How to Diagnose and Repair Transistor Circuits - No Schematics. Dynacord Powermate 600

This Dynacord Powermate 600W Powered Mixer/Amplifier is stuck in protect mode. It has mainly discrete transitor cuircuits and have I have no schematics that match the version I have. But I still need to fix it. Here is a practical example of how to diagnose a difficult fault using nothing but methodical fault finding, basic electronics knowledge, some intuition and a multimeter!
Music in this video
Barbara Pennington - 24 Hours A Day (1976)
Learn Electronics Repair #249
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Thank you
Richard

[@tomluque]

I'm 73yr old and have been doing electronic repairs on medical equipment for 45 years. I helped in improving my knowledge of circuits by watching your techniques and drawing of schematics. Thank you for continuing to post on KZbin.

[@theforthdoctor7872]

outstanding, I love that you didn't try to edit it to make yourself look like a Tony Stark style genius.Its all about the journey. utmost respect.

[@LearnElectronicsRepair]

And ain't that the truth 😁

[@DadofScience]

Couldn't agree more. I learn more from the finding of dead end and then where to navigate to from there than the final fix. Many thanks, Richard.

[@dispdisp3703]

The honesty of your channel is what I like the most. Real technician doing real repairs!

[@pauldery7875]

Richard, your videos are so awesome and authentic, why do you not have over a million subscribers. Keep up the great videos. Paul, USA

[@madkiwimuso2477]

Hi from New Zealand! I found that thoroughly absorbing, especially with the successful conclusion. I am a 'fledgling electronics repair enthusiast, and just want to express my appreciation to you for your tutorials, which are invaluable to myself and, I'm sure, to others like me. Thank you for taking the time to 'educate' us electronics philistines, and please keep up the effort you put into these videos, it is truly appreciated.

[@jimle22]

Me also. Thanks for these video tutorials.

[@seeithappen1]

Great example of real deal fault tracing, an unexpected burned path hidden under a bunch of components. They dont teach you this at school. I would not have found it. Thanks for the learning.

[@konturgestalter]

just one of the best electronics channels i currently know

[@LearnElectronicsRepair]

WoW. Thank you 😊

[@Philippians4vs4-8]

You are a wonderful technician. You bring back a lot of memories for me. I am also a troubleshooter but +70 years old now and retired years ago. It is so refreshing to see real troubleshooting nowadays, rather then just shot-gunning component's or just changing modules.

[@adambradley3284]

The ability to work without a schematic is what draws me to your videos. A Technician who can work using basic tools and logic is worth his weight in gold (as long as he's a f@t Bast**d). Thoroughly enjoyed the 2 hours..... I've always liked the analogy of water flow to describe DC through small circuits, and it occurred to me during this video that a bucket of water in a tree in East Yorkshire, +ve potential, has an equal and opposite bucket here in Australia. True Ground is not the dirt under your feet, but the centre of the earth where the water can finally rest= zero gravity, The Bucket in the gum tree is -ve potential with respect to Ground and the bucket up North. Loving the channel. Please keep it long and boring, as there are those of us that find this stuff fascinating,

[@repairfreak]

Very well done sir. After about 30 years on/off repair of consumer electronics I’m impressed. I loved how you were able to dissect the circuit down. Your lucky clue was the hot TO92 small signal transistor. But how you determined what actually was happening by your knowledgeable comparison side to side was brilliant. When it all lead to the open hidden trace and then your jumper wire correction I got excited! What a wonderful thing to see that protect light off, and then hear those relays click, it was the sound of success, “like beautiful trumpets to my ears!” The reward of hearing music in the end makes audio repair such a glorious thing. You definitely have one up on me in your knowledge of transistor configurations, and how they interact with each other. I loved how you were able to show us the bias voltages by comparing base to emitters even when dealing with a mix of positive & neg voltages, and how you were able to draw this all out and explain. I always use my flexible fiber light for inspecting pcb traces. However your break was in a most hidden place. Just goes to show how one really need to use all of ones senses, and be careful to be alert to all things/possibilities, as to not waste unnecessary time getting mislead in the wrong direction. You did this all without a schematic, I’m very impressed. I actually suspected shorted output transistors as the fault. Well done!! 😎👍

[@jonka1]

That was a quick couple of hours. Thank you for wanting to do this, I didn't see any wasted time and you got there without a circuit.

