How To Diagnose Faults In Transistor Circuits Part 2 - A Practical Example Samson TXM16 1000W Mixer
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- เผยแพร่เมื่อ 30 พ.ย. 2022
- LER #332 Here is Part two of this repair guide on Transistor Fault Finding and Diagnosis! If you watch this through you will learn a LOT about Transistor Circuits and HOW TO FIX THEM. Skip parts and you are gonna miss important stuff! Enjoy.
Mentioned in this video:
How To Diagnose Faults In Transistor Circuits 1 - A Practical Example
• How To Diagnose Faults...
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Richard
Very interesting repair project with lots of caveats, as it usually is! Any part 3 of this coming up? :)
I thoroughly enjoyed this and part 1, would love to know the outcome and if there will be part 3? It would be good to see this mixer powered up and working :)
i just found your channel last week, love it!!
You have the patience of a saint!
I do appreciate you explaining how these circuits work. With a lot of other channels, it's like watching an episode of Star Trek: "Looks like the quantum flux inverter got oversaturated with omicron particles, so we just need to depolarize the verteron emitter in the usual fashion. And there you have it, it's working again!"
That sounds like the channel SmarterEveryDay the man just talks shit for 20 minutes and doesn't explain any of the terms he is using lol.
You will love this
th-cam.com/video/Ac7G7xOG2Ag/w-d-xo.html
Surely I would replace both trimpots - either 470R or 500R - as they are mechanical parts with press-fit connections for end-points and a sliding for the mid terminal.
And I would maintain the Series Lamp (50W or larger/parallel) until things work ok for a few dozens of Audio Watts, or ‘Magic Smoke’ may appear as bad a Xmas gift.
Thanks for posting and sharing and a blessed Christmas!
I really admire the efford you have taken to analyse and try to find the fault. As I work for a company wheren we also repair audio equipment, I often am held back in spending time to find the actual fault. Therefore many devices are not repaired due to time costs. But with some love are repairable. Keep up the good work. Have subscribed!
Nice video thank you waiting for part3
Heya, nice vlog again. yes I would have loved to seen it blown (never seen that) so that would be a trill
Just started watching you a few days ago. I am a novice but your in depth descriptions and hand drawn basic operation schematics are a great help. I did notice aprox "+"20V on each side of the blown chnl. Maybe a clue to floating voltage if im not mistaken. Thaks for your chnl.
In an amp like this, I would really first run the system through a bench-PSU with current limiting and reduced voltages (say instead of +-90V) I would run it at +-30V until I have a fairly high degree of confidence that the whole amp works, but maybe that is just my style of fault finding and repair. It never is nice when things blow up when you are attempting repairs, but it can and will happen. Thoroughly love your videos, and I learn quite a bit, esp. with more modern stuff like those nee laptops.
Damn! You left off just where I always quit haha... Wanted vicarious satisfaction. Hate damn power amps.
I feel your pain ! kudos for being calm, I hope the customer wants to repair it. I noticed there is a thermal device on the left hand side of the board, if this isn't coupled could that be a problem ? but I think you're right about the output devices personally. Regarding the long tailed pair I just imagine an op amp with its pos and neg inputs and treat them like that...cheers.
NEVER power up an amplifier or any power device with the Transistors unsecured to the Heat sink it WILL kill the outputs Almost every time The only reason you got away with it at first is it had no Bias with the Voltage amplifiers decoupled . Ive repaired Vintage receivers and amplifiers for nearly 40 years and i learned at a very early stage this lesson .
