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Old 01-22-2021, 06:56 AM   #21
stj
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Default Re: Toshiba laptop charger power supply problems

half the bad charger/psu's i have had where down to broken wires in the cable because peeple keep wrapping them up.
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Old 01-22-2021, 10:14 AM   #22
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Default Re: Toshiba laptop charger power supply problems

Funnily enough I have a very similar charger I am trying to fix. What is the part number of the PWM controller chip on yours? It is hard to read on the photo. is it a NCP1351B?

https://pdf1.alldatasheet.com/datash...CP1351BPG.html

That's the controller IC that mine uses. Mine also stores a high voltage charge in the main smoothing cap, but I didn't get a shock from it due to prior experience lol

We can assume that, as you have enough voltage to give you a nasty shock on the main capacitor then the input circuitry (fuse, bridge rectifier, NTC etc) is working.

So to diagnose this fault:

First check for a short circuit on the output of the power supply. I doubt, if that was the case, that the main capacitor would hold a charge but we need to eliminate it. If you have a short here then suspect the rectifier diode(s) on the secondary of the PSU.

If there is no short on the output, (probably there won't be) then measure the voltage on the Vcc pin of the controller IC. You should be able to find the datasheet online to find the correct pin. You measure from Vcc pin to the negative end of the big smoothing capacitor.

If you have no Vcc look for some high value resistors from the Vcc pin to the positive side of the main smoothing capacitor (the shocking one!). These will typically be around 470K to 1M0 and you may have two in series. Check the resistor(s) for open circuit.

If the resistors check OK, discharge the capacitor and then measure the resistance from the Vcc pin to the negative side of the main smoothing capacitor. If this is low (a few kilo ohms or less) look for an electrolytic or MLCC capacitor connecting from Vcc pin to negative of main smoothing capacitor. Desolder it and check the capacitor for leakage or short on your multimeter.

If you still have low resistance with the capacitor removed (and there is nothing else such as a zener diode connected from Vcc to negative main smoothing cap), then change the controller IC. Obviously if there is a zener diode vcc to negative big smoothing cap, then remove the zener and test it.

If you find the Vcc goes high (say 15V-28V depending on the chip type) for a second then drops to 12V or less then go to the next step

(2) Check the function of the opto-isolator. To do that try connecting a bench PSU to the output of the power supply you are trying to repair (12V, 19V whatever it is) - taking care to get the correct polarity - positive to positive.

Now with the mains disconnected from the faulty PSU, power up the bench PSU and set the voltage a little below the correct output voltage of the PSU - for example 11V if it is a 12V PSU.

Now put your multimeter on Diode test range, and measure across the two pins of the opto-isolator on the high voltage side. You should see open circuit.

Now increase the bench PSU to a bit over the output voltage of the faulty PSU. for example increase it to 12.5V for a 12V PSU. Do you see the meter reading change? If not try again with the multimeter polarity reversed,

If feedback circuit is working correctly you should see the mulitmeter reading change when your bench PSU is slightly over the specified voltage of the PSU, then go back to open circuit when you reduce the bench PSU voltage.

If you don't see that happen I would suggest change the opto-isolator and try again.

If the opto islolator appears to be functioning in the above test, then we can try something else. However beware! The following should only be attempted if (A) you have the repair PSU connected to the mains by an isolation transformer (and really anyone fixing PSUs should be using one) OR (B) you are absolutely certain your bench PSU outputs are completely isolated from the mains and from mains Earth.

This should be true if it is a linear variable PSU and could also be true if it is a SMPS model - but you need to be sure. And check from bench PSU positive and negative to mains earth. It needs to be isolated.

Alternatively you could do this test using batteries in series. Two or three PP3 should do the job nicely.

What you need to do is connect the negative of your bench PSU to the negative end of the main smoothing capacitor. Connect the positive to the Vcc pin of the controller IC. Now power up the mains to the PSU under test, switch on your bench PSU and increase the voltage until it is near the maximum Vcc (from the device datasheet). Does the PSU start working?