[@hullinstruments]

Keep it up because although this may have hit your limit now, you are super smart and very good at adapting to a situation. The more strange things you come across and have experience with… Your knowledge will grow. Remember when we used to be in school in algebra or some other complicated class… Maybe 3 to 6 months of that class and maybe we could solve the problems but we never really understood what was going on. And then one day all of a sudden it “CLICKED”?….. that happens in electronics and will continue to happen. Obviously those instances will get fewer and far in between and spread out over time but no matter how long you stick with it you’ll have those epiphanies from time to time a few times a year… And you’re already so good at this now you will definitely improve. Especially considering the community you’re building here on KZbin of people who can assist and explain things if you have questions. I’m a complete idiot, I have my own small and humble business and metrology lab doing component level board repair specializing in test equipment and stuff for other labs. I don’t really have a great ability to adapt or deep understanding like you do. I’ve just marched ahead until I’ve had many of those “click“ instances… and watching content like yours helps me to have even more instances of deeper understanding. Definitely my favorite electronics channel I’ve discovered over the past three or four years. Don’t get me wrong I love Mr. Carlson and a few of the other highly advanced individuals… But sometimes I really don’t understand what they’re talking about, your channel and methods are a lot closer to mine and your teaching style is great. So even though I may already be experienced in some of these areas and know it well… It still sharpens my knowledge every video of yours I watch. Some of which I’ve watched a dozen times and stream in my lab when it’s quiet during the day

[@LearnElectronicsRepair]

Cheers. I did fix it so all is good, but as I said this one pushed me hard. To the limit possibly. The moment it 'clicked' for me was when I figured out that if the transistor network on one channel was monitoring 65V HT- then the ought to be monitoring 65V HT+ as well, and it was... but on the other side only 65V HT+ was being monitored and it was fairly obvious there was an open circuit track and I basically knew where it must be. If I hadn't found the broken track I would have just connected a wire from that emitter to 65V HT- (using the light bulb for protection) and it would have worked, just not quite such an elegant repair 😉 Yeah I'm still learning, like you say we all are. A friend said I should make a note in a log book of these faults and the cause, but as I pointed out to him once you fixed one like this you will *never* forget what was causing it 😋

[@reacey]

Check shango066 , repairs older stuff but hes brilliant . Big clive has probably the deepest understanding of electronics . Iv only just found this channel but this guy is right up there with the best in the business . Much respect

[@nirmanmulalic1824]

I will never forget it although I was only watching, let alone you Sir, heavily sweating.

[@theoloutlaw]

Loved it! My lack of knowledge baffled me about half way through, but glad I can always come back to these videos to review and keep learning :)

[@florinpascu4393]

hi from romania, i just love how you explain on paper ,it's much easier to understand ,thanks

[@wannabemgtow2540]

One half of it one day and saw the rest the other, and I have to admit that I did fall asleep. I have to go back and review some of the video. Anyhow it was very educational thank you.

[@markhonigschmid6116]

Thank you for your time and your helpful videos! I'm 44 and now I try to learn electronics. I'm from Austria, and so my mother tongue is German. Thank you for speaking this nice british-english! So I can improve my english at the same time. Thank you for your effort!

[@khx73]

Didn't skip a single second. Excellent work and a satisfying conclusion.

[@Colin8721971]

Just starting out on this type of repair and this is what I have been waiting for! A logical process of measuring with all the pit falls. I am making leaps and bounds in my process of self learning. Great stuff!

[@surgingcircuits6955]

I fell asleep a couple of times ... only to wake back up, rewind to where I remembered, and go forward again. It's a great way to watch a 2'10" video in over 3hrs. Ha! But... it taught me a lot and I am grateful. I also learned tenacity and diligence. Maybe more important. Thx.

[@surgingcircuits6955]

PS: I don't mind long videos. To me, it's quality over time.

[@reacey]

Absolutely love your videos mate learn more from you than i ever did at school, college or anywhere else . Thankyou , from north yorkshire ,UK

[@leopoldpoppenberger8692]

when you said an open or break I was thinking a wire connection one of my highlights otherwise you are far ahead of me took electronics 40 years ago thank YOU

[@LearnElectronicsRepair]

Yeah I've been at it a long time too. I took my city & guilds in electronics repair 40 years ago (which was a 12 month full time course) but I'm not university educated I stopped after 6th form,which is apparently grade or year 12-13 in the USA though i had to google that.

