475 With a bowed display. . . .Ideas?


Michael W. Lynch
 

I recently found a nice 475. Upon receiving the unit, I fired it up, the scope presented a strange garbled trace. Checking the various LV supplies, I found the +15V and the +15V Unregulated to be very low at about 12.2 and 16.5 respectively. Lifted the +15V jumper and there was no change, indicating that the problem was in the power supply, not the scope circuitry. Otherwise the scope looks almost new inside, no signs of tampering or repairs. All other supply voltages are spot on.

Problem was C1442 was bad. High resistance and almost no capacitance, loading the +15V supply. Replaced that cap with a new modern cap and both voltages are now spot on. Ripple checked on all supplies and we are within specs.

Cleaned and exercised all the push button switches.

Trace is bright and fairly clear for a mesh CRT.

Horz and Vert Position controls work as expected.

Focus works well

Trace Rotation R1386 controls rotation either side of horizontal equally.

Set the scope in X-Y and defocus the dot, the resulting spot is nice and round. Astig control R1397 produces the expected change in the spot. The Spot will go "oblong" in both X and Y directions in response to turning R1397.

Horizontal timing is close to right as the timing marker setting generally agrees with the Time/Div switch setting, some adjustment is needed, but the horizontal is only off about 5-7% or so.

Problem is when I put my marker signal from my marker generator into the unit and set Volts/Div knob for 6-8 division vertical markers then I center the middle marker to align with the center vertical graticule line, as you move away from the center, the markers become more and more "leaned in" in at the top. Left and right of center, the tops of the markers lean progressively more and more toward the center as you move farther from the centerline. The bottom trace line is barely, but ever so slightly bowed. The vertical markers are straight but just leaning in toward the centerline. The bottom trace bowing is not nearly as profound as the "leaning" of the markers. Trace can be adjusted to about 1/2 trace width of bow, checking at top middle and bottom of the display.

Y Align R1385 and Geometry R1390 will not bring the vertical markers into proper vertical alignment even though they do have some effect on the trace by making it less severe. Even though they never "Fix" the problem.

I have fixed several of these scopes and never encountered this issue before, where it could not be adjusted out of the instrument.

Ideas or suggestions?

Thanks in Advance.
--
Michael Lynch
Dardanelle, AR

Join TekScopes@groups.io to automatically receive all group messages.