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Marantz 2270 blowing fuses

G

gkOne

New Member
Here's an interesting one. I've got an early 2270 that's blowing fuses each time it's powered on... Unless I remove the top outer transistor on the right hand heat sink then it powers on Ok and I can hear some poor quality sound via the headphone output, but nothing through the speaker cables. It's a mystery, I've tested all other components on the output board and it all tested fine. Changed the transistors with new ones and same result. Any ideas where I could look for the culprit?
 
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You have a dead short somewhere and every time you blow another fuse the chance for further damage increases. Google a dim bulb tester, it is a very valuable tool for saving fuses and identifying the presence of a short. Cheap and won't take too much time to build. The bulb acts as a current limiter and gives you time to shut down the unit. You may not get the speaker protection relay to click with the DBT since less voltage will enter the power supply then the wall depending on the wattage bulb used.

Next, you possibly have a shorted driver or other transistor on the amplifier. I would work your way back up the signal path from the shorted output and find any bad transistors in front.
 
Many thanks for this very sound and practical advice. Will search on how to build a DBT and try that next.
 
Did you replace the mica insulators that go between the outputs and the heatsink?

If not, did you inspect them to make sure they were still good and not broken?
 
bryans12v said:
Did you replace the mica insulators that go between the outputs and the heatsink?

If not, did you inspect them to make sure they were still good and not broken?
Click to expand...

One of them has a distress mark as though it's been folded before so I'll replace it as you suggested. Now, do I need to put some of that heatsink compound or does it come pre-oiled? Noticed the original is a little oily but there's no clear sign of compound being used on it.
Many thanks!
 
I'm now checking wether the output transistors are correctly inserted. Not sure how to follow the schematics but I took a couple of pictures of the front and back of the amp board. Can anybody guide me/clues to look for in order to identify where NPN and PNP go? Or let me know what other angles you'd need to see to identify which is which.
 

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I am fairly sure the NPN outputs are on top, both sides in all variations of the 2270. You can check by tracing the connections to 759 and 760 (761 depending on when it was made) - these go to the NPN transistor.
 
@gkOne Did you ever find the issue? I have the exact same issue with an amp as I had take the transistor off the heatsink to replace the fan.. Now, after reattaching the transistor to the heatsink, the fuse blows. When not connected the amp works perfectly. I was also wondering if the thermal paste had anything to do with it or perhaps it something else. Thanks!
 
Did you use electrically conductive thermal paste like you might use on a computer? The paste needs to be electrically non-conductive. Also, did you replace the mica insulator or make sure the old one was intact?
 
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