feels good to know things can blow out in pro hands too ahah
Anyone interested in a more detailed explanation of the 'long tail pair plus CC source' input circuitry to the amplifier could do a lot worse than watching this video recommended by subscriber Spixy Cat. th-cam.com/video/mejPNuPAHBY/w-d-xo.html It serves both as a very good explanation of the circuit operation AND a perfect example of why I often simplify the circuit descriptions in my tutorials. As a repair tech you often don't need to know exactly how a circuit operates, you just need a basic understanding of what it does (or what it should do) and what to expect on the input and output
Great stuff!! Love your channel
Dang it! This was your chance to out do Electro Boom on the explosive sparking special effects! I found this video seriously interesting and I learned several things which I suppose is the point, right? Well done. Note to self, Fusable Resistors ;)
😊
I had a similar amp and gave up in the end. Your circuit explanation will make working on another much more viable. Thanks for the analysis / explanation.
the board from hell LOL thanks for all the good videos ;)
Hello from Norwich , Norfolk
Great video lots of good info thanks look forward to future repairs like this one .
Are there any markings or indicators on fuse-able resistors if you don't have a schematic to show which are wire wound or carbon standard resistors ?
Hi Richard! What I still did not get yet is why the protection kicking in when touching the drivers base tells you it is floating. Imho would be great to have a more in depth explanation of this in the video.
If I understand the following is the reason: The protection kicks in when the output is not at zero. This unbalance will cause d.c. to flow through loudspeaker which needs to be avoided. Touching the base of the driver to a multimeter set to diode will turn on hard the transistor causing an uncontrolled rise in current unbalancing the output and causing the trip.
OK, but he's using a scope there, not a multimeter in diode mode.
Anyway thanks for your comment since I think I got now what's the reason: Given Q124 (the driver of the PNP output devices) has its base connected correctly, it is somewhat biased, hence feeding -90V from its collector to its emitter and (via R144 and D114) also to the emitter of Q123 (the driver of the NPN output devices). So touching the base of Q123 with the scope (i.e. connecting it to GND via some MOhms) is enough to bias Q123, which in turn biases the PNP but not NPN output devices, i.e. generating DC on the output so that the protection kicks in.
Question: When explaining the constant current circuit, can you tell me why diodes are used instead of a straight forward voltage divider (2 resistors)?
i gurss beacuse of the tolerances of the resistors. with diodes, you make sure the voltage is constant.
Have you got a part 3 for this? Did the guy decide to go ahead?
This is a dc coupled cascade amp. The best method is to check and recheck EVERY PART OUT OF CIRCUIT. When this circuit fails - it fails in a cascade - you only get one shot to PROPERLY fix it or you will lose most or all of the parts replaced. This can get very expensive and time consuming. You have to check everything - predrivers, bias circuits, balance circuits,driver circuits, etc. If you don't --- you will let out the magic smoke. If you can isolate sections of the circuits Vcc and -Vcc and when testing with power know the voltage expected. Use your variac and monitor the Vcc and -Vcc current. The dimmer will NOT HELP.
Let the magic smoke out, it wants to come out and say hello
This is exactly what I was thinking, but since both channels are behaving exactly the same, I’d start looking at the regulated +/- 15 VDC busses. The only other supply, is +5 VDC, but that’s for logic control. This is assuming, however, that the transistors are good, the owner hasn’t had a crack at it, and the transistors were put back into place, correctly. If a customer mentions ANYTHiNG about taking the cover off, normal troubleshooting steps don’t apply, until you’re sure that the customers didn’t create any problems, or you’ve corrected what they’ve done. But, yeah, you’re right. Even a leaky transistor, in an early stage of an amp, can cause it to go nuclear, when power is applied. I have had a few amplifiers, over the years, that show a LOT of emitter current, until the Variac has been raised beyond a certain point. (~60 to 70 VAC, with 120 V mains.) Those are rare exceptions, though. I’m only 15 minutes in, so I don’t know which direction this will take, but I’d start by checking those +/-15VDc rails. What are the odds, that both channels have the same exact problem, and not have anything to do with the regulated power supply rails? I guess we’ll see! 😃👍
Here’s a link to the schematic, if anyone wants to play along, at home: diagramas.diagramasde.com/otros2/TXM16.pdf
It might be magic smoke, but in this case it is EXPENSIVE magic smoke and gets MORE EXPENSIVE every time you don't find all the faulty parts. FIRST HAND knowledge and cost of - "smoke."
