Following circuit is for 300 VDC from 12 VDC. Is it possible to convert this into 100 VDC output supply ? What changes do I need to do to the schematics ?
The simple way would be to change the transformer. However what you intend to use it for could change everything as that is a very unregulated output. Plus, that's a very old circuit.
+1. But is it worth modifying that circuit? For less than the cost of just a suitable transformer you could probably buy a complete off-the-shelf DC-DC boost converter with a suitable spec.
How much power? Current?
The output voltage for the circuit is a function of the transformer turn ratio.
How well do you want the output voltage regulated?
Also, the schematic shows a common ground for both the input and output, but that is not a requirement; the output could be completely floating. Would a floating 100 V output be better or worse for your application? The point is that if a common ground is ok and the current is low, you might get away with a simple non-isolated boost converter.
I did a bit of googling and found that the circuit was designed as a replacement for a mechanical "vibrator" as used in old valve radios when powered from 6/12v lead acid batteries.
Some amateur radio enthusiasts like to operate using original and modern recreations of a radio known as the "Paraset", so called I guess because it was used by allied agents who were parachuted in to occupied Europe during the 2nd world war.
I did a bit of googling and found that the circuit was designed as a replacement for a mechanical "vibrator" as used in old valve radios when powered from 6/12v lead acid batteries.
They were used in valve car radios, they were just oscillating relays, and basically two types, sycronous (self rectifying) and non-syncronous (needed a rectifier).