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Old 6th June 2010, 03:02 PM   #1
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Default Kenwood Kac-649s

I been working on this amp for a couple of days. The amp pops fuses when the B+ and Grnd are applied. Replaced a the fets in the power supply that were shorted. But the amp still pops fuses. Found Q47 in the (power switching) section shorted. Here is the issue, I have the service manual for this amp and the SM shows that Q47 is a 2sa733(a), but the Q47 I pulled out was a 2sa817a. So, which should I use?? The 2sa733(a) or the 2sa817a??

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Old 6th June 2010, 06:36 PM   #2
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If it blows the fuses without remote voltage applied and with the power supply FETs out of the circuit, the reverse protection diode (D41) is likely shorted.
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Old 7th June 2010, 03:18 AM   #3
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Thanks Perry, I will swap it out and go from there.
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Old 7th June 2010, 08:13 PM   #4
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Replaced Q50, Q51, D41 and Q47, applied B+ and GRD, amp did not blow a fuse. Turned on the remote and it popped the fuse. have checked Q18-21, Q38-45 and they all show to be good. D31,32 show to be ok. Any Idea on what to check next?
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Old 7th June 2010, 08:57 PM   #5
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Do you have a current limiter (built in limiter on your 12v supply, large 2 ohm resistor, automotive headlamp...)?
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Old 7th June 2010, 09:57 PM   #6
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That would be no, Been using a small fuse for testing and protection. (5 amp fuse). So, would a large 2 ohm resistor in series with B+ work??
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Old 7th June 2010, 10:10 PM   #7
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Using any of the current limiters listed allows you to power up the amp (all semis clamped tightly to the sink) and measure voltages at several points. Many times, there are faults in the audio output section of the amp (other than shorted outputs) and measuring the DC voltage across the emitter resistors tells you which channel it is.

Some of these older Kenwood amps had broken connections on the bias transistors that would cause it to draw excessive current after the remote voltage was applied. Check the solder connections on all of the bias transistors.
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Old 7th June 2010, 10:32 PM   #8
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Ok, still a little new to this, which ones would be the Bias transistors? Thanks for your help on this Perry.
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Old 7th June 2010, 11:00 PM   #9
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Q19, Q19, Q20 and Q21.
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Old 8th June 2010, 05:43 PM   #10
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A better cheap way I found is use the '10A' amperage on a cheap $10 DMM they work nice, I used to have one hooked to a big resistor all the time. A typical amplifier will pull some amps then settle to 1-3 amps for say <400rms amplifiers once the caps are full. The resistor will not let a lot of current through that could damage the 10A rating of the DMM or the amplifier if it is clamped in sink. I like to use a 2.7ohm on smaller amps up to about 300rms but that is just me. Use an aluminum one and keep my hand on it, it will warm up with more current if I am trying to run a shorted one but be careful of them lol. Otherwise just watch the meter, but in some cases I'm working/testing on the board its easier to use the hand warmer. Now I have a panel with an ampere meter in it that I use with the resistors.
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