Kirk S. wrote:
"Arfa Daily" wrote in message
...

"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

Yeah, isolation transformer or not, it's still lethal amps. I don't like to
probe with the line voltage on. I'll power it off, hook things up, stand
back and slowly bring up the power. I checked the small electrolytic cap on
the supply pin and it checks fine. I have no reference voltage from the
chip either. The symptoms were longer and longer startup times which has me
thinking that although the esr reading on that cap is ok, it may still be
dead. Might as well replace it first and see what happens. I don't get a
tweet, tweet, tweet of a cycling supply. I hate just swapping parts however
it might be the easiest thing to do...

I know that working around things like this can lead to accidents. Line
voltage is nothing to play with.

I like reading the electronics repair faq as well. Lots of good info and
constant warnings about discharging caps, using isolation transformers, one
hand. A little reminding doesn't hurt, either...

Thanks for the info!

BTW, you better power your circuit, not the scope from that isolation
transformer. That will isolate your entire construction from the hot wire
thus making your work much safer.

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