Tek 485 erratic baseline sweep all time base


Kevin Oconnor
 

Been a long time since I worked on this scope & don’t use it too often. Baseline auto sweep is erratic, choppy, and not full width. All settings of time base and uninfluenced by trigger mode or channel settings. Sometimes it settles out and works fine.
I’m looking for possible points to investigate. I have full service manual.
Kevin KO3Y


 

Kevin,

from your description of the symptoms it sounds like dirty switch contacts. Specifically, because you said that it "sometimes" settles out it suggests that this is not temperature related, and it's not an outright failed component. If you look in the service manual you will see that the horizontal sweep selector switches have a few columns that are engaged for almost all positions, and they overlap to cover all positions. If only a couple of those switch contacts were dirty it would clobber the entire sweep system, and dirty contacts do sometimes make good contact if you exercise them enough, and hold your tongue at just the right angle.

Another possibility is that one of the horizontal deflection plates is not connected, or the connection is dirty. This would be most likely if the "not full width" that you mention is in fact the right or left half of the screen, though I can imagine a way for dirty contacts on the CRT pins to produce a "not full width" sweep that manifests merely as reduce horizontal deflection.

Both of these possibilities are easy to check and address, one by cleaning the contacts in the timing drum switch, the other by checking the connections to the CRT horizontal plates.

Otherwise, someone with more skill and experience will need to advise you on what components may be failing. My only experience with loss of horizontal sweep is from a completely failed electrolytic capacitor in a 475, and that just took the sweep out altogether. It looked nothing like what you are describing.

DISCLAIMER: I've got no experience specifically with the 485, but I've worked on a couple 475s and 475As.

DISCLAIMER #2: cleaning leaf contacts in drum switch should be done with strips of 100% rag bond paper and anhydrous isopropyl alcohol.

DISCLAIMER #3: There are dangerous voltages around the CRT, including a high voltage charge that the CRT itself can retain for quite a long time after the instrument has been switch off. Exercise caution around the CRT to avoid injury from electric shock.

-- Jeff Dutky