Small Switch Mode Power Supply Repair Guide - Consumer Electronics SMPS How To Fix
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- čas přidán 3. 07. 2024
- LER #247 Let's have a look at the switch mode power supplies found in many consumer electronic devices, DVD players, Stereos, Radios etc. Learn how to diagnose and repair these circuits
00:00:00 Intro
00:00:59 Initial Tests
00:03:02 SAFETY FIRST!!
00:08:31 Reverse Engineering the Circuit
00:21:48 Understanding the Circuit
00:29:14 How It Works
00:38:41 Let’s Fix It
00:43:33 Epilogue
NOTES
at 26:06 This is NOT Chassis ground! This is hot ground, which is our reference point for measuring voltages on the high voltage side of the power supply. Do NOT connect yourself or any grounded equipment to this point!
at 31:12 The reason for the 100 ohm resistor is not to act as a voltage divider in series with the 1M resistor as I explained in the video - it is actually to limit the current flowing through the Zener diode which would otherwise be destroyed
at 36:23 If this capacitor is faulty it can cause a whole range of fault symptoms - as mentioned in the video: The faults include...
Failing to start
Starting once then shutting down until you disconnect mains power and try again
Continuous pulsing/chirping
Starts after several seconds/minutes
Only Starts when cold/hot etc
Unstable output voltages
Other common faults not mentioned in the video include
Short circuit bridge rectifier diode(s)
Blown Fuse
Short Circuit or faulty PWM Chip
Open Circuit Transformer Primary Winding
Shorted turns on the Transformer
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Thank you
Richard
Woww...the reverse engineering section is just brilliant..I had to pause the video to complement your work. Thanks.
Such a breath of fresh air! There are so many amateurs pulling stuff apart for clicks and giggles but precious few putting them back together. Every quid saved by not having to buy new electronics is a quid not going to some foreign corporation. Your video was also very easy to follow.
Richard, that was one of the best explanations of SMPS operation I have seen. Thanks for explaining the bootstrap process so clearly.
Hi Richard, although I do not understand all of it yet, you make the circuit board easy understandable by showing us to recognize groups of components and what they are doing. It also learns how to read circuit diagrams. After each of your videos, I am looking less clueless at a circuit board. Good learning video thanks for uploading.
Having spent ages trying to explain boot strap circuits to some one i am now going to send them the link to your video.
you explain things far better than i can
Glad to be of service Rob 🙂
excellent video Richard,the DVD player is not worth repairing when people have the mentality of just throwing away and buy a new one because it's cheaper . But to people like us it is worth repairing because we enjoy doing it, it's made a good educational video and it keeps it out of the landfill. Look forward to more repairs like this.
Absolutely - I always get a high when I fix something and that feeling of achievement can be worth more than the item is 😀 True it also made a good educational video. If you liked this one (and didn't find it pointless) wait till you see tomorrow's repair! 😛
Loved how you drew on the PCB this time, I’m going to do the same when figuring out what’s what - 10/10 as usual - keep it up :)
Fell asleep watching this and when I woke up I fixed a SMPS. Bad cap in the reference portion near the optos :)
Welll done Richard…the effort and method you employ in these instructional videos is outstanding. Thank you Darrell 🇨🇦
Loved it. Thank you taking the time to explain how everything works.
I do like how you explain the operation of the circuitry so as to find the problem. That's troubleshooting for me. Love these videos.
best tutorial content on youtube by far.
You have given a good insight into how a switch mode power supply works. Good stuff.
Thank you. I like and enjoy things more the way you explain and draw things out. So glad I found
You and the Discord program im learning alot .
24:21 - I appreciate that at each point in any of your videos where you discuss the main DC capacitor you remind note the severe shock hazard that exists if it has not been discharged first (and each time after powering down the supply). We all need regular reminders of this.
Thank you so much for making these videos! You are a real source of knowledge! From a Spanish living in England.😅
Absolutely briliant explanation, thank you 😊
Another good instructional and interesting video from my favorite electronic repair man. keep up the good work.
Wonderful explanation.
Excellent instruction...The most informative usefull ...and entertaining
Very good video !!! Enjoyed
Thank you for making looking at faulty PCBs less intimidating .
Just keep at it and you will find you start to recognize building blocks of a circuit rather than just a PCB
@@LearnElectronicsRepair Thank you
Another great video - very informative Richard
Yet another superb explanation video. Top man.
