When I made my first post about this supply back in 2014, I knew absolutely nothing about SMPS. 3 yrs later, I am still a novice but have learned a lot more thanks to AAC and the good people here but still manage to find things that leave me scratching my head sometimes.
The 270k resistor I changed was brand new and one that was already bad to begin with which makes me wonder if I had a bad resistor or I have something that is damaging them. No external signs of damage but definitely open.
I use an isolation transformer and a dim bulb tester in place of the fuse when I have a blown fuse. I also have 12 volt bulbs for my load so I think I am doing things somewhat right. I use a digital meter but still chicken when it comes to my scopes. Read too many stories about groundimg issues and dont want to blow my analog scopes. Anyways thats it.

 

When I made my first post about this supply back in 2014, I knew absolutely nothing about SMPS. 3 yrs later, I am still a novice but have learned a lot more thanks to AAC and the good people here but still manage to find things that leave me scratching my head sometimes.
The 270k resistor I changed was brand new and one that was already bad to begin with which makes me wonder if I had a bad resistor or I have something that is damaging them. No external signs of damage but definitely open.
I use an isolation transformer and a dim bulb tester in place of the fuse when I have a blown fuse. I also have 12 volt bulbs for my load so I think I am doing things somewhat right. I use a digital meter but still chicken when it comes to my scopes. Read too many stories about groundimg issues and dont want to blow my analog scopes. Anyways thats it.

Well if you are unsure about using your 'scope, I suggest two things to help. first, ENSURE that the output from the isolation transformer has the earth disconnected, Second, use a mains voltage15-20w filament bulb in series with your scope probe earth connection. If the bulb lights, you know there is a problem and it will save blowing up the probe lead or worse. NEVER isolate the earth on the scope, live test equipment is very dangerous.
Do have a look at the free download book I posted a link to earlier,
PDF]Troubleshooting & Repairing Switch Mode Power Supplies
www.powersupplyrepairguide.com/previewsmpsebook.pdf
it is very informative.

 

That "free download book" looks like it will cost you $39. It could well be a great book, but I hate the site. It is one of those overly busy sites that really puts me off. What it shows of the book though does look good.

 

That "free download book" looks like it will cost you $39. It could well be a great book, but I hate the site. It is one of those overly busy sites that really puts me off. What it shows of the book though does look good.

It is free to download as a pdf at

PDF]Troubleshooting & Repairing Switch Mode Power Supplies
https://readerspage.files.wordpress.com/2016/07/smpsrepairebook.pdf
troubleshooting and repairing the SMPS. 3) Some spare parts are expensive and quite hard to get from the market for example; power FETs, power ICs and ...

 

I must be missing something as all those links do is download a 9 page preview.
There may be another link on his site but it is so full of stuff I don't have the energy to hunt for it.

 

I must be missing something as all those links do is download a 9 page preview.
There may be another link on his site but it is so full of stuff I don't have the energy to hunt for it.

Hi, Ive found the direct link for it.
PDF]Troubleshooting & Repairing Switch Mode Power Supplies
https://readerspage.files.wordpress.com/2016/07/smpsrepairebook.pdf
troubleshooting and repairing the SMPS. 3) Some spare parts are expensive and quite hard to get from the market for example; power FETs, power ICs and ...

 

It looks like that book is still for sale. One of the first pages states it is not to be given away.


So I would recommend you buy it instead.
The price is not bad. I just wish he would fix his site up so it was simple and not so full of ...... stuff. I buy lots of ebook, but almost never from such sites. In my mind they just call out "scam" for some reason, but this book looks petty good.

 

I bought this book quite some time ago and have talked to Jestine a few times. He is fairly knkwledgeable an sometimes hard to u derstand with broken English. There are a lot of good pointers in the book but mostly deals with computer and TV power supplies.
My time with scope isnt understanding. Just finding enough time to play around and get good at it. I have two analog and a PC based unit. Picoscope.

 

The link to the free download is from world press, I doubt they would give it for free if they could charge for it. maybe an earlier edition than the one he currently sells??

 

Something that I do which never seems to be mentioned.
All my meter probes have long bare tips which can easily accidentally short out to adjacent parts when working in crowded spaces. I have put two layers of heatshrink sleeving over them just leaving the tips exposed.
.

One job came in, the bloke had tried to perk up the PSU and the skeleton preset was right next to the heatsink - commoned to the negative end of the bridge rectifier. The screwdriver I use for that also has HS sleeve on the shaft.

Turns out he'd been trying to correct low emission by turning the wick up - I had to replace the heater rectifier with a SB diode and add a snubber so the flyback pulses didn't kill it.

Some genius at Philips circulated a bulletin stated that with stable SMPSUs - the CRT heater only needed 6.15V. Reviving poisoned cathodes was steady work for a few years.

 

When I had the domestic equipment repair business, I used to use a B.K tube rejuvenator to give customers another maybe 2-3 years life from their colour CRT's. Although there was a risk of cathode stripping, it was probably successful 85% of the time. Earned its cost a thousand times over during the 80's.
Sometimes all that was required was to over-run the heaters by 10-15% for a couple of days to boil off the surface contamination to revive a tired tube. This is a preferred method with 'scope tubes, + a couple of milliamps from the cathode to grid.

