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?
Deep Friar: a very philosophical monk.
I don't have any useful knowledge personally here, but I can point you to the Tektronix Yahoo! group where a lot of very good Tek troubleshooters -- as well as some old Tek techs and designers -- hang out:
. I'm confident you'd get lots of useful replies there too!
On a sunny day (Sun, 7 Jun 2009 17:27:21 -0500) it happened "Tim Williams" wrote in :
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.
"Tim Williams" wrote in news:9bXWl.6733$ email@example.com:
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.
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.
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.
Deep Friar: a very philosophical monk.
Jim Yanik wrote in news:Xns9C2475B01DF2Djyanikkuanet@188.8.131.52:
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.
This is interesting and consistent with what I know about electron beam emitters used in electron beam microscopes and lithography. (See below.)
Sounds like an excellent point to keep in mind.
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.