Nice overview
Thanks and thanks again 🙂 I finally starts to understand how it works.
I really enjoyed watching this video. I also liked the way you explained it all thus found it very interesting to watch. Oh by the way your pen might need fixing next.
You strike me as a very knowledgably guy in the field of electronics and the way you explain things will make me watch all your upcoming videos to come.
Great job Explaining SWPS :) Thank you
Richard, excellent video! I've been watching numerous videos on switching power supplies. Trying to figure out how they work so I can fix one that I have. This has been by far the best video and you even mention a problem with a pulsing output which is what I have with my power supply. You are the first one that I have found to mention this issue. I have a 45 volt power supply for an LED light fixture and the lights constantly pulse. I've checked the output and it is of course pulsing. This particular power supply has a lot of components in the section you mentioned that would most likely be the issue. I think they have this power supply setup to run at multiple voltages so they have a lot of resistors in parallel and other components in parallel that they can either use or not. I did replace the capacitor in that section, since I had one on hand, but that did not fix the problem. Before watching your video, I ordered some MOSFETs since I thought it might be the MOSFET causing the problem. Now I'm thinking that was probably wrong. So far I have not found a completely failed component, but I guess I need to check the rest of the components in that control circuitry. Unfortunately I don't have any obviously blown out components like your board had.
Your the man, it's similar to washing machine main board power supply , must try and use this info on repairing those, thankyou for posting
I was given a sound box and all it was was two capacitors but it was fun repairing it and it still working . Thanks
I really appreciate your videos. I have been tinkering on electronics for years, and my work involves higher voltage equipment, but seeing your videos has given me a greater understanding of electronics, in general, and smaller applications, in particular. With that being said, can you do a video on repairing a Chinese made bench power supply? Keep up the great work. Between your channel and Mr. Carlson's Lab, I may just learn something.
A master class!
👍👍👍👍👍👍👍 the best thing on youtube!
I remember back in the early 70’s my boss asked me if I could teach some young buds basic electronics! I said ok no problem. They were
business machine engineer apprentices and all that type of technology was going electronic at that time so it was something new to them!
Well done, thank you. BTW interesting DVD 😂
Thanks again 🙏
This video is about my favorite of yours so far! Right up my alley with consumer audio electronics and common power supplies. This is the level where I'm at also in terms of learning, and THANKS for reverse engineering the schematic and explaining it by physically showing and also how it works on paper. I wish more people would explain using both methods. While the repair was simple, how you got there and what to look for along the way is what makes me enjoy 'troubleshooting', as opposed to the shot gun approach of 'just' changing capacitors. Top stuff!
These videos I come back to often and watch a few times to understand smaller details along the way.
I'm fascinated by the ability of people who can 'reverse engineer' a circuit just by looking at the board, I'd love to learn how to that more.
A pity about the 'Puss in Boots' DVD! Ahaha! Maybe one day you'll open one with 'Debbie Does Dallas' or something. :)
Thanks again for the video,
Rick.
The reverse engineering thing is a skill you can learn. Just keep at it, learning electronics and how components/circuits work and you will get there I am sure. Reaching that point is something of an epiphany and this is where you should be aiming to be as your knowledge of electronics improves, When you get there I can guarantee to will get an amazing sense of achievement. Enjoy the ride on the way,
Rich
@@LearnElectronicsRepair Thanks for your reply good Sir :)
Great video and explanation of how it works, would love to see more in depth troubleshooting for such a SMPS, many times I find all caps and diodes are good and I don't know where to look next, not sure if the IC is working or how to measure it.
Hi Richard, I got one of the Kaiweets meters through the link you gave when you reviewed them. Nice little meter. Just a tip if you don't already know, hold down the red button and it goes back to Smart Mode, just saves scrolling through.
Thanks for the info Pete - continually pressing the button to get the range you want is a bit of a pain
Thanking you sir
That reaction to the click is priceless ! You just never lose it ! It takes years of occasionally getting electric shocks, burned or something blowing up in your face the moment you switch it on to develop that response.
Linford Christie couldn't move as fast as that !
