"Kirk S." wrote in message
...
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.



The best place to connect the ground of your 'scope for primary side
monitoring, is the negative terminal of the main primary side smoothing cap.
But please, please be very careful if you're not an experienced engineer. I
know you say that you're using an isolation transformer, but the belief that
this makes you totally safe against electrocution, can lead to complacency.
If you manage to get the mains across you, even if it's being supplied by an
isolation transformer, this can still at the very least give you a very
nasty shock, and under extreme conditions, result in death. Also be aware
that under certain circumstances, with SMPS's, the mains earth on your scope
can be an issue.

If you have startup voltage, but no chopper drive output from the '3842,
suspect first any small electrolytic caps associated with the chip,
particularly the one connected to the supply pin. It's also not at all
uncommon for the chip itself to fail.

BE CAREFUL - Keep one hand in your pocket, whilst taking readings.

Arfa