Re: 475A/DM44 - intensity control locked out


ehsjr@...
 

dynasor1 wrote:

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!
I just repaired a 475 with similar problem. The cause was a broken wire
to a choke made from a few turns of wire and physically located over
the 100 volt trace on the board within a few inches of Q1338. When the
wire broke it touched the 100 volt circuit trace and killed Q1338 and
Q1332. I could not find the choke on the schematic - the copy of the
schematic I have is poor. Now to the point.

I found in troubleshooting this that a junk box transistor of the
right polarity will work in place of the proper transistor for those
two - Q1228 & Q1332 during diagnosis. You do need to pay
attention to lead orientation when you are not using the identical
part. For final repair, you want to use the correct parts if those
transistors are bad - that is particularly true of Q1338 which needs
to be very fast. The intensity control connects to Q1338 (can't
remember if it is direct or through some components), so that is a
prime area for investigation.

Ed

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