this yesterday, but in the 5
minutes this morning before heading out, I quickly put the
scope on pin 13 (vertical drive) and on the 30V supply which
connects directly to pin 8.  The 30V supply/pin 8 looks
reasonable - a bit of ripple, but no more than 5%, perhaps
less.  

Then I looked at pin 13.  It looks like a 5% on duty cycle
30V signal - 95% of the time off!  That would mean I'd be
looking at a 30V drop on forward biased D4E2.  Whatcha think
- time to replace it? :-) Seems reasonable to think it blew
when the IC blew, and the vertical drive is somehow coming
via pin 8 instead of pin 13.

Re: LA7838 Vertical IC and Mitsubishi CS-3535R
On Fri, 14 Oct 2005 09:40:32 -0400, T o d d P a t t i s t

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My memory is hazy, but I do know that I once had an obscure problem
that was cured by replacing this diode in another set. I'm sorry but I
can't recall if I observed the waveform on a scope. I'm still puzzled
by the trace going to the left. It doesn't appear in the application
diagram in the datasheet.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Re: LA7838 Vertical IC and Mitsubishi CS-3535R
If you have not washed all of the electrolyte from the boards, corrected the
corrossion damage on the traces, and checked every polar electrolytic in the
set for leakage, you are wasting yout time.   This set is well known for
having several dozen leaky caps.  If you saw smoke from the area of the
yoke, you could have a bad yoke, but with your symptom this is unlikely.
Some of these develop bad spots but when they do they usually just blow the
output IC.  If you are seeing linearity problems, you likely have a bad cap.
You likely missed the cap on the vertical supply.  We don't waste time on
these unless the client wants to pay for a complete cap job.  Litterally
every polar electrolytic needs to be checked.  You will likely find about 3
dozen bad.

Leonard

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Re: LA7838 Vertical IC and Mitsubishi CS-3535R

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I've been working on this for a while, and I've seen this
comment before - about multiple bad caps.  I've only found
about 3-4 that really seem bad.  I've washed the electrolyte
everywhere I see it, and I have repaired some traces.

LA7838 Vertical IC and Mitsubishi CS-3535R
"T o d d P a t t i s t" bravely wrote to "All" (11 Oct 05  11:34:16)
 --- on the heady topic of "LA7838 Vertical IC and Mitsubishi CS-3535R"


That kink might be due to ripple in the vertical IC's supply voltage.
Follow the vertical IC's supply lines until you get to the filter
electro and rectifiers from the flyback derived +24 volts, change
these too.

 A*s*i*m*o*v


 Todd> Xref: core-easynews sci.electronics.repair:344895

 Todd> I'm at the end of a long repair of this Mitsubishi CS-3535R.
 Todd> I've replaced the switching regulator and related caps to
 Todd> get the primary side working, the 30 volt regulator
 Todd> transistor and related zener with bias resistors that burned
 Todd> up, a burned zener and bad cap near the "black level
 Todd> corrector IC," a couple of fuses  and finally the vertical
 Todd> output IC - a Sanyo LA-7838 and all the caps in the vertical
 Todd> section.

 Todd> The initial failure involved smoke and the repair was a
 Todd> pain, as I smoked several components on the main board and
 Todd> saw smoke once in the vicinity of the CRT neck.  The last
 Todd> bit of repair was the replacement of the LA-7838, which I
 Todd> replaced with an NTE sub.

 Todd> The picture currently has a slightly brighter horizontal bar
 Todd> about 20% down from the top.  The scan lines are slightly
 Todd> closer together there, and I suspect that it is the closer
 Todd> scan lines that make it appear brighter there, but it may
 Todd> actually be brighter.  The picture is slightly compressed at
 Todd> the line, but looks OK above and below.  The caps were all
 Todd> replaced in the vertical section hoping to get rid of the
 Todd> vertical problem (which was not there prior to the power
 Todd> supply problems.)