[@Laterdays2]

17 years like you have never missed a beat. That was amazing. Thanks!!

[@CliveBagley]

Watched the lot - in two sittings- and once again this old dog learned a new trick or two. Cheers!

[@towbsmedia]

Respect, sir. Respect... Beautiful video, without edit; very helpful. I learnt a lot. So, please continue, you gorgeous man.... Cheers Siavash (from Iran)

[@cskeet1320]

Another fantastic video. I was gripped and watched from start to finish. It was not to long. Your attention to detail and logical clear explanations are great for beginners like me. You don't miss anything out and show warts and all in your journey. I learned so much as with all your videos. Thanks v much.

[@jimle22]

I have to admit I did dose off once bc I was very tired to start with, bit I just went and fixed a cup of coffee, came back rewound the tape anad continued all the way to end. Couldn't stand the mystery. Had to see how you fixed it, So glad I did, it was well worth the learning experience. Thanks Richard for another great repair video.

[@darrenbird2526]

Hi from Tasmania Australia. I'm learning electronics to do repairs and get my advanced Amateur Licence. I get so much from your channel. I'm learning so much from your channel and I think I learnt more from this diagnosis because it was so long. Absolutely loved it! I can't stop watching. Darren

[@Mcfreddo]

Hi, what do you mean by advanced amateur license? Oh, for amateur radio operator?- Talking to other people around the world?

[@marcellipovsky8222]

I made it to the end AND it was worth it. Once again a minor fault which causes a major problem. Thank you Richard for your time. I am sure that if you weren't shooting this video, you would have it fixed much sooner. (psst, don't tell the client 😁)

[@bryandraughn9830]

I've got an old Yamaha guitar amplifier with 2 big transistors on a heat sink in the back. I've changed those but the amp still just makes real spikey fart sounds. I got into the power supply and it's working fine so for now, it's furniture.lol! There's a lot of funky stuff in there. I do have an abundance of brand new spare parts. Long story. Also quite a bit of experience tinkering and building basic circuits. I think I need to run a signal through the amp in order to diagnose. For the time being, I'll keep watching your videos.

[@gregreynolds5686]

A great testament to your character that you didn't give up!

[@Cleofizoid]

Disco lives! Good job...it all started with using the IR camera. Not too long, enjoyed it.

[@fredb1130]

I love watching your videos. I always learn something. Watched the whole thing, it's out of my league but still enjoyed the whole damn thing. Thank You for sharing your journey with everyone.

[@andymouse]

This should be required viewing in colleges and Uni, great piece of diagnostics and fun to follow along...cheers.

[@spiderkhan259]

Thankyou sir, u make fault finding very intresting may God bless u and grant u long life and good health.

[@johnm4962]

Thank you so much for teaching me. I appreciate all you do. Thank you

[@ajra6805]

Very much enjoyed watching this repair. probably like you I'm in awe the trace vaporized where it did And your method of explanation allowed me to visualize the circuit and how it operates

[@ThomasIngolstadt]

A fantastic travel down the rabbit hole. For me, I don't look for (stereo) audio equipment schematics, as you always have a reference at hand. Usually i do diode mode readings around where I suspect a fault, as this is usually the faster way. Anyway, great job! Keep it up!

[@Sir.RickParker949]

I'm glad I found you, your way of explaining the various components and what their functions are is splendid. I look forward to learning as much as possible from you, since I am disabled with a crushed rt.hand I can no longer do auto repair as a living. This is going to be my new line of work since I've always had a fairly thorough understanding of electricity. Thanks again sir!

[@freeelectron8261]

Watched from start to finished. That was a tough one. Great troubleshooting lesson.👍

[@SomeOtherCrap]

Every time a customer comes in and says "It doesn't work, I think it's probably just a fuse", I'll tell them to watch this, just so they have some idea what we go through!

[@RickL_was_here]

New subscriber and that decision took about 3 seconds. I truly appreciate the way you go through everything and being a somewhat newbie with this stuff, I think I'm going to be learning a lot from you.... Cheers from Canada!

[@ngofyneface5547]

Good one. I gained a lot from this. Thanks and may God bless you.

[@stevewebb488]

Watched it all and learned so much, Excellent video and so well explained, thank you and keep em coming 😊

[@markwhitfield5412]

good job, must admit most of it i just didnt understand, ill keep watching. everything lol, u never know it might sink in, MIGHT!

