At 13:25 -0800 1/28/13, Haemogoblin wrote, and I snipped: >Hi there > >Thanks, I'm just an enthusiastic hobbyist :) > >I checked the tube in a dark room and it does glow, very dimly. Also tried a >floppy disk and found the machine will boot from it. I've checked the >brightness knob and it makes no difference on the picture, the screen remains >blank. Another thing, which i'm hoping other plus owners might be able to >answer for me. Surely the tube should glow, even without a video input? I've >never really read up on how cathode ray tubes work, so dont know what to >expect. > >James
The picture tube starts with the filament that releases electrons. It is near ground potential but might be a bit negative. Electrons are then accelerated by a metallic ring or or two which are kept at positive voltages. The geometry is magically calculated to focus the electrons on the screen which is a few thousand volts positive and attracting the negative electrons. The beam of electrons is pushed side to side and down through up using magnetic fields created by current in the wires looped around ferrite forms surrounding the neck of the tube. That's called the yoke. Back down at the starting end there is a grid through which the electrons pass while they are moving slowly. A negative voltage there causes electrons to be lost and a neutral or positive voltage lets them thru. The result is a bright or dim spot of light on the screen. All that's left is to synchronize everything with the magnetic fields to get a picture. So what can go wrong? No electrons because of too low temperature on the filament. There are two pins on the tube that are the ends of the filament. I don'y know what the voltage there should be but a typical value for a lot of tubes is 6 volts and might be AC or DC. That's the voltage between the two pins, not from either one to ground. The voltages on the beam forming rings of the "electron gun" might not be right. They might be pretty high - hundreds of volts but you won't find the correct values in anything Apple publishes. The grid might not be getting the video signal and be stuck in a negative level which removes all electrons from the beam. Somewhere there is a digital to analog converter that converts the software provided digital value to the analog voltage that the grid needs. I think I remember some tuning controls on the analog board that might adjust a bias that could turn the grid off. All of that support is on the analog board and depends on the flyback transformer which creates the horizontal deflection currents in the yoke and the high voltage for the screen It may also create the filament voltage with a few turns. The filament doesn't care if it's AC at 15 kHz. Old television sets do that but I'm not so sure about the Mac. You said you heard the decay of the high voltage and that argues for a good flyback. Right now I would bet on a bad solder joint or a broken wire associated with the analog board. Perhaps a loose connection on the tube socket or at the solder joint where the wiring terminates at the analog board. It's conceivable that a lost vertical or horizontal drive is causing the focus of the electron beam to be off the side of the screen. I doubt it. More often the deflection just fails and the result is a burned spot right in the middle of the screen. A look at the video on the grid with an oscilloscope would tell a lot -- --> Give me liberty or give me Obamacare <-- -- -- ----- You received this message because you are a member of the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/vintage-macs Support for older Macs: http://lowendmac.com/services/ --- You received this message because you are subscribed to the Google Groups "Vintage Macs" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/groups/opt_out.