101% true - that is why I left it in rather than edit it out. It's a valuable lesson IMHO but also something you either develop for yourself or you just keep on getting zapped. No other way to describe it to anyone who doesn't already have it LOL
@@LearnElectronicsRepair Back in the 1980's I was working in a lab and got distracted and a few minutes later when I came back to the job inadvertently connected mains 240 v to the input of a 7805. Instantaneously there was a loud bang/crack and the body of the 7805 bounced off the ceiling and scattered cross the lab. The three pins were still in the board !
Of course I wasn't wearing safety glasses and if I had been looking over the board I could have lost an eye or worse.
As they say, what doesn't kill you makes you stronger - or at least it frightens you enough to develop some sense of self preservation.😁
I would say to anyone getting into electronics - before you switch anything on just give it a once over in case you have done something stupid.
@@we-are-electric1445 Yeah I used to work for ICL back in the 80's in their Central Spares And Repairs workshop - with about 100 other engineers all doing component level repair. One of the product lines I worked on were large linear voltage regulators with banks of 2N3055 (or similar) type transistors on huge heat sinks and very large electrolytic capacitors the size of a Heinz Beans tin. This kit was already some years old and was built before switch mode PSUs were common. As was normal, there was a particular work bench for each product line and this one had huge linear transformers under it, some current trips (like 25A or 50A or something) and a socket where you could plug the board under test.
I had been fixing these for a couple of days, then plugged one in and the current trips jumped out. I reset the trips and plugged it in again - they tripped out. So undeterred I threw the current trips to the on position and held them there..... the thing hummed like an electricity substation for a few seconds then one of the large electrolytics exploded with a very loud bang! As I sat there under this huge mushroom cloud with little bits of white paper like stuff raining down on me like confetti, the cheer that went up from the rest of the workshop was louder than the explosion! Of course it was all over the canteen and I got ribbed about it for a couple of days.... until......
There was another guy working on the high speed 'band printers' that could chuck out about 20 lines of computer paper a second (the paper was about 18 inches wide, had green and white lines on it, and sprocket holes down the sides and perforations so it neatly stacked itself into huge reams of paper). Anyway the 'band printer' had what can only be described as a large rubber band, wider than the paper, and fixed to it were all these lollipop stick type things. Each stick had one letter or number/symbol on the end of it (A-Z 0-9 repeated many times over and over again across the rubber band). Between the lollipop sticks and paper was an inked ribbon and in front of the lollipop sticks were these hammers (solenoids), one for each character position on one line of paper.
So you can imagine how this worked, the rubber band rotated around two drive pulleys at high speed, the ribbon rotated on spools at a lower speed, and as the appropriate lollipop stick passed each hammer, it would hit it and print that letter. So basically after a short distance of travel, every possible character would be available for each hammer to strike once, and it had then printed an entire line of text, so it would then feed up the paper for the next line and continue.
Now this thing screamed like a banshee when running so it had a big soundproofed cover over it. So one unfortunate guy was working on it and had some hard to find fault on a control board so he was probing around with his meter and had the screaming banshee running with the cover off (annoying everyone else) so he could take his measurements with the machine powered up. Anyway of course he slipped with the meter probe and shorted some pins on a logic chip.
Suddenly there was this sound like a rail crash and thousands of lollipop sticks came flying out if this thing like a swarm of locusts on a biblical scale - some landing many meters away in every possible direction!!
Well after that the banshee somehow realised it was dead and stopped moving and there was this sudden silence that fell across the workshop for about five seconds.
Then we all did the only thing a self respecting engineer could do in that situation... everyone fell about the place crying with laughter and giving the unfortunate engineer more 'stick' than he had already had in his hair, clothing and god knows where else. It took about 30 mins for everything to clam down properly, and I had an easy time in the canteen after that as my little incident was clearly forgotten! The banshee incident would be raised in conversation every so often for years afterwards.
Working at ICL were some of the best days of my life.
@@LearnElectronicsRepair Aye ! Those were the days.
Every time I blew something up I was praying for someone else to have a bigger 'event' ASAP so mine would be forgotten.
Trouble is, some of the 'events' pass into fokelore and get trotted back out at Christmas parties !
😂😂😂😂😂😂😂😂😂😂
Thanks
upload a compilation of those reflexes ))
A converter like that rarely runs at more than 200 kHz. Most will be in the range of about 100 kHz to 150 kHz.
The common mode inductor or "choke" is not for keeping mains noise out. It is for keeping noise from the switcher "in."
