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Old 12-23-2009, 03:39 PM   #1 (permalink)
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Electronics repair question, any EE's?

I have a JVC receiver, RX-D302B that I destroyed last week by unplugging a speaker to clean behind it, while the speaker was playing. I know, I'm dumb... should have shut it off before letting bare speaker wires touch

The unit's behavior is: when plugged in the standby LED illuminates and when I press the power button a relay inside clicks, and then clicks again about 0.5-1 second later (exact time varies each time)

So like any guy who knows barely enough to be dangerous, I opened up the case and was thrilled to see a burned fuse on the 120v AC relay board. I bought a replacement of the same value the next day (6.3A fast blow) and inserted it, plugged in the unit (with nothing else connected) and the behavior did not change, nor did the new fuse blow.

After inspecting visually and with my nose, the rest of the unit to the best of my ability I did not find any items that appear burned or damaged. No traces on the (single layer) PCB's look burned, everything looks and smells healthy.

Next I grabbed a multimeter, plugged in the receiver to 120v again, and probed the 120v side of the main power transformer: 0vac in standby, 116vac for the brief interval when the relay clicks on after pressing the power button. OK, power is getting to the xformer.

The other side of the xformer has 7 wires which go to various parts of the main boards. 3 heavy gauge go to what clearly is the output stage, 4 others (lighter wire gauge) to a daughtercard (including one which also jumpers to chassis ground) No combination of any of these 7 wires produces any voltage reading whatsoever on the multimeter, when the "wall" side of the transformer is getting 116vac .

Next I desoldered the "wall" side wires from the xformer so I could check continuity without the relay card to which it is connected, skewing my ability to check for an open circuit. The circuit appears open, no continuity on this side of the xformer.

Based on this, does it sound pretty safe to assume the transformer is cooked open & assuming no other damage occurred to the receiver, replacing the transformer with an identical unit ($60 from JVC parts/service) would be a likely fix?

I don't want to replace the whole receiver because I like it, it fits my furniture, and my domestic partner has made it pretty clear that she doesn't like it when I change things about how the home theater works

Thanks!

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Old 12-23-2009, 08:45 PM   #2 (permalink)
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Can you post some pictures?
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Old 12-23-2009, 09:31 PM   #3 (permalink)
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That might help eh?

Sorry they are with my cell phone, I only get access to the D80 when Joni's home..

Yfrog Album

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Old 12-23-2009, 10:26 PM   #4 (permalink)
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The transformer probably has an internal fuse. It might be possible to bypass it with another one.
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Old 12-25-2009, 02:02 AM   #5 (permalink)
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Is there a part number on the xfrmr? If so , you can probably find a wiring diagram on line for it. Then, rather than checking voltages, check resistance on the primary and the various secondaries. That way you can tell for sure if any of the windings have opened.
I have seen blown output transistors from speaker cables shorting too. Last I remember(about 15 years ago) those usually have three to 5 legs. Haven't really paid attention recently.
JJ

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