Yamaha P3200 distortion - cutting positive parts of the signal

[dahopa]

I got faulty Yamaha P3200 power amp. One of the channels had most of the power transistors blown. I replaced all of them. After powering up it started to work but when I turned the volume up, distortion came as well. I connected a generator and an oscilloscope to check what kind of distortion are induced. It looks like the positive parts of the sinusoid are being cut. This happens when the load is connected. When I disconnect the load - beautiful sinusoid is being reproduced. It also happens that with no load the channel gets into high frequency oscillations". I am attaching the oscilloscope screens.:



I do not have much experience with class H amps but I guess I should look on the positive voltage control.
I would appreciate any guidance.
I am attaching the schematic as well, just in case:Yamaha-P3200_4500 pwramp.pdf

[mozwell]

start at the pwoer supply & work from there.
Are the diode bridges ok in the power supply.
Does the power suppyl DC voltage stay up, or collapse under load
Do the transformer secondaries stay up, or collapse under load.
Check all connectors are plugged in fully, and making proper contact.

The HF oscillation could be the output Zobel network, check R202 / C129 are ok.
Also check all the small ceramic disc caps less than 470pF are ok, and soldering is ok to these.

Do you have one good channel & one bad channel, if so, use the scope & compare signals through the amp.
Was it just power transistors blown, or drivers as well, did you just replace the power transistors or the drivers as well

[Enzo]

With no load it produces a nice full clean signal, that means ther amp basically works and your power supplies are present. When it collapses under load, it usually means the output cannot source the current the load demands. I suspect your positive side driver is not able to drive the positive side outputs. Comes to mind to look for opens at R126. I suspect Q109 is what is actually driving the plus side output. Is the signal present at the bases of the positive outputs? Are Q113 and 115 OK?

You replaced many transistors, each has a 0.47 ohm ballast resistor, are those open?

[dahopa]

Thank you guys for that fast reply with suggestions. I wasn't expecting it.
I will follow your advises and check all of that. Most likely the amp will have to wait till weekend as I am quite busy
I will post a result of the tests.

[dahopa]

Finally I had some time to check all of the suggestions.
The second channel for the amp works without any problem so there is no problem with power itself at all. There was only bad solder joint on C128 capacitor on the PA PCB.
Enzo, you were absolutely right, all of the ballast resistors (R146 - R149) were open. I have to order them and then we will see.
Also R126, R122, R132(the real value differs from the schematic!) were open or almost open too (could see them being burned).
Q109 seems to be ok as well as Q113 and Q115.
The R202, C129 are ok too.
Now I will order the resistors and then I will test it again.
I got hold of service manual and it will help me to set the idle current and check if it is all ok.
I will update the status when I put the resistors back in.

[Enzo]

We may have already said this, but...

Whenever your outputs fail and need to be replaced, I always change the driver transistors too, as they likely took some stress in the process, even if they check "good" on a meter. Your meter does not put real world voltages and currents on the part.

[dahopa]

Finally I had time to move forward with the amp.
I replaced 4 faulty ballast resistors and the drivers transistors. Now under load that channel reproduces beautiful sinusoid without any distortion. Now I will need do set the idle current and it should be ready to go.
I think the fans are not working but this should be easy. They are not running at all and when I disconnect the thermistors the protection & temperature lights up but the fans do not react. Maybe that was the cause of the damage - overheat?

[dahopa]

After holidays and busy work I had time to look at the amp fan control circuit. I checked it step by step - live with thermistors connected and disconnected. The fault was silly. The base of Q112 transistor had a bad soldering (almost invisible by naked eye). I re-soldered it - the fans started to work - spinning faster when thermistors connectors are unplugged. I guess that was one of the reasons why the channel blew in the first place - overheating.
Thank you all for all the advise.
I hope it will be helpful for others trying to fix that particular amplifier.

[Jazz P Bass]

Thanks for the update