SMPS: Substituting SBD/Power Diodes 1A for 1.5A and Filter Cap for +20% safe?

[Luke McKee]

This is my first post

I'm not going to be begging for help to be babied all the way. I did Dick Smith Fun-way electronics kits (Australia) as a kid, but when you are working with mains you have to be very careful making the smallest changes to a circuit design. Hence I'm asking for a few tips. Better safe than sorry.....

I've got pretty short dwell times with soldering and know how to use clamp heat-sinks, and I've usually done solid state work modifying router PCB's and setting up outdoor WiFi installation boxes for a hobby. I just don't have a lot of experience working with mains since I was a kid and built a desktop SMPS, and that was 20 years ago based of a reference design from a magazine. Also I've never got into SMT re-working.

I've got around to fixing 2x LCD PSU's as my one blew up last month. The other from an Acer (my wife's) had a very bad day described here:
https://www.repairfaq.org/sam/smpsfaq.htm
Most Common Problems

The following probably account for 95% or more of the common SMPS ailments:

• Supply dead, fuse blown - shorted switchmode power transistor and other semiconductors, open fusable resistors, other bad parts. Note: actual cause of failure may be power surge/brownout/lightning strikes, random failure, or primary side electrolytic capacitor(s) with greatly reduced capacity or entirely open - test them before powering up the repaired unit.

+ a shorted transformer (game over)

The PWM IC literally exploded and I can't identify what it was, so where I am in Vietnam I passed the whole board off to get a spare from some second hand LCD dealer for a bounty.

The other one I'm working on a Korean imported Taiwanese made 2006 21 Inch LCD PSU, it hasn't got a broken fuse. It's a **broken link removed** (no current specs online, but some similar boards specs / FCC circuit diagram for a 156 model for higher wattage)
https://fccid.io/pdf.php?id=804634

I've bought all the most likely to fail components according to various repair guides, and I can calmly and carefully based on my reference photographs and notes re-assemble the thing. However getting a couple of parts were tricky and I need to know if I can sub two components and ask 1 question.

1) Method: Of course I'll just replace all the secondary caps, but on the first attempt is there any harm in replacing the Semi's as well?

This is my decent LCD and I don't changing all the primary / secondary non-surface mount components first off because it only cost $10-15 bucks to get all of them at the Electronics Market in Vietnam. Yes all parts are correctly rated equivalent spec except for the two below. Considering I already bought the parts, should I just install them all? Let me rephrase the question: if any of the semi's are bad is that going to blow the caps again?

2) It was to difficult to track down am Samxon (tw) 82uf 400V High Temp (105 degrees C8) High Frequency Filter Capacitor (that tested faulty), but I got a new 100uf 450v 105 deg cap, that's slightly larger (but there wont be a clearance problem provided i wrap the pins in heat shrink tubing and fold them under the metal can a little (or not - instead put new heat shrink tubing on the fameproof 2W 5Ohm resister that was behind it, and have less clearance than it did before, which is probably a worse idea).

Is increasing this filter capacitor by a little going to cause any harm provided I carefully check for shorts and install it safely?

3) The bridge rectifier in this unit is 4x **broken link removed**'s & tests Ok with the mulimeter, 450 ~ 500 ohms across all of them, and open when the polarity is reversed. 1N4007's aren't recommended for new design's so can I sub them out too in old designs?
The power supply is meant to be rated for 1.5A. I've got 4x **broken link removed**'s. (marked at 2A in the shop but really 1.5A).
https://electronics.stackexchange.com/questions/37401/how-are-bridge-rectifiers-rated

Speaking of subbing diodes, is this a safe swap too for a SBD to replace a 20A for a 10A?
https://pdf1.alldatasheet.com/datasheet-pdf/view/109837/NIEC/FCH20U15.html
replacing:
**broken link removed**

4) Can I sub a TL-431A for a KIA-431A. The datasheets imply they are pretty much the same. Scratch that they are the same
https://www.ti.com/lit/ds/symlink/tl431.pdf
**broken link removed**

I'll appreciate your feedback. Thanks!

 

[spec]

Hi LMK

2) It was to difficult to track down am Samxon (tw) 82uf 400V High Temp (105 degrees C8) High Frequency Filter Capacitor (that tested faulty), but I got a new 100uf 450v 105 deg cap, that's slightly larger (but there wont be a clearance problem provided i wrap the pins in heat shrink tubing and fold them under the metal can a little (or not - instead put new heat shrink tubing on the fameproof 2W 5Ohm resister that was behind it, and have less clearance than it did before, which is probably a worse idea).

