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dlucy’s guide - how to find replacement transistors

Gain and transition frequency

Transistor gain, gain code, Beta, hFE, current gain or amplification:

For applications in a circuit where a transistor is used to amplify the signal, it is critical to choose a transistor with roughly the same gain as the original device. Or your resulting unit won’t have as much gain as it did before.

How much a given transistor amplifies is referred to as gain or Beta of hFE. I am vastly over-simplifying here, but it makes sense to do so in a simple guide to finding replacement transistors. I'm not an E.E. so take the following as coming from an amateur or hobbyist. I'm sure a more-educated AK member will correct what I get wrong below soon.

hfe and hFE are two different h parameters of a given transistor.
  • The 'f' or 'F' denotes a forward transfer characteristic, and the 'e' or 'E' indicates a common emitter configuration.
  • The 'h' should be lowercase (and I'm not aware of any uppercase 'H' characteristic).
  • When a transistor is used to amplify signals, hFE (or Beta) of a transistor measures its ability to amplify current. Higher values of hFE means more amplification of the input current.
  • When biasing a transistor to set its DC operating conditions, knowing the hFE is crucial for determining the required base current for a given collector current.
  • hfe / hFE varies between individual transistors of the same type due to manufacturing discrepancies, temperature and other operating conditions.

hFE (uppercase 'FE') is signifies the DC gain, small signal DC current gain, or DC forward current ratio (all the same thing).
  • This is the ratio of the DC collector current (Ic) to the DC base current (Ib) when vCE=0.
  • A transistor with hFE of 100, for example, means for every 1mA of base current, the transistor permits 100mA of collector current.

hfe (lowercase 'fe') signifies the small signal AC gain, small signal AC current gain, or AC forward current ratio (all the same thing).
  • hfe is essentially the same as hFE (uppercase 'FE'), but measured with changing currents and voltages, as opposed to steady (DC) conditions.

Since there is wide variation in current gain levels, the exact numbers are mostly academic. Circuit designs are made to function across a range of hFE values rather than a specific hFE value. Designers know that manufacturing will end up with transistors of varying hFE.

For practical purposes:
  • Use the hfe or hFE measurement that your tester measures. All the testers I've used show hFE (uppercase) or the current gain when vCE = 0
  • "Matching" hfe between a schematic and the devices you have available is not about getting precisely same numbers. You are aiming for being near the range of the circuit design. If the schematic calls for a transistor with average gain of 600 and your nearest available transistor type is only 300-400, that's usually OK.
  • "Matching" hfe between the actual device you'll use in one channel versus the same device in the other channel should be more precise.

The datasheet for any transistor device model should include a hFE rating. Many include sections like “hFE codes” or “gain codes” and list ranges of gain, e.g. 200 to 400, and assign “gain codes” or letters to these ranges.

Image of the gain code section of a datasheet

· hFE and gain codes on a datasheet.PNG

More info on hFE, gain, gain codes, etc.:

· What is the transistor hFE ranking? http://engineering.rohmsemiconducto...e/View/1/1/what-is-the-transistor-hfe-ranking

Transition frequency of fT:

Related to overall gain or hFE or Beta, the frequencies which a given device model will amplify are critical to match. This is transistion frequency or fT.
If you have an original who can amplify all the way up to 100 KHz, fT is 100 KHz, and you replace it with a device with a fT spec of only 10 KHz, then you’ll be not amplifying a large range of the signal you’re passing through that transistor and which you expect to hear, eventually, coming out your speakers.

There is a converse to this OK-to-be-under-but-cover-20Hz-to-20KHz point. If you replace a signal-amplifying transistor with one that amplifies much higher frequencies than the original, you may well be amplifying unwanted, higher-than-audio frequencies and making the rest of the amp work harder.

Ft in RF-signal circuits is extra-important. If your circuit, e.g. AM or FM IF or FM MPX circuit, is amplifying a RF-signal... but you choose a replacement transistor that does not amplify up to that frequency,. then you've cut off that all-important RF signal instead of amplifying it.