[@errolhibbert2390]

Sure learn! a lot about transistors there, loads of experience in troubleshooting, good skills.

[@deviantmultimedia9497]

I laughed my ass off at the end "Not including you fell asleep" because admittedly I DID fall asleep and had woken up it was to this ---> 1:37:18 . 🤣 After watching it again, I was actually keen on the subject already and you did quite well explaining it. But you've got to admit, that's pretty damn funny. And then seeing the bright green letters at the end 🤣. Anyway, jokes aside, great video. I really did go back and watch what I missed, knowing something in theory is one thing but in practice is another. This repair would've kicked my ass up and down. I would've had to give it back to the customer unfixed. Being able to watch a master in such a situation, putting the theory into practice and coming out victorious, is a valuable valuable privilege and I'd be doing myself a disservice by not watching it.

[@garypoplin4599]

2:09:46 - I like your idea of straightening up and paying attention to Richard’s pen and paper monologues; because, admittedly, I skip through his videos looking for these circuitry explanations! I do this particularly on your extra long presentations, Richard, before rewinding to watch from the beginning. I’ve learned _much_ from your “old school” technology over the years since discovering your channel.

[@absurdengineering]

I haven’t watched it to the end yet - but :) When replacing transistors in current sources or current mirrors, treat them like you would differential pairs: replace all the transistors at the same level of the circuit. In a single output current source or current mirror it’ll be two transistors. If there are more current outputs, there’ll be more transistors to replace. You want the replacements to all be from the same batch - matching them may be unnecessary, as that’s expensive to do in mass production. Another point worth making: MPSA42/43 and MPSA92/93 are linear wide-base transistors and are excellent in temperature compensation and mirror/source applications. So they don’t necessarily need to be used in high voltage circuits. I use them for low voltage log/anti log circuits and they are great for that. These transistors also make good temperature compensation diodes. If they work in a low voltage application, don’t replace them with low-voltage types. These transistors are best replaced with identical parts, unless the schematic would indicate otherwise. If they are in a diode connection, definitely don’t substitute them.

[@waylonj4078]

I always seem to learn something new from you. Great Video this time. I am actually working on a Power mixer right now myself. It keeps blowing the 3A Breaker on the unit. Everything I check is good. Maybe I have a similar issue? Trouble is, this mixer is my own... and I am just about to throw it in the bin. I have hours and hours into chasing this short and still haven't found it. I even got lucky and was able to download the schematic for it. I'll tell you this, If shipping wasn't so dam expensive, this mixer would be sitting on your bench and I could just sit back and watch and maybe figure out how I have been missing what appears to be a dead short. :) Anyway, always enjoy your videos. Keep up the good work.

[@nirmanmulalic1824]

Sir, you’ve got the brain and knowledge. Congratulations!

[@the8bitbarn836]

Well that was cool! I never really thought about why many Amplifier Manufactures went all IC based Amp chips but now I know why! It seemed that most of the struggle here was finding the offset voltages and the current between the transistors, I found that the one thing helped most was that the other working circuit could be used as a reference in pointing you in right direction. Newer equipment isn't always straight forward in that regard. Glad you got it working. Great video! Some things got lost in English to English translation but all in all you are understandable from my time spent in Telford England. Thanks Again! Cheers!

[@ruimvp]

Thank you for the lesson. What I do first, after the visual inspection, is to test every single semiconductor in circuit before trying to understand the circuit. In this case it would helped finding that bad transistor faster, I think. Of course your good analysis after finding it is the most important. Check for "viewed all video without sleep" :)

[@LearnElectronicsRepair]

Yes I agree testing every semiconductor first would have worked. It's a useful technique though doesn't make for such an interesting video 😉

[@Gutterball]

I have a difficult time with accents and understand probably 60 per cent of what you are saying, I remind myself you are speaking english. Anyway, you infared tester is of great interest to me as this began the process of fault finding.

[@frankreiserm.s.8039]

I own my own electronics repair shop Frank Reiser Video/Audio Service. You sure know more than me. Great video!

[@marinvidovic763]

OMG ... You kept me 2 h + glued to the chair. Mystery case ! If you didn't find minor fault ... this thing will End up in a Electronic wast collection BIN (* - or in a landfill... ...), ...just due to 2 mm of broken coppar traces. 👏👏👏👏👏👏👏👏👏 !!!

