Graylion wrote:Thanks Ray - yes, I've read that the possible AC heater supply problem can be checked by disconnecting it and temporarily using a 6V battery. If the hum goes, then that's the cause eh? It wouldn't be rocket scince to have an off-the-shelf stabilised DC transformer inside.
Hi, Lionel,
Unfortunately, as I recall finding out during my first year as an electrical engineering apprentice (ever such a long time ago), you can't transform DC!
All you can do is transform the AC supply to something like the required DC voltage, and then rectify it. There's a mathematical formula for working out the required AC voltage to give a particular DC voltage when rectified and smoothed.. All to do with sine-waves, sines, cosines, trigonometry, etc. I'd tell you all about it, but I learned it so long ago that I've now forgotten the detail.
JN