"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 " 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

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.