Help: NEC PJ STK conv IC fault
 

Problem is blowing Red channel output transistors in the STK392-040 IC
in the convergence board of my 6PG. The first IC blew a short to both
+/- VCC pins, the replacement (still in place) to the + VCC rail. This
is then burning the Neg FB / conv yoke return resistor.

First thing I did was run Red conv yoke from Green channel with no
obvious problem. Simple circuit and I have been right though it unable
to find anything wrong so far. The red signal has DC offset (normal)
and Red VCC is separate, so was wondering if partial short might
affect Red worse, so have turned attention back to yoke.

Measured resistance at .2 ohm which is same as other yokes, but at
limit of meter range and I know its the Q that counts anyway.

Baffled - any ideas - I have spare yoke - assume I can run it off the
tube.

I noticed preceding weeks to the original fault, the PJ came on with a
slightly blooming blank blue screen for a while before operating as
normal, this might be a red herring though.

PS. resistor I mentioned is axial 1/2 inch long of 3W, sky blue - am I
right in thinking it is std metal oxide type?

TIA. Clive.

reply by NG.

The red signal has DC offset (normal)

What do you consider normal, remember this is an inductive load. One
volt might be 5 amps, at full Vcc for that side of the IC. Anything
over =BD a volt is too much, but would not fry it immediately. Weeks
maybe. If it's 2 volts it needs fixed, 2 volts will fry it in minutes

This is not good.

It's about time you get the pinouts and start chcking it from there,
disconnecting certain things. Don't float an input pin to these ICs,
short to ground or a resistor to ground if you need to, floating won't
work in some ICs and I just don't remember if those ICs are OK with a
floating input. I think not,, but I'm not sure, gotta be grounded or
something.

Make sure to disconnect it and not blow what's driving it, some of
those drivers are really low impedance and can fry.

Not usually but it can happen.

JURB