> > So I am looking for a real 2N3904 to check, as resistor and voltage > measures on the transistor pins show right voltages and resistances; I > guess replacements burn due to excess disipation because of different > characteristics, not due to a failure in the tube circuit. > > So, if somebody has a 2N3904 laying around, please, let me know. I think it > is the only piece needed to get this compact Mac again on the road. >
Sorry to be the bearer of bad news, but you still have a circuit fault. The video circuit of classic Macs (and Mac Classics) uses the transistor as a simple inverter. Its dissipation is normally very small, and a 2N2222 would do just fine. The fact that you're still toasting every replacement says that either the load is excessive, or the base drive is too wimpy. Or the flyback clamp diode is open, letting the collector voltage fly high enough to cause breakdown. Any of these could cause the transistor to emit magic smoke. More remotely, the Baker antisaturation clamp diode (the one with the cathode connected to the collector) could be open, allowing the transistor to saturate. The slow transition out of saturation could increase dissipation, but I doubt by enough to cause rapid toasting. A more ominous possibility, given the toasted 220ohm resistor, is that your crt has an internal grid-to-cathode short. Frequently, these are undetectable when the crt is cold. If that turns out to be the problem, email me, and I'll describe a procedure for fixing it. In any case, once you've fixed the fault, you can use a 2222 or 3904 or ECG123 or any general purpose medium power NPN with a reasonable speed (say, ft above 100MHz) and a breakdown voltage bigger than, say, 40V. Btw, Gamba has generously posted my tracings of the analog circuits (including the video ckt) of classic Macs at: http://home.earthlink.net/~gamba2/schematics.html Even though the schematic is for the Mac Plus, the other b/w all-in-ones share very similar (in some cases identical) video circuitry. -- 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/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
