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dlucy’s guide - how to find replacement transistors

dlucy

dlucy

dlucy67 (Doug)
Staff member
Moderator
Subscriber
Do you have a transistor that needs to be replaced?

Figuring out what to look for in a replacement transistor can be difficult, especially if you haven’t been doing this for decades.

The following guidance may help you understand how to better find and select a modern equivalent transistor that will work well as a replacement.​

Background and disclaimers

AK has been a wonderful place for me to learn about electronics and vintage audio gear repair and now I find myself answering “how to” questions frequently. That’s a laugh in itself as I’m just a hobbyist, but I’m happy to help where I can. My replies are mostly basic, sometimes flawed, but always intended to be helpful. But these replies get buried in threads that are narrowly focused on a single piece of gear… and therefore provide benefit only to a few people.

One of the most-often asked questions is “what is a good replacement for transistor XXnnnn”. I follow the process I’ve cobbled together and picked up from other AK’ers, share my results… and then get asked “how did you determine that result (so I can apply the same process and not have to ask people next time)”.

This simplistic guide is intended to share some of the how one can look for and find the best replacement transistors… and get better over time as others add to or correct what is already gathered here. There are other threads like this one already on AK (see list below) and none of them had all the tricks I’ve learned and few of them were pedestrian or explanatory enough that *I* might pick up everything being said, so here is my go at “how to find….”

This is not perfect. This is merely background info, what I use to find replacements, laid out so less-experienced AK’ers have more information when looking for transistor replacements. More-knowledgeable AK’ers are welcome, even encouraged, to point out where I have gotten or stated something wrong. I will happily correct or add to this guide and my own knowledge… when shown my errors in a polite and constructive manner. If you just want to nit pick without adding constructively to this, please do so in a private message so this thread has the best signal-to-noise ratio… for the benefit of the AK’ers who haven’t been doing this as long as you have.​

Thanks to several editors and contributors:

And finally on to the actual point of this thread…
 
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INDEX

I will come back and post links to the various sections below so it is easier to find what you're looking for
 
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How will we find the right transistors
What are we trying to accomplish?
  • We have a transistor, a particular device, used in our unit or circuit, that needs to be replaced.
  • We want to find a reputable, inexpensive, probably modern equivalent device that we can purchase online, have delivered inexpensively, and which operates at least as well as the original device does/did.
Before we dig into all the details and the process of finding an equivalent replacement, can we find this exact device from a reputable online distributor at a price we’re OK with? If so, buy it and forget about finding an equivalent replacement device.

If not, we need to know which devices are equivalent, as per the use or application in our particular circuit. The way to find devices that are equivalent and available is to
  • Identify the important specifications of the original device
  • find (by searching) the reputable online distributors for available, cheap transistor models that will perform the same or better
  • narrow the matching results to the best results
Basically, we will be
  • Making a list of specifications used to find the right transistor
  • Fill in those blanks with the specifications of the original transistor
  • Narrowing those specs to just the ones important in our application
  • Searching through all known transistors for matches
  • Narrowing those matches to the best choices for us
 
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Going straight for a same-to-same, identical replacement
Still being manufactured and sold?
Does one of the large electronics distributors still carry the exact device we’re looking for? At a price we’re willing to pay?

Unless you want to learn how to find equivalent replacement devices and you have some time on your hands, if you can find the exact device you’re trying to replace… at a modern distributor or store… at a price you’re willing to pay… then just do that.

Example:
  • I need a 2N2222 in a TO-92 package…
  • Both Mouser and Digi-key have them…
  • But they are in the TO-18 metal can package (which is OK by me and will still fit in the original holes… so long as I line the correct leads or pins up….)
  • But the modern distributors want over $6 each… which is too much for me in this case
  • So, I could use this equivalent but I want to look for cheaper alternatives
  • Image of a website with a NOS part still available but pricey
  • NOS still available but pricey 2N2222.PNG
A modern, intentional equivalent is available
There are a few very-popular vintage devices which have been continued… as modern, slightly-renamed devices. E.g. the vintage 2SC945 and the modern KSC945

