Got an interesting problem with my bargain Tek 475. Unfortunately there is a lot of vertical noise which is resulting in very fuzzy traces. I immediately suspected the power supply so I measured the ripple on all rails with my true RMS meter and it is well in spec. So I stuffed it in X-Y mode and centered the dot and lowered the intensity and adjusted the focus and astigmatism as best as possible. Turns out there is a vertical line 1/5th of a div high. The horizontal is pin sharp but the vertical is crap. The fuzz appears on both channels so I did the Tek 465 trick of tightening up all the screws to ensure a good ground; this works very well in some cases. Alas no difference. the above was done with the channels grounded at the scope and the case on and the unit warmed up for 30 minutes. There is no near RFI as far as i can tell and nothing that interferes with its Philips colleague.
Then I noticed that if I drop the BW limit to 100MHz, things improve. Then if you drop it to 20MHz it's actually quite good but not perfect. If I sit it next to my Philips PM3217 that is absolutely pin sharp but the Tek is still way out.
Can anyone suggest any diagnostic steps or information that can help this issue at all. I'd rather use this than the PM3217 as the bandwidth is much higher and the trigger isn't made of poop.
Any help much appreciated
Edit: for ref this is a very late model 475 that has the new bridge rectifiers and no carbon comp resistors in it (thank feck)