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Old 24th February 2008, 07:41 PM   #1
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Default Kenwood L-07MII

Hello everyone,

I just purchased a pair of Kenwood L-07MII mono blocks. One works fine. The other was known to be a problem with overheating within 10 to 20 minutes of operation at the time of purchase. When I got the pair I took the one which was a known problem and opened it up. There were no noticable surface injuries (burned resistors, failed caps, etc). I have the schematic and a portion of the service manual I found out on the web which shows how to measure the voltage, dc offset and bias. The voltage was spot on at -/+ 60v and the DC off set was ever so slightly above 0v and I returned to the proper 0v. The bias was wierd. It came up VERY SLOWLY. At 25mv bias it took about 5 minutes to come up to that from start up without any load on the amp. Once up to 25mv I let it run for 15 minutes with no load. There was no noticable heat at all. I shut her down and hooked her up to my CD player and JBL speaker. Turned it on and played Dire Straits and within 2 or 3 songs she was hot to the touch on the heat sinks and since I had the top cover off I touched the transistors and they were hot enough that it was tough to keep the figures on them. Any suggestions? Unfortunately I no longer own a scope. I was thinkng about the possibilty of ocsillation at the output? Again any help would be appreciated.
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Old 25th February 2008, 09:18 AM   #2
Bigred is offline Bigred  Canada
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Yup, dollars to donuts its oscillating. These are known to have stability issues over time.
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Old 25th February 2008, 03:36 PM   #3
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Well the good think is they are easy to work on. They break down in sections with several boards set up like computer PCI cards. Since the voltage is good I am thinking my issues are not with the power supply but with the caps on the amp board. What do you think? Also what about the long time frame it took for full current to come up? Any thoughts on that?

Thanks Bigred,

AN
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Old 25th February 2008, 03:40 PM   #4
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Like Bigred, I bet in oscillation .

Inspect the small compensation capacitors for drift...
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Old 25th February 2008, 09:28 PM   #5
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Check (replace) the bias adjustment pots (anything in the bias circuit is suspect) and look for cold joints .

Mike.
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Old 25th February 2008, 10:26 PM   #6
ilimzn is offline ilimzn  Croatia
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These are known to develop oscilation problems due to unstable compensation caps, which tend to drift and become leaky with age. The short of it is, get a bunch of silver mica caps and replace them, and unlike the originals, keep the leads fairly short. There was a topic about this on the forum, use search!
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