Is increasing this filter capacitor by a little going to cause any harm provided I carefully check for shorts and install it safely?

Going from an 82uF, 400V, 105 Deg C to a 100uF, 450V, 105 Deg C capacitor, especially as it is bigger, should be OK.

With a filter capacitor the effective series resistance (ESR) and ripple current rating are important parameters as well.

3) The bridge rectifier in this unit is 4x **broken link removed**'s & tests Ok with the mulimeter, 450 ~ 500 ohms across all of them, and open when the polarity is reversed. 1N4007's aren't recommended for new design's so can I sub them out too in old designs?
The power supply is meant to be rated for 1.5A. I've got 4x **broken link removed**'s. (marked at 2A in the shop but really 1.5A).
https://electronics.stackexchange.com/questions/37401/how-are-bridge-rectifiers-rated

1N4007 series diodes are obsolescent but are still widely available and widely used. Note though that 1N400x diodes vary greatly in quality and at one time there were faulty batches floating about.

1N5399 rectifiers will be a suitable replacement.

Speaking of subbing diodes, is this a safe swap too for a SBD to replace a 20A for a 10A?
https://pdf1.alldatasheet.com/datasheet-pdf/view/109837/NIEC/FCH20U15.html
replacing:
**broken link removed**

Probably. The DC parameters are OK but the FCH20U15 data sheet is rather sparse and does not mention any switching speed.

Although the data sheet does not even indicate that the FCH20U15 are Schottky diodes, other sources on the net indicate that they are.

4) Can I sub a TL-431A for a KIA-431A. The datasheets imply they are pretty much the same. Scratch that they are the same
https://www.ti.com/lit/ds/symlink/tl431.pdf
**broken link removed**

Yes, but but the 431s do not fail that often.

spec

 

[throbscottle]

Welcome to the forum!
What an incredibly well put first post. Wish more would ask their questions in such a thoughtful way
You probably don't need to replace components that check ok, otoh, this being a smp there's nothing like being cautious regarding parts that may have been stressed by high voltage or high current.
I hope the substitute semiconductors you got are genuine parts. Cheap clone components are prone to early failure - for this reason alone you shouldn't replace anything that needs replacing if you have any doubts about the provenance of your new parts.
For testing diodes you are far better off using a meter with a diode test function. You can also test transistors to some extent using this.
2) Sounds ok. Insulate the cap leads. You should never put sleeving over a resistor unless it's for a special purpose, especially one as high power as 2W.
3) 1N4007 is pretty robust. If they test OK, you shouldn't need to replace them since they are only for rectifying the mains power. On the other hand your suggested replacements would be just fine.
4) Same device, different manufacturer. Again, if you can prove the original to be not faulty it's better to not replace it.
Hope this helps

 

[spec]

... You should never put sleeving over a resistor unless it's for a special purpose, especially one as high power as 2W...

Very good point TC.

spec

 

[Luke McKee]

Spec thanks for your helpful replies, but I know it's a once off. I've got to RTFM on how to DIY RTFM and compare the datasheets and study/brush up a little more theory so I can answer those questions myself. I don't mind going over-the top with the repair as it's my LCD of 10 years, and I'm not in a rush to get a replacement, or have it break again a few months later. Solid state has a MTBF like moving parts due to the heat-cycle problem right?

https://en.wikipedia.org/wiki/Capacitor_plague
10 year old Taiwanese PSU with Taiwanese Samxon caps is the prognosis. This post is probably just another case of the plague.

We'll lets see if my repair is just like the YouTube above, as the odds would be on it. But it appears the primary filter cap has gone dry in mine as well, that could lead to other damage. You'd think in 2006 the High-ESR ones would be better.

Ok first up I'll just replace the caps, and also use a 12V- or 24v source to test the voltage drop on the 1 or 2W 0.51Ω fusible resistor seeing my cheap Chinese (EXCEL) DMM doesn't have a range below 200Ω. With the battery being 9.0V it read's 1Ω, and 8V it's like 2.5Ω

That's a common fault too, I saw one guy on YouTube working on a PCB with the mains on and it arcing smelling up his studio, then knocked up a replacement out of 9x 1/4w ones in parallel.

Don't ask me to embed it (I can), but I give it to him he knows what he's doing, it's about as dangerous as driving where I am with the brakes half gone as I did today on my little trip to the electronics low grade component street market (yep therobscottle thanks for the tip). If it ain't proven broken, don't fix it with something worse!