The capacitance of a transistor (cob which is covered inthe next post in thiis thread) has a direct effect on the fT.

Ø Tip: if you are considering replacing a transistor in an audio circuit with one that has a much-lower fT than the original, ask AK before you order.
 
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Cob or capacitance
Here is one area that I have only a little understanding, so my guidance should be suspect… and AK’ers with a better way to explain this and help other navigate the replacement selection process, please let me know.

Every transistor will have a Cob spec, one of the kinds of capacitance the transistors exhibits, and it will be rated in pF or picofarads.

In some applications in the audio path, e.g. the early preamp stages, the Cob is not as critical and can often be ignored when looking for replacement candidates.

However, in the “VAS” or mid-stage (help me get this right here, AK’ers) and the power stage of the circuit, the Cob has a great effect on gain and needs to be met or exceeded spec-wise. In these cases, you would be looking for replacement device models with a Cob spec of equal to or lower than the original device’s Cob spec.

In RF-signal circuits, e.g. the AM or FM IF or FM MPX tuner circuits, the Cob is extremely important. Too much or too little capacitance and you're suddenly halting the RF-signal instead of amplifying or switching.

Ø Tip: if you are considering replacing a transistor in an audio circuit with one that has a much-higher Cob than the original, ask AK before you order.
 
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Can I test for a transistor’s specs in my circuit?
I don’t know of any way to simply, accurately test for these specs while the transistor is installed. AK’ers: is there a reliable way of doing so?

Can I test my original transistor?
Basic transistor testing
If you need to test the original transistor for some basic functionality, e.g. is it completely dead, you’ll have to remove the transistor and follow directions such as these:

· https://www.audiokarma.org/forums/i...lar-junction-transistor-testing-basics.43186/

· https://www.audiokarma.org/forums/i...stor-test-with-ohm-meter.372302/#post-4688953

There are many devices to use for testing transistors, they offer different kinds of testing, and some help show the device’s actual specs:

· Expensive Peak Atlas device

· Cheap $20 tester

· Simple transistor-capable DMM function

· Simpler DMM test

· Vintage Purpose-built transistor testers

· Curve tracers – vintage, DIY, in DCA75

o Interesting info on building and using curve tracers http://www.repairfaq.org/sam/semitest.htm#stitc

Transistors that have “gone noisy”
It is not easy to test to see if a transistor has “gone noisy”. You can audibly hear this when the transistor is doing amplification and it is the only transistor, but that means a special, purpose-built circuit just for testing noise. So, just because a transistor passes a simple, transistor tester “does it function” test does not tell you it has gone noisy or not.

Ø Tip: For transistor device models on the “Top Ten Worst Transistor” list by the time you’ve desoldered one, removed it from the circuit, tested to make sure it is functioning with a transistor tester and are wondering whether it is noisy or not… you can be replacing it already with a $0.20 better-than-factory replacement and know you are no longer getting noise from that spot in the circuit. Just sayin’.
 
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How to figure out the actual operating conditions in a circuit
Reading a schematic and calculating the operating conditions is a given spot is beyond my meager, current skills, so here are other methods for at least figuring out which voltages are present in some locations:

Voltage at a test point
The schematic for the circuit sometimes has voltages indicated at a few test points within a circuit. If you are lucky, the leg of a transistor you are looking at has an arrow near that leg on the schematic and “2.6 V” written at the arrow. If so, you now know that under normal operating conditions, 2.6V DC is present at that lead of that transistor.

Image of transistor on schematic with voltages

· Schematic with voltages.PNG

Voltage chart
Sometimes the circuit designer or manufacturer gives you a nice present and prints a chart or table in the service manual or on the schematic stating what voltage you should expect at every lead of some or all the transistors in a circuit. Thank you, Harman Kardon. Sometimes.

image of voltage chart

· voltage chart.JPG

Deducing the max voltage
Sometimes you are left having to deduce or guess at max voltages in a given circuit. This is less accurate, can be wrong, but sometimes is the best you can find.

