Topics

2465B CRT board values


Hirofumi Momose
 

Hello I am new to this group.
Recently I restored 2465B and which works well.
While fine tuning the CRT focus and I notice following things.

CRT cathode volt of mine is -1960V which higher from schematic (-1900V).
It should be -1924V (10V/98.73k(R1945)*19M(inside HV module)
but -1900V on schematic. (2465A sm (10V/100k*19M) is correct)
I am confusing..
Some resistor values inside HV module are different from schematic.
5.1k*2 (cathode filter) is probably 10k*2.
Slot lens -900V divider (10M/9M) is probably 7.4M/6.9M,
ratio is almost same.

Probably because of design change not of aging. But I am not sure.
Those are correct ?

Regards,
Hirofumi
hirofumi@momose.com


Chuck Harris
 

I am not sure what you are trying to discover.

Tektronix did not consider the CRT voltage to be worth
more than using 5% resistors as their control.

They let the voltages fall where they may within the
5% tolerance, and adjusted the rest of the scope to
gain the correct picture size, etc...

Do you see these "changed" values from the markings on
the parts, or are you measuring them?

-Chuck Harris

Hirofumi Momose wrote:

Hello I am new to this group.
Recently I restored 2465B and which works well.
While fine tuning the CRT focus and I notice following things.

CRT cathode volt of mine is -1960V which higher from schematic (-1900V).
It should be -1924V (10V/98.73k(R1945)*19M(inside HV module)
but -1900V on schematic. (2465A sm (10V/100k*19M) is correct)
I am confusing..
Some resistor values inside HV module are different from schematic.
5.1k*2 (cathode filter) is probably 10k*2.
Slot lens -900V divider (10M/9M) is probably 7.4M/6.9M,
ratio is almost same.

Probably because of design change not of aging. But I am not sure.
Those are correct ?

Regards,
Hirofumi
hirofumi@momose.com






Hirofumi Momose
 

Hello Chuck,

Tektronix did not consider the CRT voltage to be worth
more than using 5% resistors as their control.
In my experience servicing iwatsu scopes,
cathode voltage is specified as approx 1%.
My 2465B focus knob is far off the center at 3 o'clock.
I think this is because of cathode voltage.
But Yes, this still under adjustable range.

Do you see these "changed" values from the markings on
the parts, or are you measuring them?
Measured (in-circuit)
I think value error is little as long as caps
in the HV-module are good.

Thank you.
Hirofumi JJ2PNX


Chuck Harris
 

Resistors very often aren't the same value at high voltages
as they are at low voltages. Particularly high megaohm types.

And, it is rather hard to measure such high value resistors in
circuit. Your measurements get affected by the leakage of the
dirt on the resistors, the board, and capacitors.

I haven't found a 2465B that has its best focus at 12:00 with
the exception of the 2467B type, which has a trimmer pot to set
the best focus to the center of the range of the focus pot.

Most everything else is trimmed using the computer and stored
constants.

-Chuck Harris

Hirofumi Momose wrote:

Hello Chuck,

Tektronix did not consider the CRT voltage to be worth
more than using 5% resistors as their control.
In my experience servicing iwatsu scopes,
cathode voltage is specified as approx 1%.
My 2465B focus knob is far off the center at 3 o'clock.
I think this is because of cathode voltage.
But Yes, this still under adjustable range.

Do you see these "changed" values from the markings on
the parts, or are you measuring them?
Measured (in-circuit)
I think value error is little as long as caps
in the HV-module are good.

Thank you.
Hirofumi JJ2PNX






Jean-Paul
 

Bonjour Hirofumi-san,

very fine to have a new group member in Japan.

All the items you observe are OK, if the scope display is satisfactory.

The Tektronix service manual is online for free and has complet CRT/HV adjustment proceedure.

Chuck is a super experienced Tek veteran and gives the best advice.