[@mj12eben]

I think it takes longer to demonstrate a thought process and diagnosis as the obvious faults are eliminated. I learnt heaps.

[@christopherholmqvist321]

I'd watched it! Fantstic to be a part of your methods and problem solving. I learned a lot.

[@johnbaker26]

Damn nice job! This is the video that is going to keep me coming back to your channel

[@ExtremeBogom]

Yet again a quality video! Yes I did learn more things during this. In fact that whole thing about positive and negative voltages made that 'click' for me finally. I enjoy your fault fixing/repair videos as much as the tutorial ones. You're like Sherlock. Richard Holmes xD.

[@LearnElectronicsRepair]

I'm really happy you got the whole positive and negative voltage thing. That, to me, is one of the most important points of this whole video. And now you 'get' that it is such a big step forward that a lot of people never make. I am sure this understanding will help you a lot in future 😃

[@patodonnell2438]

Great video,I learned a lot about tracing open circuits.

[@elenium115]

Each episode is perfect. I learned so much from you.

[@LearnElectronicsRepair]

Thank you 🙂

[@mrdali67]

This is definetly REAL watts. My band blew one of these last year, and while getting it repaired we borrowed a brand new Mackie Mixer with quite the exact same specks, 2x350w in 8 ohm.. that is, the diff. Is the old Dynachord is a classic Class A/B amp, while the Mackie was a Class D amp. It also weighs a fraction of the Dynachord. And it showed. While using the Mackie we constantly ran at just below clipping, and it only delivered about half the sound volume. Just proved my impression of Class D amp designs. They may be able to somehow get an rms measurement of sometimes in the thousends of watts without weighing anything. It just dont deliver actually soundpreasure what you think the watts indicate. A traditionally Class A/B amp will always prevail. You never should under estimate a 30-40 pound 30 year old well constructed power amp of 200-400 watt stereo amp. They are miles better than most of the cheaper modern Class D amps sold today. These Dynachord Powermixers back from the 90’s was propably the best thing a small band could invest in back then. They werent especially cheap either but they had what you needed integrated in the mixer part and an extremely well build amp. They are well worth it to get repaired should they fail, instead of going out and buy some of the cheap powermixers today, if you still havn’t switched to powered speakers of a good quality.

[@stevedebeukelaer1424]

Hi Richard very nice fault finding, congratulation . The transistor if it works did it stil heats up now? I realy enjoy this one.

[@chuckbenedict7235]

I would have used the thermal camera much sooner as that 20-30 watts had to be going somewhere. But that said, the faulty transistor was not the one getting hot (if I remember right). And the burnt track...I would have never found that. That was incredible deduction buoyed with dogged tenacity. I learned a lot. Thanks!

[@ziggyironic]

Brilliant, thank you for taking the time to make this video and for sharing your knowledge. 👍👍

[@Silverlion49]

You did NOT go down a rabbit hole - you followed the readings and did a process of elimination till something made sense. Comparing one circuit to the other was logical, but the only clue I think you missed was the bad circuit NOT charging the capacitor - if cap was good then it did not have ground - all in all - GREAT JOB THAT WAS A TOUGHIE.

[@LearnElectronicsRepair]

Yeah probably I should have picked up on that a bit quicker - it was obvious once i had found the fault, but then that is usually the case 😉

[@gillesparisien4383]

Thank you from Laval in Montreal Quebec

[@LearnElectronicsRepair]

If anyone wants the schematic it is here. This is the correct schematic but I didn't realise at the time as the search option in my pdf viewer was not finding 'Q140' and similar search results. Thanks to Johnny Bravo aka check12 for his informative posts regarding this mixer/amplifier Dynacord Powermate 600 Schematic: www.badcaps.net/forum/showthread.php?t=106620 You need to register (free) on badcaps.net forum to download it - but if you are not already a member there it is a very good place to hang out, so highly recommended.

[@johnnybravo505]

Aha! ... that explains it ... yeah Dynacord/Electrovoice and some others save the schematics in service manuals as images, and of course you can't text search images (also they don't half cram a LOT into one diagram!). Not finding it though made for a MUCH more informative video I think ... :)

[@thill47]

you are a genius. Your knowlege is great.

