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Old 10th June 2009, 05:28 AM   #11
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Just registered at Audio karma so it will be a few days before I can ask a question there
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Old 10th June 2009, 07:47 AM   #12
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Well, I hang out here too.

But why you think I've got a rabbit to pull out of my hat is a mystery.

Anyway, here's the section of the board that is toasted according to your pic (circled in red):
Click the image to open in full size.

IF that hole is new, and the main symptom of the current failure, I'd venture to say that the amp oscillated itself to death and these resistors (and perhaps the .047f film cap) experienced a total meltdown...although why it didn't set off smoke detectors in the house is a mystery. Can't say that I see holes in PC boards too often.

Here's a thread over at AK about the oscillation problems in the L-07MII amp, which I believe is due to a combination of aging electrolytic caps, and especially due to the use of what I call 'Black Flag' ceramic caps used for the 'pf'-values on the driver board. I've seen these 'Black Flag' caps on several high-speed amps (Kenwood L-07MII, Sansui AU-919 & BA-F1, and a few others), and I believe they are an early design of a stacked ceramic cap that simply goes bad over the years.

Something else you may have to fight is that many of these amps sold in countries other than the US had a thermal fuse installed inside the transformer housing. When the amp self-destructs like this, the thermal fuse often dies too as the transformer overheats. I have had zero luck trying to replace this fuse on my own, but instead have sent the transformer off to a pro and let him deal with it.

The last bummer is that you may have killed the super-rare Panasonic output transistors (2SC2337A and 2SA1007A). These are what give the amp its speed and rather nice sound. The best you can probably do (if you find dead ones) is to replace them all with 2SA1116 and 2SC2607 from this guy here, who has a small supply of these original transistors which are not as fast as the originals, but make a good replacement nonetheless.
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Old 10th June 2009, 09:14 AM   #13
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You are correct, on my working amps there does seem to be the parts circled.

First thing first, what do I need to do to protect my working pair of L 07 Ms????????

Output transistors are branded NEC.

Rabbit not expected, but hoping you can point me in the right direction
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Old 10th June 2009, 09:16 AM   #14
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Oh and we pulled the smoke detectors out of the lounge room as they were constatly set off by the wood fire.
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Old 10th June 2009, 11:23 AM   #15
Stuey is offline Stuey  Australia
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Hi Echowars,

I knew you were here too...

I had already searched quickly on AK and saw a comment by you on this amp, saying that the 'II' was prone to oscillation but the 'I' wasn't. So I thought you were the man with the rabbit!

Plus you've helped me over at AK with one of my old Japanese classics...

You da man!

Cheers

Stuey
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Old 10th June 2009, 11:46 AM   #16
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OK I have to ask, which "Classic" do you own? Hmmmmmm?
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Old 10th June 2009, 12:00 PM   #17
Stuey is offline Stuey  Australia
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Oh, I guess I should just say 'old'...

I can't remember which EW helped me with; either my Onkyo A-5 or my Pioneer SX-434 or my Kenwood KA-3500. All mint, though.

Stuey
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Old 10th June 2009, 12:21 PM   #18
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OP how are the speakers coming?
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Old 10th June 2009, 09:45 PM   #19
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Quote:
Originally posted by Moondog55
You are correct, on my working amps there does seem to be the parts circled.

First thing first, what do I need to do to protect my working pair of L 07 Ms????????

Output transistors are branded NEC.

Rabbit not expected, but hoping you can point me in the right direction
Oops....yes, those are NEC trannies, not Panasonic.

To protect the amp, the pf-value caps (the 'Black Flag' caps) need to be replaced. I replace them with silver mica's. It's also a great idea to get rid of the original electrolytic caps, as their age may well contribute to the failure. I also usually replace Cf9 with a good metalized polypropylene (or metal polyester) cap.
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Old 11th June 2009, 12:29 AM   #20
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Thanks.
I will have to find help doing the work but they are worth holding onto.
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