> This problem started two nights ago. The SE/30 was > working fine, when there was a very small "click" and > the screen went from perfect to what showed in > aforementioned graphic in about half a second.
A clicking sound is not a good thing! Since you have a scope of sorts, take a look at CRT pin 2 (the cathode), and verify that it can swing close to ground, and up to double-digit volts or so. If the scope's bandwidth is low, you may not see a consistent peak-to-peak this large, but see if the cathode voltage ever gets near these limits. As long as you're seeing at least ~20 volts of swing (it should really be more than this), it should be fine. If your're getting little or no video signal, your problem is in the video generation circuitry on the motherboard, with/around U1 (the inverter-connected NAND gate in the SE analog schematics from Gamba), with the video amplifier on the CRT board, or with the 30V supply. Verify that you are getting about +30V to the collector load of the lone transistor on the CRT board (you can also measure it at P2-1). If those are all ok, then you have a problem with the crt grid biases. With a high-impedance meter (most any DMM/DVM will do, but most ordinary analog ones won't), measure the voltages on CRT pins 1, 6, and 7 with respect to ground (one of the crt filament pins is a convenient ground -- pin 4 on the Plus, perhaps the same on the SE; check this). The voltage on pin 1 should be double-digit negative, and change as you adjust the front panel brightness control. Your mileage may vary, but expect to be able to move that voltage from perhaps -25 at the maximum brightness setting, down to maybe -75 at minimum setting. The precise values are not at all critical, but should be in this general ballpark. If you can't make the voltage negative enough, that's a symptom. Possible causes there: Open C17, open CR6, open R20, leaky or shorted C19/C26. This is not an exhaustive list by any means, but should suggest some things to check. CRT pin 6's voltage should be triple-digit positive, and vary with the cutoff adjustment on the analog board. You should be able to move the voltage down to about 150V or so, and up to several hundred volts (but don't try to verify the upper end -- just eyeball that you can make the raster very bright, then adjust things back down). If the minimum voltage is greatly in excess of 150V, that's a symptom. Finally, CRT pin 7's voltage should vary as you adjust the focus control. You don't really have to measure this; just eyeball the scan lines and verify that you can move the raster in and out of focus. If you really want to measure the voltage, it should be adjustable down to zero volts, and go as high as perhaps 500V or so. And a last, remote possibility: if your crt has developed a serious cathode to grid 1 (near)short, then the raster can look very much as your screen shot shows. If everything else checks out ok, then that could be it. Good luck! -- Prof. Thomas H. Lee Center for Integrated Systems, CIS-205 420 Via Palou Mall Stanford University Stanford, CA 94305-4070 http://www-smirc.stanford.edu 650-725-3709 ph, -3383 fax -- Compact Macs is sponsored by <http://lowendmac.com/>. Support Low End Mac <http://lowendmac.com/lists/support.html> Compact Macs list info: <http://lowendmac.com/lists/compact.shtml> The FAQ: <http://macfaq.org/> --> AOL users, remove "mailto:"; Send list messages to: <mailto:compact.macs@;mail.maclaunch.com> To unsubscribe, email: <mailto:compact.macs-off@;mail.maclaunch.com> For digest mode, email: <mailto:compact.macs-digest@;mail.maclaunch.com> Subscription questions: <mailto:listmom@;lemlists.com> Archive:<http://www.mail-archive.com/compact.macs%40mail.maclaunch.com/> --------------------------------------------------------------- >The Think Different Store http://www.ThinkDifferentStore.com ---------------------------------------------------------------
