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Yamaha XM6150

While waiting for some more TA7317P devices for my previous replair I was handed a Yamaha XM6150 six channel power amplifier with the channel C protection light on.

After removing the top cover I was curious what the four huge inductors were for and wondered if I was dealing with a class D system. Some of the mystery was cleared up by googling EEEngine:

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I'm not sure I fully understand how it does what is claimed. I've never seen anything like it before in an audio amplifier.

Next I obtained the service manual. There doesn't seem to be a direct link but if you google yamaha_xm4220_xm6150.pdf the elektrotanya site will provide it.

The power amp schematic is the one at the top of the last page with the darlington output transistors.

I quickly found +55V on the orange wire to the relay board (CN806) and further tests showed, unsurprisingly, that both darlington output transistors (2SD2560 and 2SB1647) for that channel are short circuit.

Under a bright light I don't see any burned components or other damage. The D2560 has obviously been very hot as it has toasted the board a little.

I'm thinking of trying a TIP122/127 pair to see if everything works properly at low power before I blow up a new pair of correct devices.

Any other suggestions are welcome.

Old guy

Reply to
John Smith
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In amps with the output taken off the emitters I used to use like ~100 ohm resistors from E to B and see how it runs with no load. The problem is that with Darlington outputs disabled like that it may not have enough current to even drive the feedback network.

But at quiescence there should be no offset and the collector voltages should be quite low. Not sure how low but they should obviously track the input signal. It can't be really detected at the output with that configuration. You should be able to watch the audio on a scope. I would think that if the voltage control uses the same feedback loop as the main audio it might just clip up to max with low output, but you can still ascertain if the regulators are working. If the voltages are railed with no input, the failure was likely caused by a fault in the regulator control.

This configuration cannot be a real class AB because for efficiency it must turn the non driven end of the output stage fully off or very close. In the linked article it claims a better damping factor than class D, which I believe, but I don't think it is going to be as good as good class AB.

All they have really done is take the final SESAPP stage out of the class D domain. The "output transistors" actually operate as active filters, as well as polarity switches. I figured someone would come up with this sooner or later, and it's not much more (operationally) than using a commutating power supply, except that there are a near infinite number of commutators.

Since there is a limit as to just how fast this power supply can respond to the input, I imagine that sustained high output at high frequencies are hard on the outputs.

Personally I'll just stick with my Phase Linear 400 II and put a fan on it, but business is business.

J
Reply to
Jeff Urban

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I'm only familiar with Yamaha Stagepas 300 If your one is made with lightness in mind and 2 channel, check whether the audio outputs are antiphase, ie one has "tip" contact to ground, and ground contact is hot

Reply to
N_Cook

I tried a TIP122/127 pair and The DC conditions then seemed fine.

There's a new 2SD2560 and 2SB1647 in there now.

The protection light is off and all six outputs are only mV from ground.

However the channel C clip light is permanently on (It was on previously with the shorted output transistors but I didn't thnk much of it at the time.)

I haven't tried a signal yet.

The schematic is in yamaha_xm4220_xm6150.pdf whcih I got from electrotanya.

The channel C clip signal is at pin 3 of CN602 (page 59) and I found 21.6V there but 25V on the other channels.

Tracing back this comes from R454 on the driver board (also page 59).

There's 0.8V across D410 (cathode +Ve) but only 0.3V across the equivalent component in an adjacent channel. Q426 has Vbe 0.56V so no surprise it's turning on. D409 has 0.23V across it (anode +Ve).

Relative to ground both diodes are -60V but that is also true of an adjacent channel.

R457 has 0.340V across it and R460 has 0.343V across it.

moving to Q424/427, Q427 base is 46mV and Q424 base is 35mV.

So now I'm wondering what the quickest way to pinpointing the bad component is.

Thanks in advance if anyone else would like to have a look at the schematic.

Old guy

Reply to
John Smith

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