How To Find a Transistor Replacement

Sometimes you need to replace an old transistor with a modern equivalent. Let's figure out exactly what transistor we need for the clock circuit in the Amstrad CPC 464 while learning some interesting things about transistors along the way.
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Chapters:
00:00 Intro
01:11 Transistors
04:14 Clock circuit
06:54 Simple circuit
09:09 Faster!
14:16 Real signal
16:50 Testing in board
19:30 Amstrad circuit
20:47 Conclusion
Links:
Online simulator tinyurl.com/y6vfkjbw
The Art of Electronics 2nd Edition www.pearl-hifi.com/06_Lit_Arc...
Music tracks:
Funky Stars by McKlain mcklain.bandcamp.com/track/fu...
Battro OST by McKlain mcklain.bandcamp.com/track/ba...
More awesome music by McKlain: www.mcklain.com
🛠 Tools I use ➤ noelsretrolab.com/tools.html
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Mailing list ➤ noelsretrolab.com

[@megadjc192]

This gets even crazier when you want to see devices' overload characteristics and stuff. I build guitar pedals and vacuum tube sims using discrete transistor circuits. There are so many undocumented "effects" that I've learned about and used. I can say that math is great for when you are trying to nail down a specific design that requires precision. The art of using them creatively is a whole different level and it has taken me over 10 years to learn about and document a whole slew of things that aren't documented.

[@S0urceror]

That definitely answers the transistor issue. Great to see this explained. Never heard about the Miller effect. Thanks for this ‘down the rabbits hole’ video.

[@rimmersbryggeri]

Hope he cooked the rabbit while he was there. :)

[@DrHarryT]

My first thought when I saw the slow turn off time was to clamp the Base to GND with a resistor, but then again I am not an electronics engineer.

[@byronwatkins2565]

The Miller effect is a multiplication of collector-base CAPACITANCE by the amplifier gain... totally different than base charge of a saturated transistor. A Schottkey diode from base to collector will prevent saturation. This trick is employed in 74Sxx and 74LSxx logic chips.

[@VincentGroenewold]

This is gold, love the deep dives as this usually, for me at least, makes things simpler. I know people always think abstractions are better for that, I guess they are when you want to learn fast, but I always struggled with that in school. Deep diving for sure made my homework go slow, but I understood stuff way better. :)

[@danielmantione]

16:22 The small spike at the output right before the signal falls is also caused by the Miller effect: The input rises at that moment, and the rise goes directly through the parasistic capacitor to the output. The Miller effect amplifies the spike. After a very short delay, the MOSFET starts conducting and the voltage drops to zero.

[@airspeedmph]

@Daniël Mantione I used to see you frequently on various Linux forums, not much lately, what happened?

[@danielmantione]

@@airspeedmph Main reason is the retro computer renaissance, there is so much happening around the Commodore 64 and other retro computers these days that consumes a huge amount of hobby time. Some time ago SteamOS and gaming on Linux was brand new so that was where I was spending my spare time on. But currently... nothing less than wonders are happening on computers I love and I want to enjoy this.

[@Rx7man]

I was noticing that spike and it looked severe enough that it could possibly re-trigger some circuitry at the wrong time

[@airspeedmph]

@@danielmantione Totally get that, I spend my time alternating the two (Linux and retro) although I have to admit that the retro part I'm only experiencing vicariously through channels like this, since my only HW that can be called retro is a PS1 :) Lately I'm seriously consider getting real by acquiring some hardware, though I'm not sure where to begin. I'm somewhat inclined towards a Commodore 64, but a working one seem to be rather expensive. But yeah, is crazy what's going on in the retro space, new hardware+addons and even games, is impressive and alluring. And another thing that I especially find relaxing in the retro space is that everything is governed/motivated by passion, there's no in-fights, politics and ideology to be found here. Anyway, have fun.

[@DanPellegrino486]

Wow, I learned quite a bit here. Transistors are still something I get the basics of but not beyond, so this was really insightful.

[@tiemanowo]

Coffee in one hand, donut in the other and time to watch Noel's video. ps. wow. that was an interesting "eye opening" video. I didn't know about this charge build up effect on base.

[@NoelsRetroLab]

Glad you enjoyed it! I didn't know that either. I love it when I learn new stuff!

[@TaberBucknell]

I wish all electronics lectures were this good and this polished! Excellent work Noel ... thank you! 👏👍

[@mathieucuny8872]

This video is filling the gap left by the previous one! It's very nice of you to delve that deep in this subject!

[@michaelhaardt5988]

The Z80 clock signal spec is more demanding than regular TTL and I suppose they could not integrate the driver on the gate array. The inversion is just a side effect and the gate array could have inverted it for sure, but the primary use of the transistor is to improve the signal. I forgot the details, but it was quite common to use a transistor to match the Z80 clock spec. Note that they used a 74HC04, not a 74HCT04 in the second revision, which is a hint in that direction as well. MOSFETs are a completely different technology than bipolar and while you can get lucky, the considerations to drive them are very different.