With a flyback converter like that I would replace all of the output capacitors. In such a converter if one capacitor ages to high ESR the other capacitors on other secondaries tend to get overstressed with excessive ripple current due to the way current gets distributed among them. If the "main" capacitors can't accept the charge, more of it gets dumped to the other, typically smaller caps, overstressing them - exceeding their ripple current rating. This leads to rapid aging and even though they may show no overt signs of damage they may well be on their last legs. It takes very little extra time on a single-sided PCB to pull and replace all of the caps on the secondary.
No such thing as a pointless repair 👍🤓
41:12 My thoughs on cutting the leads before or after soldering are as follow :
I prefer to cut them before soldering. Why? The leads on most electronic components have a layer of anti-oxidize over the blanc material of the leads itself.
When cutting the leads before soldering, the cutoff surface is not longer protected against oxidizing , but wil be sealed off again after soldering.
If you cut the leads after soldering, you can solder the cutoff ends again, but then you have to solder each component lead 2 times, and this takes extra time,
especially if you have a big board where you have replaced lots of components, or doing prototyping.
And i find soldering the cutoff ends gives also a smoother look and cover any sharp ends. That's my 2 cents ;-) Thank you for the nice tutorial. Grtz from Belgium.
Thank you for your great informational videos. I have been trying to get into electronic repair without having to go to an overpriced school to learn a new hobby. I have learned a lot of information off of your youtube videos but some how managed to miss one important note. Where exactly are you supposed to ground off to test current voltage? I noticed different readings between neutral and board ground. This is for a PSB board in a vacuum cleaner. (Test subject) I hope to get better with electoral device repairs and will continue watching any content you put out. Keep the great work
It's a good idea to clean the flux off those joints. I have seen some occasions where it works at first, but then forms a path for current and starts smoking.
Also the glue they sometimes use to hold down the inductors or large capacitors (to help prevent them breaking away from the PCB due to vibration or physical shock) goes dark brown.black and becomes conductive. This is something else to look out for.
@@LearnElectronicsRepair Yamaha products often had this problem.
where the caps. bad, or did something else in the supply make the caps. go bad? how do u know
Cool...
Please keep in mind, that I’m a NOOB at electronics. This was yet another great video but honestly, at around the 27 minute mark, you completely lost me. It’s not your fault, it’s just my insufficient knowledge of the matter. But I watched it through, because you make it sound easy and not everybody can do that. So, please keep making videos because with each of your videos, I learn a little more - and that why I keep coming back. Cheers!
Hi Max. This isn't the easiest circuit to understand but you find it (or a version of it) in almost all switch mode power supplies so it is a very important one to get your head around. Let's try it again from the basics:
The controller chip that drives the transformer needs power to work. This power supply voltage is called Vcc.
When you first power up (and the power supply isn't running yet), the 1 Mega Ohm resistor connected to the 320V from the incoming mains supply, charges the little 10uF 25V capacitor - I made a video 'all you need to know about capacitors' if you need to learn about capacitors first. There is also a Zener diode which prevents the capacitor from over charging - again I made an 'all you need to know about zener diodes video' if you need a refresher course.
*Capacitors*
czcams.com/video/R9slJUqT6Ls/video.html
*Zener Diodes*
czcams.com/video/trjbHhLERic/video.html
Once the 10uF 25V capacitor charges up to around 15 volts, the controller chip turns on and starts working, so the controller chip starts to draw power from the 10uF 25V capacitor. The controller chip then drives the switch mode power supply which starts running and you get output voltages on the secondary windings of the transformer.
WHAT HAPPENS IF THERE IS A FAULT CONDITION
If the power supply does not start after a short period of time (due to some fault) the controller chip stops running because the 1 Mega Ohm resistor can not supply enough current to keep the 10uF 25V capacitor charged and the controller chip running. Basically, the controller chip is drawing more power from the capacitor than the 1 mega ohm resistor can provide to recharge it, so the 10uF 25V capacitor gradually discharges and when the supply voltage drops to about 9V the controller chip switches off again and stops drawing current from the 10uF 25V capacitor.
At this point the 10uF 25V capacitor starts to charge back up again via the 1 Mega Ohm resistor and when the supply voltage (Vcc) gets back up to 15V the controller chip switches on again. This property of switching on at one supply voltage (15V) then not switching back off until a lower supply voltage (9V) is called Hysteresis. The power supply will keep cycling on/off like this and often causes a chirping noise or you see the power supply LED pulsing on and off as long as the fault condition exists.
