mike@amdcad.UUCP (Mike Parker) (06/12/85)
In article <209@ttrdc.UUCP>, levy@ttrdc.UUCP (Daniel R. Levy) writes: > > Maybe Mike's memory is a little foggy. > > D. Levy > AT&T Teletype Corp. > Skokie, Ill. > > > > Engineer burnout.... Mike
levy@ttrdc.UUCP (Daniel R. Levy) (06/16/85)
Mike Parker at AMD CAD sends: > >> >cathode at a very high negative potential and the phosphor grounded to >> >minimize this effect (as you wouldn't want to get a serious shock from the >> >alternative arrangement of putting the cathode at ground potential and >> >raising the screen to a large positive potential). >> >> Not quite true. Almost all CRT's have the cathode at about ground and a >> very large positive charge on the anode, since the alternative requires some >> tricky electronics. You don't get a shock from touching your screen since >> glass is a very good insulator. I don't know myself whether the static charge >> that accumulates on the screen is from the negative electrons hitting the >> screen or stray positive charge from the anode, since glass isn't a perfect >> insulator. >> >> "There's a madness to my method." Mike Moroney >> ..decwrl!rhea!jon!moroney > >I worked in a tv shop as a youngster, I didn't understand much >of what we did at the time, but I remember a few things that might >shed some light on the subject. > >First, the static is caused by the electron beam from the cathode. >When we got a set with absolutely nothing on the screen, not even >static, we would feel the screen. No static meant no drive to the >cathode, bad amplifier. Static meant that there was no drive to the >anode, bad high voltage power supply. > >Second, I know that the cathode is not at ground. We used to measure >cathode voltage with a high voltage probe using the chassis ( ground ) >as a reference. Old B+W sets measure over 15,000 volts, new color >sets measure as high as 35,000. For those who question my terminology, >the cathode voltage is measured at one of the pins on the back of >the neck, the anode is under the suction cup. The anode is also >not at ground, it's the part that Dad always said would kill you >if you didn't know what you were doing. > >So, let's move the discussion to net.analog and see if someone >can explain the facts. > >Mike I also worked summer jobs as a TV tech when I was in undergrad school but some of the things Mike says here sure sound strange. Measuring cathode voltage with a high voltage probe? The 15,000 to 35,000 volts referred to are to be found at the ANODE, the 'suction cup' connection. I don't see how the cathode could ever get to that potential (it is at moderate voltages on the order of a couple hundred volts at the most in tube type sets) unless the CRT socket were pulled off with the cathode hot and the voltage measured at the pins right away (the cathode would assume the high anode voltage). Also you should feel static on the screen (feeling with the back of the hand works best) when high voltage is present at the anode because of the induced charge on the screen (the screen, at a high positive potential, creates an electrostatic field which attracts electrons to the other side of the glass if a source of them, such as your hand, comes near). This happens even if there is no cathode beam (turn the brightness all the way down on your TV, suppressing the beam, then try it). There cannot be a beam unless there is a potential difference between cathode and anode of the picture tube. At least that was my experience with TVs. Maybe Mike's memory is a little foggy. Now (as has been discussed further in net.consumers) with oscilloscopes it is usually a different story: the cathode is way negative and the screen is only moderately positive. It's easier this way for the electrostatic de- flection drivers, which do not need to deal with ultra high voltages. It does, however, create difficulty in trying to modulate the beam, done with the first grid of the tube, which is very near the cathode in potential. A.C. coupling (through a high-voltage capacitor) or special voltage level shif- ting for D.C. coupling is required for this modulation, for retrace beam blanking and fancy 'z-axis' modulation input. D. Levy AT&T Teletype Corp. Skokie, Ill. > >