Takasago KX-210 power supply

[Asuka]

Can't tell from the picture if it's visually broken.

No, you are exactly right, you can't tell from that photograph. That's why I shone a light bulb through the circuit board for this photograph:

https://www.eevblog.com/forum/repair/takasago-kx-210-power-supply/?action=dlattach;attach=392503;image

The point is, I don't have a camera which is good enough to take a photograph showing what I can see with my eyes, which is that the enamel wire is broken.

You sure you didn't measure on the enamel of the magnet wire, yeah?

Yes, I am very sure, because I was able to measure continuity from the transformer lead which I indicated with the pink circle on the back side of the board to the plus lead of the large capacitor, but not to any other leads on the transformer or any other parts of the circuit board except for that large power track which goes from the plus lead of the capacitor towards the transformer. That was the measurement which I thought was very odd, which is why I had a very good look at the transformer and found this. As for measuring on the enamel wire itself, I'd like to see someone try it.

Next stop is to desolder the transformer, and try to fix that tiny little wire.

[Armadillo]

You know, for example switching transformer EMI screen is a screen [copper plate or wire] with 1 lead, normally connected to ground and nothing else.
But you can take it out and investigate.

Just don't investigate "too long" because these wires maybe fragile. 

[Asuka]

You know, for example switching transformer EMI screen is a screen [copper plate or wire] with 1 lead, normally connected to ground and nothing else.

You are absolutely correct again. Please see the attached photographs.

But you can take it out and investigate.


Just don't investigate "too long" because these wires maybe fragile. 

You are always very helpful and concerned about everything, I feel glad to get your attention.

[Asuka]

I was able to remove the transformer from the board after quite a lot of effort with soldering wick. The through holes seem to be able to swallow a lot of solder and it was rather hard to get them out. I have attached two pictures of the broken wire. It is about 0.2 mm in diameter I think. At first I was planning to repair it using heat sink tube and 0.8mm diameter solid core wire from a doorbell, which you can see in the third photograph. That doorbell wire was just too thick to solder on. In the end I used 0.35 mm enamel wire which you can see in the fourth photograph. Wrapping the other end of the enamel wire around the transformer's leg was a rather delicate job but fortunately the wire and the solder joint survived that. I was able to resolder the transformer to the circuit board and I have confirmed continuity of the connections, as well as non-continuity of the things which aren't meant to be connected. The newly-soldered wire has a resistance of 9 ohms, which seems like a lot, but it is extremely thin wire. Unfortunately removing the solder with the wick left a lot of mess on the back side of the board, so I'm going to clean this up and then reassemble the power supply tomorrow. I don't have anything suitable for cleaning the board at the moment.

[Armadillo]

  Well done, you found the fault.
With the transformer removed, can see the broken wire clearly.

[Asuka]

Well done, you found the fault.
With the transformer removed, can see the broken wire clearly.

Thanks very much for your help.

If it starts working with this fixed transformer, I might write to the manufacturer and ask for a replacement for the transformer just to be on the safe side. I haven't replaced it back yet, since it's been a long day. I'll post an update when it's all back together.

[Asuka]

I put the power supply back together today and tried powering it up. Unfortunately the unit did not operate. I noticed the following symptoms: LD1, the pink circle on the left in this picture, lit up, and HS3, the heat sink behind the FET on the right, also circled in pink, got extremely hot, so that a smell was noticeable.

It's possible that the transformer failed again due to my repair breaking, or that I should have cleaned the back of the sub-board more carefully, or that I damaged something putting it all back together, or I failed to connect something properly, or there is another problem in the item somewhere.

[Armadillo]

Did you manage to get a current draw this time? or the brightness of the lightbulb?

If its hot and smell, something is shorted. This is a easier repair than other types.
I would dismantle the connectors to all the outputs [power input is needed].
Check all the mosfets, diodes and caps.

The next time you want to run it, make sure the lightbulb in series method is used. When the light is bright, turn it off immediately.

[Asuka]

Did you manage to get a current draw this time? or the brightness of the lightbulb?

I didn't have a light bulb in series, so I've spent some time rigging up an isolation transformer and current limiter.

If its hot and smell, something is shorted. This is a easier repair than other types.
I would dismantle the connectors to all the outputs [power input is needed].
Check all the mosfets, diodes and caps.

Yes, I will dismantle the unit, then clean it up, then check the continuity.

The next time you want to run it, make sure the lightbulb in series method is used. When the light is bright, turn it off immediately.

Got it. I have rigged up the light bulb as shown in the picture. Isolation transformer also made from back-to-back audio amplifier transformers.

[Armadillo]

Ehmmmmmm....... You would need to know the KVA and the voltage drop. I don't know but I assume you did your maths.

[Asuka]

Ehmmmmmm....... You would need to know the KVA and the voltage drop. I don't know but I assume you did your maths.

I did the maths of matching the transformers and checking the resistance of the coils, and I tested it out with a 12V AC power supply.

If I have a 100 W light bulb in series with the transformers, the most amperes that could possibly flow is 1 amp, and these are big audio amplifier transformers which take mains inputs. Both weigh about two kilograms with outputs around 50 volts, so I don't think there is any problem.  The output of the transformers is exactly the input voltage, 100V, if the bypass is switched on, and reduces to 95V when the light bulb is in series. I think it's adequate. I've also put a 3 amp fuse in between the transformers and the mains input.

