Fake MOSFETs!!

[dicky]

Came across this one tonight and I wasted a good couple hours trying to figure it out! So this is just a quick heads up for the few others here who like to repair stuff.....

I have a load of Amps here I am fixing - and the first one had a short circuit MOSFET on the amplifier output - device number 2SK1058

That was easy enough to diagnose, so I ordered a replacement from a UK seller on ebay.

It arrived a few days back and this afternoon I got back to repairing the amp.

I fitted the replacement, checked all around the Amplifier PCB for any other short circuit transistors. All looked good, so I fitted new fuses and powered the amp up. However as a precaution I always connect the mains via a 60W light bulb in series with the live wire until I am happy all is safe. This is so that if anything bad happens, the worse you will see is the bulb come on bright, and not something go bang!

Anyway - the bulb came on full brightness and I got a mains hum on the speakers.

Hmmm.... something is short circuit, but for the life of me I couldn't find a short.

So I powered up the amp again (via the lightbulb) and checked the plus and minus voltage supply rails - about +3V and -3V (should be a good 30V or so)

What you have on this amp is 2x 2SK1058 N-Channel Mosfets connected in parallel from the positive supply and 2x 2SJ162 P-Channel Mosfets in parallel from the negative rail. The Source pin of all four devices connect together and to the +ve terminal of the speaker. The speaker -ve goes to 0V (ground)

There was no voltage on the Gate of any of the Mosfets, so with all devices effectively switched off (no gate drive) then you would expect to find zero volts on the Source pins. But I had about +2.5V - half a volt less than the 3V on the positive supply rail. One or both of the 2SK1058 was conducting when it shouldn't. So I removed them both again and tested on my Atlas component tester. Both test OK as N-Channel Mosfets... hmmmm. I then powered up the amp without the 2SK1058 and it powered up with no current flowing as such. I had found my 'short'

OK so now I soldered one of the 2SK1058 back into the PCB - it was the good one of the original pair. I powered the amp up once again and got a quick flash on the light bulb then it went out. This looked good, so I tried to play some music, and the amp worked! The bulb was flashing with the bass beat and obviously the amp was clipping and distorting as the supply rails were being current limited by the bulb - but it did work. Next I removed the good Mosfet and soldered in the other position on the PCB and it worked again

Right so now I was onto something, I soldered back in the new 2SK1058 I had bought. And immediately I got a full lit lightbulb. The short was back. Took out the new 2SK1058 and checked it agin on the component tester - it tests OK.

Now this was getting really silly so I decided to have a look at one of the other amps (same type) This also had a couple of short circuit mosfets, but one of the 2SK1058's tested OK. I was gonna solder this into my faulty amp and see what happens, but I suddenly had an odd thought..... I tested the new 2SK1058 I had bought from ebay again, then the one from the second amp, and suddenly realised the pinout on the new device was reversed!!

It should be 1: Gate 2: Source 3: Drain

This damn thing was 1: Gate 2: Drain 3: Source!!!!

My only guess is it is a bloody fake!!!

Anyway I just messaged the seller with the attached pics and let's see what they have to say about it. The component was less than a fiver but I have wasted several hours and lost a little sanity before I figured out what the problem was. On the plus side I proved my abilities to fix stuff have not wained lol

Rant over!!

Rich

[Nakatomi]

Hahaha Rich that's quite a find! Faulty FET I can find. FET wrong way round I'm not so sure about.

Nothing would surprise me these days - just goes to show that maybe Ebay isn't the best place to buy components. Fake chips are all over the place now but the worst thing I've ever seen was on a PC motherboard back when cache RAM was external to the CPU. The board I bought was cheap-ISH but not massively so, and I couldn't get it to work right for love nor money. Anyway, I took it to work to give it a once-over to see if it was anything obvious. I thought "ooo ello, what's this?" as I looked at the solder joints of the j-leaded QFP cache ram chips... all the pins were shorted together & the top of the chip had gaps at the corners. A little more investigation revealed that there was nothing inside the package at all. Just 144 or however many legs of the chip sandwiched in the casing which had all the right numbers written on it.