What is the max supply voltage being supplied to a given circuit?
If you know a circuit is supplied with only X volts from the power supply, then you know there won’t be any higher voltages in that circuit… and you can reduce the potentially-higher transistor specs down to that max voltage.

Example

· A 2SC945 transistor is used in the FM tuner section of a receiver, and

· That tuner section only has one power supply input and that is 13 V DC, and

· The 2SC945 device model has a spec of Vcbo 60 V and Vceo of 50 V, so

· Your replacement search might find few candidates who also have a Vcbo of 60 V and Vceo of 50 V, but

· You know there is nowhere near that voltage running around in this particular circuit, so

· You could reduce your replacement specs to something greater than the 13 V supply voltage but far less than the max Vcbo and Vceo specs of the 2SC945, and

· Find a suitable replacement that only handles 30 V max which will be fine.

What are the max values of the capacitors immediately around each leg of the transistor?
In a similar vein, you can also guess at the max voltage in an area of a circuit by observing the max working voltage of the capacitors used. If one lead of a transistor leads to a capacitor specified as 6.3 V DC on the schematic, you could guess that that lead of the transistor will never see more than 6.3 V.
 
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Transistor pin out or pin order
Getting a good replacement device chosen and purchased but then ruining it by getting the pin order wrong is a very common experience. I say that because I did it many, many times… and many have shared that same experience, so getting the pin order right in a replacement is important… and can be tricky when an exact pinout match isn’t possible.

What pin out is your existing PCB expecting?
Each use of a transistor in your circuit expects the pin out or pin order of the transistor to be sometime specific. Could be ECB, CBE, or EBC.

When you replace the original transistor with a replacement transistor, you must match the original pin out or the transistor simply won’t work. And, if you have my luck, it blow as soon as you apply power. So, you must put the E emitter pin or lead of the replacement transistor in the original transistor’s E emitter hole, the C collector lead in the C collector hole, and B base pin in the B base hole.

The original transistor’s datasheet should depict, somewhere, what the pin out order of the original device is. Should. Doesn’t mean they all will. Also, there are a few rare instances of the datasheet showing a different pin out order than the device actually installed in your circuit. You’ve been warned.

If you’re lucky, the PCB you’re working with will have the E, C, and B holes for each transistor marked. Unfortunately, just because they’ve been marked at the factory when the unit was made, doesn’t mean those markings are correct. You’ve been warned.

Image of PCB with ECB markings

· PCB with markings.JPG

In some instances, tiny diagrams showing the PNP or NPN style of the transistor have been silkscreened on the PCB so you can tell, by reading the diagram, which hole is to be which lead. Just like with the potentially-wrong E, C, B holes markings, just because the factory spent time and money to put a cute little diagram on the PCB does not guarantee it is correct. You’ve been warned.

Ø Tip: After getting the pin out order wrong a few times and after being burned by misleading, incorrect silkscreened markings, I’ve started photographing the original device before removing it and using a transistor tester to tell me what the actual device’s pin out order is. Regardless of what the datasheet or PCB says.

Ø Tip: If you have space on the PCB and you have a steady hand and you have a Sharpie permanent marker with a fine point, you can label each PCB hole with the identified E, C, or B marking. This has saved me hours of mistakes in the past. However, many cleaning products, e.g. IPA, will happily remove this smart marking you’ve just made, so you may want to snap a photo before you douse with IPA or just draw a simple diagram of the circuit and holes and pinout order before accidentally erasing it.

The cheap component tester, for me, is also very useful in making sure I've got the pinout of the original device to match the pinout of the replacement device. Getting this wrong can blow your replacements quickly. Or cause the circuit to not function... until you get the pinouts matching.

When possible and practical, you want to replace an original transistor with an equivalent transistor with the same pin out order. That’s the best starting point.

Getting help with pin out order https://www.audiokarma.org/forums/index.php?threads/hk-930-tech-advice.757415/page-2#post-10334877
 
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A different pin out order
It is possible to replace a transistor with one with a different pin out order, but that can get tricky, so take care.

