"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 " 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:345160

Todd "Asimov"
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.