If not, switch off. Wait for the main smoothing capacitor to discharge - or discharge it yourself but not with your fingers - then turn the bench PSU back on and finally turn the mains back on to the test PSU. Does it work now?

If it does start working, then you should look for a secondary winding on the SMPS transformer that supplies power back to Vcc pin of the controller IC via diode and possibly a resistor and another zener. Check all these for open circuit (and the zener for short circuit).

The idea of this circuit is that the one or two high value resistors mentioned earlier (470k or 1M0) only supply enough power for the the controller IC to start.

Once the controller IC starts, it draws current and drains the capacitor which is connected from Vcc to negative of the main smoothing cap. The 470K/1M0 resistors can't recharge the capacitor fast enough to keep the controller IC running.

What should happen is the PSU starts up and then supplies Vcc from a secondary winding on the SMPS transformer - and this takes over from the high value resistors.

If the SMPS does not start up for whatever reason, the voltage on Vcc will drop and the controller IC will shut down. Depending on the controller IC type, some will try to restart every few seconds (often causing a chirping sound) and others will latch off and you need to switch off and discharge the main smoothing cap before they will try to power up again.

By using the bench PSU to power the controller IC you are bypassing this Vcc supply circuit so if it works with the bench PSU you should look in that area for faulty components.

If after all that, the PSU will not start with your bench PSU powering Vcc then check the Power FET driving the SMPS transformer. this won't be short or the main smoothing capacitor would not stay charged but it could be open. Really you need to desolder it check with a semiconductor analyser, or you can google how to check a FET with a multimeter,

If the FET is OK, refit it and check for continuity from main smoothing capacitor positive to one of the pins on the FET (if there is no continuity it could be an open circuit SMPS transformer or just a bad soldered joint)

If that reads OK check from main smoothing capacitor negative to the one of the other pins on the FET. If that reads open look for a low value resistor (typically less than 1 ohm) connected between the FET and main capacitor negative. This could be open circuit

Lastly use the datasheet and find the PWM output pin of the controller IC. Use your multimeter and make sure this is connecting to the gate of the FET, possibly via a resistor.

If all those checks are good, and the PSU will still not power up. Change the controller IC

This is not meant to be an exhaustive method of repairing small SMPS power supplies, but it is a good start and will identify most of the problems you are likely to encounter when you have a small PSU where the main smoothing capacitor holds a nasty high voltage charge, and the PSU does not power up.

You don't need anything other than a bench PSU, a multimeter, a semiconductor analyser (optional) and an Isolation transformer and you can diagnose most small SMPS. All these things should be in any repair workshop to be honest.

You can even get away with a selection of 9V and 1.5V batteries to replace the bench PSU - in which case you don't need an isolation transformer either - in which case you just need a multimeter and the ability to work step by step through the guide.

Rich

Last edited by dicky96; 01-22-2021 at 10:35 AM.. Reason: corrections
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Old 01-22-2021, 12:58 PM   #23
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Default Re: Toshiba laptop charger power supply problems

dicky96

Very nice thanks
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These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

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All of these had CAPs POOF
All of the mosfet that are taken out by bad caps
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Old 01-23-2021, 06:48 PM   #24
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Default Re: Toshiba laptop charger power supply problems

I agree with Sam, very nice write-up dicky96!

It probably captures over 90% of the failure scenarios in PSUs with current-mode PWM controllers.

In regards to injecting voltage on the Vcc pin of the primary-side controller with batteries or bench PSU... I've actually done that with a 2-prong non-grounded power adapter. It can be switching type or linear (line-connected transformer) one - just has to be a 2-prong adapter. Typically, these are labeled as "double insulated" (and a symbol with two square in each other.) So that's a cheap way of injecting a low voltage on the primary without an isolation transformer.