I don't think it's even possible to get high grade low esr fast switching capacitors retail where I am, unless I order online in .vn, and the'd tell me to rack off unless I buy 1k of them per order.

Is another other thing worth testing (a common SMPS fault?) is a big schottky diode SB530 or SB540, but I'll have to take it out of the circuit to get it's specs and test it's voltage drop, because all I can see on the top side is the "SB".

My electronics experience is not all that technical but as a teenager I used to fix Westinghouse 1632/1642 running 1024B/C networking RS422 protocol to Cisco routers all the way back to the airlines mainframes via Sita.aero & supported by their service wing when it was ITS Pte Ltd, before it became Equant IS, before it became part of Hutchinson Telecom's network. (Sita's network division, in Y2K they had more bandwidth than the internet on some POPs). Now why did I mention that, ah yes... My SITA type-B pre-internet-email address was SYDMQXS from memory. I think the cargo industry still uses that.

Before the Internet we'd get our monthly DBASE EFIL ("Electronic Fault Finder's Information Library") updates for tips, and work of common faults.
Half of those Westinghouse's were so old that they had linear power supplies. And I did cop one of them with a day old charge on a capacitor through the right thumb. That's got to hurt. Hence why I have respect for AC, and feel the need wear sneakers when there's no Isolated ground or double insulation. Without at least an isolation transformer I wouldn't work on anything mains with it plugged in. Has someone compiled a decent (not for too great profit) very comprehensive guide to SMPS repair?

Does the internet have a replacement for that free or not (just as good categorization) ? A database of common faults for boards, or it's just google + sites like these.
--------------------

"Supply dead, fuse not blown - bad startup circuit (open startup resistors), open fusible resistors (due to shorted semiconductors), bad controller components."

^^ That advice motivated me to get the semi's at the market today ^^ Should I replace the main PWM IC?

https://www.mikesarcade.com/cgi-bin/spies.cgi?action=url&type=info&page=psrepair.txt

^^^ seems like another good guide from the BBS days.

Seeing I lack a capacitance tester (I borrowed one for the filter cap today at the market) I was thinking playing around with Eagle or some other strip-board designer to make myself a good old fashioned ESR meter as my next project. This one looks nice, and it can feed into my crap multi-meter too. the DMM uses resistance or voltage to measure the ESR? (diagram on the link). If the VN shop can't find a replacement board, I'll try and fix the other one too and order the shorted transformer from abroad.

https://www.qsl.net/iz7ath/web/02_brew/15_lab/06_esr/index.htm

But I saw others on YouTube who got kit's with pre-programmed PICs, that takes away even more of the fun. This one looks cheaper and more practical for those with limited parts availability in their neck of the woods.

 

[Luke McKee]

P.S.

Spec: An advantage for changing the Dual Common Cathode Schottky rectifier for the ones I bought would be thermal properties? The heat-sink on this board for that is 1/3 the size of the PWM IC (you can see that YouTube video). This PSU run's pretty hot, and the back side of the LCD case has some thermal damage from the heatsink (grayed PVC plastic), and it's been a hot running since the day I bought it.

The same logic would apply for the 1N4007s vs the 1N5399's because they dissipate less heat per watt right? The first 1n4007 has left a carbon build up below it like a shadow on the board though it tests ok in circuit and seems to get a bit too warm too.

Also for cleaning the green PCB board... I couldn't get isopropyl alcohol from all the chemists here, as they are big on using ethanol 90% instead since Wikipedia said also comes under the "rubbing alcohol" umbrella. I used a little bit of that and it left white scarring on the bottom of the PCB (dissolving it?) so I stopped immediately. So all I'm doing is just using 60/40 solder and a solder pump.

That's about all my concerns, I should be able to get the job done now If she blows again I'll have to order from from Overseas a "kit" of low esr/hi-freq caps for the second stage.

 

[Luke McKee]

... You should never put sleeving over a resistor unless it's for a special purpose, especially one as high power as 2W...

Very good point TC.

spec

Yeah the old sleeve (heat-shrink) I cut off to see the value got turned into soft rubber on that half ohm resister that's 1cm long from 10 yrs of use.

Damned if you do damned if you don't. What would 240VAC do to the metal cap of a filter electrolytic capacitor that's also live? The resistor was mounted vertically and pins can bend in shipping and transit. The manufacturer had to decide if they wanted to avoid arc's.