 Todd> I've gotten advice from some here whose advice I really
 Todd> respect, and the consensus is that it's probably not the
 Todd> vertical IC, despite the fact that I used a sub.  However,
 Todd> having replaced all the electrolytic caps in the vertical
 Todd> section, I'm not sure where to go now.
[,,,]

... 'Keep the smoke inside.' -- 1st Rule of Electronics.


Re: LA7838 Vertical IC and Mitsubishi CS-3535R

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I've been through this area a lot, but I could have missed
something.

The SMPS produces 14V, 30V, 130V and 220V.  There were
problems in all, but particularly the 14V and 30V.  I looked
quickly with a scope this morning at the 14V supply to the
chips electronics.  It goes through a pair of zeners and
filter caps to get dropped to 11.5V.  It has a 100 mv ripple
kink on it at vertical scan frequency. I suppose I could
check to see if the ripple is at the kink.  Does that sound
excessive?

The 30V provides the drive for the vertical output on the
IC.  I haven't looked at it yet.

I was wondering if there was a safe way to use the vertical
output to drive something other than the yoke to see if that
kink is caused by the yoke.  Just a resistor (specs say the
IC produces 2.2A max and the supply is 30V) or maybe a yoke
from an old multisync monitor?

It seems to me it's got to be:

1) The yoke or related components are bad.  There's a board
with some diodes, a cap, etc on the CRT.  Possible since I
saw smoke near the yoke.  Yoke had lots of dust on it.  I've
inspected the yoke and nearby components carefully, and
don't see any signs of damage.

2)  the new sub vertical IC chip is bad (unlikely)

3) The supply voltage (there are at least two) to the chip
is poorly filtered.  You'd think a bad filter kink this
would drift over the vertical scan wouldn't you?  Mine is
always at the exact same spot 20% down from top. Possible
since I had lots of SMPS problems.

4) Something wrong on one of the feedback systems/pins.
Hmmm.

(sorry, thinking out loud here.)

Re: LA7838 Vertical IC and Mitsubishi CS-3535R
Since the B+ for vertical deflection circuitry flyback derived and is
powered by the flyback it won't have the normal "drift over the
vertical scan" like in the power line derived B+... ...by the way, 100
mv ripple is terrific, only about 1%, so I don't think that is an
issue.... if you think it is, tag on an additional filter cap after the
series diode coming from the flyback.... say 470uf at 35 volts.... see
if that makes a change for the better.
Are you certain that the yoke is not defective.... did it ring test OK?
The LA7838 or NTE subs are fairly cheap, go ahead and replace so you
can confirm that the problem is not with the chip.... the NTE subs for
the Vert Delf chips have always works fine for me.  The small cost for
the chip is a lot cheaper than many hours pulling out your hair.
Look for leaky diodes other than the zeners.
It is possible that one of the new replacement caps has marginal ESR...
 I always check two or three of them in a package and select the very
best one for critical circuits.   Not all new parts are always good.
electricitym
.
.


Re: LA7838 Vertical IC and Mitsubishi CS-3535R

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I don't think this is right.  I've got part of the circuit
and the 30V drive comes from the main transformer driven by
the switching regulator IC, the output of which is half wave
rectified and fed through a zener/transistor regulator.
Both the switching IC on the primary side and most of the
30V regulator circuitry burned out in the initial failures.

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No.


I've ring tested a flyback before, but even that's been a
while. I figured disconnecting it in some way and testing
the waveform output from the chip would be easier.

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I was on this road early, but hit two snags, first, those
with far more experience than me conclude this is very
unlikely, and second, I can't seem to find the Sanyo LA7838.
I know it's common, but Mits Parts does not have it, local
efforts have all failed, and the online I checked all have
NTE subs only. I don't see a lot of value in adding the same
sub part.  If I'm going to redo it, I might as well stick in
the original.   I'm sure someone here will tell me where I
should have looked, but .....