Also, I've seen the diode on the primary-side auxiliary winding fail a few times. It's not a common failure mode, but it can happen. On older Delta PC ATX PSUs with a 2-transistor 5VSB, for example, that diode always darkens the PCB around it. Seems like Delta's design just pushes it too hard (and it's a GP diode too - typically 1N4002, which isn't even a fast-recovery type.) Still, I actually haven't seen this diode fail in the old Delta ATX PSUs.

Quote:
Originally Posted by stj View Post
half the bad charger/psu's i have had where down to broken wires in the cable because peeple keep wrapping them up.

Same.

It's not the problem that they wrap them up, but how they do it. Most do it just too tightly around the adapter and cause too much strain - especially on the part that goes in the adapter. I've probably seen more shorted cords close to where the wires are soldered onto the PCB vs. at the plug end.
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Old 02-06-2021, 12:40 PM   #25
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Default Re: Toshiba laptop charger power supply problems

hey guys I'm very happy you like it

Thanks to your kind comments I would like to expand this tutorial a little (also I'll add momaka's comments - yeah I've seen that diode fail a few times as well)


------------
We can assume that, as you have enough voltage to give you a nasty shock on the main capacitor then the input circuitry (fuse, bridge rectifier, NTC etc) is working.

So to diagnose this fault: (Main capacitor charges and stays charged but PSU does not start)

(1) First check for a short circuit on the output of the power supply. I doubt, if that was the case, that the main capacitor would hold a charge but we need to eliminate it. If you have a short here then suspect the rectifier diode(s) on the secondary of the PSU.

(2) If there is no short on the output, (probably there won't be) then measure the voltage on the Vcc pin of the controller IC. You should be able to find the datasheet online to find the correct pin. You measure from Vcc pin to the negative end of the big smoothing capacitor.

(A) If you have no Vcc look for some high value resistors from the Vcc pin to the positive side of the main smoothing capacitor (the shocking one!). These will typically be around 470K to 1M0 and you may have two in series. Check the resistor(s) for open circuit.

If the resistors check OK, discharge the capacitor and then measure the resistance from the Vcc pin to the negative side of the main smoothing capacitor. If this is low (a few kilo ohms or less) look for an electrolytic or MLCC capacitor connecting from Vcc pin to negative of main smoothing capacitor. Desolder it and check the capacitor for leakage or short on your multimeter.

If you still have low resistance with the capacitor removed (and there is nothing else such as a zener diode connected from Vcc to negative main smoothing cap), then change the controller IC. Obviously if there is a zener diode vcc to negative big smoothing cap, then remove the zener and test it.

(B) If you find the Vcc goes high (say 15V-28V depending on the chip type) for a second then drops to 12V or less then go to the next step

(C) If Vcc is around 15V-28V and steady, change the controller chip


(2) Check the function of the opto-isolator. To do that try connecting a bench PSU to the output of the power supply you are trying to repair (12V, 19V whatever it is) - taking care to get the correct polarity - positive to positive.

Now with the mains disconnected from the faulty PSU, power up the bench PSU and set the voltage a little below the correct output voltage of the PSU - for example 11V if it is a 12V PSU.

Now put your multimeter on Diode test range, and measure across the two pins of the opto isolator on the high voltage side. You should see open circuit.

Now increase the bench PSU to a bit over the output voltage of the faulty PSU. for example increase it to 12.5V for a 12V PSU. Do you see the meter reading change? If not try again with the multimeter polarity reversed,

If the feedback circuit is working correctly you should see the mulitmeter reading change when your bench PSU is slightly over the specified voltage of the PSU, then go back to open circuit when you reduce the bench PSU voltage.

If you don't see that happen then we need to look at the other side of the opto isolator.

Set your multimeter DC volts range and connect it to the two pins on the low voltage side of the opto isolator (it doesn't really matter which way round you connect the meter).

Once again set your bench PSU to a little below the rated voltage of the PSU under test. You should read 0V or there-about on your multimeter.

