"The Schneids" wrote in message
...
Kirk S. wrote:
Hi,

I'm trying to figure out a dead monitor power supply that uses a UC3842B
controller. I'd like to hook my scope to the chip and observe any
signals
during startup, etc. Since this is connected to rectified line voltage
on
the ground plane and a large power resistor on the vcc line, I'm unsure
where to hook my ground for the scope probe. I'm using an isolation
transformer, variac and series light bulb in addition to the scope.

I'm just a tinkerer used to scoping clock signals on TTL circuits and
hesitant to do something that will toast my scope.

Any suggestions? Oh yeah, no schematics for the monitor either. I'm
looking at a similar schematic using the same control chip. I see the
startup voltage of +16vdc and no reference voltage or output strobe
signals.

Help!

Kirk S.



I've worked on these a lot - I don't use the scope anymore.
When I did, I made sure the ground of the scope was floating -
use one of those 2 prong to 3 prong adapters to isolate the
ground. BUT - since the IC isn't working - there really aren't
any signals to see. The +16v is good - suppy voltage. No ref
voltage says the chip is shut down. Also, if memory is good, the
16v has to go up over 18v to turn the chip on. It gets this via
feedback from the flyback [usually]. No feedback voltage - chip
doesn't turn on. Check all the diodes around the 3842 chip. And
all the other electrolytic caps around the 3842 too.

The next thing I'd check would be all the rectifying diodes on the
secondary of the power supply circuit. If one of those is bad - no
low voltages - no flyback voltage - chip doesn't turn on. Then I'd
check the horizontal output transistor.

There were a couple of times I actually had to replace the 3842; but,
usually it was something else that caused the 3842 to shut down. They
used to be pretty cheap at Dalbani; but, it looks like they've gone
out of business....????

later, The Schneids

Interesting to see that you run your 'scope with the ground disconnected. I
do too, but hesitated to suggest that the OP did this, because unless you're
well experienced, as I am, and clearly you are too, it can be a dangerous
practice.

I actually have the ground wire disconnected from inside the plug, and
brought out through the crack between the two halves of the plug, so that it
can be clearly seen to be disconnected, and I also have a Dymo label on the
plug top saying " Caution. No Earth "

One day, we're both gonna fry .... |:-(

Arfa