Transistor Substitution

[Allison]

>
>Subject: Re: Transistor Substitution
>   From: ard at p850ug1.demon.co.uk (Tony Duell)
>   Date: Tue, 09 Jun 2009 18:53:39 +0100 (BST)
>     To: cctalk at classiccmp.org
>
>> 
>> 
>> Okay: I admit it, I am sometimes pig-ignorant about basic hardware 
>> questions.  :/
>> 
>> I have a couple of DEC machines which I need to replace a few components 
>> on, and also stock up spares of others. With the transistors and diodes, 
>> however, I often can't find a direct replacement - and don't know how to 
>> figure out what a modern substitute is.
>> 
>> For a 2N3009, for example, I can find basic information and a datasheet 
>> online easily enough - but as for choosing a functional, available 
>> substitute for it, I'm honestly not even sure where to begin.
>> 
>> Is there a basic resource for determining modern equivalents for older 
>> transistors and diodes? Can someone helpfully provide me with a clue 
>> here?  :)
>
>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

Way too much information..  What he needs to know is what can he buy now that
should work to replace a 2n3009?

Answer: the 2N2222A (metal can not the plastic PN2222) has the same or close 
enough FT, Ic and breakdown voltages.  It would be my first choice if I could 
not purchase/salvage the exact part. Also the 2n2222A is available and usually 
cheap.  I buy them usually in for groups of 25 for about $0.08US each.

DEC used a lot of 2n2907, 2n2905. 2n2219, 2n3553 and 2n2222A equivilents under
different marking schemes.  These parts are still very common.

To assure the substitution it would help to know how that 2n3009 was used
but from working on a lot of DEC gear the 2n2222 is a good bet.

The only case where they type transistor is a bit fussy is some of the faster
flip chip cards (logic) and SMPS.


FYI: Older DEC machines like straight-8 and friends used a lot fo the ceramic 
epoxy parts similar to the 2n3638 and 2n3563.  Those are ceramic headers with 
a block of epoxy covering the die and are prone to popping the epoxy and the 
symptom works when cold or mechanically intermittent. 


Allison