Design questions regarding LM2735 (boost-converter) for DC-DC
OK, from the service manual, here is the circuit you're driving:



Since the 9VAC is used to feed a voltage doubler in order to produce the +12V bus, you do indeed need AC. However, the waveform is not critical &mdash; a square wave (with some filtering to eliminate RFI) would work just as well. Also, the frequency is not critical either, unless something connected to the "User Port" cares about it.

So, I'll modify my previous comment. You could just feed pins 6 and 7 with complementary square waves that swing between 0V and 12V using your H-bridge, and the circuit will be happy. No need to boost the voltage, and no need for isolation. Nothing will be damaged.

The "+9 unreg" rail will be close to 12V, but that's the nature of "unreg". It probably was that high anyway, assuming the 9VAC input is an RMS rating.