Tek 465 trace brightness variation during sweep

[Al Schapira]

On my Tek 465 scope, I notice a peculiar variation of the intensity of
the trace at fast sweep rates. The intensity decreases during the sweep
for about 200 ns near the beginning of the trace and returns to normal
for the remainder of the sweep. It is most noticeable using ALT sweeps.

Please see the pictures at

http://home.att.net/~a.schapira/pix/Tek465/scope_.05us.jpg
http://home.att.net/~a.schapira/pix/Tek465/scope_.1us.jpg
http://home.att.net/~a.schapira/pix/Tek465/scope_.2us.jpg
http://home.att.net/~a.schapira/pix/Tek465/scope_.5us.jpg

You can clearly see the intensity drop off for about 200 ns and come
back up. For each picture, the inputs are the same; only the sweep
rate was changed according to the name of the picture file.
Its as if some of the sweep or trigger signal is leaking into the Z axis
circuitry.

If you could suggest a cause (and remedy) for this I would be grateful.
I do not have a service manual or schematic (I would like one), but I am
generally good at troubleshooting and repairs. I just don't have any
experience troubleshooting Tek scopes.

Thanks for any help you can provide with this.

-Al Schapira, KC2HRH, [email protected]

 

[N Cook]

On my Tek 465 scope, I notice a peculiar variation of the intensity of
the trace at fast sweep rates. The intensity decreases during the sweep
for about 200 ns near the beginning of the trace and returns to normal
for the remainder of the sweep. It is most noticeable using ALT sweeps.

Please see the pictures at

http://home.att.net/~a.schapira/pix/Tek465/scope_.05us.jpg
http://home.att.net/~a.schapira/pix/Tek465/scope_.1us.jpg
http://home.att.net/~a.schapira/pix/Tek465/scope_.2us.jpg
http://home.att.net/~a.schapira/pix/Tek465/scope_.5us.jpg

You can clearly see the intensity drop off for about 200 ns and come
back up. For each picture, the inputs are the same; only the sweep
rate was changed according to the name of the picture file.
Its as if some of the sweep or trigger signal is leaking into the Z axis
circuitry.

If you could suggest a cause (and remedy) for this I would be grateful.
I do not have a service manual or schematic (I would like one), but I am
generally good at troubleshooting and repairs. I just don't have any
experience troubleshooting Tek scopes.

Thanks for any help you can provide with this.

-Al Schapira, KC2HRH, [email protected]

Something out of kilter in the bright-up/blanking/fly-back suppression
circuitry ?

 

[Jim Yanik]

On my Tek 465 scope, I notice a peculiar variation of the intensity of
the trace at fast sweep rates. The intensity decreases during the sweep
for about 200 ns near the beginning of the trace and returns to normal
for the remainder of the sweep. It is most noticeable using ALT sweeps.

Please see the pictures at

http://home.att.net/~a.schapira/pix/Tek465/scope_.05us.jpg
http://home.att.net/~a.schapira/pix/Tek465/scope_.1us.jpg
http://home.att.net/~a.schapira/pix/Tek465/scope_.2us.jpg
http://home.att.net/~a.schapira/pix/Tek465/scope_.5us.jpg

You can clearly see the intensity drop off for about 200 ns and come
back up. For each picture, the inputs are the same; only the sweep
rate was changed according to the name of the picture file.
Its as if some of the sweep or trigger signal is leaking into the Z axis
circuitry.

If you could suggest a cause (and remedy) for this I would be grateful.
I do not have a service manual or schematic (I would like one), but I am
generally good at troubleshooting and repairs. I just don't have any
experience troubleshooting Tek scopes.

Thanks for any help you can provide with this.

-Al Schapira, KC2HRH, [email protected]

Well,first thing to check is power supplies;voltage and ripple.
Then,you will need a service manual with schematics;you need to scope the
Z-axis waveform to see if it's flat at the time you get the intensity drop.
The Z-axis amp and DC restorer is where I would be looking.

 

[James T. White]

On my Tek 465 scope, I notice a peculiar variation of the intensity of
the trace at fast sweep rates. The intensity decreases during the sweep
for about 200 ns near the beginning of the trace and returns to normal
for the remainder of the sweep. It is most noticeable using ALT sweeps.

Al,

Could I can't remember if a 465 does this or not, but HP scopes of that era
would highlight the section of the trace that would be "magnified" by the delay
when you had the delay mode switch set to normal and the delay ring of the
timebase anywhere but off. Try adjusting the delay vernier to see of the
highlighted section moves.

 

[krikkit]

Al,

Could I can't remember if a 465 does this or not, but HP scopes of that
era
would highlight the section of the trace that would be "magnified" by the
delay
when you had the delay mode switch set to normal and the delay ring of the
timebase anywhere but off. Try adjusting the delay vernier to see of the
highlighted section moves.

I'd say James is right on the money, this looks like the scope is in "B
intensify" mode.

 

[Jim Yanik]

I'd say James is right on the money, this looks like the scope is in
"B
intensify" mode.

Then others here who own a 465 could duplicate the "problem".

 

[Al Schapira]

Thanks to all who replied.

. The A INTENSIFIED and B DELAYED controls work as expected. The
scope was not in either mode when I took the original pictures.

. The problem seems to only appear in ALT sweep mode. When using CHOP
to display the same waveforms, the intensity is uniform.

. Depressing the BEAM FINDER results in a pretty uniform intensity.

. When using A INTENSIFIED (by B) another similar anomaly occurs.
After the intensified portion of the trace is supposed to return to
normal intensity, it also drops off (dimmer than normal) for about 100
ns and then returns to normal. This, of course, happens, not just at
the left edge, but anywhere the B DELAY is set to.
From this I conclude that the Z Axis signal is overshooting/ringing,
but only in the direction to dim the trace.

Since I don't have another scope fast enough to troubleshoot a 200 ns
pulse (my other scope is a Tek 535A) I'll have to do it the hard way.
Can anyone suggest which components might be involved?

Thanks again.

-Al

 

[Jim Yanik]

Thanks to all who replied.

. The A INTENSIFIED and B DELAYED controls work as expected. The
scope was not in either mode when I took the original pictures.

. The problem seems to only appear in ALT sweep mode. When using
CHOP
to display the same waveforms, the intensity is uniform.

. Depressing the BEAM FINDER results in a pretty uniform intensity.

. When using A INTENSIFIED (by B) another similar anomaly occurs.
After the intensified portion of the trace is supposed to return to
normal intensity, it also drops off (dimmer than normal) for about 100
ns and then returns to normal. This, of course, happens, not just at
the left edge, but anywhere the B DELAY is set to.
From this I conclude that the Z Axis signal is overshooting/ringing,
but only in the direction to dim the trace.

Since I don't have another scope fast enough to troubleshoot a 200 ns
pulse (my other scope is a Tek 535A) I'll have to do it the hard way.
Can anyone suggest which components might be involved?

Thanks again.

-Al

First,I'd try to find out if other 465's do this.It may just be a quirk of
the scope.It might just be the way your CRT grid bias is calibrated.