[Armadillo]

So, if its supplying 1A then theoretically zero volts is left for the DUT???
What is the Watts rating of the KX-210?

Edit: But if your objective is just to test for shorts and bypass thereafter, I suppose it should work for you.   

[Asuka]

So, if its supplying 1A then theoretically zero volts is left for the DUT???

I am surprised that the same person who suggested using this is asking this question. The bulb acts as a 100 ohm resistor in series, so the maximum current which may flow is 100V / (100 ohms + load), which is always less than one amp. It only goes to 1 amp through the light bulb if there is a short circuit.

My design is based on the one in this video:

What is the Watts rating of the KX-210?

Maximum input is 5.5A depending on the load.

http://www.takasago-ss.co.jp/english/products/power_electronics/dc/kx/kx.html

That would require a 600 W light bulb in series. I'm not planning to run this under load anyway.

[Armadillo]

I am surprised that the same person who suggested using this is asking this question.

Don't be surprised. We are just trying to guide.   

The important thing is the Operating Voltage of the Takasago. What is the minimum voltage that it can operate so that you can check the circuit.
For example 0.5A X 100 ohms = 50V, so 50V remains for the Takasago. Will the transistor switch at 50V?
And how about at other starting or operating currents?
But always, never try never learn, aside from that the transformer will also drop some voltage other than the bulb.
I am not good at 100V supply. 

Edit: You can always parallel another 100W incandescent bulb if not enough. 
But i am not sure of your audio power transformer back to back configuration especially effect on voltage drop.

[Asuka]

But i am not sure of your audio power transformer back to back configuration especially effect on voltage drop.

The output voltage is 100 volts, identical to input voltage. I have measured it with a multimeter. Anyway, I ran it for a half an hour with a 100 W light bulb and the correct-way-round transformer didn't even get warm. The wrong-way-round transformer got a little bit warm after half an hour. I don't think it's going to be any kind of problem.

Today I had some time and took the thing apart again. I cleaned up the mess I made last time, and started measuring continuity. There is continuity right across the gate and source of the mosfet which was getting so hot before. The part number of this MOSFET is 2SK1808, which seems to be this:

https://www.renesas.com/en-eu/products/power-mosfets/mosfets-for-switching/device/2SK1808.html

(link to English-language page). The gate of the mosfet is only connected with a 200k-ohm and 22kohm smd resistor,


so I don't see how it can be short-circuiting like that. Perhaps I misunderstood something. According to the data sheet there is a function like two zener diodes between gate and source, but I just get continuity with a voltage difference of less than one volt. I also measured the resistance, which was much less than one ohm. The package doesn't seem to be physically damaged at all.

[Armadillo]

You are right to use ohms to measure shorts. if it is less than one ohm both ways [red and black probes reverse] it is then a definite shorts.
This warrant you to take out [desolder] the mosfet and confirm it externally that it is dead sure shorts before you order things around.
If you are able to take out the transformer, I am sure you can take out the mosfet. Just remember to retain any mica insulating sheet or the plastic screw sleeve.

Is this mosfet part of the auxiliary power supply? Seems rather huge?   

[Asuka]

You are right to use ohms to measure shorts. if it is less than one ohm both ways [red and black probes reverse] it is then a definite shorts.

It's about 0.5 ohms on this multimeter, but it's not an EEVBlog multimeter but just RadioShack.

This warrant you to take out [desolder] the mosfet and confirm it externally that it is dead sure shorts before you order things around.
If you are able to take out the transformer, I am sure you can take out the mosfet. Just remember to retain any mica insulating sheet or the plastic screw sleeve.

Hopefully the mosfet will be easier than the transformer.

Is this mosfet part of the auxiliary power supply? Seems rather huge?   

It's the same one as in the picture before, where I circled the heatsink in pink.

If the mosfet is faulty, it seems like I need to find a new replacement since this is no longer in production.

Putting V_DSS, V_GSS, I_D into Mouser I find STF4N90K5 from STMicroelectronics and FQPF4N90C from ON Semiconductor, and a few similar things at DigiKey. Is that a good way to choose a replacement, I wonder?

[Armadillo]

I would agree with the STF4N90K5 in TO-220FP package.

I suggest you remove the mosfet and measure again to confirm no other shorts.
From any DC capacitor after the rectifier, you measure ohms to make sure that the capacitor charges up to high enough ohms and not shorted or low ohms. If there are still shorts, the DC capacitors will show a short, meaning cannot turn power on. But still cursory measure each mosfet to be double sure, no more shorts.

Have you already study the auxiliary power supply at least it can output the voltage at the DC capacitor after the regulator or rectifier of the auxiliary supply?
You can isolate the main switching side by taking out some fuse or connector. Just test the auxiliary power supply for function.
Now you have the isolation transformer ready, you can use it to test it.
Be safe and take your time.
I always will forget to unplug and discharge capacitor after test. I think I need to tie a red string on my thumb for reminder.   
Wear a rubber glove and a safety goggle to add confident level.