Kikusui 5020 analog scope with wierd and cool sweep error

[oz2cpu]

a few day ago i tested and repaired this old analog scope
Kikusui oscilloscope 2ch cos 5020
when testing the mag10 feature at high sweep speed and input at 10MHz or so,
input is a sinewave, I got the picture attached

save your time go directly to about 27 mins :-)
where the curve is most crasy

https://youtu.be/DQMB-F0ed4I

[Chris56000]

Hi!

This mess looks very much like ripple from the power supply or timebase amplifier itself is getting around the horizontal deflection amplifier circuit due to dried up decoupling/smoothing capacitors, begin with C623 (47  μ 250 V) on the power supply board, also C624 (2μ2 250 V) and C625 (1μ0 160 V) as well – C624 is a local decoupling capacitor for the h.t. line and C625 is a negative feedback coupling capacitor to feed ripple that may be left on the h.t. line back to the error amplifier U602B pin 5 for ripple cancellation!

Also make sure that the series pass transistor Q603 (2SB861) isn't defective – this is in the negative h.t. return between chassis and the negative side of the h.t. bridge rectifier – you can use a TIP42C to replace this if necessary!

The bridge rectifier CR621 might have a diode duff causing half–wave rectification, if you suspect this, use one of those small round W08G types to replace it!

If these components all test OK you'll have to go on to the various smoothing and decoupling capacitors across the various l.t. supply lines!

(Glossary: – h.t. = "high tension", l.t. = "low tension")

Chris Williams