Willliam J
Yeah this is a problem with power amplifiers. I fixed quite a few, normally if the light bulb comes on then goes fairly dim, the amp will be OK once I take the light bulb out of circuit. As you can see with this one, once I replaced the open circuit 4.7 ohm resistors, the light comes on bright then goes dim, but not quite so dim as it was without drive to the output transistors.
Can you suggest which components I should have tested and didn't? I show everything I did during the two videos on this amp. The 15V rails were tested in part 1.
I had just removed and tested all the transistors and diodes on the channel that failed again. Note on the video I then mention I turn the input signal (coming from the sig gen) down lower - on the mixer sliders - (so just a couple of LEDs on the VU meters are lit). I also don't have any load (speakers) connected to the amp either. As the light bulb was fairly dim at this point, it's 65W halogen, I guessed the amp plus the mixer were drawing about 20W - which should be OK to take the limiter out check the wave forms before I put everything back together properly and drive it harder. From past experience this method has proved safe...
And for a minute or two it worked, then something clearly gave up on one channel but the other was OK - both were driven by the same sine wave from the mixer. I strongly suspect it was one of output transistors on the PNP sided that failed, as these were the original ones that somehow survived the initial fault of shorted PNP side drivers - if you saw the first part video
th-cam.com/video/JokusaT7eok/w-d-xo.html
I will take your advice and use my Variac next time 🙂 Also I'll fit just one output pair per channel at first, until I know the amp is basically operating correctly. All in all though, yeah it is a bit of a hassle having to replace the blown components, a few of which also failed the first time, but it makes for a very educational video, yeah?
@@Willam_J
The mode selector was in BRIDGE MODE when observed. This makes it 2 for 1 - so you even have to check the mixer, preamp, etc for both channels and it would be good to scope the dc power supplies for ripple. Somewhere the fault circuits are getting triggered
Great video. I have been following you for a while and am trying to learn these stuff. Where do you get your schematics from
Fair costs
I take it when you are repairing amps, it is not uncommon to take a step forward and then few steps backward? If you run on low current until you verified all the repairs would that prevent the burnout?
Great video and explanations as usual. Did the client want it repaired? ☺
When you discovered the fusible resistors blown about half way into the video, I was hoping you were going to balance the trimmers (emitter) before installing new ones. Not sure if it would cause the meltdown but might be worth thinking about.
@Wobble Wall Yes several others have suggested the same thing. This will be a topic for part 3
Is there a part 3?
And i was looking for that transistor checker, what model?
Dont you just love DC coupled amps?
About as much as switch mode power supplies 😡!
@@davidv1289 Yeah!!!!!!
No, its a love hate relationship and I love to hate them. :-)
Note that it is better to change all the output transistor because they are driven in parallel: that means that if one of the 3 transistor is much slower (or worst case one is faster than the other 2) means that the faster 2 transistors are each getting another say 50% load from the slower. So it is important to have identical transistors which is often achieved by using all new ones. I hope you understand a bit of the explanation
Hi @Spixy cat, Totally agreed. None of the original output transistors tested faulty when I was first asked to repair the amp (which was total short circuit/partial short circuit in three of the driver transistors, two PNP one NPN) th-cam.com/video/JokusaT7eok/w-d-xo.html
So I left the output transistors in there and I think that is ultimately the cause of the second failure
Next time I test it though, I will fit just one output transistor pair per channel until I am happy it is OK to fit the other pairs and test it under load. Keeps the cost down, which I guess you will also agree with 😉
@@LearnElectronicsRepair yeah that is also a smart way to fit just one for testing.
I watched this channel that explains the combination of the long tail pair and current source combination quite well: th-cam.com/video/mejPNuPAHBY/w-d-xo.html
But i have to admit: you're doing a pretty good job explaining it also. Every explanation explains it a bit differently which helps to understand the concept a bit better.
