How to substitute transistors?

[Uncle Bulgaria]

As part of the ongoing CR 240, I have a number of transistors that are different from the parts labelled on the circuit diagram, and some that have failed. I have no idea how to discriminate between the myriad options given on websites that purport to offer alternatives, and some of the numbers aren't even recognised on the usual supplier catalogues.

The diagram for the board I'm interested in has BC183, BC238, S7267 and BC308. The ones removed from these positions are actually BC547C, BC548B, S7302 and MBC558B.

I understand the BC548 is the current production update of the BC238, but the S7267 has no datasheet I can see (although an Internet search shows someone repairing a Uher has used a BC368, this seems to be a model that is also discontinued).

As someone without an electrical engineering background, is there any easy way to find substitutes for what appear to be common transistors, when I don't know which parameters are important in the circuit position?

I've trawled some datasheets on alltransistors.com and come up with BC547B and BC548BTA from CPC, assuming the gain suffixes are not critical for what I think are just switching applications.

Most transistors of the same material (silicon/germanium) voltage rating (the higher the voltage usually means less gain) polarity (PNP/NPN) and physical size (power dissipation) will work in most circuits. The only real gotcha is power transistors, the older (1970's) ones have a much lower frequency capability, new ones in some circuits can "take off" at VHF and go bang.

[G6Tanuki]

In times-past almost every electronics-tech had a copy of the books we all called "T.I.T.S." - Towers International Transistor Selector - to help you out with the characteristics of transistors and their interchangeavbility/substitutability.

The ones you list with a "S"-number sound like some sort of "house numbering" scheme - which could mean they were specially-selected ones (higher gain, lower noise?). In the past it was normal for a manufacturer to produce a batch of transistors then test each one for some specific characteristic (noise, gain, frequency-response) and grade them accordingly - that's why a transistor like the BC548 or BC547 comes in different versions with the letter-suffix to indicate the 'gain-group' they fit into. Other manufacturers used colour-coded marks to indicate different quality-grades.

(The manufacturer would charge a bit less for a low-gain 'A' and a bit more for a high-gain 'C').

[paulsherwin]

There is much less difference between silicon planar transistors than you might expect, and they are mostly interchangeable in typical small signal circuits.

See https://en.wikipedia.org/wiki/BC108_family for an overview of the common BC types. Other very common general purpose types are the American 2N3904/6 and the Japanese 2SC1815/2SA1050.

You just need to ensure that the voltage and current ratings are adequate for the circuit, and the transistor isn't doing anything strange.

[Maarten]

That wiki page has some types I didn't know about, but I miss the BC182 and BC407 subfamilies. On further inspection, I'm not sure whether the BC317 family belongs in there as they might or might not user other dice.

[paulsherwin]

I'm not sure of the origins of the BC18x/BC21x family myself. They seem to be basically BC108s with higher voltage ratings and a different numbering convention. Do you know if it was a Philips/Mullard design?

[David Simpson]

Best bet, UB, would be to follow G6Tanuki's example and get hold of a 2nd hand TITS, &/or an AVO Valve Data Manual. Plus one of the cheapo modern Chinko transistor testers, like the Atlas. Especially if you intend to do more vintage transistor circuitry projects in the future. If your intended replacement transistors reveal under test similar characteristics as TITS show for the original item - then bobs your uncle. Gain, Noise, Vce, Vb, etc. are the important things to look for. These modern cheapo testers automatically determine the lead connections for Coll. Em. & Base, plus their Si or Ge construction. Also, they can just simply say - Fail.
Maybe there is a Forum chap down your way who could mentor you in transistor testing, and has a tester himself ?

Regards, David

[paulsherwin]

It's certainly worth getting hold of a copy of Towers if you can find one - it's a comprehensive source of key transistor parameters, and I use mine (update 4) all the time. The suggested equivalents are less useful, because the suggestions have often become obscure and obsolete themselves since the edition was published, but it's easy enough to identify your own substitutes.

[Skywave]

Towers International Transistor Selector: recently I have seen this book for sale at eBay.

Al.

[Uncle Bulgaria]

Top tips, thank you.

I've bought a TITS - I found a 1989 edition on Abebooks for 80p!