Now increase your bench PSU again to a little over the rated voltage of the power supply - and you should see about 1.2V-2.5V (plus or minus depending how you connected your meter).

If you do then I would suggest change the opto isolator and try again.

If you don't then you should look for resistors, zener diodes and possibly a transistor(s) connecting between the output of the PSU to the low voltage side of the opto-isolator. Check these for short (capacitors or zeners) open (resistors) or faulty transistor(s).

If the opto islolator feedback circuit appears to be functioning in the above test, then we can try something else. However beware! The following should only be attempted if (A) you have the repair PSU connected to the mains by an isolation transformer (and really anyone fixing PSUs should be using one) OR (B) you are absolutely certain your bench PSU outputs are completely isolated from the mains and from mains Earth.

This should be true if it is a linear variable PSU and could also be true if it is a SMPS model - but you need to be sure. And check from bench PSU positive and negative to mains earth. It needs to be isolated.

Alternatively you could do this test using batteries in series. Two or three PP3 should do the job nicely.

What you need to do is connect the negative of your bench PSU to the negative end of the main smoothing capacitor. Connect the positive to the Vcc pin of the controller IC. Now power up the mains to the PSU under test, switch on your bench PSU and increase the voltage until it is near the maximum Vcc (from the device datasheet). Does the PSU start working?

If not, switch off. Wait for the main smoothing capacitor to discharge - or discharge it yourself but not with your fingers - then turn the bench PSU back on and finally turn the mains back on to the test PSU. Does it work now?

If it does start working, then you should look for a secondary winding on the SMPS transformer that supplies power back to Vcc pin of the controller IC via diode and possibly a resistor and another zener. Check all these for open circuit (and the zener for short circuit).

The idea of this circuit is that the one or two high value resistors mentioned earlier (470k or 1M0) only supply enough power for the the controller IC to start.

Once the controller IC starts, it draws current and drains the capacitor which is connected from Vcc to negative of the main smoothing cap. The 470K/1M0 resistors can't recharge the capacitor fast enough to keep the controller IC running.

What should happen is the PSU starts up and then supplies Vcc from a secondary winding on the SMPS transformer - and this takes over from the high value resistors.

If the SMPS does not start up for whatever reason, the voltage on Vcc will drop and the controller IC will shut down. Depending on the controller IC type, some will try to restart every few seconds (often causing a chirping sound) and others will latch off and you need to switch off and discharge the main smoothing cap before they will try to power up again.

By using the bench PSU to power the controller IC you are bypassing this Vcc supply circuit so if it works with the bench PSU you should look in that area for faulty components. Quite often the diode connecting from the secondary winding to Vcc fails. Also there is often a capacitor and zener diode here from the secondary to ground and they can go short.

If after all that, the PSU will not start with your bench PSU powering Vcc then check the Power FET driving the SMPS transformer. this won't be short or the main smoothing capacitor would not stay charged but it could be open. Really you need to desolder it check with a semiconductor analyser, or you can google how to check a FET with a multimeter,

If the FET is OK, refit it and check for continuity from main smoothing capacitor positive to one of the pins on the FET (if there is no continuity it could be an open circuit SMPS transformer or just a bad soldered joint)

If that reads OK check from main smoothing capacitor negative to the one of the other pins on the FET. If that reads open look for a low value resistor (typically less than 1 ohm) connected between the FET and main capacitor negative. This could be open circuit

Lastly use the datasheet and find the PWM output pin of the controller IC. Use your multimeter and make sure this is connecting to the gate of the FET, possibly via a resistor.

If all those checks are good, and the PSU will still not power up. Change the controller IC

This is not meant to be an exhaustive method of repairing small SMPS power supplies, but it is a good start and will identify most of the problems you are likely to encounter when you have a small PSU where the main smoothing capacitor holds a nasty high voltage charge, and the PSU does not power up.