Example:
  • I have a blown 2SC945 in a TO-92 package
  • Every modern distributor is long out of the 2SC945, but I’ve learned the KSC945 is the same thing
  • The KSC945 is available, they are cheap in small quantities, and dirt cheap in large quantities
  • So, I order qty 100 of the KSC945 at $0.06 each and I’m happy

List of modern renamed original device model numbers (e.g. KSC1815 for 2SC1815)

NOS or new old stock is available from a reputable distributor
If the original device is no longer being manufactured, can we find this exact device from a reputable online distributor who still has a cache of new old stock at a price we’re OK with? If so, buy it and forget about finding an equivalent replacement device.

Search at some of the well-known, reputable NOS distributors for availability and a price you’re willing to pay.

Example:
  • I’m looking for a specific Sanken output transistor in MT200 package, many AK’ers have had good luck with B+D Enterprises, they have the exact one I need, they’ve still got a pile of them from 1978 when they first bought them, and it’s worth it to me to pay $12 each for the four outputs I need. Plus shipping.
  • Image of a website with a NOS part still available
  • NOS still available 2SC2240.PNG

List of reputable NOS distributors
 
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“original” or NOS or new old stock is available from eBay or other online sources
If you search on eBay or just do a Google search for the original device you need and which you can’t find at Mouser or DigiKey, you may find a good number of sellers who offer the exact one you need. You may want to consider lots of AK advice and the following:

· If none of the big, reputable distributors carry the 2SC2240BL that you need and a dozen eBay sellers offer it… in any quantity you need… you may wonder why the big, reputable distributors don’t just buy a bunch from these eBay sellers

· The eBay sellers state “original device” and offer a large quantity for almost nothing (e.g. 100 pieces for $2.99)

· The eBay sellers are in China or Hong Kong… and offer free shipping… and free returns

· It is unlikely, not impossible but unlikely, these super-cheap, almost-free, original devices are, in fact, what they say they are. If it seems too good to be true, then it might well be untrue.

· But you can always buy them anyway and test them (get a curve tracer and compare the new device’s curves to the original device’s datasheet curves) before you put them in the circuit

· Or just throw caution to the wind and try them out without testing in your circuit. They might work… or they might blow out more components in your vintage circuit.

Image of a cheap knockoff part from China on eBay

· Cheap knockoff from eBay.PNG
 
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NTE (and ECG) equivalents for original devices

Edit: everywhere below where you see “NTE” applies also to ECG.

NTE sells equivalent devices. This is what they do. And they have very nice, fairly comprehensive cross-reference charts you can find on the web… and find a NTE equivalent to many of your vintage, original devices. So, it is pretty easy to search for a vintage original part and find a seemingly-perfect NTE equivalent part for it. But there are potential problem with this:

What NTE does is

· They choose a common set of characteristics or specs from several original devices

· Choose a NTE number to slap on those set of characteristics

· Publish a cross-reference between many vintage devices and this new NTE number because the NTE specs cover the specs of the cross-referenced original devices

· Locate one or more manufacturers who can supply new devices that fit somewhere under the NTE specs for this device. Buy a bunch and warehouse them.

· Sell you the NTE device that you’ve crossed your original to.

The hidden problems behind this are

· You aren’t crossing your original device to a NTE device; rather you’re finding a set of NTE specs that kinda match your original device’s specs

· NTE is not guaranteeing each of the NTE devices they sell you is the same as the other NTE devices in the same order. You may need four 2SD1313 transistors, order four of NTE’s equivalent devices, and get two, three, maybe even four very different devices in the same box. Each one of them meets the NTE overall spec, but no two in that order match each other.

· The quality of each of the NTE devices you receive is not NTE’s quality; rather it is the quality of the original supplier to NTE, of that device, at that time.

· Example NTE replacement might be bad

NTE isn’t bad, per se. You just need to understand their cross-referenced parts are not a magical, perfect fit for your original part, the quality and exact device can vary one part to the next, etc. regardless of how great is sounds.
 
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Even genuine NOS new old stock parts might not be what you want
Is a new old stock one of the original device good enough? There are sometime reasons why you want to avoid new old stock.

