"clifto" wrote in message
...
Arfa Daily wrote:
That is the most important thing that you must do before going any
further.
If not properly bypassed, 78 series regulators will *readily* oscillate
violently, particularly if you have long leads attached to them. Connect
0u1
between "in" and ground, and 4u7 in parallel with 0u1 between "out" and
ground. It is *essential* that these are soldered *directly* to the pins
of
the device, not some distance away down the connecting wires.

Might as well add the back-bias protection diode with anode at "out" and
cathode at "in" while he's at it.


Yeah, if you like, though probably an unnecessary complication for this test
setup. Going back to the story of how this unfortunate situation arose, I
think that there is a valuable lesson for Robb to add to what he's learning
here, and that is "If it ain't broke, don't fix it ..." The fact that it was
working after replacing the VFD driver chip, should have been an end to the
job. It obviously didn't matter that the clock was a bit scruffy, as
observed on the 'scope, as the item was managing to use it satisfactorily.
Depending on how fast the clock is, and how good a probe was being used on
the 'scope, the signal may not even have been 'wrong' in the first place. It
might just have *looked* as though it was. It's pretty likely that any clock
inputs on the chips are internally schmitt'd anyway, to make sure that a
clean signal is being passed on into the chip's circuitry.

Arfa