[@RetroUpgrade]

Hi Richard hope you are doing fine . I see you are still working really hard :D keep it up you will make it far :D as for me i have had a lot to do and have not been able to post videos unfortunately the atari clone video is in the works :D

[@LearnElectronicsRepair]

Hi Carlos, yep running a YT channel takes a very large amount of time that is for sure. I'm looking forward to you posting some more videos.

[@Dark_Knight_USA]

Greetings: I was the guy shouting, but not loud. Ur methodology was OK. I was that way when I started vin the early 70s on Audio and Communications. Now I would have saved U N hour or 1.5hrs on the Vid, no telling how much real time. Those circuits typical. Ck around the outputs, if good then the protection. Another thing. I do not warranty outputs ( B it audio or RF) on a 2nd warranty without my verification of everything connected outside. I encourage my testing of such after the 1st repair. Operator error is not my expense. . I used 2 get that stuff a lott. Including the cheap junk worse than Dynacorp. I did not bother 2 cross the Mexican outputs. I figure them 2 B married 3055s. They want it fast and cheap. Sometimes I would have told them " U should have purchased good and expensive". Either way, I like these better than the newer junk. That was about the last of tolerable stuff. Not a bad show old chap. It was a refreshing trip down Memory Lane.

[@henrybartlett1986]

That was a hard one! Well done. I’m just starting so this was great to see, if somewhat challenging.

[@LearnElectronicsRepair]

Yeah this is the way it goes sometimes. If it is any consolation I also found this repair challenging

[@Mcfreddo]

Yes, I watched the whole video. Thank you!

[@Diebog]

I love your videos. A hour goes by and I don’t even realize it! The diagnosing out loud helps me understand what your thinking or how I can use the same principals. Keep up the good work! I’ve been wanting to offer repair for pcbs and had done so for a time but always had an issue with how do you go about figuring what to charge? Hour? by job? What do you do if you don’t or can’t fix the issue but have hours diagnosing? Just curious how you run the monetary side of a repair business. I realize you don’t make millions. But where is the fine line where you make a living but don’t overcharge somone? Thanks in advance

[@aarontyrrell2931]

Nice fix, wish I had that know how to throw in my back pocket!!

[@jeremywilcox]

Not sure it would have found it here, but a 10x eye glass is invaluable. In my old work environment we found a good, close, visual under bright lite found more taults than you would ever expect.

[@Puner54]

I Fell Asleep...Truly!

[@peterbrooks539]

Fascinating and very engaging.

[@allenfleckney5969]

Brilliant repair , you stuck with it , loads of techs wouldn’t , it was a mystery how the amp wasn’t reading short but the protect was kicking in , so I learned that protect can kick in with a rail down rather than fault , also good on you for desoldering items from the good side so the viewer got to see a complete comparison , I’m guessing the thermo camera does not have a sd record option otherwise you could have edited that into OBS ?

[@LearnElectronicsRepair]

A subscriber sent me the thermal camera and I just grabbed it by chance on this video, I'll make a proper review of it.

[@metusa666]

Excellent job thanks for the video always new to learn every time i watch

[@1dandandy1]

At 01:16:30 I would have swapped the right and left transistors to see if the hot followed the swap. I do not think it would have saved any time finding the blown trace but it is my simple minded way of thinking. Awesome video. Thank You. 👍

[@fredfabris7187]

I’m really enjoying your content. Please keep it up! I love anything audio and switch mode power supply. I don’t know what a GPU is😂, I guessing I’ll learn.

[@errolhibbert2390]

That's a good learning experience you have shown me, you are very good at probing circuits. I am always on this repair chanell.

[@twobob]

In the end a ridiculously thorough inspection of every millimeter of the board with a light would have found the issue. Typical. LOL. Now you will always be tempted to make such a gargantuan task on the off chance you don't have to wait a week. Haunting. Great catch. Nice detective work.

[@twobob]

I had just been checking out such a constant current source on "TheAudioPhool"'s channel if I recall correctly. He is worth a watch for some approachable basics of circuit design. Complex little beastie that mixer, near identical to oh so many other me-too mixers now, with more, or less, effort made.

[@LearnElectronicsRepair]

@@twobob Thanks for the info do you have a direct link to a video? That constant current transistor with base-collector shorted is a new one for me and I would like to understand it a bit better myself.

