LA7838 Vertical IC and Mitsubishi CS-3535R

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.'

Reply to
T o d d P a t t i s t
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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.

Reply to
Mark D. Zacharias

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

Reply to
Leonard Caillouet

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

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.

formatting link

Reply to
T o d d P a t t i s t

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.

Reply to
T o d d P a t t i s t

"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> From: T o d d P a t t i s t 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.

Reply to
Asimov

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.)

Reply to
T o d d P a t t i s t

On Wed, 12 Oct 2005 09:38:54 -0400, T o d d P a t t i s t put finger to keyboard and composed:

What about the corresponding diode? Could it be leaky?

-- Franc Zabkar

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

Reply to
Franc Zabkar

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 . .

Reply to
electricitym

"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> From: T o d d P a t t i s t 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!

Reply to
Asimov

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.

Reply to
T o d d P a t t i s t

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.

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.

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 .....

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

Reply to
T o d d P a t t i s t

"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> From: T o d d P a t t i s t Todd> Xref: core-easynews sci.electr >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.

Reply to
Asimov

I did - I posted a link to the trace.

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

That's what I suspected.

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

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:

formatting link

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.

Reply to
T o d d P a t t i s t

On Wed, 12 Oct 2005 09:38:54 -0400, T o d d P a t t i s t put finger to keyboard and composed:

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.

Reply to
Franc Zabkar

"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> From: T o d d P a t t i s t Todd> Xref: core-easynews sci.electr > 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...

Reply to
Asimov

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.

Reply to
T o d d P a t t i s t

...

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.

formatting link

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?

Reply to
T o d d P a t t i s t

On Fri, 14 Oct 2005 09:40:32 -0400, T o d d P a t t i s t put finger to keyboard and composed:

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.

Reply to
Franc Zabkar

"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> From: T o d d P a t t i s t Todd> Xref: core-easynews sci.electronics.repair:345528

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>

formatting link

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.

Reply to
Asimov

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