PDP 11/45 partial success.

[Tom Uban]

Eric wrote:

>Jim wrote:
> > after a few days frustration, I have got a little further with my 11/45.
> >
> > The power supplies checked out OK with respect to ripple and output, but a
> > couple have them have the odd VERY fast spike on them - is this normal?
> > (I'm using a 100MHz 'scope - I suspect they may not be visible on a
>
>They are switching power supplies, so there will be some high frequency
>noise.  I'm not sure what the typical switching frequency is, though I'd
>guess that it would be in the 100-200 KHz range; maybe Tony Duell knows.
>Anyhow, one would expect some very high frequency spikes occurring at
>the interval of the switching frequency.  But the spikes shouldn't have
>very high amplitude; if they do, it's possible that you have some bad
>filter capacitors.

The switching frequency of these supplies is in the neighborhood of 10KHz,
which I'm guessing at based on my memory of the last time I listened to
one whine/whistle.

Are you sure that you don't have a ground loop in your scope setup? Have
you isolated your ground plug wire on your scope? What is the amplitude
of the spike?

--tom