Path: news.weeg.uiowa.edu!news.uiowa.edu!hobbes.physics.uiowa.edu!zaphod.mps.ohio-state.edu!caen!uunet!cs.utexas.edu!qt.cs.utexas.edu!yale.edu!yale!not-for-mail From: elvey-matthew@cs.yale.edu (Matthew Elvey) Newsgroups: comp.sys.apple2 Subject: Re: Solid red monitor screen Date: 30 Nov 1992 17:22:51 -0500 Organization: Yale U. Lines: 111 Distribution: world Message-ID: <1fe47rINNsch@SCORPION.ZOO2.CS.YALE.EDU> References: <9211231450.AA00178@TIS.COM> NNTP-Posting-Host: scorpion.zoo2.cs.yale.edu Here's a more detailed possible solution: I still have to get around to doing this. The following message is from last year sometime. From: dennis mcclain-furmanski I never did have time to get to my RGB monitor. But I found this on AOL this week: "It isn't easy, being green..." Kermit, ca. 1982 I'd started to think my Apple 2GS RGB monitor was immortal. It had always worked for me just as it did when I first turned it on in 1987 or thereabouts. Then without any precursor warnings my monitor's screen lit up with a hurt-your-eyes green raster and no video. 50-dollar bills flashed before my eyes. I'm no stranger to tv servicing, so there was no question but that I 'd give it my best effort before spending the big bucks for repair or replacement. I hit the wall without service data. The local electronics-supply dealer told me, "Nah, there is no Sams Photofact for the Apple 2GS monitor... Apple never did sell many 2GSs, so Sams didn't do anything for the 2GS!" I drew myself to my full height, started to tell him about the millions of Apple 2GS computers still in use then said, "Ok..." and left the store. I'm tired of arguing the point. I made my own Sams by tracing out the pc-board circuitry. A boring but necessary step. The paper schematic helped to make sense out of the voltage readings on the pc board's various test points. The excess greenness was the result of an open peaking coil. Peaking coils are small inductors used to extend the frequency response of video amplifiers. A transistor amplifier has unavoidable capacitance that bypasses high-frequency video components. It's these high-frequency signals that enable the CRT to change brightness quickly as it forms sharp edges. Inductance, such as that found in a peaking coil, cancels capacitance. If you choose just the right amount of inductance you can compensate for the unwanted capacitance, which is exactly what happens when a peaking coil is doing its thing. The open inductor in my monitor's circuitry allowed the green cathode on the CRT to drop about 100 volts lower than normal, and this action biased the green gun into saturation. It was obvious that something was amiss once I'd determined that the peaking coil had100 volts across it's terminals. I had two good peaking peaking coils for comparison in the red and blue circuitry. Each of these good coils measured about 10-ohms resistance. The flakey coil was open. Shorting the open coil for a test eliminated the excess green on the screen but at the expense of poor green bandwidth. How do I get a replacement peaking coil? The local electronics supply dealer was about as helpful as he'd been with the Sams Photofact deadend. "Peaking coils almost never fail. You have to go back to the manufacturer to get a replacement! Forget it!" This time I stood up to him, asked to see his stash of peaking coils. I needed a 20-microHenry inductor, and he did have a 22-microHenry unit. $1.95. The 10% excess made no noticeable difference and my monitor seems as good as new. Being green is not so bad if your monitor can also be red and blue. *** I wrote him and asked if the part was labeled L6G2, a small blue component with a red and black dot, just above the transistor Q6G2, on the board hanging on the back of the CRT. This is the reply: Subj: Re: RGBlues Date: 92-08-26 11:25:01 EDT From: AFA DonB To: DynaSoar01 You have the right part... It's a small blue component with one black dot on the side, a black and a red dot on top... Measure the dc voltage across the component. If it's open (defective), you'll measure somewhere around 100 volts... If it's good, the voltage drop will be negligible. Another way to check: Measure the voltage at each of checkpoints on the CRT socket. All three should measure about 175 volts, referenced to the ground trace on the pc board that rides on the back of the tube. If the peaking coil is open for one of the three color circuits, that color's testpoint will measure about 70 or 80 volts... This is turning out to be a really common problem. Your report is not the first I've heard! Fortunately, once you know what to look for, it's an easy fix! Don (Apple Hardware AFA) -- Matthew Elvey Print "Hello, World!" elvey@yale.edu or elvey@minerva.cs.yale.edu or Input (response) elvmatr@yalevm or Matthew_Elvey@quickmail.ycc.yale.edu if response = nil Box 5186; Yale U.; New Haven, CT 06520-5186 (203)436-1251 Print "Uh-Oh!"