After a perusal of a copy on the Internet Archive I thought it was a jolly good plan. I've been following the ideas on here meantime, and noted the relevant parameters from datasheets to find what I hope are useful equivalents. I'm surprised that for all the ones listed as 'obsolete' or 'end of life' on the big suppliers, they haven't got 'use this instead'. Cricklewood helpfully offer that. I've got some of the common types on the CR 240 boards on order so I have a stock in case any more are bad, or fried by my cack-handed efforts.

That's also a good idea to find someone local. If I can't break this logjam (though biting off more than one can chew is the best way to learn, it can sometimes get too much) I'll throw out a plea!

[ionburn]

There is a copy here: https://archive.org/details/TowersIn...sistorSelector

[toshiba tony]

I have used BC107\8\9's in more apps than I care to remember. I remember the old Tower books, as for audio outputs BD135\136. I used to knock my old Darlingtons together, things were tight financially.

[Radio Wrangler]

In most cases, in substituting transistors, you can get away with murder, though not with genocide.

Popular transistors got repackaged several times in different cases.... TO-18 metal into several shapes of plastic (including the triply-damned lockfit) Then other makers did their own versions and clapped on their own style of type number.

If you're not fussed about the details of the package or the order of the litle wires, you get a lot of choices as substitutes.

You run into narrower choices once you get to specialist parts. TV line output for example as well as switch-mode power supplies and other high power applications can be a bit touchy.

Power parts leave you wondering how to re-jig heatsinking as well.

Look out for high voltage types as well.

Don't just swallow whatever gain figures you're given, look at what collector current they apply at.

Even across parts of the same numerical type, there is a lot of variation from part to part across batches. Canny designers know to design to accept this range, and in doing so, they give you margin to find substitutes.

There are lots of little exceptions, but of the immense number of types ever made, you can get away with surprisingly few.

David

[russell_w_b]

We have several versions of the TITS books at work, and also the Semicon International Index volumes, which do bipolar transistors, FETs, diodes & SCRs and ICs too. Very useful for direct comparison of parameters.

Used to be available from RS but will be kicking around second-hand, likely.

['LIVEWIRE?']

I have one edition of the T.I.T.S. book, plus several Mullard Data Books, a 1970 Mazda Data Book (when I can find it?), plus two DAV (The 'a' should have a umlaut over it) data books bought from Willow Vale back in January 2001. These latter volumes comprise one which lists Transistors from A-Z, and a second covering devices from 0-u. They are very comprehensive, running to 568 pages each, with base diagrams, suggested equivalents, and other data, and were published by 'Deutsch-Osterreichische Buchandels, Munchen(German-Austrian Book Publishers of Munich). I've never come across any later editions, though they may exist. They are smaller than Towers, but, IMHO, just as useful/

[Radio Wrangler]

THere were also the D.A.T.A books.

They were VERY expensive back in the day, but gave one line's worth of parameters for an awful lot of devices. Rare as hen's teeth nowadays. I wonder if they've appeared on-line?

David

[Uncle Bulgaria]

Slightly OT - when substituting the transistors, is there a rather better way to do it than sucking each leg in turn? It seems slow and unlikely to completely clear them of solder, meaning jumping from each leg to the next while wiggling with a third hand. Not great for keeping pads attached or transistors cool!

[paulsherwin]

There is no single technique. I generally just melt the solder and pull the wires out, reversing the procedure to fit the new part. Silicon transistors are pretty heat tolerant, though I normally leave longer leads than would be left in the factory so the heat can dissipate.

[Scottart]

I occasionally refer to The Avo Transistor Data Manual - owned it for many years.....but not the Avo Tester!
Stephen

[Uncle Bulgaria]

Another book I could put on my list! Thanks, Stephen. I'll see what the TITS throws up when it arrives. I imagine that for really obsolete types, getting a 1989 equivalent will enable me to get a modern equivalent.

Paul - do you find you can melt enough on a single lead to leave the part unattached? I find I can't suck away enough solder to prevent having to hold the board and component while pulling it and applying the iron to all three pads in quick succession to get a little movement on each leg!

I've tried the magic of a heat gun on an old board I wanted parts from, but it's not suitable for single components!