The 180 degree rotation
It is common, and easy, but not foolproof to replace an ECB or BCE pin out transistor with a replacement with the opposite order pin out… because you can simply rotate the replacement transistor 180 degrees to “get” the same pin out order as the original. But you have to remember to do this and you have to know what the real pin out order on the PCB is.

Example

· You have a 2SC458 in a circuit, you hate the crackling noise it introduces into the signal, and have decided to replace it

· On the way out of the board you record which leads go to which holes, and

· You use a transistor tester to identify the 2SC458 pin out order is ECB, but

· It looks like it is BCE because of the dumb way transistor was encapsulated in plastic with the “shithouse” TO-92 package,

· You see the KSC1845 is often recommended as a replacement and order a few,

· You use the transistor tester to identify the KSC1845 pin out order is also ECB, and

· You install the KSC1845s with the ECB leads in the same ECB holes even though the package makes it look like it is 180 degree spun around from the original device, and

· Your amp now sounds clean and beautiful

Twisting the leads
If your original device is one pin out order and the only practical, available, affordable replacement device has a different, not-exact0opposite pin out order, then you can still use it but you’ll have to insulate and twist the leads and that perilous and is not recommended. But sometimes is it the only option.

To do so, you have to insulate the replacement transistor’s leads (e.g. heat shrink left un-shrunk), twist the leads so that can go in the correct holes, and carefully solder them in.

This is not a great solution. You have been warned.

Ø Tip: If you find yourself having to twist the leads to make a replacement work, then take a break and ask AK for a sanity check.

Reverse tracing to identify the pin out holes
You can, if you have to, read the actual schematic and follow each transistor’s lead’s hole to the next component on that path… to identify which hole is which pin or double-check previous guesswork. This is not easy for beginners (like me) and is not foolproof, but it is an available technique if you must.

Sometimes the pinout order is different, like center-collector
Some modern devices have “center collector” versions, so an EBC modern device might not be an ideal candidate for a replacement except for the fact it has a “center collector” version that would make it perfect replacement for the ECB or BCE original device. See “center collector pinout” section above.
 
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The “TO” package types
Which package types will work in your application?

When searching for replacement devices, you may need to broaden your search beyond the original transistor package style.

Through hole “TO” package definitions and images
There are a large number of package style for transistors. These different styles have come and gone in popularity and use, so when you search for replacements and include “same TO package style as the original device”, you may find few to none matching candidate replacements.

It is often easy, sometimes required, to replace an original device with a new device that has a modern transistor pack instead of the old, ancient one. To understand what you have and what is available, there are a bunch of online sources for TO package definitions, some of which are right here on AK:

· https://www.audiokarma.org/forums/index.php?threads/common-transistor-case-styles.116021/

· https://www.eesemi.com/to-types.htm

· http://www.interfacebus.com/semiconductor-transistor-packages.html

· https://en.wikipedia.org/wiki/List_of_integrated_circuit_packaging_types

It is perfectly OK to replace a TO-92 original package with a modern TO-92L… or modern TO-126, so long as the important electronic specs are met and you have enough space. More on this later.

There are a good number of similar, perfectly-OK replacements of a new package type for an old, no-longer-manufactured package type.
 
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Replacing big metal can packages with plastic ones
The “modern” TO-220 plastic transistor package was designed to replace the “ancient” TO-66 and TO-3 metal can packages, so don’t despair when you search for available replacements for your TO-3 device only to find few to none. In these cases, you can also search for TO-220 packaged transistors and use them… with a little bit of modification during installation.

