gst@gnosys.svle.ma.us (Gary S. Trujillo) (10/07/90)
In unix-pc.general <1990Sep26.030226.1112@zorch.SF-Bay.ORG> scott@zorch.SF-Bay.ORG (Scott Hazen Mueller) writes: > I was wondering if there is a way install a switch to *safely* turn the power > to the monitor off, while leaving the UNIX-PC up and running. The following article, from late last year, discusses this subject, plus a few others having to do with monitor failures. I have not installed a switch on my monitor myself, but am considering doing so. Can anyone think of any reason *not* to do so? Gary ------------------------------------------------------------------------------- Path: ...!gem.mps.ohio-state.edu!ctrsol!lll-winken!scooter!neoucom!wtm From: wtm@neoucom.UUCP (Bill Mayhew) Newsgroups: comp.sys.att Subject: Re: 3b1 Terminal & Scrreching Noise Summary: Usually not just the flyback is bad Keywords: Flyback transformer replacement? Message-ID: <1837@neoucom.UUCP> Date: 23 Nov 89 04:30:53 GMT Organization: Northeastern Ohio Universities College of Medicine I've had more opportunities to look at sick monitors over the years than I would like to have had. Most of the time when the flyback transformer is bad there are other components bad too. It seems that most monitors (90%??) have the flyback capacitor coupled to the horizontal output transsistor via a cheap electrolytic capacitor. The flyback of the horizontal scanning voltage from the right side of the screen to the left sets up a ringing oscillation in the flyback transformer, generating the high voltage for the CRT anode supply. What usually happens is that the electrolytic capacitor fails by shorting out. There is a fairly high AC current waveform flowing in the capacitor. A lot of monitors use power supply filter type capacitors that aren't designed for heavy currents. If the coupling capacitor shorts, the flyback is essentially connected directly to the DC power supply; this in turn burns out the flyback winding, and maybe the damper diode too. Some high frequency monitors, such as 31.5 KHz monitors used for VGA displays don't need electrolytic capacitors; they use nonpolarized mylar caps for coupling. At least this is what I noticed when I disassembled a Samsung VGA monochrome moniotr for preventative maintenance. I was impressed. My recollection is that the monitor on my 3b1 has the typical old ugly electrolytic cap. I have seen at least two types of CRTs on Unix PCs, so yours may be different. The monitor on my machine has a small glass fuse where the power supply wires connect. The monitor on our Netnews fowarder Unix PC back at the office has no fuse in the monitor and a different PC board layout. With monitors' penchant to fail in ugly ways, I opted to install a a separate power switch on the 12 volt power lead feeding the monitor on my 3b1 at home. I soldered a wire to the fuse and another wire to the fuse holder and put a piece of tape between the fuse and its holder. A switch is then attached to the free ends of the wires. As such, the switch can be used to shut off power to the monitor while I'm not at home. I drilled a 3/8" hole on the right bottom of the case for the shaft of a small Raido Shack toggle switch's handle. On my monitor, there was plenty of room to install the switch. Note that it is important that you put tape under only one end of the fuse when installing the switch so that the switch is wired IN SERIES with the switch so that you have continued fire protection. Of course, if you are going to work on the monitor, unplug the machine from the wall and leave it off for 30 minutes before opening the case. This is necessary to assure that any residual high voltage dissipates; even then, you should still presume that high voltage charge may remain. Never work on anything electronic alone. Have someone present to call for assistance if you are subjected to electrical shock. One more safety issue is the display tube. Hitting the tube may cause it to crack and subsequently impode. Very serious injury can be caused by the flying glass of the implosion. I have seen a CRT implode; it is VERY ugly. You don't want to be in the same room if it happens. If you have the type of monitor without the fuse holder, you'll need to splice into the power lead itself if you want to install a switch. Summing up, it is best to replace the whole monitor. The flyback transformer and associated components are involved in generating the high voltage for the CRT anode. Said voltage determines the amount of X-rags emitted by the CRT (yes, the CRT itself can give off X-rays). If you affect repairs, you have to use exact replacement parts to assure continued safe operation. One source of Unix PC monitors is Discovery Electronics. Check an Issue of Unix World for their address. Discovery used to be known as Digital Data Systems, I believe. One might be able to use something like a NEC multisync GS for a Unix PC monitor when/if the supply of "real" monitors dries up. It would, of course, require consulting the hardware reference manual to figure out the wiring. Bill wtm@neoucom.edu or wtm@impulse.UUCP -- Gary S. Trujillo gst@gnosys.svle.ma.us Somerville, Massachusetts {wjh12,bu.edu,spdcc,ima,cdp}!gnosys!gst