Tektronix 7905 Oscilloscope
 

I bought a used non-working 7905. Have almost everything fixed except:


When increasing the trace brightness to an acceptable level there is a
halo present. It is not a uniform halo, but is displaced slightly up
and to the right of the trace, probably 20% brightness at normal
brightness of main trace.

The focus and astigmatism controls work properly.
Your help will be appreciated,
Thanks,
Emil

"egb" wrote in
oups.com:

I bought a used non-working 7905. Have almost everything fixed except:


When increasing the trace brightness to an acceptable level there is a
halo present. It is not a uniform halo, but is displaced slightly up
and to the right of the trace, probably 20% brightness at normal
brightness of main trace.

Could be a bad CRT.
Can you display a signal of amplitude enough to cover the entire graticule
and check to see if it's distorted in any way? I used to use the 50Khz ref
signal from a SG503 leveled sine wave generator.


The focus and astigmatism controls work properly.
Your help will be appreciated,
Thanks,
Emil



Well,you could have a 7904(4 holer) or a 7903(3 hole rack mount),but there
is no 7905 TEK scope.

--
Jim Yanik
jyanik
at
kua.net

Yes, you're absolutely correct, excuse me. It is the 7904. Thanks for
the correction.
Emil

Jim, thanks for the reply. The display is just fine in every other
respect. Single trace,or 4 trace everything works, regardless of
amplitude.

"egb" wrote in message
oups.com...
I bought a used non-working 7905. Have almost everything fixed except:


When increasing the trace brightness to an acceptable level there is a
halo present. It is not a uniform halo, but is displaced slightly up
and to the right of the trace, probably 20% brightness at normal
brightness of main trace.

Are you sure all of the CRT voltages are within specifications?

"egb" wrote in
oups.com:

Jim, thanks for the reply. The display is just fine in every other
respect. Single trace,or 4 trace everything works, regardless of
amplitude.


Some CRTs had more halo than others,and depended on trace brightness and
sweep speeds.Also Z-axis unblanking could be a problem.

--
Jim Yanik
jyanik
at
kua.net

"Charles Schuler" wrote in
:


"egb" wrote in message
oups.com...
I bought a used non-working 7905. Have almost everything fixed except:


When increasing the trace brightness to an acceptable level there is a
halo present. It is not a uniform halo, but is displaced slightly up
and to the right of the trace, probably 20% brightness at normal
brightness of main trace.

Are you sure all of the CRT voltages are within specifications?




Yes,like the cathode V,-2960V IIRC.(which also determines the anode V)
The thick-film network can go out of spec.(no new replacements available)

It could also be a electron gun element knocked out of alignment,and part
of the beam striking it and diffracting.
Thus the test I mentioned for geometry in another post.

--
Jim Yanik
jyanik
at
kua.net

Jim and Charles,
Thanks for all the help. I'll go over everything you gentlement
suggested this weekend.
Emil