O. Sharp schrieb: There exist books which provide cross references replacement types for obsolete components. Often manufacturers also provide lists in the Web which recommend their own modern components for such old transistors. Google is your friend there. The point, however, is that one should carefully look at the circuit itself to find out what the transistor is actually used for. Take a basic amplifier, for instance (two resistors at the base, and one each at the collector and emitter, resp., AC-coupled). This circuit is built around certain transistor characteristics; IIRC, the AoE book describes the formulas. Unless you also replace the passive components around it, you should have a replacement with the same hfe21, Ube, Ic, Ib, and maybe even fT, etc. But often, this whole work is not required at all. The 2N3009 is not a "high speed switching NPN" with respect to modern devices - it may have been 30 years ago, and the other parameters do not really suggest it is something really uncommon. For some HF transistors, there may be a problem (cf. amplifiers!), but unlikely here. In digital DEC machines, such a beast is possibly just used for driving display bulbs, in the power supply, or converting voltage levels - I don't know the circuit you refer to. In such a case, I have no problems to take some modern standard NPN with almost the same characteristics (Ic, Uce, P, hfe) which I find in my semiconductor box. -- Holger

It depends on what the transistor is being used for.. In general transsitors used in the logic (particulalry low-speed ones, for things like lamp drivers) and transsitors used in linear PSUs are fairly easy to substitute. Transsitors used for high energy pulse work (SMPSU choppers, horizontal output transsitors in monitors) are a lot more critical, and the data sheet may bot help. Basically, you need to get the following right : Tpye/polarity (NPN or PNP etc) VOltage ratings high enough to stand voltages in the circuit (if the ratings of the substitute exceed those of the original, you should be OK) Current ratings (particularly Ic, collector current). Again, if your substitute is better than the original it should be OK Gain (hfe and all its varients) is not too critical in most classic computer applications, particularly for switich transiustors which are driven hard into saturation. But try not to use a transistor of too low a gain. Max frequency (Ft, etc) is something you should look at in logic transistors, particularly in higher speed circuits, clock oscillators, etc. But my experience is if you pick something of the same polarity and similar characteristics, it'll work in most classic computer circuits (except for SMPSUs and horizontal output stages). Probaly 99% of small-signal transistors can be replaced y 2N3904 (NPN) and 2N3906 (PNP) :-) Diodes are evem more generic. Most switching diodes can be replaced with 1N4148 :-). For PSU rectiifers in linear PSUs (again, SMPSUs are another story), choose something with sufficient votlage rating (PIV -- peak inverse votlage) (2*sqrt(2)*output voltage should be safe, say 3 times the output voltage or more) and sufficient current rating (If, forward current). When there are similar diodes of different PIV ratings, I normally buy the highest. It's not much more expensive, and I'm on the side of safety. -tony