Transistor cross reference database & rules for finding equivalents

[Script]

I need to replace a few obsolete transistors. A quick search on the net gave me these pages.

http://www.alltransistors.com/  [last update: 2005]
http://nte01.nteinc.com/nte/NTExRefSemiProd.nsf [2011/2013: only NTE parts!?]
http://english.electronica-pt.com/db/cross-reference.php [??]
[And, of course, Mouser e.g. has that "parametric search" function.]

Most of the transistors I'm looking for are designated as "general purpose", used in the (motor) control section of an old CD player (i.e., 2SA933 = 2SA733, 2SA970, BC556 -- so probably all fine).

But there are a few I can't find equivalent matches for that look close enough. I haven't been able to source a free copy of the schematics yet, so not sure exactly what the transistors do, but I thought I might give it a try anyway. However, even if I had the schematics, I still wouldn't really know how to find close enough substitutes.

Is there a general rule for value deviance (I'm thinking hFE and power dissipation: if higher no problem?) or does it exclusively depend on the circuit the transistor is used in?

All input is highly appreciated.

[ADDED: Is there anything online that I could/should read to learn more about this?]

 

[JohnRoberts]

The rules for substituting is pretty much the same as the rules for initially selecting transistors, while it's a little like replacing nuts and bolts in a car, you want the replacement to be strong enough and fit in the hole.

In circuit design there are (were?) many transistor applications where a design engineer used the GP NPN or PNP device that he already has in stock.  I still have hundreds of the 3 legged buggers sitting in my back lab. i even had a few GP pairs (NPN and PNP). A low voltage low current GP device, and higher voltage GP parts, and very low noise parts for preamp front ends.

Just like you can replace a bolt with a stronger bolt, your can replace a transistor with a stronger (better) part 99% of the time. The obvious parameters to satisfy are breakdown or max collector voltage,  next is beta (hfe), finally for more specialized applications noise, or gain bandwidth.

To properly answer this question involves understanding the circuit design. If you don't have a clue, try to get the exact replacement part. if you understand the circuit, test your legs to select a new part and see how it works.

JR

 

[Script]

Got hold of the service manual (in Japanese) and found most transistors (except for one, cos all long obsolete Japanese parts) in an obscure component shop.

 

[Script]

Thought I'd add this Choosing transistor replacements tutotial page.
Basically the same as JR already explained, plus some additional info.