Tony , look in the circuit corresponding to the output that blew again and
there are driver transistors in that circuit that drive the bigger power
transistors ... check them for shorted or open or for a bad gate ... its a
typical output stage and when you lose the main outputs it usually takes out
the pre-drivers with it , not always but sometimes ... i would almost bet
you that one of your pre-drivers is shorted ..... always compare one channel
to the other with a meter at different points , i usually meter all drivers
transistors as its a good sign when all the drivers are reading good , then
i proceed to the final output stage then i will meter and compare the final
stages against each other before i fire it back up ..... hope this helps


regards


"Tony Heslington" wrote in message
oups.com...
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