Replacing Chopper Transistors on Switching PSU - QSC Says to Have it Factory Calibrated!

[Mark & Mary Ann Weiss]

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/library/schems/Discontinued/PowerLight Ser
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?


--
Best Regards,

Mark A. Weiss, P.E.
www.mwcomms.com
www.basspig.com
-

 

[Arfa Daily]

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/library/schems/Discontinued/PowerLight Ser
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

 

[rebel]

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.
(snip)

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?

only gullibility ....

 

[Mark & Mary Ann Weiss]

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.

After speaking with QSC's engineer, there is really only one thing that
*can* be critical, and that's the transformer balance (two 4552's driving
two banks of IXFN55N50's which drive two primary coils on T1), but I reckon
it would have to be off by quite a bit to pose a problem. The amp has all 1%
and 0.1% tolerance components throughout, which should be very stable.

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.

I was able to order a repair kit from QSC, which contained all of these
components. So, not wanting to take any chances, I replaced everything
associated with the failed device--the drivers, the diodes, the resistors,
the cap, the input rectifiers, etc. I also used ridiculous degrees of care
in cleaning every trace of flux off the board.

QSC admitted that their genius engineer who designed the PSU used some
'bleeding edge' implementation that was not approved by the device
manufacturer. Their only means of addressing it was to come out with a
higher rating of device and replace older devices. He also agrees with me
that it is likely a voltage-induced failure, series spikes exceeding the
junction breakdown voltage. I suggested a 400V MOV type device across the
junction to blunt those spikes and to isolate the pairs from eachother with
..05-ohm resistors on the drain terminals.

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

Because there's a space in the filename, the whole link didn't get parsed
when clicked on. You have to shade over the whole thing, copy and paste into
the browser to get the right doc.

MY blog has photos and a detailed blow by blow of this repair:

http://www.aamserver.dnsalias.com/basspig/Blogs/2007/Jan.htm#Jan 26

scroll down to Jan 26 to see the story.


--


Take care,



Mark & Mary Ann Weiss



VIDEO PRODUCTION • FILM SCANNING • DVD MASTERING • AUDIO RESTORATION
Hear my Kurzweil Creations at: www.dv-clips.com/theater.htm
www.basspig.com The Bass Pig's Lair - 15,000 Watts of Driving Stereo!
Business sites at:
www.mwcomms.com
www.adventuresinanimemusic.com
-

 

[Jim Land]

Because there's a space in the filename, the whole link didn't get
parsed when clicked on. You have to shade over the whole thing, copy
and paste into the browser to get the right doc.

Just for the record, putting a space character in a filename or web address
is considered bad practice. It's easy to use an underscore _ or hyphen -
instead of a space. FWIW.

 

[Mark & Mary Ann Weiss]

Just for the record, putting a space character in a filename or web address
is considered bad practice. It's easy to use an underscore _ or hyphen -
instead of a space. FWIW.

I know. But I'm not the webmaster for QSC's web site. ;-/


--


Take care,



Mark & Mary Ann Weiss



VIDEO PRODUCTION • FILM SCANNING • DVD MASTERING • AUDIO RESTORATION
Hear my Kurzweil Creations at: www.dv-clips.com/theater.htm
www.basspig.com The Bass Pig's Lair - 15,000 Watts of Driving Stereo!
Business sites at:
www.mwcomms.com
www.adventuresinanimemusic.com
-