4 Jul
2006
4 Jul
'06
11:53 a.m.
On Tuesday 04 July 2006 12:53 am, der Mouse wrote:
Right. I'd forgotten about that. Those 4.7M resistors are to maintain the DC
balance of what's normally driving the plates. You might check and see if
they are indeed 4.7M.
I know, bad form to follow up to myself....
It is? :-)
[...'scope
which doesn't seem to have the beam on-screen...]
...I turned out to be in luck.
On closer examination, this 'scope has a bank of screw terminals which
are labeled (in black-on-dark-brown, not obvious except on close
inspection) as being suitable for jumpering in certain ways to get the
X and Y deflection plates as coming direct from inputs or coming from
the X and Y deflection amplifier circuitry. I removed the insulator
panel on which these terminals are mounted, and it is clear that four
of the terminals go directly to the deflection plates (two for each
axis), with a resistor (4.7M) hardwired from there to the
deflection-amplifier terminals. (The direct-input terminals are capacitor-coupled to
the plates if the
jumpers are in the direct-input positions; if the jumpers are in the
amplifier-fed positions, they short out these resistors, with the
direct-input terminals doing nothing at all.)
So I removed the jumpers entirely and, using alligator clip leads,
grounded both X deflector plates (to the chassis, that being where all
other grounds go). Voila, I can find the trace! (Well, spot, under
the circumstances.) Using a voltmeter on the amplifier-output
terminals indicates that one is driving about +100V and the other about
+400V (of course, these may be reading low because of meter impedance,
but they clearly are nowhere near equal).
That's a heck of an imbalance, there.
Grounding the deflection plate corresponding to the
+400V line and leaving
the other alone gives me a spot near one edge of the display; the other way
around and I get the symptom I wrote about.
So now it's just a matter of tracing circuitry back until I find the
problem. (Heh. "Just".) Sorry to bother everyone when the answer was
right there waiting to be discovered all the time.
Probably leaking capacitors coupling at the amplifier output?
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin