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Anyone remember the semi-old Teks?

T

Tim Williams

Jan 1, 1970
0
Tektronix 475 in particular. Mine has a weak picture. Intensity cranked
all the way, it's pretty much reaches regular viewing level. It's not just
that grid bias or whatever is low: as you turn the knob, it gets brighter,
then brightest when the control is about halfway up, then doesn't get any
brighter, only more smeared (adjusting focus helps a little with that).
The -2550V TP measures as -2500ish with a 10Mohm probe, and deflection is
still to scale, so it doesn't seem to be a high voltage issue?

Tim
 
J

Jon Kirwan

Jan 1, 1970
0
Cathode pollution?
The old TV CRTs you could reactivate by putting +300 V on the first grid,
in series with a 25 W light bulb, for a few seconds, until the cathode was burned clean and the light bulb lit up.
Done it many times.
But never with a scope CRT.

Measure beam current, if it tops it is that.
If you kill the scope doing it...
Do not blame anyone but yourself.
475 is a bit antique anyways.

Could also be a leaking cap in the brightness control, or a resistor gone high there.

I think I may have 6 tubes in storage. I'll have to go look, at some
point. Maybe too late for this, but I'll put it on my list to find
out.

Jon
 
J

Jim Yanik

Jan 1, 1970
0
Tektronix 475 in particular. Mine has a weak picture. Intensity
cranked all the way, it's pretty much reaches regular viewing level.
It's not just that grid bias or whatever is low: as you turn the knob,
it gets brighter, then brightest when the control is about halfway up,
then doesn't get any brighter, only more smeared (adjusting focus
helps a little with that).
The -2550V TP measures as -2500ish with a 10Mohm probe, and
deflection is
still to scale, so it doesn't seem to be a high voltage issue?

Tim

it's called "double-peaking",and a good indiction of end-of-life.
when you turn up the intensity,the tired,worn cathode emits from a larger
and larger area,and thus the gun elements cannot focus it tight.

incidentally,you can make a nice HV probe with a string of 10M resistors to
make 90M and the DMM provides the last 10M for a 100M load.I put the string
into a old tuning tool,one thats a 3/8" red tube with a 6/32 hex spacer and
screw inside for a screwon TEK hooktip.

10M on the HV supply is really too much of a load.
 
T

Tim Williams

Jan 1, 1970
0
Jim Yanik said:
it's called "double-peaking",and a good indiction of end-of-life.
when you turn up the intensity,the tired,worn cathode emits from a larger
and larger area,and thus the gun elements cannot focus it tight.

So the tube's pooched for sure? Definitely not a EHV problem?

No way I can get into the HV box, not without removing all the circuit
boards, but I can get at the 2nd anode lead. I could measure that with up
to 3 x 10M 5W. Don't have any more than that. I can easily measure with
two combinations and do the Thevenin conversion.
incidentally,you can make a nice HV probe with a string of 10M resistors
to
make 90M and the DMM provides the last 10M for a 100M load.I put the
string
into a old tuning tool,one thats a 3/8" red tube with a 6/32 hex spacer
and
screw inside for a screwon TEK hooktip.

10M on the HV supply is really too much of a load.

Ah, but since it still measured within 10%, I'd say not only can it supply
the load, but the HV regulator is in excellent condition as well.

Tim
 
J

Jim Yanik

Jan 1, 1970
0
So the tube's pooched for sure? Definitely not a EHV problem?

Yes,you need a new CRT.
No way I can get into the HV box,

the 475 HV multiplier is potted and has a HV lead with socket coming out of
it.you can't get at the anode(safely) without making an adapter from old
CRT anode and multiplier leads.
not without removing all the circuit
boards, but I can get at the 2nd anode lead. I could measure that
with up to 3 x 10M 5W. Don't have any more than that. I can easily
measure with two combinations and do the Thevenin conversion.


Ah, but since it still measured within 10%, I'd say not only can it
supply the load, but the HV regulator is in excellent condition as
well.

somehow I don't have a lot of confidence in your measurement. ;-)

DONT try measuring the anode lead without a real HV probe,like the Fluke
that is 1000M ohms.
 
Q

qrk

Jan 1, 1970
0
Cathode pollution?
The old TV CRTs you could reactivate by putting +300 V on the first grid,
in series with a 25 W light bulb, for a few seconds, until the cathode was burned clean and the light bulb lit up.
Done it many times.
But never with a scope CRT.

Measure beam current, if it tops it is that.
If you kill the scope doing it...
Do not blame anyone but yourself.
475 is a bit antique anyways.

May be antique, but is still a darn good scope. I still use a pair of
465 scopes for everyday use. Can't do low level analog design on a
digital scope since it buggers the noise of the amplifier.
 
J

Jim Yanik

Jan 1, 1970
0
Yes,you need a new CRT.

the 475 HV multiplier is potted and has a HV lead with socket coming
out of it.you can't get at the anode(safely) without making an adapter
from old CRT anode and multiplier leads.


somehow I don't have a lot of confidence in your measurement. ;-)

DONT try measuring the anode lead without a real HV probe,like the
Fluke that is 1000M ohms.

BTW,it was discovered by TEK CRT manufacturing that CRT life is directly
related to filament voltage.As little as 0.1V too high,and CRT life
suffers. A lot of 2213/2215 CRTs were replaced under warranty for this
reason,and the HV xfmr had a half turn removed from it's filament winding.

since most TEK CRT filaments get their supply from the HV
oscillator(excluding the old 500 series..),regulation and proper HV voltage
setting is essential.
 
J

Jon Kirwan

Jan 1, 1970
0
BTW,it was discovered by TEK CRT manufacturing that CRT life is directly
related to filament voltage. As little as 0.1V too high, and CRT life
suffers. A lot of 2213/2215 CRTs were replaced under warranty for this
reason, and the HV xfmr had a half turn removed from it's filament winding.

This is interesting and consistent with what I know about electron
beam emitters used in electron beam microscopes and lithography. (See
below.)
since most TEK CRT filaments get their supply from the HV
oscillator(excluding the old 500 series..),regulation and proper HV voltage
setting is essential.

Sounds like an excellent point to keep in mind.

Thanks,
Jon

P.S. Emitters are coated to promote thermionic emission, then a
Wehnelt to enhance the field effect and shape the emission, and so on.
But the aging of such emitter systems is HIGHLY dependent on the
temperature at the emitter itself. This, in turn, depends on large
part with the current used. Variations in lifetime of emitter systems
I worked with were from 1-2 months to as long as 20-24 months. Same
units in every respect except for the procedures used. Many of these
systems used expensive current control systems to set the emitter
heating current, but that really wasn't enough. The systems that ran
longer were ones where regular protocols were used with disappearing
filament optical devices to observe the _temperature_ at the tip, not
the current, and where that was done regularly. The very best were
those where the disappearing filament devices were calibrated on a
regular basis using NIST calibrated tungsten filament standards driven
by expensive current controls and getting new calibration standards
every 100 hours of calibration use (or less.) Those consistently got
the longest lives out of their 'heads.'

In these cases, getting the absolute MOST out of the emitters was
important, though. For example, lithography happens faster (which
means more production value) if the electron beam is more intense. So
clients want to operate at the absolute ragged edge. That's different
than an oscilloscope requirement.
 
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