· Tin whiskers – due to the materials used inside a transistor or on a lead, changes can occur over time that reduce a device’s performance or halt it all together. These effects occur with time, not whether it is installed in your circuit or sitting on a shelf in some warehouse. These do seem to be uncommon, but here is more on this: Examples:

o

o https://nepp.nasa.gov/whisker/background/

o https://nepp.nasa.gov/whisker/

· Lead corrosion – the legs or leads of a transistor can “go black” or “get tarnished” or otherwise oxidize. This can happen to the NOS devices, too. Whether or not this is a genuine issue, something you should avoid in NOS and just stick to modern equivalents, gets debate and people on either side of the “yes, it is bad” and “no, it is fine” discussion.

o https://audiokarma.org/forums/index.php?threads/transistor-leads-that-turned-black.180725/

· “going noisy” – some devices are famous (or infamous) for producing lots of noise when they have aged quite a bit. Some of this is caused by poor encapsulation (burying a tiny transistor inside a black plastic package with big leads sticking out of it) and oxygen slowly creeping inside the transistor over decades. This particular kind of going noisy affects transistors installed in your circuit as well as those lingering on a warehouse shelf.

o https://www.audiokarma.org/forums/i...ilure-prone-whatever-and-replacements.731653/

o

o http://www.eng.auburn.edu/~wilambm/pap/2011/K10147_C011.pdf
 
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Kinds of transistors
What kind of device is the transistor you need to replace?

The starting point for understanding how to replace a transistor is understanding the transistor you need to replace. There are several kinds of kinds of transistors and a basic understanding of some of these kinds of kinds will help you navigate replacement sources and narrow candidate devices.

Discrete transistors
When you are looking up available parts at a distributor’s site, their menu will likely be broken down with a top level or parent category… followed by some lower level or child categories

Transistors will be listed under semiconductors… as will diodes, thyristors, SCRs, and such

The transistors found in most vintage audio gear will be discrete devices, meaning the physical part you buy only has one transistor inside it… maybe two… but not several or hundreds like an integrated circuit IC chip has.

Bipolar junction transistors or BJT
The transistors covered in this guide are known as or called bi-polar junction transistors or BJT for short.

You may also find JFETs or MOSFETs listed under transistors, but those won’t be covered here… as I have zero idea what I’m doing when trying to replace FETs. We need another AK’er to guide us, me mostly, in how to properly find modern equivalents for vintage FETs.

All bipolar junction transistors will be either of type PNP or NPN. These are the only two polarities of BJTs and you’ll need to understand which polarity each device you’re trying to replace is: NPN or PNP. A NPN transistor cannot be replaced by a PNP and vicea versa.

Image of both NPN and PNP transistor and diagram

· PNP and NPN transistor diagram.PNG

Germanium versus Silicon
The material the transistor is made of is critical as well. This guide only covers silicon transistors you are trying to replace, not Germanium transistors. Germanium was the material of choice for early transistors and you are likely to find those used in vintage gear made in the mid-seventies and earlier.

Images of transistor specs for Germanium and Silicon

· Transistor specs Germanium.PNG

· Transistor specs Silicon.PNG

You cannot replace a Germanium transistor with a silicon transistor… without significantly redesigning the circuit. Germanium transistors come in PNP and NPN, they have the same set of operating specs, but the way they perform in operation, what you get out of a germanium is different enough from what you get out of a silicon transistor so much that it is impractical to replace them this way. Yes, this means if you’ve found an awesome Kenwood TK-140U receiver for pennies and you just need to replace the outputs transistors and those are Germanium… then this guide won’t help you much other than the bits about how to find NOS sellers of the original Germanium devices or equivalent Germanium devices. Sorry. I’ve shed tears over this fact, too, so I understand how you feel if this applies to you.

If you are desperate… or just curious, here are some sources of Germanium-to-Silicon conversions:

· http://www.hawestv.com/transistorize/germanium1.htm
 
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Darlington transistors
There is one kid of BJT, silicon, discrete transistor that is commonly put in a separate category and that is the Darlington transistor.