This resistor should be "flame proof", but I don't see any special markings to denote that it looks like a normal 2W metal film. A case of made in China (or "ROC"). With the new cap installed there will be even less clearance.

 

[spec]

... You should never put sleeving over a resistor unless it's for a special purpose, especially one as high power as 2W...


Yeah the old sleeve (heat-shrink) I cut off to see the value got turned into soft rubber on that half ohm resister that's 1cm long from 10 yrs of use.

Damned if you do damned if you don't. What would 240VAC do to the metal cap of a filter electrolytic capacitor that's also live? The resistor was mounted vertically and pins can bend in shipping and transit. The manufacturer had to decide if they wanted to avoid arc's.

This resistor should be "flame proof", but I don't see any special markings to denote that it looks like a normal 2W metal film. A case of made in China (or "ROC"). With the new cap installed there will be even less clearance.

Hi LMK,

The problem with putting a sleeve on the resistors is that the sleeve acts as a thermal barrier which means that the resistor cannot lose heat by convection.

By the way, if you mount components, like the 1N4007 diodes, off the board that will allow air to flow under the diodes and cool then better. Also, the board will not overheat.

spec

 

[Luke McKee]

Spec, Oh i'm just reporting on what the people did in the Taiwanese owned Chinese factory 10 years ago
Because it's only switching at 50Hz in Asia doing the AC/DC conversion the 1N5399's will cope fine, and dissipate less heat and allow up to 1.5A vs 1.0.

Now that I've got home I've done the poor man's capacitor tester (using a 9v battery) and the filter capacitor was definitely buggered. The new 450V can hold the voltage, the old one is/was most definitely completely open circuit, and yet all the caps in the secondary stage passed this rudimentary test.

I was considering also waking in the $1 STR-W6251 before powering it on the first time. Throbscottle: You may be right about the providence of the parts. The silkscreen of the part number on this replacement is about as tilted as the date stamp on Obama's f**ed long form birth certificate. Different tilt angle per line, so I'll hold back and do some functional testing with just the new caps, power diodes.

No harm done to the new cap if I power it up. Is 800Ω between Ground and VCC on the PWM IC a short / nothing when reversed? That would be about right?
It's behaving like a diode then tested in circuit but to test things properly you need a breadboard and a desktop PSU to make the "measurement circuits"...
https://media.digikey.com/pdf/Data Sheets/Sanken/STR-W6251D.pdf

To answer these troubleshooting questions by myself, I have to RTFM on spice (I'm a Linux geek). That's definitely my next challenge to use that as a learning tool.

 

[spec]

Hi again LMK,

I suppose you know all this but here goes.

With this type of board it is best to thoroughly clean it and the components both sides and inspect the board for physical damage: cracks in the copper traces and components. Also look for any corroded of crystallized solder joints. Very often it is best to reflow all solder joints.

And check wires for corrosion, especially where the wires are soldered.

spec

 

[throbscottle]

Looks like you're solving stuff by just talking about it, LMK. Amazing how it clarifies the picture, that.
I'm guessing you probably already know, but in case you don't, a handy quick capacitor test with an ohm-meter is it will look momentarily like a short, and the resistance will gradually go up, more slowly = bigger capacitor. If it's short obviously the reading will stay low, if it's open (uncommon) you won't get that initial "kick". More important to have a cap that works than worry too much about ESR (unless it's stopping the regulator working).
If you put caps in parallel the values are summed, but the formula for parallel resistance applies to the ESR. Possibly useful in your situation.
You should be able to test the voltage drop of the diodes with a battery and resistor in series with the diode. Just measure the voltage across it. Calibrate your readings against known good diodes, then you should be able to get a good idea of whether an unknown is likely to be Schottky or not.
Don't forget, your PSU was built to a budget. Putting in better spec'd components will likely do no harm, and may get you better efficiency. The only issue you might have is introducing new stress further down the line, so test, test, test!.
(edit) You didn't mention originally that the resistor had sleeving to start with. In which case it would have been chosen to be operated like that originally. Still better to let it "breath" though.
Here's a link to a capacitor ESR tester I haven't got round to building. It's in French, so Google translate is your friend unless you happen to speak it. It took me a long time to work out that the original "fils" which Google translates as "son", actually means "wire"!
https://forums.futura-sciences.com/...testeur-de-condensateurs-electrolytiques.html

 

[throbscottle]

Beg pardon, missed last two posts whilst writing that!
And now I have dog walking duty...