 

When I had the domestic equipment repair business, I used to use a B.K tube rejuvenator to give customers another maybe 2-3 years life from their colour CRT's. Although there was a risk of cathode stripping, it was probably successful 85% of the time. Earned its cost a thousand times over during the 80's.
Sometimes all that was required was to over-run the heaters by 10-15% for a couple of days to boil off the surface contamination to revive a tired tube. This is a preferred method with 'scope tubes, + a couple of milliamps from the cathode to grid.

You could at one time buy CRT heater transformers with a + 10% tap, for both heater cathode shorts and boosting low emission.

The last of the CRT TVs had scan derived heater supply - if the TV was clapped out anyway, I'd add a turn or 2 to the heater winding and with any luck; the tube lasted long enough for the set to die of natural causes.

 

Yes I remember them! Many times I would see early B/W tv's with the mains voltage selector set to a lower voltage to boost the tube. Hey, those were the days eh!

You could at one time buy CRT heater transformers with a + 10% tap, for both heater cathode shorts and boosting low emission.

The last of the CRT TVs had scan derived heater supply - if the TV was clapped out anyway, I'd add a turn or 2 to the heater winding and with any luck; the tube lasted long enough for the set to die of natural causes.

 

You guys are old. Just kidding, I am sure I am pretty close to the same age give or take.
Back to my topic for a second. On these SMPS, what do you guys do to test the small transformers? Static testing for continuity is one method but is it accurate? I like dynamic testing so should I put power to it and see what it does?

 

It is fairly unusual for the transformers to fail I think. Testing for shorted turns could be a good idea, but I'm not sure how easy it is to build a tester. Swapping for a good one is an easier thing to do.

 

Well I built a ring tester which is invaluable in checking smp inductors. Far from being rare to fail, I probably have had 1 in 100 smp faults caused by an inductor. I discussed this with my former partner who took over my business when I retired. When servicing equipment for a living, you need to accurately diagnose as quickly as possible which components are faulty.
Lets say you found a shorted switching fet and associated parts, replaced them, power up, then BANG!!! Oh dear, wasted time and money because now you have to start all over again. An hour or two later you have traced even more blown parts, you've checked the resistance of the transformers, maybe even checked the inductance and they appear ok. then you wisely slowly increase the input power (which you should have done the first time) and it is drawing excessive current. At this point, you are really frustrated, what could be wrong??
Now this is why a ring tester and oscilloscope will save the day! By ring testing, you will be able to confirm whether or not there are shorted turns in the primary windings. If you had done this at the start, much time effort, money and frustration would have been saved.
Just recently I was asked to repair a laser printer with no power and blowing fuses. The switch driver i.c had internal shorts and a capacitor looked suspect. I removed both and the ring tested the transformer, sure enough, shorted turns. As this was a cheap printer and no spares available, it was consigned to the bin!

Later today, I will post some pictures of the set up and method I use. After 55 years in electronics, 22 of which were in servicing, I think maybe I might just know what I'm doing lol.

(My interest in electronics started at around the age of 5, by 7, i new all the colour codes, could read circuit diagrams etc. made an oscilloscope when i was 10 using ex W.D parts. so guess I've actually been in electronics 64 years!!)

 

It is fairly unusual for the transformers to fail I think. Testing for shorted turns could be a good idea, but I'm not sure how easy it is to build a tester. Swapping for a good one is an easier thing to do.

But you need a good one to swap with, you can build a ring tester for £20.00 and then you don't have to spend £hundreds on transformers most of which you won't need, or have to wait days or weeks to arrive.

 

But you need a good one to swap with, you can build a ring tester for £20.00 and then you don't have to spend £hundreds on transformers most of which you won't need, or have to wait days or weeks to arrive.

A tester would be a good idea. Can you put up a link to a good design to make one?
But he has a good supply and a faulty one so can swap for the good transformer to test the faulty supply.

 

A tester would be a good idea. Can you put up a link to a good design to make one?
But he has a good supply and a faulty one so can swap for the good transformer to test the faulty supply.

Here is one, but mine is slightly different, I will try and locate where i got the schematic from.
http://rudzwijaya90.blogspot.co.uk/2014/08/DIY-Blue-Ring-Tester
or just google flyback tester schematic diagram

If i remember correctly, mine was based on a design by Dick Smith. I added an output bnc connector to leave a 'scope connected to the "hot" output via a 22k resistor.
Prior to building it, the way we used to Ring test Line output transformers in t.v's was to use the timebase output from the scope and see if the ring was damped by shorted turns.




.

 

Here is the original, I made mine a long time ago and run it from a 9Volt pp3 battery not 6Volts, can't remember why I did that though. Probably because it was what I had to hand at the time. Just add a bnc connector and connect to "Hot" via a resistor of 22-100k and connect it to your scope. Later I will post some pictures to demonstrate.