Hi all;

Well, got another one. Loses all convergence correction after running awhile.
At first, no problem, threw in the STK730 on the convergence PS, but it failed
as I was putting the back on. I sort ruled out the rectifiers in this case
because of two reasons. For one, IIRC if a diode were to short, it should maybe
take out the STK, if not it should be squealing in current limiting. For two,
when I went into the menu for customer convergence adjustment, it started
working again.

Ordered the convergence board and after installing two defective ones, I
finally got a good one. Thonpson's market share in our area is such that we
actually do not have a chipper checker. What I did was to put the EPROM from
the original board into the new one, and all I had to do was to touch up the
vertical lines at the sides using V80. This is expected due to slightly
different slewing rates of the outputs. No problem.

Now the way I understand it, the EPROM data is loaded either at startup or
powerup, doesn't matter, noboby was fiddling with convergence when it failed
for me, therefore it should not be the EPROM at fault. If I am incorrect please
enlighten me, but so far I am at a loss.

Reason I ask is that the set is about to be a recall, we got the call today.

I read in the training manual that certain supplies are monitored and will
result in convergence shutdown if they drop. In the print I have seen the
detection circuit, it only detects a drop in volage of the lower
(non-commutated) supplies. This is as they describe it. If this is happening,
does access by the customer menu reset it ?

The way I see it I have one of two ways to go, either I got a shorted or
intermittently shorted convergence yoke, or one those diodes in the PS
secondary is opening up.

If a yoke has a shorted winding all the time, it's unlikely for the set to have
good convergence, which it does when it works. The ones I've seen might
possibly be able to align with one shorted turn, but looking at your average
convergence yoke, I think it unlikely. An intermittent short really would have
to be pretty bad to trigger shutdown since the system is current feedback.
Although it would require less voltage, thus loading the low output supply
more. It wouldn't really wait long though, it would be bad right away.

If it's a rectifier opening up, why would it come back when you access the menu
?

As with some things I work on, it seems to be haunted.

A penny for your thoughts, heck three pennies !

Thanx in advance and thanx for reading my dog day blues.

JURB