You don't need anything other than a bench PSU, a multimeter, a semiconductor analyser (optional) and an Isolation transformer and you can diagnose most small SMPS. All these things should be in any repair workshop to be honest.

You can even get away with a selection of 9V and 1.5V batteries to replace the bench PSU - in which case you don't need an isolation transformer either - in which case you just need a multimeter and the ability to work step by step through the guide.

Take your time to work through it, and good luck. If you get a bit lost trying to follow the guide, or you got to the end and it still didn't fix the problem, then post on the forum and for sure some kind folks will help you.


I italicized the changes. And I think that now covers a few more percent of the faults people are likely to encounter. I didn't set out to write a repair guide but if you like it and the gods of sticky threads would like to stick this guide or copy/paste/improve it so it doesn't get lost in the depths of old threads, then I for one can certainly cope with the fame LoL


Last edited by dicky96; 02-06-2021 at 12:52 PM..
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Old 02-06-2021, 01:46 PM   #26
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Default Re: Toshiba laptop charger power supply problems

I have a question about this part of your guide

Once again set your bench PSU to a little below the rated voltage of the PSU under test. You should read 0V or there-about on your multimeter. ”

This is the part I am little confused about

“ Now increase your bench PSU again to a little over the rated voltage of the power supply - and you should see about 1.2V-2.5V (plus or minus depending how you connected your meter). <—[ see below ]

If you do then I would suggest change the opto isolator and try again.
” <— please explain this in more detail is this because you are assuming that it is bad I do not want to assume the wrong thing with troubleshooting a circuit are you talking about the input pins on the optic sensor where you are taking the measurements

If this is the case I just learned something very important thing about troubleshooting a power supply

Last edited by sam_sam_sam; 02-06-2021 at 02:10 PM..
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Old 02-07-2021, 01:05 PM   #27
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Default Re: Toshiba laptop charger power supply problems

Very nice and precise. I'll print this and post it next to my work bench
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Old 02-07-2021, 01:37 PM   #28
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Default Re: Toshiba laptop charger power supply problems

Very nice and precise. I'll print this and post it next to my work bench.

My steps I usually use are as follows.

1. Visual inspection of front and back of board for burned areas, damaged or cracked caps.and other components.
2. If burned components I try to determine why they failed and replace them.
3. I test the output cable for continuity and also check for dry or disconnected solder joints.
4. I test fuse, bridge diode, 3 pin transistor on primary side usually KB3569 for short. Also test 3 pin diode on output and also both 8 pin ic for short.
5. usually above steps I find the problem. If not I use a Serial bulb circuit / lamp for short circuit test that way If I have more than one short or I didn't fix the problem I don't burn components. When the light bulb stays on I know I have a short somewhere still.
6. After the above, if the cap is still holding a charge, I check all the components on the top side of the board one by one.
7. If I still have a short or charger doesn't work, I continue to check with the serial bulb circuit and I now know I have a problem on the SMD components on the back side.of the board. I test each and everyone.

This is where dicky96 method comes to the rescue.
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Old 02-07-2021, 01:51 PM   #29
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Default Re: Toshiba laptop charger power supply problems

Quote:
Originally Posted by dicky96 View Post
hey guys I'm very happy you like it

If that reads open look for a low value resistor (typically less than 1 ohm) connected between the FET and main capacitor negative. This could be open circuit

L
I've had this a couple of times usually 0.15, 0.18, 0.25, 0.28ohms resistors. Is it necessary to replace with the exact value? Also after this resistor there are sometime very small blue colored resistors (usually under the large cap) does this value also has to be same? I am asking because I have a charger that burned the resistor so I can't tell color code and there is no schematic.
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Old 02-07-2021, 02:10 PM   #30
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Default Re: Toshiba laptop charger power supply problems

I trying to modify a ATX switching power supply and I going to be using your troubleshooting guide to figure out which optic sensors are on which power supply rail 5 and 12 volts