Image of a Darlington transistor diagram

· Darlington transistor diagram.PNG

These transistors have very high gain, when compared to most non-Darlington, single, BJT transistors… because they are two BJT transistors in series inside an otherwise single-transistor package. These show up under the discrete BJT category as they are mostly used like single transistor device, just much higher gain than what you’d get from just one.

If you have a very high gain, say 400 or higher, discrete single transistor to replace, you may need to broaden any equivalent search to include this side-category of Darlington. Similarly, if you have a Darlington transistor you need to replace, you might be able to find a true single transistor discrete BJT that does what you need. I’ve had very little experience with Darlingtons, so I’ll leave it there… until a fellow AK’er jumps in and teaches us both more about these.
 
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What is the model number of the transistor?
So, here is the first potentially-tricky bit: the model number, if you will, of the transistor you need to replace.

Images of transistors with differing numbering schemes

· Transistor closeup 2N3904.JPG

· Transistor closeup 2SC1345.JPG

· Transistor closeup BC560.JPG

· Transistor closeup MJE15034.JPG

· Transistor closeup HP.JPG

In a perfect world, you have the schematic for your vintage unit, you have the service manual for it and it includes a parts list, the printed circuit boards (PCB) inside the unit have been labeled with silkscreening of the component number, that component’s device model number, and the pinout of the device (which legs of the transistor go into which holes)… and all that nice, concrete information is actually accurate.

The above is rarely the case. Schematics are printed well before the units are put into mass production, parts are sometimes swapped with equivalent parts during a production run, the service manual was made for one particular variant of your unit but not 100% the same as what you have, the silkscreening is wrong, etc. The short version of this is you often have to rely on the lettering printed on the actual device that you pull off the PCB.

Ø Tip: Write down exactly what the lettering says when you pull each device off the board. I like taking several digital pictures, each from a different angle, of the devices before I pull them so I always have a record of what they said before I pulled and lost them, which pins went where, etc.

Image of transistors on board before removal

· Photos or transistors before removal.JPG

And it remains tricky, even when you’ve pulled the device and are looking at it. Because there is very, very little room on the device’s face to print anything…. And the manufacturer wants to cram more than just the model number on that tiny face. The printed letters and numbers aren’t necessarily the exact model number.

Example:

· A transistor in a tiny black, half-round package has “A733” printed in the top line,

· Has “E” in a circle, then some space, then “7812” on the second line

· But the model number is not “A733-E-7812”

So, we need to understand a little about the different model numbering schemes in order to understand how to figure out what is actually printed on the device itself.
 
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JEDEC versus JIS versus…
There is more than one numbering scheme for identifying the “model” number of any given transistor, so an important starting point is understanding which scheme your device follows… so the markings or printing on the device itself makes enough sense to help you figure out what it is.

The short version is “there are two simple schemes… and then there are a bunch of manufacturer-specific ones”:

· Model numbers following the pattern 2Nnnnn

· Model numbers following the pattern 2SAnnnn, 2SBnnnn, 2SCnnnn, 2SDnnnn

Update: OnSemi has changed some devices to be marked JCCnnnn and JCAnnnn as of 03/02/2022 as described in this post and this post. Thank you @GlasgowGrip

· Model numbers following the pattern ACnnn, ADnnn, BCnnn, BDnnn

· All other model numbers like TIPnnx, MJxnnn, MJxnnn, etc.

There are excellent, better sources for more detail on these schemes, so if you need more than the above, visit:

https://www.electronics-notes.com/a...nts/transistor/transistor-codes-numbering.php

Ok, now that you know more about the numbering schemes, you’re better-prepared for translating what is printed on the device to what the model number actually is.

If your device has a fully-formed model number somewhere on the face, then that is likely the actual device number.

