2235 with 360 kHz 100mV P-P ringing on +100V PSU rail

Vaugha Brewchuk

Good day,

This is my first post about my first scope that I purchased recently. I am also not an electronics technician, so please be gentle with me :-).

The scope is in excellent condition and has been recently calibrated. However, when using the scope I noticed occasional small ripples in the traces. I opened the scope and tested the power supplies and determined that the ripple on all the rails was 2 to 3 times larger than the limit quoted in the service manual. Consequently I replaced all electrolytic capacitors in the power supply, except the C906 input filter capacitor, with Nichicon PW series capacitors. I used the exact same capacity components where the originals were +/-20% and the next larger capacity where the originals were -10% to +30, +50 or +100%. I also replaced the four 0.01uF 3000V capacitors with CDE 940C30S1K-F polypropylene capacitors.

Unfortunately the occasional ripples have not completely disappeared after the overhaul even though the traces seem to be a bit sharper. The ripples on all the power supply rails have significantly reduced, with the exception of the +100V rail. Here the actual low frequency ripple seems to be in check at approximately 30mV P-P, but every second ripple peak exhibits damped high frequency ringing at approximately 350 - 360kHz with 100mV P-P amplitude. I attached an image showing the ripple and the ringing on the +100V rail.

In addition, I also noticed that all of the supply rails exhibit intermittent noise peaks overlayed on top of the ripple waveform. These are perhaps 2mV P-P on the low voltage rails and closer to 20mV P-P on the high voltage rails. I am guessing that this and the 100V ringing is not normal and that this is likely the root cause of my trace ripples.

I would very much appreciate any words of wisdom from the experts regarding this issue. I spent some time studying the service manual schematic, but the power supply design it is really beyond my level.

Thank you very much in advance!

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