.
WHAT HAPPENS IF ALL IS OK
If all is OK, the secondary on the high voltage side of the circuit (the one with the diode and two 100 ohm resistors) starts to generate voltage and this also charges the 10uF 25V capacitor. Now the capacitor has sufficient voltage supply (current) supply to stay fully charged and power the controller chip indefinitely, so the power supply keeps on working as long as you have it plugged into the mains.
See if that helped you, then try that section of the video again: If it's still clear as mud let me know and I'll try to think of some other way to explain it. Once you master this part of the circuit you can repair just about any switch mode power supply.
@@LearnElectronicsRepair My goodness, and thank you so kindly for your reply - it’s truly appreciated. Yep, this is a LOT to wrap my head around, but I don’t give up easily. I will watch this video many more times, as I often do with your videos, because your approach seems so… natural, to me - I wish you were my uncle. And I won’t hesitate to ask more questions, you don’t have to worry on that front [I can be quite annoying]. Anyhoo, thank you so very much, for your help. Keep making videos, good sir. Cheers!
Have watched many of your videos Learn Electronics Repair, some several times ! I wish to purchase an isolation transformer, as you have advised often, for safety. My interest is SWMP on washing machines, etc. What size should I get ?
Hi Richard unclear about voltage divider and charging capacitor. I calculated the voltage across the 100R is 0.031V. Can you explain how the capacitor charges to 15V
So the first lessons were basic electrical components so resistors, capacitors etc! I had gone over the safety issues and came to capacitors! Capacitor values and working voltages were covered strongly. I explained if a capacitor working voltage was exceeded it could explode! Some of them laughed at this so I thought demonstration needed here! With the revelant safety (goggles) I setup a electrolyte capacitor behind a safety glass connected to a DC power supply! So a 40 volt electrolyte was the first candidate selected. As the meter approached the 40 volt the milliamperes started to increase! still it crept up over 45 volt, nothing happened “he is speaking dross I heard” 5 minutes later it was near 70volt, they were all drinking coffee by this time! BANG!!! Some of the coffee was dropping off the wall! “ JEEZUZ CHRIST” I heard! It went off like a shotgun! Now you will remember the danger! I said! If you had been working on a bench that could have cost you your eyesight! My boss agreed!
The dc negative rail is what you were calling hot ground?
Sir, you completely left out the most important part, i.e. feedback loop with voltage reference IC102 (e.g. MCP1525) and transoptor IC105 (as far as I can read).
hi richard ive got a strange thing happening with my switch mode power supply i am repairing from a mixing desk - after removing i could see the two 1000uf caps bulging obviously failed so i replaced with new ones but when i power up the power supply just ticks and the voltage across one of the caps is over 30v which it shouldnt be as caps are rated at 25v - is it an issue with the feedback circuit causing it not to adjust its PWM properly? could it be a faulty opto isolator or the tl431? it looks like a surface mount tl431
if i put the old knackered capacitors back in it powers up?? and mixer works apart from slight hum
Heya still it is very learn full to differant home elektroinics being repaired
The pwm ic is 8 pin ic why is it in your illustration is only have 3 pin what is the part number of the 8 pin ic sir tnx
Good day to you sir
I've always wondered if a computer's PSU after about 10 years of use, that seems to be working fine, Would there be something that's gone bad?
I'm asking because I didn't replace my PSU (Seasonic SS-750JS) for about 10 years and might upgrade the PC next winter without replacing the PSU.i
It is actually a real Seasonic or a PC power & Cooling which at that time was getting their power supply from Seasonic ?, I have since 2009 about 18 PC power & Cooling PSU and some of them fail due to I gess aging electrolitic capacitors inside
Check in BIOS what the voltage readouts are - as long as they read above 12v, 5v and 3.3v then the psu is good for powering your PC, when under load some of these voltages can droop but if they get too low then there isnt enough voltage to power your PC, Seasonics are very good - doesnt suprise me its fine after 10 years if its been lightly used - the other thing to look for would be weird voltage spikes - like 5v spiking to 6.7v that could also be bad for the PC - what you want is nice stable voltages at say 12.3V, 5.1V and 3.34V
@@NOMADCREATIVESOLUTIONS Thanks for the info.