Example

· A transistor in a tiny black, half-round package has “KSA922” printed in the top line,

· Has “F”, then some space, then “1801” on the second line

· The model number is probably “KSC992”

If your device has the trailing letters and numbers of a fully-formed model number somewhere on the face, then that is likely the actual device number… once you add the probably-correct preface letters on the front:

Example

· A transistor has “D313” printed in the top line,

o That is a single letter in the beginning position, that letter is either A, B, C, or D

o The trailing numbers are three or four digits

· Has “D”, then some space, then “504” on the second line

· The schematic says “2SC1061 C or 2SD313 D” for the transistor in this position

· Then you’re safe assuming the “D313” part is the shortened-for-space version of “2SD313”

· And 2SD313 is the model number of your original transistor

If your device has numbers of a fully-formed model number somewhere on the face, then that may be actual device number… once you add the probably-correct preface digit and letter on the front:

Example

· A transistor has “2222A” printed in the top line,

· then “8311”

· The schematic says “2n2222” for the transistor in this position

· Then you’ve likely got a 2n2222 or 2n2222A model device

Once you know what model number your original device is, you can proceed to finding the specs of that model.
 
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Variations of a transistor and its model numbers
There are often intentional changes to a device model number that can affect how you look for replacements.

Gain codes
Many transistor models, e.g. KSC1845, are available in different “gain codes”. The basic physical design of a KSC1845 yields transistors that amplify at different amounts. One KSC1845 might amplify with a hFE of 201 while the next one amplifies significantly higher at 496. Since designers need to know, roughly, how much a chosen model will amplify when it is put to use, the actual devices are segregated by how much they amplify into groupings of gain, typically a range.

So, when manufacturing KSC1845 devices, those who amplify between 300 and 600 may be given a “gain code” of F and all the devices that amplify between 600 and 1200 be given a different gain code of “U”. The datasheet from the manufacturer will list these gai codes and the ranges of amplification they represent.

When looking at a schematic or an actual original device, you may see a gain code.

Example

· The schematic states “2SC1061 C or 2SD313 D” for a given transistor position in the circuit

· This means you may find either

· A 2SC1061 transistor with gain code of “C”, or

· A 2SD313 transistor with gain code “D”

Example

· A TO-92 package 2SC458

· Has “C458” printed as the top line on the device, and

· Has “E” inside a circle, then some space, then “7342” printed as the second line on the device, and

· It is a 2SC458 transistor, with

· Gain code “E”

Adding a gain code to a transistor model number
Sometimes, only sometimes, the gain code of a given transistor may be added to the device’s model number. Why they do this is beyond me.

Example

· Mouser lists two separate parts for the KSC1845

· The “KSC1845” part has some gain code, but you’d only know it by looking at the datasheet and the device you actually receive

· The “KSC1845F” is not model number “KSC1845F”; rather it is model number “KSC1845” and this particular batch of these devices has a gain code of F. They are telling you what the gain code is by slapping that on the end of the model number.

Image of Mouser part listings with both KSC1845 and KSC1845FTA

· KSC1845 and KSC1845FTA.PNG

Ø Tip: when searching for a specific transistor model, search only for the xSCnnnn part and intentionally exclude the trailing gain code or packaging type that someone else has written down or posted for you. You’ll find more results that way and can intelligently weed out the ones you don’t want… instead of never seeing them and thinking “oh no, Mouser is all out of KSC1845s” when they are actually only out of the KSC1845FTAs because you included the gain code and packaging type at the end.
 
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Except when you see an “A” at the end
Sometimes you’ll find a device model number that ends with the letter “A”. I have not encountered any instances of this indicating the device’s gain code; rather this is indicating an updated, higher spec version of the device.

Example

· The original KSA1220 is spec’ed at only 120 V Vcbo and 120 V Vceo, but

· The updated model KSA1220A is spec’ed at 160 V Vcbo and 160 V Vceo, so

· The KSA1220A does everything the KSA1220 does, but it can do so for a bit more voltage

In my experience, this has always indicated an increase in the device’s maximum voltage (one or more of the three voltage limits) handling of the device without a change to the performance of the device. So, they’ve just added a “hey this one handles more” suffix code letter of “A”.

