Sorry I haven't posted back on this forum sooner!

To answer some questions: the 'amplitude' I was referring to was the
pulser trimpot - as the coil audible oscillations got louder I had
assumed amplitude.

With regards the fitted 2N3904 I was mystified that after having
worked for years I'm only now getting a problem (even when replaced
with 2N3904) and caused me to question if that was all that was
going on.

.. If exceeding Vce is all that is happening then I'll swap them out
for an 80V Vce ONSemi pin and function equivalent.

Regards

Rod

--- In randrdesulfatorforum@yahoogroups.com, "linesrg48"
<courtiestown@...> wrote:
>
> Rod,
>
> Yours was an early example and I cvan't honestly be sure of the
> specification of the 2N3904/ 3906 devices originally fitted.
>
> I do now use the 60V rated ones available from Digikey.
>
> Rick has suggested trying a 2N2222/ 2907 combination?
>
> Regards
>
> Richard
>
> --- In randrdesulfatorforum@yahoogroups.com, "rod.badcock"
> <rod.badcock@> wrote:
> >
> > Hi,
> >
> > I have one of the Courtiestown PIC pulsers that has worked
flawlessly
> > for a couple of years.
> >
> > .... until now that is!
> >
> > I'm having problems with the FET drive section of the board,
with Q2
> > and Q3 (NPN 2N3904) blowing, causing the FET to remain in
continuous
> > conduction and blowing the fuse.
> >
> > Any suggestions from here on what might be going on? (seems to
be
> > related to amplitude)
> >
> > Rod
> >
>