[@juggler1977]

I agree. I came to understand the Z80 clock input requirements designing a computer. Z80 is generally TTL level but clock is special.

[@SebastianXpander]

Such a great hands-on guide to this issue I have been struggling with. Much of this is relevant with fet-based audio compressors from days gone by. Because the characteristics of the fet are what determine the some of the 'attitude' of the compressor, it is important to know how to find a modern match with similar properties. This now seems less daunting, thank Noel!

[@mogwaay]

Big love for the deep dives here, never leave a problem unanswered, it's just too interesting having you track down the solution. Oh and your head planting the Art of Electronics book was a great shot!

[@retrolabo]

Awesome video! Thank you for taking the time and fully deep dive into subjects like this!

[@pauldourish]

Great video! Thanks for taking the time to follow this one right through and then share it with us. Very enlightening!

[@VintageProjectDE]

Very nice research on the transistors. Thank you for the effort showing the process! As for the 2nd edition of AoE, I recently said "What I learned from technical books: Contrary to popular belief they in many cases do not get better with revisions. It's often worth keeping earlier ones around." I'm glad I'm not the only one who found that. It's definitely worth it to keep older revisions.

[@75slaine]

It’s this kind of content that sets your channel apart Noel, love it. Thanks for putting the time into this video.

[@falksweden]

This was a really great and interesting piece of information, Noel! Well done!

[@nednettapp]

Awesome video. Very interesting but also easy to follow those of us without a background in electronics. I'd love to see some similar content looking at different types of capacitors.

[@peterlinddk]

Excellent video - this is why I love this channel, rather than just telling us the solution, you take us on the journey of learning and experimenting, showing both failures and successes (reminding that a failed experiment is also a succes, because you learned something new). It is really inspiring, thanks for that!

[@hagen-p]

Never have I been more happy to have an OLDER version of a book. (My "Horowitz & Hill" is the Second Edition.) Thanks!

[@JeremyNasmith]

Awesome video!! Using a real-world example as the motivation for this deeper dive into transistors really makes it easy to understand.

[@MichaelEhling]

15:50 Hoorah! I'm happy you opted for fun. Deep dives into things we love are fun. Thanks, Noel.

[@wolcek]

Transistor in the clock circuit is to adjust the high voltage level to what Z80 needs.

[@rockyhill3]

That was a fantastic video! It had meat on it's bones and you explained it in a very clear manner. Coincidentally I also had to find some modern replacement transistors for a board I've been working on. I loved the experimentation, keep it up!

[@amaiorano]

Fantastic video, Noel! Love these deep dives on electronics. You definitely are a great teacher.

[@djmips]

I've really wondered about this topic myself. Great practical problem to explore this!

[@Fredthefat]

Good video. I'm not much of a hardware person, but am picking up a bit these days thanks to you and others working on retro kit. This was a great insight to transistors, which I was always a bit scared of in my distant youth. You are very skillful at explaining things, you have the ability to get into detail, but still keep it enjoyable enough to carry on watching - something my old statistics lecturer was sorely lacking! I also like that you want to know why things work, or don't work. Some would have just found a solution and moved on, likely to be something I would do. But your curiosity has given us another great video.

[@wherami]

This was amazing. Your deep dive was a great story

[@TRONMAGNUM2099]

Excellent work! Really great deep dive on implementing the proper transistor for this application.

[@Seftdelmer]

Brilliant, just the right depth. I always wondered how to determine the correct replacement transistor and this provides a great starting point. More like this please Noel!

[@didiersee]

Congratulations from Belgium for the simplicity of the explanation which allows everyone to understand without having studied engineering, thank you for sharing.👍😉

[@JVHShack]

I loved this video. Thanks, Noel! This one will come in handy for years to come!

[@Leahi84]

This was fascinating. Thanks for this.

[@robtaylor2045]

Great video as always. Love the deep dive stuff, really informative.

[@RudysRetroIntel]

Excellent work and thanks for sharing. I fully agree that older books are always good to have around. Was doing the same or similar experiments about a month ago.