I can only add two old American sayings...
"If it ain't broke don't fix it"

"leave well enough alone"

Kind Regards

Jon


Hirofumi Momose
 

Hello Chuck, Jon

Chuck wrote:
I haven't found a 2465B that has its best focus at 12:00
Oh.. My unit is just 'normal'. Understand.

Jon wrote:
"If it ain't broke don't fix it"
"leave well enough alone"
Now works. Best focus point of trace/readout is a bit different.
But no problem to read.
Yes ,no more tune and leave it.

Thank you.
Hirofumi JJ2PNX


Chuck Harris
 

Hi Hirofumi,

Scopes, like the 2465B, have only a single electron beam to
draw all that is on the CRT screen. They do this by time
multiplexing the beam between the readout, and the traces.

If the CRT only displayed a single intensity, and hence a
single electron beam current, it would be easy to just set
a single voltage on the focus lens, and get best focus on the
screen. But they don't.

So, tektronix made a special non-linear circuit that took as
its inputs horizontal sweep rate, beam current, and focus control
voltage, and used it to warp the voltage driving the focus lens
into a pretty good compromise between high intensity and low
intensity trace focus... but still a compromise...

And, since a sharp bright main trace is the money maker for a
scope, it gets money spent on its own adjustment for high
brightness sharpness, and the readout focus gets compromised.

If you do the CRT adjustments in the CAL section, you too can
have a chance to decide on how to do that compromise. Pay
particular attention to "HIGH DRIVE FOCUS".

The CRT adjustments section of the calibration can be done on
its own, and only needs a 50KHz sine wave.

-Chuck Harris

Hirofumi Momose wrote:

Hello Chuck, Jon

Chuck wrote:
I haven't found a 2465B that has its best focus at 12:00
Oh.. My unit is just 'normal'. Understand.

Jon wrote:
"If it ain't broke don't fix it"
"leave well enough alone"
Now works. Best focus point of trace/readout is a bit different.
But no problem to read.
Yes ,no more tune and leave it.

Thank you.
Hirofumi JJ2PNX






Hirofumi Momose
 

Hi Chuck,

Scopes, like the 2465B, have only a single electron beam to
draw all that is on the CRT screen. They do this by time
multiplexing the beam between the readout, and the traces.
Flicker less readout with single gun displaying multiple
traces, various timebase is a magic to me.

Especially, I feel a bit difficulty to get best result from
CRT adjustment is interact of geometry and edge focus.
To minimize the vertical edge curveture(CAL 08)
causes the edges to being some degree of defocus.
Focusing on edge causes the center to defocus...
All of these are interact at some extent and to get best result
needs 'dialog' with the scope.
But now, got satisfactory level.

Best regards,
Hirofumi JJ2PNX


Chuck Harris
 

Hi Hirofumi,

One thing to note is that Tektronix didn't expect that you
would be able to get perfect results in the outer most
major divisions of the CRT graticule. I always try for
perfection, but if you have to have a little geometric
distortion, or less than perfect focus, that is where it
should happen.

-Chuck Harris



Hirofumi Momose wrote:

Hi Chuck,

Scopes, like the 2465B, have only a single electron beam to
draw all that is on the CRT screen. They do this by time
multiplexing the beam between the readout, and the traces.
Flicker less readout with single gun displaying multiple
traces, various timebase is a magic to me.

Especially, I feel a bit difficulty to get best result from
CRT adjustment is interact of geometry and edge focus.
To minimize the vertical edge curveture(CAL 08)
causes the edges to being some degree of defocus.
Focusing on edge causes the center to defocus...
All of these are interact at some extent and to get best result
needs 'dialog' with the scope.
But now, got satisfactory level.

Best regards,
Hirofumi JJ2PNX






Hirofumi Momose
 

Hi Chuck,

I understand that scope CRT is not a 'picture tube'.
Getting best result is just personal satisfaction.
Sharp trace make me happy.
Thank you.
Hirofumi JJ2PNX