[@twobob]

@@LearnElectronicsRepair I was one of his many "current based" ones. However kzbin.info/www/bejne/r2rElI2CptqMgq8 This gentleman describes the effects clearly if slowly, demonstrating how the values become fixed.

[@twobob]

@@LearnElectronicsRepair books.google.co.uk/books?id=hB6kDwAAQBAJ&pg=SA1-PA61&lpg=SA1-PA61 another write up

[@lameckarika1949]

Omg that is the worst rabbit hole I have Good job. I would have chucked it away 90 min mark

[@1dandandy1]

Wow! Blown traces are the toughest. 👍

[@kerrycomm1702]

Thanks Richard good to learn 👍

[@davekendall9749]

800 likes. yes I did fall asleep for a bit but I still enjoyed it. may be a new career may be in order. I couldn't resist putting heat-shrink on those speaker cables.

[@richardw3294]

Logical thinking got you to gold.

[@johnnybravo505]

Nice vid, nothing wrong with the length :) ... very similar to the 1st time I worked on one of these (it was me that eventually replied to you late on Badcaps) ... I've seen a few of these now (Dynacord Powermates, Dynacord S900, S1200, Electrovoice Q44, Q66, Dynacord CL1600, Electrovoice CP2200, CPS2.8 etc.) are all same basic class H amp design with more or less output devices and added efficiency control circuits in the more expensive models. Just to add a little extra time saving info for others, if these amps are stuck in protect with NO blown output devices & NO DC on centre line (very common), the problem has always been in the small section at the amp output, in the overcurrent & DC/HF protect circuits (Left channel Q135 - Q145 or Right channel Q335 - Q345 ... note the component numbers can change slightly between models in these sections) or the 2x (Left either i102, i103, i104 , Right either i302, i303, i304 ... numbers change slightly for different models) TL074 quad op amps that control the limiter ... or both, I've seen a couple stuck in protect and limiter on with no input ... :)

[@LearnElectronicsRepair]

Hi Johnny nice to see you here 🙂 Thanks for the advice, it would have got me there a bit quicker if I had returned to badcaps to look for late answers. Would you be interested in joining the LER discord server - it's free and kinda like a cross between a forum and chat room if you didn't try discord before. There are sections for all sorts of repairs there, an active repair community and quite a lot of schematics too, everyone is welcome and knowledgable guys like you are always handy to have around. Learn Electronics Repair is now on Discord! Come and join the fun and let's fix stuff together, it's free and a nice place to be. discord.gg/vam6YC8vwU

[@johnnybravo505]

@@LearnElectronicsRepair Yeah that blown trace in vid was interesting, the video is good because it shows the details of a realistic process and all the real valuable info is in the detail. I get frustrated all the time watching edited vids where all the REAL good info has been discarded or even worse no conclusive outcome. I've seen a few blown tracks on these myself and carbonized pcb causing problems but that was different faults to the one in the vid. The last one I sorted was in protect due to DC on the centre line caused by a MICROSCOPIC carbonized pcb short between C & E on a MJE340 in pre-driver section, which I eventually found with funnily enough the same torch that you use in the vid (although I've converted mine to an 18650 and only need to charge about once a year now) LOL. I'll give the discord server a look when I've got time but I've never really got used to the constantly changing subject format & find it much harder to follow than a forum ... :)

[@mikeuk1954]

Fascinating stuff. Watched the whole video. 👍

[@goodsingers]

Thanks so much for a good explanation

[@davesdigitaldomain]

thanks Richard that really helped. Please correct me if I'm wrong but the way I would have approached this repair would be to inject a signal into the base of the preamplifier and then using oscilloscope to signal trace through the main amplifier. Would this have worked with this particularly repair? Looking forward to the next video.

[@LearnElectronicsRepair]

Probably not. The amplifier itself was working correctly (though your method would likely have proved that quicker than the way I did it by looking for an imbalance in the DC voltages (bias) on each channel) and therefore if the amplifier is good then the actual fault must be in the circuitry that detects an error condition (triggers protect mode). The protect mode circuit was quite complex and I worked out as much by intuition as anything else that the circuit protecting the 'good' channel must be monitoring both +65V and -65V rails amongst other things (which I proved was the case), and the circuit monitoring the 'bad' channel had no connection to the -65V rail, hence the erroneous fault condition and the amplifier stuck in protect mode. Once I got that far it was not so difficult to find the problem and fix it.

