I came across this forum thread where the person asks how to find and equivalent transistor. First off I would like to say that if you substitute another transistor and it burns up in a few seconds, like it did in this case, then you should certainly consider that the circuit is bad in some other way and no matter how exact the replacement is, it is not going to work until the problem is fixed.
I faced this ‘sub’ question many, many times years ago when I did a lot more TV and receiver and audio repair, and found out how important it was to get the right transistor. Someone gave me a good quality stereo amplifier, and said he didn’t want it anymore because he had it repaired but it sounded awful. I put the ‘scope on the output and I found that it was oscillating at 2 MHz! If I had put a hundred feet of wire on the output, I could have had a transmitter!
I learned a lot about what the transistor datasheet is and how to use it. The manufacturers used to publish a data book, and they gave them away freely for just asking in a letter with you company name (a friend and I made up the company name Solid State Systems). So I got many transistor manuals and data books for every type of electronic component.
Nowadays, much of the data cane be found online by googling for 2N3055 datasheet for example. You will get many hits for places that will let you download the datasheet .PDF for free, as long as you look at all their annoying adverts.
But then what do you do with this datasheet? It’s full of technical jargon that the average person doesn’t understand. It took me years to learn enough about this to make a good judgment on what’s important to know in order to make a correct substitute.
In this forum thread, people give all sorts of advice, from good to totally incorrect. I once told someone that his advice was, to be tactful, less than accurate, and you know what he told me? “The advice you get is worth exactly what you paid for it! $00.00!” And he was right. If you get free advice, you can expect that it’s worthless!
But I try to give worthwhile advice. Not many care, though. They have no clue as to whose advice they should take. As a result they go wrong and are very often discouraged enough to throw the device out and go buy another new one. I think if more people would try to repair the broken equipment, it would save a lot of resources and the world would be a better place. But that’s a discussion for another time.
Back to transistors. in the case I cited, he could get replacement 8050 transistors from Mouser or Fairchild, called the SS8050. They are very cheap, maybe a nickle apiece, even less in quantity. But the shipping might be many times the cost of the part.
There is another part that might work. NTE sells replacement parts at ten or more times the cost from a distributor. They sell the NTE11, which is a very high current transistor, for about a dollar fifty. It’s in the same package as the 8050. But the gotcha is that the pinout is E C B instead of the 8050’s E B C – it’s the same as other similar Japanese transistors. So you will have to slip a short length of insulation (stripped off a solid wire) over the collector lead so you can cross it over without it shorting against one of the other leads.
There are other substitutes, but those two are readily available. You can go to Radio Shack and buy some transistors from them, but the salesdroids don’t know any more about substitution than their customers, so buying a transistor from them is most likely hit-and-miss and a waste of time and money.
Substitution in General – I have a few publications that are my favorite references when it comes to substitution. I used the National Transistor Databook so much that I wore it out, and had to get a replacement on eBay or Amazon, I forget which. The great thing about this book is that it tells you which process the transistor came from. Then you can read about that process, find out what the chip looks like, and find which other transistors use the same process. For instance, you might find that the 2N4124 uses a certain process, and 2N3904 comes from the same process. So if the circuit demands are modest and the packages are the same, then one transistor could be substituted for the other.
Motorola Transistor and Diode Manual – This tome (it’s 3 inches thick!) was the reference for just about any U.S. transistor or diode made up until its publication in 1969. This still covers a lot of the devices used today. After that there was not so much growth in the number of discrete devices (transistors, diodes, etc.) and more growth in integrated circuits. Today the number of transistors registered with the 1N and 2N numbers is still under ten thousand. There are other transistor makers that published transistor manuals and crossreference manuals, one was Sylvania, AKA ECG.
NTE – Another way to get a rough idea of a substitute is to use the NTE catalog. You can also go online and search their web pages. Having the catalog on paper is easier and faster. There is one gotcha. I have done lookups in the NTE catalog, and they seem to think that the NTE123A or NTE123AP will substitute for just about any small transistor, when I know that they will clearly not substitute. But the catalog will give the reader a good idea of what package their substitute transistors are in, and what their rating are. For instance if the transistor is a power transistor, you can find the package and how much power, etc. it will handle. NTE sells a line of replacement transistors and parts, but their prices are very high. If you have an immediate need for a single part, then they may be a worthwhile solution. But I find that it’s much cheaper to buy a bag of a hundred transistors from Mouser, DigiKey, etc., for the price of one or a few NTE transistors. The NTE numbers are the same as the Sylvania ECG numbers, so NTE123AP is the same as ECG123AP.
There was a lot of growth in the Japanese (2S numbers) and the Pro Electron (AC, BC, BD etc. numbers) series. These were used in a lot of the equipment manufactured in Japan, Europe and other overseas countries. Very many of, or possibly most of consumer electronics equipment has Japanese numbered parts. These numbers usually start with 2SA, 2SB, 2SC, or 2SD, however the transistor package has so little room that the numbers are usually abbreviated by dropping the 2S. For instance the 2SC1815 transistor is marked C1815. So if you look up the substitute for C1815, also look up the sub for 2SC1815. Similarly, sometimes the European makers dropped the B, so a BC547 transistor may have C547 on the package. The BC547 number is one of the Pro Electron number series, which originally started with A when the transistors were germanium. Now most transistors are silicon, which starts with a B.
Many transistors have numbers similar to the JEDEC 2N numbers. One good example is the 2N2222A, which comes in a metal package. Manufacturers used the same chip in a different plastic package to make them much cheaper, and gave them a similar name such as MPS2222A or PN2222A. Also, the same transistor chips can be found in surface mount packages such as PZT2222A or MMBT2222A. There are also integrated circuits that have four transistors, such as the MPQ2222A, which may be very difficult to find. So if there is room, four individual PN2222A transistors could be used.
Circuit Design – I should also discuss circuit design. In the early years of transistorized equipment, the cost of the transistor was higher and designers tended to design equipment with few transistors. The ultimate transistor radio “reflex” receiver used a single transistor for first amplifying the radio frequencies, then after detection, the audio was fed back through and amplified by the same transistor! Consumer equipment used coils and capacitors for tuned circuits, and these had to be adjusted or “aligned” at the factory for proper operation. At one point the makers started using “SAW” filters which did not require alignment, and this elimination of labor saved a lot of money and time. So designs became more complex as automation made assembly much cheaper and less dependent on labor. The result is that if you are replacing the transistor in older equipment, the substitute most likely needs to be more compatible, because it is likely that the equipment will have to be re-aligned. More modern equipment may be much more tolerant of transistors that are not as close a substitute.
Back to experimenting;…
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