"Mark & Mary Ann Weiss" wrote in message
...
While I've managed to order a repair kit directly from QSC for my
Powerlight
6.0 PFC, QSC keeps urgently pressuring me to send the amp in for
recalibration.

I am replacing the chopper transistors, Q1-Q4 (Q2 is what failed) that
drive
the primary of the transformer and you can find this circuit on page 12 of
this schematic, available from QSC's web site:
http://www.qscaudio.com/support/libr...werLight%20Ser
ies/pl6.0PFC.pdf

Frankly, I don't see where this non-linear device, replacement of such,
would necessitate a recalibration of the controller, since this is simply
a
binary (on/off) process, chopping pulstating DC off the input rectifiers
and
producing the initial 180KHz powerwave that gets split up and filtered on
the secondary.

Are the variations among the same part number IGBT so great that a factory
recalibration of the entire unit be performed, using "proprietary test
equipment and procedures"?

My impression is that there is little to nothing to adjust for--I'm just
replacing a switch with a new switch. Performance should be the same. Am I
missing something?


Can't say in 25 years of repairing switchers on a daily basis, that I can
remember ever having workd on any particular variety which called for a
'recalibration' after replacing the switching element - or indeed any other
component on them. One slight caveat. If any such PSU has any adjustment
pots for output voltages, then yes, the rails should be checked and reset as
necessary using an accurate DVM after the repair is completed, but this
should be done as a matter of course anyway. I can only assume that they are
just being cautious and treating such a failure as a good 'excuse' to
subject the equipment to some routine checking and maintenance.

My feeling is that you should be good to go on your replacement, but I would
also recommend that before you turn it back on, you carefully check all of
the small electrolytics on the primary side, using an ESR meter, and maybe
check that there is drive to the chopper gates, with them disconnected, as
the drive circuitry appears quite complex on this item. If the drive was
stuck "on" due to a faulty 4552, I would have expected Q1 to probably have
blown as well, but worth checking anyway. Also, the gate drive resistors, as
these used to be favourites on a supply that I used to do lots of, after
failure of the chopper FETs.

Incidentally, your link appears to be incorrect as it comes up "page not
found", even filled in manually. I am assuming that it is " /PL6 " rather
than " /p16 " ??

Arfa