We had a similar problem.  There was a short in the source.  Checking with
an ohm meter on the main connectors, we pushed the source gently and noted
that the current was shorting out.  We reinserted the source making sure it
was not shorting out and the problem was solved.  I was told this was a rare
occurence but I suspect it might be the same with yours.  I had replaced the
filament with 2 different filaments until we discovered the short.  Best of
luck!  Les, Price