How the TO-220 is suppose to replace TO-3 and TO-66 packages https://audiokarma.org/forums/index...dard-3-lead-transistors.843145/#post-12092870

How to heat sink a TO-220 in a TO-3 or TO-66 position https://audiokarma.org/forums/index.php?threads/to-220-substitute-for-a-to-66.434154/#post-5559740

Upload PDF with tech specs

o TO-3 or TO-66 conversion to TO-220 AN1040-D.PDF

Link to or images of the TO-3 to TO-220 exchange

o Retrofitting TO-66 and TO-3 with TO-220 01.PNG

o Retrofitting TO-66 and TO-3 with TO-220 01.PNG

o Retrofitting TO-66 and TO-3 with TO-220 01.PNG

Replacing small metal can packages with plastic ones
Back in the good old days, during the cold war, during the space race, and before cost-cutting finance departments got serious about profit margins… many small transistors were made with strong, metal cans to house the transistor instead of cheap, easy-to-automate-manufacture-of little plastic boxes.

Many of these earlier, stronger package designs aren’t needed any more or simply are too expensive to mass produce. Luckily, most of these can be safely replaced with plastic packaged transistors.

Example

· The original 2N2222 device in your circuit is in the small, metal, gold-leads TO-18 package style,

· You can’t find a replacement 2N2222 with metal can package style for less than $12 each, so

· You search for replacements in the TO-92 or TO-126 plastic package style and find many for $0.03 each

However, there are applications of these metal can packaged transistors where the metal can was needed to either dissipate a lot of heat or to shield the audio signal from higher-frequency signals or to shield space-faring circuits from cosmic rays. In these cases, you’ll have to investigate the replacement options further.

Through hole versus surface mount
Unfortunately for us in the vintage audio gear domain, there are fewer and fewer discrete, through hole transistors being manufactured each year. Most manufacturers are halting through hole production while continuing to produce surface mount devices (SMD) that do the same thing.

The SMD device packages are significantly different from the through hole ones and the package designations are completely different.

https://components101.com/articles/different-ic-package-types-and-which-one-should-you-select

Some images of SMD packages versus TO packages

· SMD vs TO packages 01.jpg

· SMD vs TO packages 02.jpg
 
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SMD to TO package adapters
However, there are possibilities in using modern, available SMD devices in place of through holes devices.

I know very little about adapters that will allow a replacement SMD device to be used where an original TO-packaged transistor was place, but I do know such things exist… so keep that in your back pocket for the unusual circumstance of you can’t find a suitable replacement in a through hole, TO style package but you can find one in a SMD package.

Some images of SMD to TO adapters

· SMD to Through-hole adapter 01.jpg

· SMD to Through-hole adapter 02.jpg

· SMD to Through-hole adapter 03.jpg

Vendors of SMD-to-Through-Hole adapters

· Cimarron Technology https://www.cimarrontechnology.com/product-category/surface-mount-adapters

Ø Tip: it is common for a particular transistor to be available in more than one transistor package style, so be careful when you order, read the datasheet carefully, double-check the specs on the actual part you are ordering.

Adapters for NLA (no longer available) MT-200 packages

The MT-200 "big wing package" transistors are all but gone and only bad fakes are available now, so what to do when one or more of your MT-200 transistors die?

An interesting idea is to use a different-package, equivalent-or-better transistor in a TO-3P or TO-247package... mounted on a replacement heat sink that makes it similar to the MT-200 package:


hoppesbrain.com

MT-200 to TO-3P / TO-247 Transistor Heat-Spreaders / Adapters

MT-200 to TO-3P / TO-247 Transistor Adapters / Heat-Spreaders
hoppesbrain.com hoppesbrain.com
 
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Heat sinks and package type and pin out
Don’t forget about heat sinks… and the pin out of devices needing a heat sink

Don’t forget about modern plastic equivalent TO packages when replacing old metal can panel mount devices
 
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Searching for equivalent devices
Alltransistors.com

Manually in a distributor’s parameter search
 
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Choosing the best result from many results
Narrowing a huge number of results

Choosing a subset of viable candidates

Manufacturer
 
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How to proceed when you have few or uncommon choices
Carefully consider the parameters that limited your matching result set to so few. Do you really need 180V Vceo? Or will 150V Vceo be fine for your application? There are more transistors and more-common transistors at the 150V Vceo level.

Packages size alternative to TO-92 are TO-126, To-220, TO-xx

Cob

Is fT transistion frequency important in your application?

