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

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

(snip)

only gullibility ....

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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.

would

as

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.

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:

26

scroll down to Jan 26 to see the story.

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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:
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"Mark & Mary Ann Weiss" wrote in news:rSwuh.29363$ snipped-for-privacy@fe02.news.easynews.com:

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.

address

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
-