Kenwood KA-1500 transistor fault? - diyAudio
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Old 11th February 2010, 10:22 AM   #1
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Default Kenwood KA-1500 transistor fault?

Hello gents, ive a ka-1500 amp thats outputting raw power to the speakers, similar to that of a damaged input lead. Theres no trace of a signal, just massive buzzing noise..

I read on this forum this fault or similar faults are usually in the 'differential stage' transistors. I have basic understanding of electronics&components but am new to amplifier architecture.

Is it a good start to replace all transistors & capacitors if unable to source the fault? What components should i be testing with multimeter, bearing in mind iv only a volt/resist/current meter? Can you test internal components in a solid state amp without an output load?

Any help much appreciated!
Colin
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Old 12th February 2010, 05:16 AM   #2
big jim is offline big jim  England
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Hi colin the fault will be the out/put transistors 2SD525 and 2SB595
ive had this fault in the 80s 100+ times.
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Old 12th February 2010, 06:17 PM   #3
big jim is offline big jim  England
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Hi here is the circuit hope it helps

regards jim
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File Type: zip KA1500.zip (424.4 KB, 217 views)
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Old 13th February 2010, 11:22 AM   #4
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Jim thank you so much! My plan was to test them and replace as many transistors&caps as possible, this narrows it down.. Whats the cause though, other than old age? Excessive heat?

Also do you possible know what the secondary winding voltage should be?

Cheers
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Old 13th February 2010, 12:24 PM   #5
big jim is offline big jim  England
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Hi out/put transistor failure is normaly caused by the speaker wires being accidently shorted and the mains transformer secondry winding =2x 24.3 as stated on the circuit good luck.

regards jim
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Old 13th February 2010, 12:37 PM   #6
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thanks again, one more question if i may.. Can one test the circuits while powered up without a speaker load attached? Should some form of resistance be inserted on the outputs?

Cheers
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Old 13th February 2010, 01:27 PM   #7
big jim is offline big jim  England
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Hi yes no load thats the best way. or a 8ohm resistor rated 25watts min
The KA1500 as no protection circuit.

regards jim
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Old 13th February 2010, 03:06 PM   #8
Bonsai is offline Bonsai  Taiwan
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Amazing - it is so easy to add protection, and yet they chose not to do this on a high volume amplifier.
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Old 14th February 2010, 06:53 AM   #9
Mooly is offline Mooly  United Kingdom
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Using a mains filament bulb (100watt) in series with the incoming supply can save a lot of frustration. If these's a major fault/bias error etc then the lamp lights rather than the outputs blowing.
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Old 14th February 2010, 04:10 PM   #10
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Mooly, you say a 100w bulb in series with incoming supply, i assume you mean after the transformer but before the speaker outputs? Or in place of the speaker load?

Jim,the power transistors i see are marked 'T D525' and 'T B595', are these the same as 2SD525 and 2SB595?

Does anyone have a suitable protection circuit i could incorporate?

Thanks for the help
Colin
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