Attempting audio amp (Pioneer A-9) repair - any tips?

One tip might be to reverse your speaker connections left to right and see if the problem remains in the same channel.

It might not be a problem with your amp....

jak

Maybe it's a problem with the speaker? Have you tried switching the speakers? Carefully checked the speaker wires? Maybe the right speaker is bad. Maybe there is a frayed wire. maybe even a short at one of the ends. A frayed end just barely touching the other wire could be causing the problem, and if it is allowed to continue like that you'll blow the channel.

I did switch the speakers and the problem stayed on the right side.

thanks, andrew

Jumpster Jiver wrote:

Try feeding a 400 HZ square wave into it and see what comes out (compare channels)

If the leading or falling edges look funny describe the bad one.

I'm not familiar with the A-9 but some discrete amps used to use fuses on the speaker terminals so if the amp shorts it won't set fire to the speaker cones. The fuse holders or fuses would corrode and cause distortion when the current was higher (speakers versus phones).

"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" 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.

I've seen mainly controls and switches need cleaning on these, including the speaker switches, and speaker relay.

Mark Z.

an open output transistor or resistors. Chuck

My thoughts exactly....

jak

Yes, the outputs, one or the other on the affected side is open in such a way that allows the driver to power the output. The driver will pass enough current to operate the headphones, but not the speakers... which quickly clip the output of the driver. Check the outputs with a vom and you'll find one of them open. Replace in pairs, adjust bias, and offset voltage measured at speaker outputs. Check associated resisters for opens. Power up with variac, watch current rise at low voltage to ensure everything is good before applying full line voltage.

Yup, I've been unable to use the balance and tone controls due to dirt or corrosion on this amp for years. I can only run it in "straight-through" mode where it works fine.

And after cleaning the connector that goes to the power amps everything works fine again.

Thanks! Andrew

Thanks to everyone who replied. My amp is working again but the solution was a bit anticlimactic. I took everything apart and when I hooked up the scope the signals looked ok and the sound over the speakers was clean again.

I think the problem was in the connector between the main amplifier board and the board with the power amp for the right channel. After cleaning the connector everything started working again.

Andrew