Delivery-packaging suffix letters
And some distributor add even more letters to the end of a device model number… to tell you what kind of packaging the devices you buy will be delivered in. This is not the TO-92 transistor package style versus the TO-126 transistor package style; rather it is for the BULK (all the parts loose in a baggie) style versus the CUT TAPE (all parts taped together for automated installation) style versus the REEL (parts taped to a cardboard feed tape for automated installation), etc., etc.

The outcome of this is

· The device model KSC1845 may be listed as several different parts at a distributor

· The part listed as KSC1845FTA is the exact same model as the KSC1845 (nothing) and the KSC1845BU,

· But they are delivered to you either loose or on a reel or taped together and therefore have separate part numbers in the distributor’s catalog

· But they are the same device model

Center-collector pinout
Some manufacturers of modern device models with “EBC” or “CBE” pinouts also offer the same model but with a collector pin as the middle pin. When they do this, they sometimes add a “C” to the end of the otherwise-normal device model number.

Example

· KSC945 and KSC945C are the exact same insides (a KSC945 discrete transistor), but

· The KSC945 has a pinout of EBC (emitter as pin #1, base as pin #2, and collector of pin #3)

· Therefore it fits in PCB holes, roughly, which are in the sequence of EBC or CBE, and

· The KSC945C has a pinout of ECB (emitter as pin #1, collector as the center pin or pin #2, and base as pin #3)

· In order to allow the KSC945 to be used in PCB holes which are in the sequence of ECB or BCE
 
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Original Device specifications
What are the original device’s specifications or limits?

We are finally prepared enough to look for the specifications of the original device for use when we look up device that have equivalent specifications and therefore are good replacement candidates.

How to find the original device’s specs
Original datasheets
If you are lucky, when you use Google to search for “device model number” and the word “datasheet”, you may get a result or two at the top of the list which are PDFs from the original manufacturer with the exact specifications and operating characteristics of your device model.

Image of a datasheet

· Datasheet example.PNG

Curators of datasheets
Quite often the links at the top of the search results go to websites who collect transistor datasheets… and show you ads for other things in exchange for helping you find the datasheet you’re looking for. Nothing wrong with that, except…

Often these sites will list more devices’ datasheets than you actually searched for.

Example

· You search for 2SC458 on Google,

· The top result in the results list says “2SC458 datasheet”,

· Clicking that link takes you to a site that lists datasheets instead of the datasheet itself,

· And that site also lists datasheets that are not what you want:

o 2SC4581

o 2SC1458

o 2SC4501

And that just means you need to be careful when chosen which datasheets to view or download since a curation site may have thrown extra, spurious ones in there in its attempt to be helpful.
 
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Alltransistors.com
There is an excellent source of information about the specs for most transistors available at alltransistors.com.

· Go to the site

· Type your device model number in its search box

· Carefully review the result list and chose the appropriate model number

· You’re presented with a bland list…

· Of the device model’s specs… mostly

The top dozen or so specs, like Si versus Ge, NPN versus PNP, Vcbo, Ic, etc. are mostly correct and definitive. However, there are sometimes specs, such as package, that are not always correct or relevant when coming from the alltransistors’ database.

Example

· I’m holding a 2SC853 in my hand, it is obviously a TO-92 package, and

· I search for 2SC853 on alltransistors.com, and

· It shows 2SC853 and 2SC853A as the results list, then

· I choose 2SC853 instead of 2SC853A, then

· It displays all the specs, and

· It says the 2SC853 is in a MSOP-8 package when the one in my hand clearly is not. That’s a really big difference.

I count on alltransistors.com data every day; however, I sanity-check some of the specs before basing a replacement search on them.

Manufacturer’s catalogs and cross-reference guides
If you’re old school or OCD, like me, you may have some vintage, printed softbound books that are manufacturer’s catalogs listing their transistor back in the day or cross references that show which transistors can be substituted for which models.

Both of these sources often contain the specs that we otherwise count on from datasheets on the web or in alltransistors.com’s database.

NTE and Weisd
When you search for transistor device model numbers on Google, you often will see results that link to NTE or Weisd. Neither of these are bad, neither typically has a PDF of the datasheet, but both often contain the very basic specs that you need to search for candidate replacements… elsewhere.