I actually ran HW monitor and the lowest vdroop I got under load is 11.7 volts, spikes only to 12.4 the 5v is stable never goes under 5 and 3.3v is stable as well.
@@enriconapolitano4321 It's a Seasonic PSU bought alone, the model is SS-750JS.
11.7v is starting to get low but it probably will still power.. starts getting to 11.5v I'd maybe look at replacing it..
Smart board....noise elimination from left to right.
Is this video hidden or something? I somehow stumbled across it in one of your playlists, but it shows that it was uploaded 13 hours ago… However if you go to your channel and click on most recent it’s nowhere to be found. Like I said I accidentally stumbled across it in a playlist. Looks like there’s no views comments or likes except for my own
You shouldn't be able to find it yet. I always upload as unlisted until I complete any notes, chapters, thumbnail etc. It will be live very soon
It never occurred to me it would appear on the playlist until it was public. Now I know, I wont add videos to playlists until they are ready 😉
@@LearnElectronicsRepair
Probably the first time I’ve ever been the first view, first comment, and first thumbs up on any CZcams video.😂😂😂
guess I was tossing and turning in the middle of the night and I’m always looking for new troubleshooting and electronics videos to listen to 😂 and as I think about the repair process while I’m listening… sometimes it distracts me enough so that I can drift back off to sleep.
How about some videos on those cheap ebay meanwell / Chinesium SMPS's, idealy the 12 volts 30+ amp versions. I am specific with the specs because every other video on these are for the 10 amp versions and / or how to jack the voltages up to 24 volts or make them variable. The videos that go into higher amp models are either not in English or the repairs are not explained well, no good video of the repair, lack of proper knowledge of electronics, etc.
Benteq pwr supply seems “bent”.
Pretty sure that SMPS is in every cheap DVD player with a hard on/off switch ever made - and has the same output caps that fail in the same way 😂🤦♂️
99% of the time it is one of the rectifier diodes burnt. If not then the ecaps are next to check...they dry out in old units.
Are you referring to the power supply in this particular example, or in switching power supplies in general? In my particular experience of dealing with many different varieties of power supplies, the primary failure is one of the switching components, either mosfets used for the actual power supply switching, or the PFC(power factor correction) mosfets or a combination of those with the driver ICs and coupling components for the mosfet switches. Only about 10% of the time do I see the bridge/main rectifiers go. And, if they do go, it's usually because something upstream already failed. The typical PSU failures are mostly the cause of some kind of abnormal stimulus to the PSU, such as a high temperature/inadequate cooling, a dead short on the output from something external to what the supply is driving, or at the end of life for the electrolytic caps and their leaking or exploding. Your comment is certainly an easy very fast first check with a continuity meter but also, while in diode check, quickly one can go for all the mosfets and switching diodes. But the rectifiers are certainly not 99% of the time with switching PSUs in general, believe me, I'd love for them all to be so easy to fix and have some nice quick checklist to get these things fixed fast. But it's not so easy many times with more complicated designs especially with ones using unusual and weird switching ICs without available datasheets and schematics.
Hi, are you related to "Pig Pen" from Peanuts? Everything so dusty and dirty.... 😉
Hehe no - but this is Gran Canaria and though it almost never rains in the south of the island (where I am) we get a lot of dust due to the Calimas which we get quite often
www.lag-o-mar.com/lanzarote-hit-by-high-temperatures-what-is-a-calima/
@@LearnElectronicsRepair 😂 i know the island...( vacation ) But i was thinking more of the blue carpet pad and the power supplies
@@allinclusive5762 Hmmmmm.... Maybe I should get the wife to clean my workbench 😅
@@LearnElectronicsRepair That was my thought! 😉👍
The quick simple fix is usually to look for the electrolytic capacitor that's close to a heat sink. Planned failure.
There's 2 of them with domed tops at the edge of the board. They're obvious as soon as the lid is off.
@@1magnit Isn't that exactly what I showed at 04:25, immediately after explaining the *MOST* important thing first - which is to check the main (bulk) capacitor does not have a dangerous high voltage charge in it before looking for anything else?
@@LearnElectronicsRepair Sorry, I was skipping parts of it. Just look for the obvious first before getting out the meter. Often with those things there's a power transistor mounted on a heat sink and one of the small coupling capacitors is right next to it. Those dry out but they don't get a bulge in the top so test them in circuit with the ESR box. It's pretty much ALWAYS a capacitor that's failed in those things