"andrew queisser" bravely wrote to "All" (23 Nov 05 12:28:03)
--- on the heady topic of "Attempting audio amp (Pioneer A-9) repair - any
tips?"

aq From: "andrew queisser" andrew.queisser at hp com
aq Xref: core-easynews sci.electronics.repair:349443

aq My trusty Pioneer A-9 has a problem on the right channel. If I listen
aq on the headphones the sound is great, on speaker the sound is fuzzy
aq but only on the right channel. The headphone ouput actually comes from
aq the speaker output, not the preamp, so it seems that fundamentally the
aq signal is getting through but under load it gets fuzzy. I can hear the
aq distortion in the headphones when I click the speakers on but when I
aq click the speakers off the sound is fine.

aq The power supplies feed both channels in parallel so I doubt it's a
aq power supply problem.

aq I have a scope and a little audio probe gizmo that lets me listen to
aq audio signals with an earphone. I can generate signals from my PC and
aq feed them into the amp, if that helps. I also have the schematics so I
aq can trace the signal but I'm not quite sure how to go about it.

aq Any tips?

aq Thanks,
aq Andrew Queisser


Connect the scope to bad channel and inject a 400Hz triangle wave test
signal, increasing the amplitude until the buzz is obvious. If the
triangle wave is mishaped at the zero crossings then it is crossover
distortion. This is most likely due to a defect of idle bias
circuitry. If instead the triangle wave is clipped then positive
clipping means a defective NPN output, and negative clipping a
defective PNP output. Perhaps intead one of the drivers or supplies.
A triangle wave is a good test signal because it consists of straight
lines and it also stresses the output devices less than a sinewave or
squarewave.

A*s*i*m*o*v

.... A stereo system is the altar to the god of music.