Kenwood KA-1500 transistor fault?

MexicanGringo · 11th February 2010, 10:22 AM

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

big jim · 12th February 2010, 05:16 AM

Hi colin the fault will be the out/put transistors 2SD525 and 2SB595
ive had this fault in the 80s 100+ times.

big jim · 12th February 2010, 06:17 PM

Hi here is the circuit hope it helps

regards jim

MexicanGringo · 13th February 2010, 11:22 AM

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

big jim · 13th February 2010, 12:24 PM

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

MexicanGringo · 13th February 2010, 12:37 PM

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

big jim · 13th February 2010, 01:27 PM

Hi yes no load thats the best way. or a 8ohm resistor rated 25watts min
The KA1500 as no protection circuit.

regards jim

Bonsai · 13th February 2010, 03:06 PM

Amazing - it is so easy to add protection, and yet they chose not to do this on a high volume amplifier.

Mooly · 14th February 2010, 06:53 AM

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.

MexicanGringo · 14th February 2010, 04:10 PM

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

11th February 2010, 10:22 AM	#1
MexicanGringo diyAudio Member Join Date: Feb 2010	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

13th February 2010, 03:06 PM	#8
Bonsai diyAudio Member Join Date: Aug 2006 Location: Taipei, Taiwan	Amazing - it is so easy to add protection, and yet they chose not to do this on a high volume amplifier. __________________ bonsai http://www.hifisonix.com/

14th February 2010, 06:53 AM	#9
Mooly diyAudio Member Join Date: Sep 2007	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. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

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12th February 2010, 05:16 AM	#2
big jim diyAudio Member Join Date: Feb 2006 Location: london	Hi colin the fault will be the out/put transistors 2SD525 and 2SB595 ive had this fault in the 80s 100+ times.

13th February 2010, 11:22 AM	#4
MexicanGringo diyAudio Member Join Date: Feb 2010	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

13th February 2010, 12:24 PM	#5
big jim diyAudio Member Join Date: Feb 2006 Location: london	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

13th February 2010, 12:37 PM	#6
MexicanGringo diyAudio Member Join Date: Feb 2010	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

13th February 2010, 01:27 PM	#7
big jim diyAudio Member Join Date: Feb 2006 Location: london	Hi yes no load thats the best way. or a 8ohm resistor rated 25watts min The KA1500 as no protection circuit. regards jim

14th February 2010, 04:10 PM	#10
MexicanGringo diyAudio Member Join Date: Feb 2010	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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