On 8 Jul 2006 14:13:54 -0700, "Tony Heslington"
wrote:

Hello fellow techno-junkies!


I've always been a little of an electronics junkie. I was that kid
who took his toys apart, brought radios and tv's home from the
garbage... Just one of those.

I've been given a Kenwood ka5700 amplifier and have been attempting
to repair it my brother who been given the amp had replaced the fuses
and found they blew almost right after power was applied.

Being so inclined I quickly accepted the gift and put my nose right in
it. I examined the amp deeply and found no obvious signs of damage. I
removed the 4 main power transistors and found 2 had failed.

I replaced them just the other night and with my brother on the phone
the switch was thrown and 1 of replaced transistors managed to smoke
before the fuses blew.... (Note the volume was all the way down and
the speaker select switch was off.)


I have purchase the service manual for the amp and I simply am at a
loss for what to next.

I removed two main jumpers from the amp separating the power supply
from the reset of the amp. According to the service manual power after
the bridge rectifier circuit should be 40v dc. I measure at this point
50v, it's not to spec but is this to high?

What should I do next? I am thinking I should purchase a variac to
introduce a lower ac voltage to the system that may help to determine
the problem. Of course I don't know the trick to repair with this
device. Do I simply apply half the AC power and expect half the
voltage in the circuit from what is listed in the schematic and trace
the components from there.

Do I simply start removing every component and test - then repair and
replace until I've gone through every bit?

It's not so much that I want to repair this particular item, I am
just enjoying the chase.

Thank you for your kind advice. Perhaps someone could recommend a
book?

Tony



As someone who repaired a large volume of these amps, I would suggest
putting a 60 watt bulb in series with the hot side of the AC input.
Then if I was inexperienced in repairing amps, I would replace the
driver, output, and bias transistors and check the values of resistors
in the related circuitry. Sometimes on this amp, the differential
input transistors would fail causing a dc offset on the output. After
replacing these parts, turn the amplifier on and see ,after the
initial brightness of the bulb, if the bulb dims and if the amplifier
works. If it doesn't you will at least know there are other problems
and most likely won't damage any components. Once the amp is
functional, plug it directly into the wall and set the bias and offset
voltages. This amp isn't too shabby and is much easier to work on
than the multichannel wonders on the market today. Chuck