Transistor substitution

You could start here:

One thing to watch for with old circuits is whether they use germanium transistors. Those aren't so common anymore, there was mostly a switchover to silicon transistors. And the biasing requirements are different for germanium transistors. If nothing else, they will work at a lower voltage than bipolars generaly do.

Another thing to watch is whether the devices are NPN or PNP. That can be adapted toa certain extent, but it defines whether the base and collector voltages are positive (NPN), or negative (PNP). Though in the early days, that may not immediately be obvious, since the circuit might instead have ground as positive (rather than the more common ground as negative), so they could ground the collector even though it is an NPN transistor.

Another consideration is that while some stages can be quite fussy in regard to parts substitution, lots of other stages aren't. Indeed, magazines tended to use the same types of transistors. It was never clear who or how someone picked a given device, but once it was out there, others would use them too. And those devices were easier to get through hobby channels, precisely because they were being used. So you'd see a lot of 2N706s in the sixties, while later 2N2222s and 2N3906s. IN some cases, people may not have even built with the specified device, just some general type they had handy, then they list a common part so others can easily get that part.

The first few projects I tried never worked. I'd copy out the list of parts from the magazine, go to the store, and they'd make substitutions that might not have been right or had some different pinout that I didn't know about. Who knows, lots of variables at the beginning. But the projects that first worked were when I learned enough about the parts so I coudl reuse things I'd taken off circuit boards. So I was using generic parts, but knowing enough to know what might work and what might not. You cant' just make substitution, but once you have some background, it's easier to see what can be changed and what can't be.

Since I was generally stripping junk equipment and reusing the parts, I early on got a replacement guide for one of the Big Replacement Lines. These were a smaller set of parts that could be carried by stores, and then the substitution guide would lead you to a part that was kind of close.

I never bought the expensive replacement line parts, I just used it to look up transistors in a circuit, and then look up trnasistors I had, and then I'd find matches to the common replacement, or look up the specs of the replacement and see if I was "close enough". Since many circuits were just using transistors they had, "being close" was often good enough, so long as I knew enough that that VHF circuit would need better transistors than that audio amplifier, and so on.

It seems daunting going in, but it didn't take that long when I was a kid, reading as much as I could get my hands on.

Michael

If you posted a link or a picture of the circuit with part numbers. Someone here may be able to give you advice on replacements.

George H.

I'd start by looking to the hobby/surplus places for components -- for silicon transistors, you may just find exactly what you want, instead of equivalents.

All Electronics MPJA Electronics Jameco Electronics Search on "surplus"

For germanium transistors, look on da bay -- I'm sure there are parts out there.

For resistors and caps, for the most part, anything with the right value will work.

And, as has been said -- if you're having trouble you can scan the schematic, post it on some image-sharing site, then ask questions with a link to the schematic and we'll help you out.

Thanks for the links, JW, that really helps.

Michael, I find your comments extremely helpful! Thanks you

Tim, I think I remember from your previous posts that you are from Oregon. I live up in Sandy, OR. California transplant as of 11 years ago. Thanks for your help.

In many cases I am looking for popular items like the "2N2222". Could I simply place a 2N2222 in an LTSpice circuit, study the outputs, and then substitute something similar (close) and compare outputs? Trying to build (breadboard) simple projects like sine, square and sawtooth generators so I can play with my newly acquired (very old) Heathkit oscilloscope. Please don't laugh, it only cost $10, and is able to display 60Hz, 120V AC. Of course it might make a lot more sense to abandon these old schematics and build from newer design, possibly available on the web, using currently available parts.

Thank you eveybody!! Your answers have inspired me. Ivan Vegvary

you can use your soundcard to generate all kinds of waveforms.

We're neighbors, then. I'm about 30 minutes from downtown Sandy, via Estacada.

But my oldest Clackamas County roots go back a bit further, to the 1840's.

The 2N3904 will substitute for the 2N2222 in just about anything. There's also a "PN2222" that's supposedly a 2N2222 die in a TO-92 case.

Or 2N4401/ 4403 for pnp.

George H.

Somehow I fell into liking the 2N3904 / 2N3906 pair -- I don't know why, and at this late date I can't justify it.

(Actually, when I'm prototyping with through-hole parts I have a small jar marked NPN and another one marked PNP. 9 times out of 10 I just shake one out of the jar, plug it in, and it works. The tenth time I actually have to select something specific.)

These days if there's a transistor in one of my circuits, it's almost always a dual or prebiased, it's in the smallest package that I feel up to dealing with, and I just look for what's available on DigiKey.

I found abag of 2N4123 (NPN) (30V 200mA) that were damaged in a loading machine. The leads all have little nicks and some leads are bent and ready to fall off so I solder extention leads. Never counted them but they half fill a coffee can. Maybe 2000 or so and the price was right at $3 for the whole bag.

--- news://freenews.netfront.net/ - complaints: snipped-for-privacy@netfront.net ---

Motorola used to sell those general purpose NPN and PNP transistors by weight... it was too expensive to count them ;-) ...Jim Thompson

weight... it was too expensive to count them ;-) "

HAHAHAHA.

So what now, they got a machine to count them ? Or does some Chinese guy do it for 12 cents a day ?

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

I bet you don't have one of these ----: Toshiba MG240V1US41, 240 amp, 1700 volt, N channel IGBT, weighs 1 pound.

--- news://freenews.netfront.net/ - complaints: snipped-for-privacy@netfront.net ---

Nope.

Heaviest I ever got into was 600V NPN off-line switching devices at a few Amperes... fractional ounces. ...Jim Thompson

It's only a N channel on the input and that only drives the base or bases of the actual bipolar transistor, which is the part handling all that current and voltage.

So one must understand that the V(ce)saturation voltage isn't going to be low as you would expect otherwise.

Jamie