 

[Luke McKee]

Thanks guys. I've tested lost of stuff by myself (e.g. the NTC) by RTFMing it was good to have a helping hand for a little bit.

Basically what I was doing was trying to find all the most common faults with SMPS and test those first, especially what needs checking if it dies with no fuse blown and no visible damage to the board. Hence the brainstorming before the plug and pray.

I did an ultra poor mans test with the 1 or 2 W current limiting resistor at .3 amps 7V using a short from a 9v battery, and measured the voltage drop which showed the low ohms resistor had a 140Mv drop which was what I was expecting.
I'll do better for the diodes if it doesn't start & I'll strip an old phone charger and use that for a 5V source assuming it's stable, and make sure I use a current limiting resistor to keep the output below what the charger is rated for. Use the rig for testing the voltage drop on diode's too. Of course with it being yet another SMPS ad those capacitors to filter out some line noise.

https://www.robotroom.com/Measuring-Low-Resistances.html

I shied away from the poor man's cap resistance test because that works best if you have an analogue meter not a DMM. DMM's don't really run a real-time OS, they tend to sample only every second or so and wait until the measurement value stops fluctuating. I have a cheap Chinese one remember - no capacitance testing (and most DMM's are not accurate at that anyway) You can't really watch and rapid changes on a digital multi-meter, you only get a random measurement until it goes out of the range of the DMM, that's useless for comparison.

https://forum.arduino.cc/index.php?topic=173708.0

^^^
Another way fun way to measure capacitors is by a software-CRO by turning my PC's almost useless sound-card (due to RF from mains - and the lack of any appliance in Vietnam having EARTH wired up - still hunting an IEC to strange US plug with a circle instead of a tab for ground). Maybe I could just run the software to visualize the line noise before I build the kit to know what I'm up against. I wonder if someone could make software for that for your PC soundcard CRO kit. Could be a fun project there! My capacitance tester is on github (no not github they banned me over something politically incorrect) so lets say bitbucket.org

OMG, you could make it an android app with a kit that plugs into the stereo jack on a mobile phone in theory. They have pretty good sound-cards with low RF due to their battery power source. There would be limitations / limited tolerance at the low ranges (pf / nf) but it would be interesting to investigate.

Update: That would be a crazy idea for a kit. Make a CRO probe for android / iphone & do some app development to have it automatically analyse capacitance.
Make the kit very cheaply - and have it get extremely accurate results (within it's tolerance range) on a big pretty LCD.

Having capacitance tester function in new app that automatically interpret the results. See if it's more accurate than the low end DMMs. Could become a popular kit / app. I wonder if anyone has already done this. It would more popular if it's open source we make a raspberry PI/PC version too. You'd need to add a 9v battery and some kind of regular or maybe one of those 12V cells that the combination ESR/Cap testors use and the protection circuitry.

The Raspberry PI's though have horrible 12Bit audio ADC/DAC chips on die in them though so it would work better with a phone/PC application due to the quality.

 

[Luke McKee]

throbescottle: I came here to learn a thing or two rather than just changing the caps and praying, and now there is more to it than just the same cap / voltage value I see now. Because you told me to check the specs on the caps, I did and boy did I learn something. YOU WERE RIGHT - FAKE PRODUCTS FROM CHINA ARE ALL THROUGH NAM! Sometime's fiddling around before getting your hands dirty pays dividends.

Original: **broken link removed** KM(M) marking - GENERAL not customized for SMPS
Original Vendor Series Chart: **broken link removed**
Replacement: https://www.nichicon.co.jp/english/products/pdfs/e-he.pdf HE(M) marking "Long life"
Replacement Alleged Vendor Series Chart: https://www.nichicon.co.jp/english/products/alm_mini/index.html
HE series only goes to 100V. Oops I have a "450V" one HE(M) marked one.
And what's on every Nichicon's product page?

ALERTS......Customers Beware
Counterfeit Nichicon Products
To all Nichicon customers,
Recently it has been reported that counterfeit Nichicon capacitor products are turning up in the market. These counterfeit products are not produced by nor guaranteed by Nichicon. They are of inferior quality and could cause end product failure as well as possible safety hazards.

In order to protect your company, do not use counterfeit parts. Only use genuine Nichicon products purchased from an AUTHORIZED Nichicon distributor or Nichicon sales office.

If you have any concerns as to the authenticity of any Nichicon product please contact your local Nichicon sales office for support.

