475A/DM44 - intensity control locked out


dynasor1 <dynasor1@...>
 

I was watching the screen when it happened - suddenly went from a
normal trace to super bright. Now the intensity control doesn't
reduce the brightness at all, although it does alter the background
scatter a little. After fiddling the controls to obtain a trace that
wouldn't burn lines in the screen (x10 mag, high sweep rate), I was
able to ascertain that all other scope functions still work properly.
In other words, a working scope, but with no means of controlling the
intensity (naturally, the trace is now a full minor division wide as
well, even when set for optimum focus, the retrace is visible, and
the screen background is somewhat lit up). Interestingly, the scope's
Delay Time Position control and the Delta Time control on the
attached DM44 still shift a perceptibly intensified portion of the
already-intense trace, so I don't guess the trouble is in the "A
Intensify" circuit path.

The z-axis input has no effect when connected to an amplitude
calibrator and varied, so I suspect either the z-axis amplifier or DC
restoration circuit. I did a visual inspection of these areas and
wiggled all the associated transistors in their sockets hoping to
find an intermittent contact, but no luck. I've also "bounced" all
the switch contacts, to no avail. Before tearing into this further
and testing individual components out-of-circuit, has anyone
experienced a similar situation, or is there a known failure mode
that corresponds to this problem?

HELP - this was my only working scope!

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