powerampfreak wrote in message
...
On 14 Juni, 12:08, JW wrote:
On Sun, 13 Jun 2010 14:16:36 -0700 (PDT) powerampfreak
wrote in Message id:
:



I was given an oscilloscope, a Fluke PM3082 from a retired industrial
electronics engineer.
He said that during a measurement, the oscilloscope's powersupply just
bang and a flame came out of the rear panel. After this, the scope was
left for several years...until I got it.
The mains fuse has really exploded. Shattered glass in the fuse
holder. A brief examination of the primary side of the smps reveals no
shorts. The smps is an ordinary flyback supply. Mains rectifier ok,
switch ok, snubber, line filters all ok. Really don't know why the
mains fuse went.
I just don't want to put a new fuse in and try again, not before I
investigate the cause.
Could an incorrect measurement setup cause the mains fuse to explode?
Anyone familiar with these scopes or perhaps has a service manual for
it?
Best regards
PAF

IF you are sure there's nothing wrong on the primary, check for shorted
diodes and caps on the secondary side of the switching transformer. You
mention that you checked the "switch". Do you mean the power
transistor(s)
that drives the primary side of the switching transformer?

As Jeff said, isolate the power supply from the rest of the scope if
possible. You should be able to use a lower rated fuse at this point to
help minimize any damage if you don't have any way to current limit the
AC
input.

http://www.jetecnet.com/claims to have a service manual for $10.

Might also want to search for free manuals for PM3084, PM3092, or
PM3094.
They may share the same power supply.


I've examined the primary side of the powersupply very carefully.
Absolutely NO signs of damage anywhere.
The Y caps measures correct 2nF (2pcs of 1nF) also X cap ok at 0.22uF.
A NTC inruch limiter measures 15ohm, correct according to the text on
it, primary switch transistor ok, mains rectifier diodes ok all of
them...
But the curiousity took control over me. I cleaned up after the blown
fuse. Put a new one (1.6AT) - connected it to mains - and it works!
All channels and functions seems to operate just fine.
There are really no sign of flames anywhere. One thing which I haven't
paid any attention yet, is the mains filter which is an integrated
block of some coil and filtercaps encapsulated.
I will change the Y and X caps to make sure they're ok.
I've never seen a shorted secondary side make the mains fuse explode.
Just primary shorts has the power to do this, in my opinion based on
hundreds of smps repairs.
Anyway, it appears that I've got another nice looking four channel
oscilloscope on my bench! :-)









What are the primary side HV caps like? I would replace or at least remove
and inspect underneath.