"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 " 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!