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Fuses on Kenwood L-O7M mono amps
Fuses on Kenwood L-O7M mono amps
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Old 17th October 2020, 12:31 AM   #1
simoon is offline simoon  United States
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Join Date: Feb 2010
Default Fuses on Kenwood L-O7M mono amps


This is more for just curiosity sake, not really a problem.

I've owned these 70's beauties for years, had them checked out a couple of years ago, and they still meet factory specs. they are built like battleships). And they sound great too.

On the bottom side of the amps (under a annoyingly pain in the a$$ panel to remove), there are 2 fuses; a 5 amp and a 1 amp. The 5 amp fuse has never blown for me, but if the speaker cables touch accidently when the amp is on, the 1 amp blows.

When the 1 amp blows, the amp still outputs audio, but it is a few db lower, and highly distorted. Sounds almost worse than a blown speaker driver. It almost sounds as if one were to continue playing the amp like this, it could damage the speaker.

My question is, why the hell does this fuse being blown still allow the amp to output such a distorted signal? Why not just disable all output?

For those that are unfamiliar with these amps, they were part of Kenwood's high end line, and their (successful. IMO) attempt at competing with the best of the US high end audio products.

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Old 17th October 2020, 12:50 AM   #2
rayma is online now rayma  United States
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Join Date: Apr 2011
The 1A fuse is likely to be within the feedback loop, so when it blows there is still a path, but at a lower level.
Shorting the amplifier output is a big no-no, regardless. I would make whatever changes are necessary
in the speaker wires for the output to not be so easily shorted. That is not normal or benign. The fuse is
telling you to change your ways. You don't want the amplifier output transistors to start telling you that too.

Last edited by rayma; 17th October 2020 at 01:02 AM.
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Old 17th October 2020, 02:09 PM   #3
Chamberman is offline Chamberman  United States
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Join Date: Jun 2001
Location: USA, Dallas/Ft Worth, Tx
I rebuilt a set of these L-07m's a couple of years back for a local audiophile.

The 1A fuse you're referring to is showing as a 100mA fuse in the schematic. It supplies power to the main AC power relay circuit. This function is selected via the rear panel switch as Remote/Mains.

When this fuse blows the amp should power down.

If this fuse is blowing and you still have sound then this says that the mains relay is likely stuck on.

I second what Rayma said about shorting the amp output. Your bigger concern is not "why do I still have sound when the low amperage fuse blows?" but more "why are my speaker cables shorting?" You are damn lucky that a blown fuse is the only problem you have especially if it has happened more than once. You need to figure out what modification you need to make to ensure that the speaker cable will be able never short again.

My first suggestion,
Never physically move the speakers, speaker cables, or amplifier with the amplifier powered on. The only time in 30+ years of audio listening I've ever come close to having the issue you describe of speaker cables touching is when I tried to move or reposition a piece of gear. I learned long ago to have everything powered off before trying to move anything connected to the amplifier. Most of my amps are on the floor in my audio room for easy access to allow for swapping and I do not even walk around behind them with the power on.
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Old 17th October 2020, 10:32 PM   #4
simoon is offline simoon  United States
diyAudio Member
Join Date: Feb 2010
Thanks for the answers!

The speaker cable shorting was a very quick, accidental issue, 100% user error. Not an ongoing problem.

I already made a small correction on my setup which will prevent it from happening again.
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