Where I asked you a question in post # 26 because I have not found a circuit diagram for this FSP power supply I found one that is similar but not exactly the same so I have to reverse engineering this power supply but I am wanting to learn more on how to troubleshooting ATX switching power supply’s and I figure that I will learn a lot from this exercise—————>

I am working on a proof of concept this is another one that I have seen several times on the internet and I have seen different was to make a adjustable bench switching power supply out of a ATX power supply

Dicky 96

I have also printed your guide on how to troubleshooting a ATX and going to be by my work table and and few other forum members I going to print there as well and put them all together in a note book

But here is another case where you see some things and ask the question will this work they way you see on YT so well we will see

Last edited by sam_sam_sam; 02-07-2021 at 03:01 PM..
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Old 02-07-2021, 10:03 PM   #31
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Default Re: Toshiba laptop charger power supply problems

Quote:
Originally Posted by SuperAman View Post
I've had this a couple of times usually 0.15, 0.18, 0.25, 0.28ohms resistors. Is it necessary to replace with the exact value?
Well, those resistors allow the PWM IC to sense current going through the MOSFET and shut it down in case of over-current. Hence, these resistors provide over-power protection for the primary side. Using a resistor with lower value that original will increase the power limit and using a resistor with higher value will decrease the power limit. But in all likelyhood, as long as you're within 20-30% of the original value, the adapter will probably work fine without any noticeable difference. Any more than that, and the power limit may get set a bit too high or too low, causing the adapter to not be able to reach the stated power output or not shut down when more power is drawn than the adapter is rated for. So try to match the value... but if not, just try to get at least somewhat close. Depending on design, some adapters may not work right, though.

Quote:
Originally Posted by sam_sam_sam View Post
I trying to modify a ATX switching power supply and I going to be using your troubleshooting guide to figure out which optic sensors are on which power supply rail 5 and 12 volts
If the PSU has optocouplers, these are *not* for the 5V and 12V rails. Usually PSUs with optocouplers use forward converter design with the PWM IC being on the primary side. In such case, one of the optos will be for enabling power to the PWM controller, and the other is feedback for telling the PWM controller to crank the PWM up or down (based on load)... which will bring up or down all rails at once - not individually. As to whether or not you can change the 5V and 12V rails independently, that depends on design. With group-regulated PSUs, there just isn't much you can do about that, unless you start re-winding transformers and output toroids. The only "easy" fix that might give a slight tweak in voltages is to swap Schottky rectifiers for FR ones and vice versa to get higher or lower voltage drop respectively, in order to better "balance" the output. 3.3V is typically independently regulated - either by a mag-amp circuit or linearly with a MOSFET. And if the PSU is *not* group-regulated, then main PS will produce 12V, from which 5V and 3.3V will be generated from buck regulators.

Also, making an adjustable PS out of an ATX PSU is not so simple, depending on design and ICs used. With half-bridge designs that use an all-in-one IC, such as AT2005b, ATX2005, SDC2921, and SG6105, you may have a hard time adjusting the voltage without making the PSU loose regulation or oscillate. And for others, you will have to disable all protections (OV, UV, and SC), which could make for great fireworks show if you short any of the outputs.

So all in all, it's not very practical to convert an ATX PSU to an adjustable one via these mods. I know a lot of videos on YT show it, but many of them are just copy-pasta of someone's mods without too many technical explanations. And I don't recall anyone hooking up an o-scope to one of these to check ripple and noise, which could easily shoot through the roof if the PSU is not regulating properly or starts oscillating. Basically, there's a lot of "untold" truths to these mods, so it's best to not follow them. When it comes to technical info, there are very few people on YT that actually know what they are doing exactly and can explain everything well. Most are "I saw this mod on here and here and did this and that to get it working, so here's a video of it."