New replacement I found:

Nippon **broken link removed** (mfd 2010)
Original Max Ripple Current: 440mA * 0.80 (adjustment due to 50Hz / < 200uf as per data sheet)
Replacement Ripple current 660mA * 0.81 ("" "" )
OK
Original Dissipation Factor 0.24
Replacement Dissipation Factor 0.20
OK?

 

[throbscottle]

I'm afraid I've reached the limit of my capacitor knowledge, but I'm sure spec or others can advise you about df.
Re: post #9. The de-facto spice application that people on this forum (and in a lot of other places) use is LTSpice. You can download it for free from Linear Technology's website. It runs well on wine (I use Arch Linux btw). It's easy to learn, has a huge support community, and is fairly easy to add non-LT models to.
I would expect the pwm chip to exhibit resistance in both directions between vcc & gnd. 800R in one direction and short in the other suggests something is wrong. OTOH this particular chip might happen to be like that. You can check the resistance between other pins, it's normal to get some kind of resistance reading between any pins of any chip unless you know different for that device. The output mosfet should read o/c in one direction and like a diode in the other (parasitic diode).
A tip for when you are testing - I've never done this myself - but when testing something mains powered you can connect a light bulb in series with the mains, this will limit the current since incandescent bulbs are constant(ish) current devices.

If I have written garbage I apologise, it due to Jim Beam...

 

[throbscottle]

I think you may be a able to jury-rig a test circuit with the chip in-situ.
Since Vcc is a low voltage you should be able to power the chip by tacking a low voltage supply from a wall-wart to it in place. The chip's oscillator should run which you can then test. You obviously have the imagination to figure out how (and I am drunk so I wouldn't trust anything I say). It'll try to produce maximum output because there's no high voltage going into the transformer primary.

 

[Luke McKee]

duplicate
**broken link removed**.
Cable attacked by those pesky great white sharks again. I have lag.

 

[Luke McKee]

Oh happy ending. The PSU was fixed. Filter Cap and fuse less-resistor primary fault. Replaced the 2W fuseable with 2x 1ohm 2W resistors, and I'll have to make sure power-saving is on for safety. The extra surface area on two resistors with double wattage waiting and no heat-shrink tubing on them should do the job.

For good luck it needed a change of the secondary stage caps as well. My next step is to fix an IMPI-111C that's had it's PWM IC blown up with a blown fuse, if the local LCD repair shop can't find a spare board.

Your Jim Beam binge was very useful because you did inspire me to check the data sheets on the caps, and I found out the first attempt at buying a replacement was confirmed to be a Chinese fake load of crap, so you are forgiven

A way to spot a fake is it has a 450V rating in a series of capacitors that only goes to 100V.

I use Gentoo.org Linux. I have respect for Arch. It's those debconf nuts are our sworn enemy. They use obsolete source half the time and patch the crap out of it with crap that doesn't cleanly go against mainline.

 

[throbscottle]

Your Jim Beam binge was very useful because you did inspire me to check the data sheets on the caps,

I really don't see the connection - but good work! Or did my mention of spec the person make you think of spec the specification?

Anyway, well done on the fix.
I used Debian a long time ago (in the early 00's). Had a good time with it. Never tried Gentoo - I think I'd switched to Arch before I really looked at it. I went something like Red Hat 6.0 - Suse - Mandrake - Slackware - Debian - Ubuntu - Arch. Or something like that. Only trust the ends of the list!

 

[Luke McKee]

Oh here's another small question. Again it's worth asking and will lead to yet another repair - yay! My dad sent over a touch screen XPS Dell laptop for my daughter to play with (those old scummvm games for the little ones etc), and it had problems starting now it doesn't start. Now as you know I'm an IT person and this is a hardware issue, either a rectifier (in the unit) or a the external SMPS.

The external SMPS is 18.5 volts without load, and the laptop is expecting 19.5V. From my PC experience I know how sensitive the solid state components are to 2/3 threshold voltage, and that tolerance is much much less with modern CPUs

I was watching another youtube video and a SMPS without a load is meant to have a higher voltage (at least the power-packs), and others have very low or no voltage without load.

First step of course is to hunt around for another working similarly rated 19.5V supply and transplant the plug on it. My question is - is 18.5V without load a sign that it's definitely gone? I've seen a few youtube videos that imply that these external laptop power supplies or 12V power packs normally output a higher than rated voltage (by .5-1V without load). Is this true?