Image of WEISD “datasheet”

· WEISD datasheet example/PNG

I have often been unbale to find a proper datasheet for a device, but been able to find the Vcbo, Vceo, Vebo, and Ic on Weisd… and subsequently been able to locate a suitable replacement.
 
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Application-specific specifications
At this point, you should have the basic, published specs for the original device you need to replace.

How is this original device used in your unit or circuit?
Every transistor model, e.g. 2SC1815, has a unique set of operating characteristics and the circuit designer chose each transistor for how it would be used in their circuit. An example of a common transistor for low level amplification (e.g. a preamp circuit) is a 2SC945. A designer may have chosen to use a 2SC945 in an audio preamp because they needed a certain amount of gain and the 2SC945 could provide that gain and handle the 0.1 V to 1 V signal passing through that transistor. The original circuit designer reviewed the operating needs of that circuit, specifically how a transistor would be applied or used in that circuit, and then looked up transistors whose operating characteristics or specifications or limits would meet or exceed the application needs of that transistor in that particular spot in their circuit.

Image of transistor on schematic with voltages

· Schematic with voltages.PNG

When you consider replacements for that 2SC945 transistor, you will find may places where a KSC1845 is recommended. The KSC1845 is a great NPN small signal transistor for audio signals, but this is sometimes a bad choice… because of how the original 2SC945 was used in this particular circuit.

For instance, a 2SC945 can handle 150 mA of current while a KSC1845 can only handle 50 mA of current. If you replaced a 2SC945 with a KSC1845 in a circuit that only passed 40 mA of current through the transistor, the KSC1845 would operate just fine. However, if you replaced a 2SC945 with a KSC1845 in a circuit where the 2SC945 was regularly getting 120 mA of current, the KSC1845 would likely fail… because it wasn’t designed to handle that much current.

Ø Tip: a recommendation of transistor XXX is a good replacement for transistor YYY is not necessarily a good recommendation if it does not include the application for which the original transistor was used. This is why many questions on AK about “which transistor should I use as a replacement” are often answered with the question “where in the circuit is that transistor used”.

So, there are two sides to this how-it-is-used-in-this-circuit information:

· When you are looking for a replacement devices, you can find the datasheet for the original device… and use all of its operating characteristics or specs to find all the candidate replacement devices which meet or exceed every single one of the original device’s specs… or

· You can find the datasheet for the original device, review how that device is used in this particular spot in this particular circuit… to understand which subset of specs the replacement device actually needs to meet or exceed.

For instance, an original device used to amplify audio signals, those between 20 Hz and 20 KHz, might have a fT (frequency transition) spec of 200 MHz and you might rule out any replacement candidates that don’t reach al the way up to fT of 200 MHz… but then you’d be missing may great candidates, ones that would work fine in this spot in your circuit, but which only amplify up to 100 MHz, for example, and would have been discarded as candidates if you were trying to match every single one of the original device’s characteristics.
 
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The datasheets for the device and the application of that device in a circuit will dictate which parameters or limits or characteristics are needed for proper performance… and what you’ll use to search for in replacements… and what you’ll have to research. If we lump all these into “specifications”, you’ll find there are quite a few that every transistor has:

· Signal versus power

· Heat dissipation

· Voltages: Vce, Vbe, Veb

· Saturations

· Current

· Gain or hfe or Beta

o AS mentioned on schematic

o As printed on original device

· Common versus low noise versus very low noise

· fT or transition frequency

· Physical mounting and pin outs

· And many more….

There are several sites on the web where these terms and specifications are explained:

· Upload the PDF from Toshiba defining these terms

If you’re interested in how the values for these specifications are determined, what they reveal about a circuit or design, etc. you can find some answers in the older transistor manuals or product line catalogs from manufacturers such as GE

Images from GE transistor manual

Links to online version of GE transistor manual

Other sources for learning about transistors, the specs, and formulas

Which of the original device’s specifications or limits are not important in the search for a replacement?
Every transistor will have values for each spec, but those specs are not always put to use in the spot where that transistor is used in your circuit. If you know how the transistor is being used, you can ignore some of the specs of that transistor and potentially open the field of replacement candidates much wider.
 