Cheers!
@@therealspixycat Cheers I will take a look at that link. I agree with you that looking at different explanations of the same thing often causes a 'Eureka' moment and then the whole thing falls into place and all the previously slightly puzzling explanations suddenly make perfect sense
37:03 - I would think that the scope would not load the circuit to the point of engaging the protection circuit. Am I missing something here?
I've seen something similar - I think in the previous video you had moved the bias current setting pot, perhaps wasn't in the same place, and had some overcurrent thermal runaway. Had it on a B&O Beomaster 5500 with some not perfect output transistors - could watch the current climb slowly and then very rapidly and if not caught in time, bang, stuff goes pop :) May be worth using thermal camera to spot such things before the component cost increases... IMHO, E&OE etc 😜
That sounds quite possible, also IMHO, two cents worth, etc 😉
@@LearnElectronicsRepair Particularly as there was also an input signal which would speed up the heating process somewhat opposed to quiescent :) I hope the chap feels willing to replace the transistors - either way it'd be nice to get this one working again just for the satisfaction factor 😌
@@budgiefish Yeah I agree - actually I am happy to forego any further labour charge, just the cost of parts to pay. I feel that is the decent thing to do.
37:48 How would putting a high resistance meter (10 to 20 Mohms) activate the protection circuit? I would think the impedance of the meter would be way too high to introduce a problem!
The trimpot sets the bias current, not the balance. If it fails or is removed the output bias current will be small or zero, which cause non-linear amplification, which must be compensated for by the input stage.I am not sure it this could cause your output transistors to be overdriven - probably not.
Love your video's. I am a hobbyist & i have the same Sampson mixer amp. & love the way you explain Things. is there a chance you could send a copy of the schematics for this amp
Just got all the parts delivered & using your method for fault tracing am ready to dive in & the schematics would be a great Help. keep up the good work. I have subscribed & will be sending a donation for the service you are giving. keep it up.
I would like to see a repair of a power inverter. preferable 24 volt to 120 volt.
I very much suspect the 470R trimmer (VR101) you didn't changed. It is on the base of the first transistor (Q120) in the chain which blow up. All transistors in front of it are okay. So may be the trimmer is nearly open as well which would cause a high voltage on the base of Q120.
@THOM Brick That is quite possible, though it tested OK in part one, or at least the resistance was less than 470R to the wiper and it changed as I altered the position. I'll just change it ready for some new transistors, no point in not doing that really, I probably should have done that already. Still, we all learn as this goes along. 😉
I haven't missed part 3 have ????
at timecode 57.07 i think it happend because of the bias not right as you changed the vr 101 and you have not adjusted it to the right value
Dutch Linux Yeah this has also been suggested on other comments that this trimmer was already intermittent/faulty and I should have changed it when I changed the other obviously open circuit one on the channel that then did not blow up. It's quite possible these trimmer pots were the cause of the original failure of both channels. Seems like it's 'live and learn' on this video, self included, which makes it so much worth publishing when other channels may not have done quite possibly because they are afraid of looking less than perfect😉
Nothing worse to work on than a dc coupled amplifier, as others have stated. There is no dc isolation between any stages, so one fault throws the who circuit out. So it can be as they say removing every component and checking them. You might get away with breaking the feedback and applying a dc voltage in its place to the long-tail pairs.
Yep.
Guessing… just guessing… the owner decided not to go ahead & wanted the mixer back 😂
Ouch!
part 3 ?
looks to me like the four 47ohm resistors were designed to fail first in order to save the outputs
3hr estimate $1000.00 repair!
I just want to know...What did your wife (or lady in front) do when it sparked?
Actually, Missus LER was not in the office at the time. Of course, if she is present for this sort of occurrence, she does the only thing any self-respecting wife would do, and falls off her chair laughing! 🤣😂🤣