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Harman Kardon PM640, what is this burnt thing?

Hi folks, I have a PM640 as well. Worked perfectly for years and then suddenly blew the power fuse when I turned it on the other day. Took it apart and replaced F1 with a 4amp 250v. Blew again when I powered it up. I also notice the same transistors were “burnt” looking. I’m a newbie to electronics, but wondering if there are any suggestions of where to look to fix my problem? Thanks.
 
So after reading a bit more here, I’m now thinking my issue is with the output transistors.

once I determined where they were I tested them for continuity. The set on the left were all open. I’m assuming that’s bad, as it didn’t match the set on the right. I’ve ordered new ones.
 

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Hi:
This is an old thread (begun in June 2008, yikes!), might be worth your while to start a fresh post.

That being said, in your picture (I added red boxes) there looks like water stains on the PCB:
hk.jpeg

Are you original owner or know complete history of this unit?
 
Thanks for your reply. I’m new to the forum, I’ll post a thread if/as this thing progresses.

I’ve owned it for about 5 or 6 years, always worked great. I use it in my garage with a couple of ‘70s Sony speakers and two newer Polks.

Never had a problem until a few days back when I hit the power button and she lit up for a second and then just shut off.

I’ve got some experience with electronics, did a few courses in high school and my dad repaired tube radios, but not near enough to go very deep on this.
 
bnorman said:
So after reading a bit more here, I’m now thinking my issue is with the output transistors.

once I determined where they were I tested them for continuity. The set on the left were all open. I’m assuming that’s bad, as it didn’t match the set on the right. I’ve ordered new ones.
Click to expand...
Welcome to AK!

Best way to test transistors is with the diode test function on your DMM. Here's a great tutorial from a member who used to post here:
https://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/

Also build a dim bulb tester. This is crucial to avoid blowing the replaced transistors in the event that there is more wrong than just the outputs... and with direct coupled amps, it's virtually always more than just the outputs.
https://audiokarma.org/forums/index.php?threads/dim-bulb-tester-build-and-how-to.808399/

BTW, the ones that read good continuity are probably bad; the failure mode of output transistors is a dead short. And speaking of output transistors, where did you order the ones you've got coming? They're obsolete, and the chances of finding good original stock is extremely rare. Money isn't the only thing counterfeited these days.
 
Thanks so much for your reply, I’ll definitely check out the link on testing transistors. I’ve had somebody else suggested I make a dim bulb tester as well, I will do that when I get a chance. As for the transistors, I googled one and found somebody selling a few of both of the ones I thought I needed together, coming from China so as you say… Good chance they may not be real. We will see, they didn’t cost a lot of money.

By the way, I’ve started a new thread on this here:
https://audiokarma.org/forums/index...ld-cause-an-output-transistor-to-blow.965091/

Thanks again for your reply.
 
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