[@TheSimTetuChannel]

A very humbling video! I recently repaired a friend's Commodore 1541 drive where a resistor went bad and had to use an equivalent NPN resistor. I thought I had done my homework right by comparing specs and it was a success, but you now make me realize I probably just got lucky! 😁

[@thebyteattic]

Fantastic video, Noel! You're surpassing yourself. And I was delighted to see that you also had to battle the Bibl... err... the Sedra as an undergraduate. So did I! :-) Regarding the clock inverter, if the gate array uses the uninverted clock signal internally, it makes sense to invert it before passing it on to the CPU: the 180-degree phase shift helps account for, and even take advantage of, the propagation delay of the other signals shared with the CPU. The Atari line of 8-bit computers makes liberal use of this kind of phase shift to ensure proper synchronization between CPU and other active components, and it effectively doubles the speed of the system, since you get two clock tics per clock cycle, each in a different part of the system. Back in the day this kind of hack mattered. I also would be very careful about biasing any switching transistor without a base resistor. You got lucky (or perhaps you knew it) that the gate array had an internal pull-up. Otherwise the pin wouldn't be able to source the kind of current you get through a diode to ground, and the chip might have been ruined.

[@NoelsRetroLab]

Thank you! Yes, the inverted clock signal makes a lot of sense. The thing that surprises me is that the Gate Array doesn't invert it before outputting it since nothing else on the board is using the uninverted one. I guess they either completely ran out of space in the GA or it was a "fix" they had to do afterwards.

[@thebyteattic]

@@NoelsRetroLab That was probably because of a limitation of the specific process technology used in the gate array, which may have prevented either enough buffering capacity (i.e. cannot drive or sink enough current) or fast-enough switching at the required current. It surely makes sense. An external transistor can use very different process technology than logic and can have larger die space than what could be done in the gate array.

[@Xoferif]

My analogue electronics lecturers championed Sedra & Smith, and I was heading towards failing the course... Then one of my classmates showed me his copy of The Art Of Electronics, and suddenly everything was alright. =)

[@malcolmhodgson7540]

Wow. What a comprehensive video. Great information

[@podunkpennsylvania292]

Very informative. Thank you Noel

[@mertuckan]

This is very useful. I'm not just saying that. Please keep making more component videos. Really helps. Great Job!

[@WacKEDmaN]

Great deep dive Noel... very infomative

[@radman999]

Excellent video, Noel. You have a great way of explaining things!

[@horusfalcon]

I hadn't heard the words "Miller Effect" in over forty years when you uttered them here. Wow... wotta blast from the past, man. Thanks!

[@gertsy2000]

Great info, and a great video Noel.

[@NathanClarke]

Great video Noel. I had a similar issue when building my Sixty Clone with transistor in Q2. I eventually found one with help from one of the component suppliers, but at the time I thought a lot about transistor replacement for these replica boards.

[@weirdocollector]

"Practical Electronics for Inventors" ! My favourite book (even more than "Art of.."). Great video as always!

[@johnsonlam]

This topic is really not popular but very useful, I have some retro cards laying around, and some components are not in production anymore, thanks to Noel which dig deep in the topic.

[@MrMurdock2021]

This was so interesting it nice to hear that there still people who appreciate the retro computers enough to research find even small things that to make to make the whole system whole again for a new generation 😀

[@NoelsRetroLab]

Thanks so much for the kind words! Glad you enjoyed it.

[@lindoran]

Amazing, I really love your attention to detail.

[@YogSothoth1969]

Very informative, thanks for sharing your experience 🙂

[@TheFicktion]

The book 'Art of Electronics,' in Polish known as 'Sztuka elektroniki,' is excellent. It contains a lot of content and is easy to understand. I've been using this book throughout my entire high school, or as we say in Poland, 'W Technikum.' :)

[@FrancoCatrin]

I don't plan to use transistors, even I don't have plans of working with electronics at this level but, oh man, I enjoyed this video A LOT! Your videos are always full of high quality material, presented in a way that everyone can understand. Great work!

[@JendaLinda]

Thank you for covering this topic. BC547 is just cheaply made transistor intended for low frequency amplifiers and similar applications. There are also specialized high frequency transistors available. A metal can package was actually pretty popular in the past. For example, KC507 transistor, a BC547 clone, was produced in metal can as well.

[@Haplo-san]

Thank you, it was very informative. I again learned something new that I would never use in my life but what can I say, I love the learning part.

[@EtoileBleu52]

a really good and instructive video thank you

[@buzz1973]

Great informative video Noel 👍🏻

[@RacerX-]

Very informative. Thanks, and keep up the good work.

[@SobieRobie]

Very interesting episode!

[@skeleton_craftGaming]

Older books may have relevant information to restoring older electronics? Yeah that checks out to me and ironically maybe the best advice to people getting into this hobby that I've heard for a while.

[@Inject0r]

This is some really useful information. Thanks!!

[@MrFixiit]

Very facinateing and educational thanks noel.