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Power supplies
Transistors, when used in the power supply circuit of a vintage audio unit, are used for switching on and off or voltage regulation. They are not used to amplify anything, so the specs related to amplification are unimportant.

Examples

· The Hfe or gain spec is unimportant in a transistor when that transistor is being used to regulate voltage instead of amplify. So, when searching for replacement device models of a transistor used in a power supply, you can ignore the hfe or gain spec.

· The fT or transition frequency is similarly unimportant when a transistor is used in a power supply circuit. Since fT tells us the frequency at which this transistor model stops amplifying and we aren’t using the transistor to amplify in the power supply, you can ignore the fT in replacement devices and yield more candidates that match the remaining specs.

· Noise or noise factor or NF is not an issue (that transistors need to care about) in power supply circuits.

AK’ers: any more specs one can mostly ignore when choosing transistors for use in a power supply circuit (versus an audio signal circuit)?

Which of the specs can tolerate a lower-spec’ed transistor?
Often an original transistor with great specs was chosen for the design, but one or more of the specs was overkill… and that can limit the replacement candidates.

This one is a little tough to consider when you aren’t an E.E. or a circuit designer or professional tech. This is included for completeness rather than broad applicability.

Example

· A 2SC945 transistor is used in a preamp circuit within a receiver.

· The 2SC945 is in the audio signal path and amplifies the incoming signal some amount.

· The KSC1845 transistor would make a great replacement for this transistor in this spot of the circuit because it has far less noise, but

· The KSC1845 has an Ic current max of 50 mA instead of the 150 mA Ic spec of the KSC945, and

· Therefore, the KSC1845 would not normally be in the result set of a search against the original 2SC945 specs, but

· Because we know this particular preamp circuit never sees more than 10 mA in the audio path, then

· The KSC1845 would work fine as a replacement for the 2SC945, and

· It would introduce less noise into the signal than the 2SC945.

Example

· An original transistor’s specs has a Vebo of 7 V, and

· Searching through several sites shows there are very few modern, available transistors who can meet that 7 V voltage at the base, but

· We know the circuit this original transistor is used in never has more than 2 V flowing… anywhere, so

· We can safely reduce the Vebo spec in our search for replacement candidates as the replacement will never see voltages at the base higher than 2 V

So, there are some times when some replacement devices might be able to be included if you know the actual operating conditions in the circuit where the replacement would be used. But it is a bit tricky, so take all this with a grain of salt.

Ø Tip: when you are unsure, ask AK.
 
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Which of the original device’s specifications are critical for the replacement device to have?
Voltage handling
The voltage handling of any replacement device is pretty straight forward. If the replacement candidate is not spec’ed to handle as much Vcbo or Vceo or Vebo as the original or as actually used in the circuit, then that particular replacement is a bad choice.

Current handling and power/dissipation
The current handling and power dissipation specs are closely related.

· If the replacement candidate device has a lower current handling spec than the original, it is likely the replacement transistor will heat up too much or fail due to excessive (for it) current flowing through it. Sometimes this failure is spectacular. And fuses can be expensive. Just sayin’.

· The power handling / heat dissipation ability of the replacement candidate can also be a disqualifier. If enough current is run through the replacement device, even when it can handle that amount of current, the device may heat up more than it can stand or more than it can perform correctly under.

The transistor package style usually indicates a rough mount of current handling or heat dissipating ability… and can be used to “move up to the next level” spec.

Example

· A TO-92 device (tiny black semi-cone) is used in a spot in the circuit where a lot of current, near the Ic max of the device model, and you have few to no matching replacement devices with a TO-92 package style.

· In cases like this, one option is to replace the TO-92 device with a transistor that meets or exceed the electrical specs and which is packaged in a higher-current, higher-power / heat-dissipation TO package. A mid-sized TO-126 package or a larger-sized TO-220 package would be OK replacements for a TO-92 original package… if you don’t mind drilling out the lead holes in the PCB and you have a bit of room for the larger device body.
 
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