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I'm sure the new parts test OK with the ESR - better than
what I took out.



Re: LA7838 Vertical IC and Mitsubishi CS-3535R
"T o d d P a t t i s t" bravely wrote to "All" (12 Oct 05  14:52:53)
 --- on the heady topic of "Re: LA7838 Vertical IC and Mitsubishi CS-3535R"

 Todd> Xref: core-easynews sci.electronics.repair:345032

 Todd> The SMPS produces 14V, 30V, 130V and 220V.  There were
 Todd> problems in all, but particularly the 14V and 30V.  I looked
 Todd> quickly with a scope this morning at the 14V supply to the
 Todd> chips electronics.  It goes through a pair of zeners and
 Todd> filter caps to get dropped to 11.5V.  It has a 100 mv ripple
 Todd> kink on it at vertical scan frequency. I suppose I could
 Todd> check to see if the ripple is at the kink.  Does that sound
 Todd> excessive?

100mv is about 1%, that isn't much but it might be a clue.


 Todd> The 30V provides the drive for the vertical output on the
 Todd> IC.  I haven't looked at it yet.

What are you waiting for?


 Todd> I was wondering if there was a safe way to use the vertical
 Todd> output to drive something other than the yoke to see if that
 Todd> kink is caused by the yoke.  Just a resistor (specs say the
 Todd> IC produces 2.2A max and the supply is 30V) or maybe a yoke
 Todd> from an old multisync monitor?

I think I've seen a manufacturers suggested test circuit. Can'd
remember where now. Perhaps a handbook...


 Todd> It seems to me it's got to be:

 Todd> 1) The yoke or related components are bad.  There's a board
 Todd> with some diodes, a cap, etc on the CRT.  Possible since I
 Todd> saw smoke near the yoke.  Yoke had lots of dust on it.  I've
 Todd> inspected the yoke and nearby components carefully, and
 Todd> don't see any signs of damage.

A visual inspection won't necessarily turn up which part let out the
smoke. This is basic stuff.


 Todd> 2)  the new sub vertical IC chip is bad (unlikely)

 Todd> 3) The supply voltage (there are at least two) to the chip
 Todd> is poorly filtered.  You'd think a bad filter kink this
 Todd> would drift over the vertical scan wouldn't you?  Mine is
 Todd> always at the exact same spot 20% down from top. Possible
 Todd> since I had lots of SMPS problems.

 Todd> 4) Something wrong on one of the feedback systems/pins.
 Todd> Hmmm.

Might be a small value electro used for feedback from the yoke socket.


 Todd> (sorry, thinking out loud here.)

Who doesn't. But look here, you have a permanent symptom and a scope,
it should be child's play. If the output from the LA7838 is a nice
trapezoid without a kink then the yoke is questionable, especially
since you actually saw it smoke!

 A*s*i*m*o*v

... If all else fails, hurl it across the room a few times!


Re: LA7838 Vertical IC and Mitsubishi CS-3535R


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I probably qualify as better than a "child" but it's still
slow going for me :-)

I tried disconnecting the vertical part of the yoke to see
if I get the "nice trapezoid without a kink" I think I
should see, but there's no output from the chip then.  I
presume I can't just pull the entire yoke without risking
burning the center of the tube (or some auto shutoff
protection)  That's why I'm here - looking for more
experience.   My next thought was to just put a high wattage
resistor on the output, limiting to say 1/4 amp at 30 volts,
then look at the output, but I was hoping someone here would
suggest that or something similar.  I figure there's no need
for me to come up with creative ways of testing when I know
others have already come up with them and, probably, made
the stupid mistakes for me.


