Crossover distortion in Yamaha G100-212

[Nyqusit5]

Hi all,
I am repairing a Yamaha G100-212 first series that had its power transistors shorted. The 2SC1586 were replaced with MJ15003s. The amp powers up ok but there is a very strong crossover distortion on the output: the power transistors won't turn up soon enough. The ballast resistors measure ok, the idling current pot does change the voltage on the bases of the 2SA483 and 2SC783, but the crossover notch remains. Any ideas?

Schematic (I can't insert files anymore...):
https://elektrotanya.com/yamaha_g100.../download.html

[glebert]

Here is a service manual that might have better information re: adjustment.

Yamaha G50-112_G100-112_115_210_212_sm.zip

[Nyqusit5]

Thank you Glebert but this service manual is for version II. The PA circuit is completely different.
The schematic I gave the link to at Elektrotanya is the right one.

[DrGonz78]

Are you adjusting the bias with or without a load?

[Nyqusit5]

Without a load.
But you remind me that I forgot to mention that there is no notch without a load, and no noticeable voltage accross the .22Ohm ballast resistors (when there is no signal, obviously, there is with a signal).

[DrGonz78]

I would scope it with a load attached and then perhaps you'll see the notch. Also measure those ballast resistors again with the load attached.

[Nyqusit5]

I can see the notch on my oscilloscope so i know it is there. My main problem is to remove it.

What do you mean by measuring the ballast resistor with a load? They can't be measured if the amp is powered up.

[strato56]

Can you post the DC-voltages from the power-transistors and the drivers ?

[Enzo]

If the waveform looks good with no load but fails under load, it means the output stages cannot feed sufficient current for the load. With new outputs, did you also replace the drivers? I always replace drivers when I replace outputs. Plus of course the 150 and 10 ohm resistors.

[J M Fahey]

Hi all,
I am repairing a Yamaha G100-212 first series that had its power transistors shorted. The 2SC1586 were replaced with MJ15003s. The amp powers up ok but there is a very strong crossover distortion on the output .....

I can see the notch on my oscilloscope so i know it is there.

YOU know and see, we donīt.
Please post 2 scope captures, one putting out, say, 5V RMS, no load, then same with load.

The ballast resistors measure ok, the idling current pot does change the voltage on the bases of the 2SA483 and 2SC783,

And such voltage IS??????
Please measure driver base to base voltage at both extreme CW-CCW settings of Bias pot.

but the crossover notch remains.

Those bases are very far from speaker out, as in 2 transistors and 2 junctions away so that alone tells us very little.
Repeat above test (fully to each side) and measure/post Vbe for top driver and Power , same for bottom ones, and also across top and bottom ballast resistors.
Also DCV from speaker out rail to ground, before the output coupling cap.

[Nyqusit5]

Thanks Enzo,
The 2sa483 was replaced (the amp did the crossover thing beforehand), but not the 2sc783 (can't find them and the one installed seems good). The 10 Ohm was replaced and the 150Ohm on the collector of the 2sc783 driver measures ok. As a matter of fact, the resistors in the idling current circuit measure all ok.

[Nyqusit5]

Let me start by saying I am sorry if I offended you with my posts Mr Fahey. You seem quite angry but I have no clue as to why. So again, sorry.

There is no voltage accross ballast resistors at both extremes of the bias pot. The speaker out rail to ground is about 50V. I haven't measured the rest yet.

[J M Fahey]

No, not angry AT ALL.
Just asking for data to be able to suggest something useful insted of something generic.

And solve your problem in 4 or 5 answers at most, instead of 24.

Fact is *you* have the amp, in your bench, and can measur eit, scope it, look at PCBs, notice smoke, red hot parts, smell transformer fumes, etc.

NONE OF US CAN, so in the interest of helping you, I ask you to do same things I would if it were on my bench.
I think itīs fair.

[DrGonz78]

What do you mean by measuring the ballast resistor with a load? They can't be measured if the amp is powered up.

I guess I was thinking there would be a small millivolt reading across those resistors to indicate that there was a nice dribble of idle current.

[J M Fahey]

In the interest of clarity, Iīll self-quote.

There are 3 answers, each made out of 2 parts.

The second part on each, highlighted in red, is a very clear and definite Technical question needed to solve the problem.
The first half of each, highlighted in violet, is the explanation as to why the second part is needed.:

YOU know and see, we donīt.
Please post 2 scope captures, one putting out, say, 5V RMS, no load, then same with load.

And such voltage IS??????
Please measure driver base to base voltage at both extreme CW-CCW settings of Bias pot.

Those bases are very far from speaker out, as in 2 transistors and 2 junctions away so that alone tells us very little.
Repeat above test (fully to each side) and measure/post Vbe for top driver and Power , same for bottom ones, and also across top and bottom ballast resistors.
Also DCV from speaker out rail to ground, before the output coupling cap.

I am at loss to understand why technical questions and their justification can be considered "offensive"