Tektronix 7905 Oscilloscope

[egb]

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

 

[Jim Yanik]

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.

 

[egb]

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

 

[egb]

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

 

[Charles Schuler]

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?

 

[Jim Yanik]

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]

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.

 

[egb]

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