Is HFE gain really importan in your application?
 
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Pairs and thermal coupling
How to deal with transistor pairs in a single device

Replacing transistors… in pairs
When replacing transistors in the audio parts of an amplifier or tuner, it is good practice to replace them in pairs. Meaning, if you have a dodgy transistor in the right channel at a place like TR706 and there is an identical transistor doing the exact same thing in the other channel of that circuit, say TR705, then you want to replace both of those at the same time... even if one is working fine.

The reason for this is every transistor model will operate a bit differently. An original 2SC1815 when replaced by a modern KSC1845 will operate just a little bit different than that replacement, so you want to make the same changes in both channels at the same time.
 
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hFE or gain matching transistors pairs
When you can, when you know a transistor is doing some kind of amplification, you want to replace the right and left channel versions of that transistors with a closely-matched pair. This simply means that you take a larger number of replacements than you need, say a purchase of 20 KSC1845's when you only need 2, and you test each of the 20's device for their hFE or Beta or gain. When you've measured many of them and found two that are identical, like hFE 396 in replacement transistor #3 and hFE 396 in replacement transistor #11, or closely matching like hFE 410 and hFE 412, and use those two in your circuit as they amplify nearly the same in both channels.

Ø Tip: using a transistor tester, test many of the samer transistor in a row and record the hfe of each. If you’ve bought the transistors in a tape or reel, write the hfe on the tape under the transisotrs so you will know what that transistors measured later. If you’ve bought the transisotrs in loose or bulk fashinon, cover the leads in a bit of nmasking tape and write on the masking tape instead. If you’re testing TO-220 pacakaged transistors, write on the back side ot the device. If you’re testing metal can transistors, write directly on the metal can.

All so it is easy and fast to compare multiple hfes to each other when looking for a matching pair

Image of hfe written on tape

Image of hfe written on masking tape

Image of hfe written on backside of TO-220

This is one of the times when a w cheap component tester will come in handy. Use it to test each new transistor, at least up to the point where you get two matching hFE values, and use those two.

Not needed in power supply circuits
When replacing transistors in a power supply circuit, the hFE doesn't matter as you are not amplifying the signal. Transistors in power circuits are typically turning on or off... or regulating the amount of voltage or current flowing through them... instead of amplifying it. So, the hFE or gain matching is irrelevant.
 
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Gain matching complementary pairs
In some circuits, a transistor in one spot will have been chosen because it is acting as a complementary transistor to one other transistor, nearby, in that same circuit. The ones I’ve seen were always one NPN and a complementary PNP (or the other way round).

Image of complementary pair on schematic

· complementary pair 01.PNG

When finding replacement candidates for one of the transistors in a complementary pair, you should plan to replace the complementary transistor as well.

· https://www.audiokarma.org/forums/i...nsistor-subs-important-for-comp-pairs.757654/
 
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How many to buy
Shipping costs… each order… is expensive, more expensive than the transistors

Easy to damage with heat

Sometime need to gain- or hfe-match… and therefore you need a larger set to draw from

How qty 100 is sometimes crazy-inexpensive

How manufacturers sometimes run out temporarily
 
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Other threads on “how to find replacement transistors”
  1. List of common replacements https://www.audiokarma.org/forums/index.php?threads/transistor-substitutions.688515/
  2. Pointers on how to read transistor datasheets https://audiokarma.org/forums/index...ut-transistors-pleeease.912474/#post-13773779
  3. Choosing Transistor Replacements https://www.electronics-notes.com/a...nsistor/choosing-replacements-transistors.php
  4. Example 1 of finding a replacement transistor https://audiokarma.org/forums/index...r-substitution-question.754182/#post-10253865
  5. How to Select Transistor Replacements Using Datasheets - a Guide
  6. Bipolar Junction Transistor Testing Basics, by Echowars
  7. Common Transistor Case Styles, by Echowars
  8. Transistor substitutions
  9. Top Ten Worst Transistors - noisy, failure-prone, whatever... and replacements
  10. Once more Sansui transistor substitutions
 
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