DEC Transistor Substitution

Hi Michael, At 05:37 PM 2/1/2014, you wrote: --- The datasheet I found for the 2N2894 shows breakdown voltage at 12V. The B602 operates on supplies of +10V and -15V. While I didn't immediately find a schematic of the B602, I did find the B360, which seems similar except that a delay line is included. It isn't obvious to me, whether or not the PNP devices see more than the 15V that a simple inverter circuit would. If it were just 15V, the chances are good that the 2N2894 would work, because actual breakdown would be much higher than the 12V minimum spec. You can easily test the breakdown voltage of a sample (non-destructively) by applying it through a (say) 1K resistor to the collector and emitter, with the base open, while monitoring the voltage across the resistor. Raising the voltage, the point at which you see perhaps 1V or so could be considered the breakdown (BVceo). [The datasheet uses a much higher current but I would be more conservative.] If you see almost nil at 25-30V, the device should work okay. If it's 15-20V, it depends on the circuit as mentioned above. DEC part substitutions at http://so-much-stuff.com/pdp8/repair/subst.php showed a large number of entries but DEC-6 was given as "NONE". Looking at the turn-on, turn-off times for the 2N2894, I see max figures of 60, 90ns. From the 10MHz ratings of the B-series modules and the mention of a typical 40ns input pulse, those figures don't give me a warm feeling. On the other hand, speed is very dependent on drive, so actual performance could be much faster. Also, looking at a more detailed datasheet, showing curves of actual timings, I see that operating around 10mA, switching is in the 20-30ns region. You could setup a working module with a 40ns drive pulse, while observing input and output on a 2-channel scope. Measure the delays of both initial and trailing edges. Then observe the module with the substituted devices and see if there is a significant difference. Casting about at Central Semiconductor for alternatives, I found these: (2N2894 - 12V, 60/90ns for comparison) - 2N3829 - 20V, 35/65ns - 2N3073 - 60V, 40/100ns - 2N3209 - 20V, 60/90ns - 2N3250A - 60V, 35/50ns - 2N3251A - 60V, 35/50ns high beta --- So it looks like there are better bets out there. Of course, locating them in distribution may be an issue. I have found Octopart is a good place to start: http://octopart.com/ I found 2N3250A at American Microsemi for $7.59: http://www.americanmicrosemi.com/products/search/index.php Steve L.