swr 550x blowing power amp chips and fuses....?

[acorkos]

swr 550x bass head...the owner said he was playing a gig with the back opening of the rack case pushed up against a heavy curtain, probably blocking air flow. the panel-mounted 8 amp mains fuse blew after an hour, and 4 out of 6 of the 2sc3264 sanken power amp transistors were shorted. a new fuse held with the jumper to the power amp assembly disconnected. i checked the other components on the power amp assembly and they all seemed to check out ok.

anyway, i replaced the 4 shorted 2sc3264's, and the amp worked fine when i tested it....i played around with it for about 10 minutes. the owner called back after receiving the amp and said the fuse blew again after only 2 minutes. i checked it again and the panel mount 8A fuse was intact, but the internal 10A fuse in series with the 8A panel mount fuse was blown, and all 6 of the 2sc3264's were blown.

i've ordered 6 more 2sc3264's, but i'm wondering if i missed something else, or if the owner is doing something wrong. any ideas of other things i should check before i possibly blow 6 more power amp chips? thanks for your help!

aris

[Rick Erickson]

First thing I would do is crack open one of the blown replaced output devices and look at its substrate. It doesn't matter where you bought them - there are a LOT of counterfeit transistors on the market and they will not handle the current demands of a pro audio amp. For more info search Google for "Counterfeit Transistors". Beyond that, check to be sure all the other components are good, that the current limit transistors are functioning properly and that the idle current is stable. Solder anything that looks like it might be loose, etc..

RE

[acorkos]

what does authentic substrate look like compared to counterfeit?

[Rick Erickson]

Did you do the search I mentioned? Sometimes you have to crack a few nuts to get the answers you seek.

RE

[Enzo]

Where DID you get the transistors? For the record, I have always got good ones from MCM. So far...

If all the outputs are blown, replace them. Replace the drivers, yes even if they test OK.

Are ALL six emitter ballast resistors intact? None open?

How did you set the bias? Is the bias adjust control functional?

They will never hear a little crossover, so do the amp a favor and set the bias a little cold.

When the amp appears to be working, run it up to clip with a sine wave - I use 100Hz - WITHOUT a speaker load. Scope it. Does the output waveform extend pretty much all the way up to rail voltages? Do both positive and negative peaks clip about the same? They should.

Now add the speaker load - resistive is fine - and run the amp up half way. Let it sit there like that for a while. Have the cover on the amp so the conditions aer real world. Scope the output. Waveform still pretty? If the amp can handle 3/4 power for an hour, THEN I might start to trust it.

[acorkos]

ok...sorry. i just did a google search and checked the counterfeit sanken pics. i cracked a couple of the failed chips open and these appear to be authentic.