Re: LA7838 Vertical IC and Mitsubishi CS-3535R
"T o d d P a t t i s t" bravely wrote to "All" (13 Oct 05  09:55:22)
 --- on the heady topic of "Re: LA7838 Vertical IC and Mitsubishi CS-3535R"

 Todd> Xref: core-easynews sci.electronics.repair:345160

 Todd> wrote:

 >Who doesn't. But look here, you have a permanent symptom and a scope,
 >it should be child's play. If the output from the LA7838 is a nice
 >trapezoid without a kink then the yoke is questionable, especially
 >since you actually saw it smoke!

 Todd> I probably qualify as better than a "child" but it's still
 Todd> slow going for me :-)

 Todd> I tried disconnecting the vertical part of the yoke to see
 Todd> if I get the "nice trapezoid without a kink" I think I
 Todd> should see, but there's no output from the chip then.

No, leave the yoke plugged in and scope the output.


 Todd> I presume I can't just pull the entire yoke without risking
 Todd> burning the center of the tube (or some auto shutoff
 Todd> protection)

It is easy to miss the obvious: no image, no burn spot.
(a.k.a. Turn the brightness & contrast down.)



 Todd>  That's why I'm here - looking for more
 Todd> experience.   My next thought was to just put a high wattage
 Todd> resistor on the output, limiting to say 1/4 amp at 30 volts,
 Todd> then look at the output, but I was hoping someone here would
 Todd> suggest that or something similar.

A resistor is not a good load because the proper current waveshape is
formed by the inductance of the vertical coils. You could use a known
good yoke externally. However, as a replier wrote: ring the "yoke",
not the fbk.


 Todd> I figure there's no need
 Todd> for me to come up with creative ways of testing when I know
 Todd> others have already come up with them and, probably, made
 Todd> the stupid mistakes for me.

I think the kink points strongly to a marginal electrolytic capacitor.
Perhaps related instead to the the vertical oscillator. I now recall
there is a small electro approx 47uF (or 4.7uF?) in the oscillator
section of the LA7838 that should be replaced even if it tests good.
Perhaps off pin 6 or pin 3, but possibly the size control. This might
be something to try if you can't find any obvious cause for the kink.

 A*s*i*m*o*v

... Reactance: your imaginary friend.


Re: LA7838 Vertical IC and Mitsubishi CS-3535R
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I did - I posted a link to the trace.

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I've already tried disconnecting only the vertical yoke, but
there's no output.
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That's what I suspected.

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This would require some research on the procedure - no
problem, but sometimes these things require some experience
to correctly interpret.

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Thanks.  I've replaced every electrolytic cap near the
vertical, and checked to see if any have similar numbering.

If you are interested, here's a partial of the circuit in
this area:

http://delpet.com/images/VerticalCircuit.jpg

Replacing them had no effect at all.   I don't have all the
circuit, but someone who does has told me there's another
board that processes the vertical signal, so I'm looking
there now.  I have to admit I'd love to know if the problem
is associated with the yoke or the circuit before the yoke.

Re: LA7838 Vertical IC and Mitsubishi CS-3535R
"T o d d P a t t i s t" bravely wrote to "All" (13 Oct 05  16:51:57)
 --- on the heady topic of "Re: LA7838 Vertical IC and Mitsubishi CS-3535R"

 Todd> Xref: core-easynews sci.electronics.repair:345193

 Todd> wrote:

 > However, as a replier wrote: ring the "yoke",
 >not the fbk.

 Todd> This would require some research on the procedure - no
 Todd> problem, but sometimes these things require some experience
 Todd> to correctly interpret.

A ringer is basically a pulse generator the output of which is applied
to the coil to be tested through a relatively high impedance. When
observed on a scope, testing a good inductance, the resulting waveform
should show a number of fading resonnant oscillations. If there is
one or more shorted winding in the inductor this robs the magnetic
field energy and dampens that natural oscillation. So instead of 4 to
6 oscillations there may be only 1 or 1-1/2 and depending how badly
shorted, there may even be none. Shorted turns behave like the
secondary of a transformer and rob power from the self inductance.