[@davesdigitaldomain]

@@LearnElectronicsRepair thanks Richard for your detailed reply. I see what you're saying. I did learn a few helpful tips and tricks . I really want to learn electronics repair and your channel is the best on the Internet and I think a lot of guys that watch your videos would say the same. I look forward to your next video

[@johnnybravo505]

@@davesdigitaldomain Hi David just to add to what Richard said, no signal tracing would not have helped in this case ... when these amps are in protect audio signal only gets as far as the last NE5532 op-amp (i101B Left channel & i301A Right channel) before the pre-driver section & I've never seen a fault up to that point. Signal tracing becomes more useful if you get the amp out of protect and there are still issues ... e.g. with the limiter circuit (which is quite common with these). Richards method is about as good as it gets under the circumstances here because these amps have a complex and easily triggered protection system, which is good because you don't see many with blown output devices but can be frustrating and time consuming to diagnose.

[@thisismyuniquestory]

@@LearnElectronicsRepair INTUITION Richard comes to the fore. Your insight helped that customer greatly as your watchers have agreed. No landfill and no stitched up customers. Best wishes

[@thisismyuniquestory]

@@johnnybravo505 yep Johnny. I've been around since 1963 and a tech school full time school education 13 16 in the UK. A system of education thought up by a Civil Servant by 1943. A man, Mr Beveridge, probably a Liberal Party politician. He had great insight expecting the Allies to win the war. The Tech Schools system was for boys who failed the Grammar Schools entrance at age 11. Britain dumped the greatest aid to the UK economy with kids equipped to start work with three year's of workshop experience. Technical Schools were Building, Woodwork and Metalwork, I still remember the forms of building design of the Parthenon in Athens, Doric columns, perspective from the age of 13. Teachers who had fought a war and survived. Each tech school was related the local industries which for me was Westland Helicopters Ltd. Accordingly my first proper lathe after a Myford with a 600mm bed was a brand new Colchester with an eight foot bed, maybe longer. Intended for use by Westland apprentices manufacturing aircraft part's, jigs and tools, including machining rotor blades. I didn't take up an offer of a Westland apprenticeship because the training for your classified trade was decided by the firm not the employee. I must have done something right even if I chose to work in electrical maintenance as an apprentice, the most modern tannery in the world. The advent of electronics in general manufacturing and the purpose built electrical panels were built on site, so panel design his stuck with me into retirement. I may not have chosen aircraft manufacture for a career option, working for a manufacturer of industrial heating at 21, PowRmatic, a US company with no lines of demarcation. Heating, ventilation, air con. Working as a company service engineer allowed me two new company cars a year because of the high mileage required. The access into hundreds of MOD sites, the Army,, the Navy and Air Force, so many famous locations for work, thinking back over many war's and peace time by the seventies it was still the Cold War which was creepy seeing what was planned in the event of a nuclear war.

[@mtoddvan]

Lol some shouting, but enjoy the depth of explanations you go through, interesting to watch you progress 😉 through the circuits.

[@MsKelvin99]

my new favorite channel

[@peterlarkin762]

Brilliant! Nice diagnostics.

[@Sv3TL10]

Watched without skipping a second, very interesting and useful! I want to ask a question, maybe stupid one, but how is the lamp connected and how it is preventing a damage?

[@LearnElectronicsRepair]

It is in series with the mains and it prevents damage by limiting current. I'll make a video about it.

[@Sv3TL10]

@@LearnElectronicsRepair thank you :)

[@00Skyfox]

Great work on this project! I wish I could send you my function generator for a diagnosis. It's been dead for years and I can't figure out why. It powers up but it has no output, and tracing backwards from the output BNC I can't even figure out where the signal should originate from.

[@g8sbs944]

wow what a job i watched it all and did learn a few things thanks great video

[@LearnElectronicsRepair]

That's great to see you learned a few things on the way. Thank you.

[@kriswillems5661]

Great fix. Not an easy problem.

[@kriswillems5661]

I think the small transistors are a DC fault protection circuit for the speakers. They switch off the relay to prevent damage to your speakers should a DC voltage appear on your speakers. The relays also have a delay, so you don't get popping at start up that might damage your speakers.

[@LearnElectronicsRepair]

Yes, you are correct. I eventually figured that out and if you watched the video all the way through you will know that this is where the fault was (a false protect signal)