"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


The variac trick only works if the amp is not electronically switched. If it
is, you need to get to at least 75% of the line input voltage before the
drive to the power switching relay will come up. By this time, the rails
will be up high enough to blow your transistors again. The way around this
is to bridge the power relay, but you need to understand the schematics and
what you are doing, to do this correctly and safely. The variac is
definitely the right approach for this type of fault on this type of
equipment, assuming that you possess one. You are correct in your surmisal
that half the input voltage equals half the secondary rails, but half is way
too much. If you understand what you are measuring as you bring the input
voltage up, 10 - 20% is plenty enough to find the fault.

In going for a repair on a DC coupled amp, which has a failed output stage,
you have picked a notoriously difficult area. Some of these amps can have
even the most experienced of us, who work on them all the time, tearing our
hair out.

Before investing in a variac, assuming that the other channel works ok -
prove this by removing the bad o/p Tr's, fitting new fuses, and retrying -
you might try just doing comparison checks of resistance between the good
channel and the bad. I would suggest an analogue multimeter for this, as it
tends to give a quicker and more meaningful display than a digital. Set the
meter to a standard ohms x 1 range, clip the black lead to deck, then do
comparison checks between corresponding points on the two channels. As soon
as you find a significant difference, chase this down to arrive at the
faulty component(s).

I'm willing to bet that as well as your bad output transistors, you also
have at least one bad driver transistor. Also check for open circuit
resistors. Typical places for these after an output stage failure, are the
low value high power emitter resistors for the output pair, and base
coupling resistors, which are often tiny low value types.

I wish you luck with it but, based on many years' experience, I fear that
you may not get very far.

Arfa