 Todd> I have to admit I'd love to know if the problem
 Todd> is associated with the yoke or the circuit before the yoke.

Well there's always the pincushion correction circuitry. Could be bad
diodes there. However, pincushion affects the bowing of the sides and
a single kink is more a vertical ramp anomaly. Either the output adds
the kink or the vertical oscillator's ramp generation does.

Did you check if the signal at the ramp pin of the LA7838 is linear?

 A*s*i*m*o*v

... Is reactance then illusory?  No, it just appears that way...


Re: LA7838 Vertical IC and Mitsubishi CS-3535R
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...

This is fixed. The problem was the 30V "regulator" circuit.
It fed the vertical drive and had too much ripple.   For
comments, I've posted it here.

http://delpet.com/images/30VRegulator.jpg

The unregulated input had better regulation than the
regulated side.  At first glance this seems like a plain old
30 volt zener holding the base at 30 volts and the emitter
thereby being held 0.7 volts lower.  But note the collector
voltage - 29.2 volts.  It's feeding the zener through the
zener bias resistor R4H1.  There's not enough voltage to
drive the zener into breakdown, particularly if there's any
current through R4H1 to feed the zener or base of the
regulator transistor Q4A1.   I used an NTE sub transistor
NTE382 IIRC) and subbed the 30V zener. Tweaking the drive
current and adding a larger filter cap eliminated the
horizontal line.  I probably could have just gotten a better
sub for both.

Of course, as soon as I fixed that, I noticed a dozen faint
vertical lines that I eventually tracked down to ringing on
the horizontal from an exploded 33 uf 250 volt cap on the
CRT board.  Repair is done at last.

Anyone want to comment on the 30 V regulator circuit?

Re: LA7838 Vertical IC and Mitsubishi CS-3535R
"T o d d P a t t i s t" bravely wrote to "All" (17 Oct 05  15:34:47)
 --- on the heady topic of "Re: LA7838 Vertical IC and Mitsubishi CS-3535R"

 Todd> Xref: core-easynews sci.electronics.repair:345528

 >I'm at the end of a long repair of this Mitsubishi CS-3535R.
 >I've replaced ... the 30 volt regulator
 >transistor and related zener with bias resistors that burned
 >up,
 ...
 >The picture currently has a slightly brighter horizontal bar
 >about 20% down from the top.

 Todd> This is fixed. The problem was the 30V "regulator" circuit.
 Todd> It fed the vertical drive and had too much ripple.   For
 Todd> comments, I've posted it here.

 Todd> http://delpet.com/images/30VRegulator.jpg

 Todd> The unregulated input had better regulation than the
 Todd> regulated side.  At first glance this seems like a plain old
 Todd> 30 volt zener holding the base at 30 volts and the emitter
 Todd> thereby being held 0.7 volts lower.  But note the collector
 Todd> voltage - 29.2 volts.  It's feeding the zener through the
 Todd> zener bias resistor R4H1.  There's not enough voltage to
 Todd> drive the zener into breakdown, particularly if there's any
 Todd> current through R4H1 to feed the zener or base of the
 Todd> regulator transistor Q4A1.   I used an NTE sub transistor
 Todd> NTE382 IIRC) and subbed the 30V zener. Tweaking the drive
 Todd> current and adding a larger filter cap eliminated the
 Todd> horizontal line.  I probably could have just gotten a better
 Todd> sub for both.

 Todd> Of course, as soon as I fixed that, I noticed a dozen faint
 Todd> vertical lines that I eventually tracked down to ringing on
 Todd> the horizontal from an exploded 33 uf 250 volt cap on the
 Todd> CRT board.  Repair is done at last.

 Todd> Anyone want to comment on the 30 V regulator circuit?


Only that the 7838 uses +24 volts but I've seen as high as 29.
And the moral of this story is: when in doubt blame the supply!
I'm filing this one, thanks...

 A*s*i*m*o*v

... A couple of volts below threshold.


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