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Woops! Shorted transistor during testing...blew fuse

Homer4beer

Homer4beer

Well-Known Member
Hi guys,

I've been trying to trouble shoot my Marantz PM7200 amp with the help of member MBZ (he's been AWESOME). Well I stuffed up and slipped while testing the emitter of a transistor with an oscilloscope. Yea I slipped and shorted it out with some minor sparks. I removed ASAP and turned the amp off.

When I turned it back on the main fuse blew :(:(:(

I thought I'd stuffed the transistor however I pulled and tested it----it's fine. So I have no idea what other damage I may have done.

It was transistor number 7256 on the board.

Guys, seriously, you'd be saving my bacon if you can suggest what else I MAY have fried in the process. I was testing for an audio sine wave at the collector when I slipped. I may have shorted the emitter.

Cheers,
Homer

Thread here;
http://audiokarma.org/forums/index.php?threads/bought-a-marantz-pm7200-dead-channel.791273/

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You might not have damaged anything, if the fuse did its job. Are you sure it blew when you turned the unit off and not when you shorted the transistor?

Have you tried replacing the fuse?
 
Homer4beer said:
It blew when I tried to turn it back on. I have not replaced the fuse yet.
Click to expand...
Ah. You wrote, "when I turned it off the main fuse blew". :)

The usual failure mode is melted semiconductors. Start by checking nearby transistors and diodes for dead shorts, i.e., 7252, 7254, 6258 & 6260, etc.
 
Thanks Dave. I fixed my original post. This amp has been a real pain to work out why the right channel is not working.
 
From your description, there is a good chance that the fuse melted partially when you shorted the transistor. Then, when you powered up again the normal initial current surges were enough to melt it the rest of the way. Good to be cautious, of course, but you might not have any more of a problem than that.
 
DaveVoorhis said:
The usual failure mode is melted semiconductors. Start by checking nearby transistors and diodes for dead shorts, i.e., 7252, 7254, 6258 & 6260, etc.
Click to expand...

You were right Dave. 3 cooked resistors so far and a couple of bad resistors. I'm going to check every component on the right channel now for damaged components.

Good times :no:
 
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