Onkyo M-510 parts

it might sound strange but from time to time some parts, main boards etc are for sale from onkyo owners...

I’m guessing that what you are really saying, is that you want to do a transformer tear down and get a ’build look’ at the actual transformer build.

Possibly for the purposes of copying the intellectual property? Knock-off transformers? The map is not the territory so any given look at the innards will have nothing to do with the mind and thinking that created it..nor any design aspects behind the build.

One possibility, as it is a bizarre and unusual request. If it is not that, I apologize.

From the vintage Knob website, on the description for the Onkyo Integra M-510 amplifier:

The secret inside lies in the two secondary transformers : these are "In Phase" transformers which echo the Signal In-Phase Filters of the Integra P-308 high-end preamplifier. In Phase transformers are to cope with the reactive load of loudspeakers, and not only their resistive aspect ; these transformers are to avoid the phase shift between voltage and current in the amp-to-speaker signal path.
Most pronounced around the speaker’s resonance frequency, this phase-shift also happens between the voltage and the charging current in the amplifier’s power supply ; these charging currents may start fluctuating along the low-frequencies contained in the musical signal. Out-of-phase charging currents can generate electromagnetic flux which in turn often induces voltages of the same incorrect phase in the nearby driver stage (through which the audio signal passes) ; the problem is then, naturally, sent to the loudspeaker... muddy bass and blurry soundstage.
Real Phase transformer are inserted between the positive and negative charging currents of the the power-supply and the capacitors : as the positive and negative currents pass through the transformer’s two windings, unwanted peaks and dips cancel each other out - in phase ! You can see the schematic of the Real Phase system by clicking the "more" button below - click !

If you want one as you have a blown one, well, that will still likely require an entire amplifier purchase. I just can’t see (imagine/visualize) any spares of that type being left sitting in stock anywhere.