While on the topic, though, if you really want to learn, dig up some old threads from Th3_uN1Qu3 in the PSU Forum section. He did a lot of write-ups on how to design/build/modify lots of PSUs, including ATX ones. He even posted some calculators and whatnot. So if you want to learn, this is a much better resource than Youtube. On that note, here are a bunch of interesting threads I pulled from my above search (in no particular order, BTW):
https://www.badcaps.net/forum/showthread.php?t=14990
https://www.badcaps.net/forum/showthread.php?t=12588
https://www.badcaps.net/forum/showthread.php?t=13836
https://www.badcaps.net/forum/showthread.php?t=14287
https://www.badcaps.net/forum/showthread.php?t=16086
https://www.badcaps.net/forum/showthread.php?t=13575
https://www.badcaps.net/forum/showthread.php?t=22599
https://www.badcaps.net/forum/showthread.php?t=14136

Quote:
Originally Posted by dicky96 View Post
hey guys I'm very happy you like it
...
I italicized the changes. And I think that now covers a few more percent of the faults people are likely to encounter. I didn't set out to write a repair guide but if you like it and the gods of sticky threads would like to stick this guide or copy/paste/improve it so it doesn't get lost in the depths of old threads, then I for one can certainly cope with the fame LoL

That would be nice, indeed. Perhaps a mod can do this and split the post into its own thread (&stickied?) I've already bookmarked it and probably will link it everytime someone is asking for help on a flyback SMPS - should save me time from having to explain what to check too.

Thanks again, Rich!

Last edited by momaka; 02-07-2021 at 10:09 PM..
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Old 02-08-2021, 12:12 PM   #32
dicky96
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Default Re: Toshiba laptop charger power supply problems

@SuperAman
Yeah of course there are other scenarios - burnt components, blown FETs etc - Rather than a full guide I was trying the answer the OP question where he had a SMPS where the main capacitor charged up and stayed charged, making it a high voltage shock hazard. But it could be expanded to cover those types of faults.


@Sam Sam Sam

OK what I meant in the guide.... First use the multimeter on Diode Range and measure across the high voltage (the photo transistor) side of the opto couple. Use your bench PSU connected to the output of the faulty PSU. Now vary the voltage a little above / below the rated voltage of the faulty PSU and you should see the opto couplers transistor turn on/off.

If it does then you can pretty much assume the feedback circuit is working. Yes there could be some faulty component or connection between the opto couple and the FB pin on the controller IC but I don't recall ever finding that happen.

If it doesn't, then we should check the LED side of the opto couple to see if it gets any drive voltage. Go to VOLTs range and measure the voltage directly across the low voltage (LED) side of the opto couple. Again vary your bench PSU a little above/below the rated value of the faulty PSU. What you should see here is the voltage vary between 0V-ish, and something like 1.8V+. If you see that happening, then you are seeing the forward voltage drop of the LED built in to the optocouple.

If this isn't happening (no 1.8V or so) then you need to look at the circuit between the output of the PSU and the optocouple (resistors, zener, transistor, short circuit opto coupler LED etc) because the opto coupler is not getting any drive voltage, and it should be.

If are seeing the voltage switch between 0V/1.8V or so but there was no change when you measured on the high voltage side (if there was you shouldn't be in this part of the guide anyway), then we are seeing that the diode in the optocouple is being driven as it has a forward voltage drop in the range we would expect, but it either isn't illuminating or the photo transistor isn't reacting - therefore change the opto couple. Also an open circuit LED in the opto couple would probably cause you to see the full output voltage across the opto coupler LED pins.

I hope that clarifies it.

By the way ATX PSU use a different topology and need their own guide really. Typically they have a small power supply to provide 5VSB (this guide will sort that one), a more powerful power supply for the main voltages (frequency modulated half bridge), and a PCF section (PWM driven)

@Momaka - just THANKS!

Rich

Last edited by dicky96; 02-08-2021 at 12:39 PM..
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Old 02-08-2021, 07:30 PM   #33
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Default Re: Toshiba laptop charger power supply problems

Yes to both of you for clearing this up for

Thanks
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