amp distortion

[bigtom]

KENWOOOD KA-31 Amplifier

the left channel on this amp sounds distorted at lower volume and cuts
out at med to high volume.
i tried swapping the speakers but its definitely the amp
ive esr'd the caps on the main board but theyre ok.
there are 4 big transistors on heatsinks and 4 small ones next to them
they all test ok on a dmm set to diode test(readings are not exactly
the same, but close)
what should i be looking at first?

 

[Leonard Caillouet]

uh, maybe an oscilloscope?

Leonard

 

[Mark]

probably an open transistor,

did you take them out of the circuit to test them?

are there any large resistors like 0.47 Ohms near the output
transistors, check them for opens also


Mark

 

[Mark D. Zacharias]

KENWOOOD KA-31 Amplifier

the left channel on this amp sounds distorted at lower volume and cuts
out at med to high volume.
i tried swapping the speakers but its definitely the amp
ive esr'd the caps on the main board but theyre ok.
there are 4 big transistors on heatsinks and 4 small ones next to them
they all test ok on a dmm set to diode test(readings are not exactly
the same, but close)
what should i be looking at first?

Solder connections, relay contacts, speaker switches...

Maybe a transistor has an open junction.

A 'scope would help. Often these types of problems are intermittent, and
pushing, poking, tapping around can lead to a suspect...

Mark Z.

 

[bigtom]

found it, open junction on one of the small transistors (C556)
had to take it out to test it properly.

 

[Dave Moore]

: found it, open junction on one of the small transistors (C556)
: had to take it out to test it properly.
:

One trick is to look for approx .65 volts on the
base emitter junctions while the unit powered up.

Almost all of the transistors in most "vanilla"
SS power amps will be biased "on",
so, you can usually expect to see this forward junction
voltage on almost all of the transistors except for the
two over-current protect transistors which would be
biased off in the abcense off an output over-current
protect condition.

This sort of check probably would have sniffed
out that open junction




:
: Mark D. Zacharias wrote:
: > bigtom wrote:
: >
: >>KENWOOOD KA-31 Amplifier
: >>
: >>the left channel on this amp sounds distorted at lower volume and cuts
: >>out at med to high volume.
: >>i tried swapping the speakers but its definitely the amp
: >>ive esr'd the caps on the main board but theyre ok.
: >>there are 4 big transistors on heatsinks and 4 small ones next to them
: >>they all test ok on a dmm set to diode test(readings are not exactly
: >>the same, but close)
: >>what should i be looking at first?
: >
: >
: > Solder connections, relay contacts, speaker switches...
: >
: > Maybe a transistor has an open junction.
: >
: > A 'scope would help. Often these types of problems are intermittent, and
: > pushing, poking, tapping around can lead to a suspect...
: >
: > Mark Z.
: >
: >
: