Re: 475 hor.jitter


Richard Gagnon <richg_1998@...>
 

Thank you Dean. As soon as my manual arrives I will
look into this. I thought the fan was maybe a 12vdc
fan.

Richg




--- dhuster@pb.k12.mo.us wrote:
Rich, the 465, 465B, 475, 475A, 464, 466 and 468 had
identical fan
modules. And by the way, this has nothing to do
with bearing noise,
but if you have a fan that has a dead spot and won't
always start up
and you don't know whether to blame the motor or the
driver chip,
just put the motor on the dead spot and then pull
the driver chip and
reinstall it "backwards". It has a symmetrical
pinout, and if one of
the transistors in the driver chip is bad, you'll
have moved it to a
different spot in the switching sequence, the motor
will no longer be
on a dead spot with the reversed chip and will
probably start up when
power is applied. If the dead spot is still in the
same place, then
you can blame the motor.

And Rich, the 475 has had several reliability mods
over it's life,
and if you have a lower serial number, it may need
some of them.
The -8v P/S should have a new rectifier that's much
larger than the
others (i.e., it's NOT a 152-0488-00) and will be
laying down rather
than standing up in order to clear the case.

There's also a bluish 1% resistor (with the scope
upside-down as you
look on the main board with the front panel facing
you, it's along
the left edge maybe 2 or 3 inches from the front)
that overheats,
turns brownish and changes value, goofing up the
timebases. If
there's a black resistor in that spot, the mod has
already been
made. Otherwise, it needs replaced with a
higher-power resistor.

And the one mod that may concern you most is down
near the hole where
the horizontal deflection plate leads go down to the
jug. An
unmodified scope has a silver-colored, axial-lead,
tantalum, 1000�F
capacitor just to the front of the hole. If it's
there, get rid of
it, for it's underrated in voltage and will cause a
jittering
problem. A modified scope usually has a
physically-larger, black
aluminum electrolytic with a higher voltage rating
installed in it's
place.

Dean



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