[@boblewis5558]

For that particular transistor I have no problem: 1. I used to work for the manufacturer (Ferranti) 2. I have many ZTX312's and many other ZTX series devices from 45-50 years ago! 😱😳 3. I have several Towers' transistor selector books that detail the critical specs of the transistor and also give equivalents from that time (mid '70's!). I often find that modern equivalents quoted are actually NOT and finding an equivalent of the original equivalent works much better. 😎 Nice explanation of a really good approach though. 👍👍👍

[@MarkoTkalcic]

Fantastic video, thanks a lot. I checked the AoE 3rd ed book and found the Miller effect in Sect. 2.4.5

[@tony359]

really really Nice, thank you for the great video!!

[@juanmiguelcortarello6823]

Me devolviste a la facultad con este video. Espectacular la investigación. Excelente video! Gracias.

[@MoreFunMakingIt]

Brilliant video Noel! I'm still finding my way and a lot of this is miles over my head. But I learned a lot today watching this. Thank you!

[@Flatwoodsdad]

I'm retired now and legally blind. But a great book that helped me back years ago was "Getting Started in Electronics by Forrest M Mims". It's free now as a pdf. For this 8 bit stuff it will help a lot.

[@MoreFunMakingIt]

@@Flatwoodsdad thank you very much! I will look that up.

[@johnsmallberries3476]

Very instructive...thanks!

[@bitoxic]

Excellent analysis on transistors... learnt a lot on this video! 😁👍

[@volodymyrgutsu7011]

Awesome dive into 👍

[@retroredrum]

I didn't watch the video yet, but I know it's going to be good and help me in my repairs. +1 Thumbs up !

[@brettb.345]

Thanks for the info!

[@TotoGuy-Original]

great video nice story behind it to solve the problem

[@edgeeffect]

I'm more into microcontrollers etc than retro stuff (although I do have a great nostalgia for the 8-bit days)... so this video was really really useful... more useful than some of the electronics channels... thanks!

[@NoelsRetroLab]

Glad you enjoyed it!

[@raybin6873]

This guy is awesome. I'm trying to repair an audio power amp - voltage regulators failed - no exact replacements available so seeing if alternatives can be used. 👍

[@purrator]

What an awesome timing, just as I need to find some replacement transistors :)

[@charlesjmouse]

Very good! Thank you.

[@henriquepereira4058]

You have taught me a lot about transistors that I didn't know before

[@ford1546]

it was very interesting. Thanks 👍

[@Cabalist3131]

I love this. Thank you so much

[@atec9228]

How interesting, depending on the model, the transistors can affect the signal that they can obtain at their output. great video Thanks

[@CoolerQ]

I *just* encountered this delay effect with a transistor circuit a few days ago. I solved it by replacing the transistor with a different part, but now I know I can add a capacitor or read TAoE. Thanks!

[@galier2]

Thanks a lot for this gem of a video.

[@miriamramstudio3982]

Great video. Thanks

[@erwindewit4073]

Very interesting, as I ran into this a couple of months ago. My NAD 5000 CD speler stopped working, and despite everyone telling me it was the transport (or laser), I didn't think so. It just didn't make sense. So I started reading, and found that after 25 years 2 transistors tend to burn out because they're not sufficiently cooled. So, I opened up the CD player and the burned spot told me that they were right. But then the search started.. Fortunately, in the end the local (well, 40 minute drive away) electronics store knew the player and what modern replacements WOULD work. And indeed, they did and it works like a charm again. But still, I was lucky there. This makes looking for replacements a bit easier... Thanks!

[@rastislavzima]

Thanks, it was very educational video.

[@bobsretroelectronicswork-bn4qc]

Great transistor experiments!!!!

[@rpapp]

Awesome video!

[@albertdelafuente9977]

Muchas gracias por la excelente clase sobre transistores y sus compatibles, ya se algo más, buen trabajo.

[@SidneyCritic]

Good detective work.

[@minombredepila1580]

Where were you when I was in the Uni?. Oh man, another masterpiece from Noel !!!!

[@LanceHall]

Brilliant!

[@podunkpennsylvania292]

The fact you stated about Base saturation made sense in switching times

[@TheDiveO]

In the past I once accidentally picked a BC308C PNP instead of a BC238C NPN for driving a ~20mA LED from an 74Nxx output. I wondered for several minutes why the LED lit up when the TTL output was low. Until I checked the printing again and noticed my mistake. But this wasn't just a PNP-NPN mistake, this also meant that the PNP had the C-E current running against its natural direction. The BC308C was still very happy and worked in other breadboard setups.

[@freeeflyer]

Very interesting.. Thanks..

[@DerekHerbst747]

I suspect that this application would find the best replacement transistor by matching the emmitter-base capacitance as a priority. Nice video!

[@Dextermorga]

This video was the best!

[@Bianchi77]

Thanks for sharing, like it :)

[@Nairod2]

its always fascinating to remember all computers run on frequencies, just like electrons protons and neutrons spinning inside atoms.

[@atec9228]

Miller effect !!! Very interesting

[@Savan_Triveda]

Wow, I learn a lot here.