Tektronix 2445 problem

[moby]

Any Tekxperts out there ?. I have a 2445 with a wierd trace problem.
Trace 3 and 4 are fine, but 1 and 2 have a wierd modulation on them -
independent of the attenuator setting but fixed frequency (changes with
the timebase setting) in the ultrasonic region. Oddly, if the traces
are positioned halfway up the tube, they are ok, but as moved higher or
lower they get worse the further from centre. The amplitude gets less
as the scope warms up but it doesn't go away. It looks like this :
http://homepages.kcbbs.gen.nz/moby/Tek2445.JPG
Any insights appreciated
Cheers
M

 

[Phil Allison]

"moby"

Any Tekxperts out there ?. I have a 2445 with a wierd trace problem.
Trace 3 and 4 are fine, but 1 and 2 have a wierd modulation on them -
independent of the attenuator setting but fixed frequency (changes with
the timebase setting) in the ultrasonic region. Oddly, if the traces
are positioned halfway up the tube, they are ok, but as moved higher or
lower they get worse the further from centre. The amplitude gets less
as the scope warms up but it doesn't go away. It looks like this :
http://homepages.kcbbs.gen.nz/moby/Tek2445.JPG
Any insights appreciated

** Looks like SMPS noise on one of the DC rails.

The improvement on warming up strongly indicates bad ( high ESR) electos.

Got a Bob Parker ESR meter ??

If not, try bridging likely PSU electros with new ones.



........... Phil

 

[Jamie]

Any Tekxperts out there ?. I have a 2445 with a wierd trace problem.
Trace 3 and 4 are fine, but 1 and 2 have a wierd modulation on them -
independent of the attenuator setting but fixed frequency (changes with
the timebase setting) in the ultrasonic region. Oddly, if the traces
are positioned halfway up the tube, they are ok, but as moved higher or
lower they get worse the further from centre. The amplitude gets less
as the scope warms up but it doesn't go away. It looks like this :
http://homepages.kcbbs.gen.nz/moby/Tek2445.JPG
Any insights appreciated
Cheers
M

looks like a weak capacitor.