[dicky]

You are quite right Justin and a lesson learned for me too - hence I thought it worth the post here to warn anyone else into repairs

Stupid thing is I tested the new device on the Atlas before fitting it and when it came up as N-Channel MOSFET I thought no more of it

If I hadn't the previous expertise/knowledge to use a 60W light bulb as a current limiter (learned that trick the hard way a very long time ago) this amp would have seriously gone BANG most certainly destroying the replacement MOSFET and possibly some of the P-Channel ones as well and anything else the current surge may have taken a grudge against -and then left me wondering A: what the hell I did wrong and B: why I now need a new pair of trousers!!

And never having the evidence to be able to get to the bottom of it. I may even have checked everything over and ordered another one and tried again!! :rolleyes:

Will be interested to see what the vendor has to say about it

Rich

[dicky]

This supplier is now taking the mick!

--------------------------
Sent to supplier

Hi
I have a serious problem with the 2SK1058 you sent me - it appears to be fake

Once I soldered it into the amplifier I was repairing, I powered the unit up via a current limiter (to prevent anything nasty happening) and my limiter immediately reduced power to the unit

The device you sold me apparently checked OK as an N Channel Mosfet on my atlas. This caused me a lot of messing about for about two hours as I could get the amplifier to power up via the current limiter and work if I fitted the good one of the original pair of 2SK1058 in either position on the PCB (there are two in parallel) on it's own.

But with your device I got current limit with it soldered in either position. I couldn't understand this as both Mosfets checked OK on my Atlas tester - until I suddenly realised the one you sent me has the wrong pin out!!!

It should be 1 Gate, 2 Source, 3 Drain (google the datasheet) but you can clearly see on the attached pics it has the Source and Drain reversed.

See Pics!!!
(Same pics I posted on the Opening post here)


--------------------------
Reply from supplier

Hi,
You are, probably, doing something wrong, because I sold 17 pieces from the same batch without any complaints.
Maybe something else is causing problems in your audio.
Regards,

--------------------------

My response to supplier

Doing something wrong? Not in the slightest - since messaging yesterday I removed a working 2SK1058 Mosfet from another amp (I have quite a few of these faulty amps) and fitted that one to the the amp I was repairing and it now works fine

Please suggest what I can possibly do wrong that would cause the Drain and Source pins of a Mosfet to magically swap around?

I already sent pics from my Atlas componenet anayser which clearly shows the incorrect pinout.

Here is the datasheet for 2SK1058

http://pdf1.alldatasheet.com/datasheet-pdf/view/63145/HITACHI/2SK1058.html

You will clearly see

Pin 1 Gate
Pin 2 Source
Pin 3 Drain

This is correct.

The device you sent me

Pin 1 Gate
Pin 2 Drain
Pin 3 Source

This is wrong.

I wasted several hours of my busy life only to discover this!

Also on the datasheet you can clearly see the internal flyback diode between the Drain and Source pins. This is what was conducting and causing the 2.5V on what should have been the source pin when there was 3V on what should have been the drain pin - but if course it was back to front so the internal flyback diode was forward biased and conducting hence the 0.5V drop I could measure between the positive supply rail and the speaker out connection.

I suggest you test one of your remaining stock yourself, I am sure you will find the pinout is incorrect for a genuine 2SK1058 the same as I did.

Most likely the reason you have no earlier complaint is because the purchaser fit the new part, it didn't fix the problem (it probably went BANG as most engineers do not have enough knowledge and experience to use a current limiter when repairing amps) and the buyer just assumed they did something wrong.

AS I already suggested - please test one of your remaining stock if you don't believe the photos I sent you showing the reverse pinout

Then you will see that I am correct, these are probably remarked/badged devices of the wrong type, the pinout does not match the 2SK1058 datasheet

Rich

--------------------------------


Let's see what they say now :p

[Nakatomi]

I'd say maybe the seller knows squat all about electronics & they're on course for being forced to refund you Rich.

If it was me I'd replace all the fets with their modern equivalents which are likely cheaper anyway

- - - Updated - - -

I'd say maybe the seller knows squat all about electronics & they're on course for being forced to refund you Rich.

If it was me I'd replace all the fets with their modern equivalents which are likely cheaper anyway

[ckpr2]

I have always bought from RS Components and Farnell and never had any problems with components.

I have seen a thread on another forum where they took apart a fake transistor to compare with an original one.
The fake chip inside was about a quarter of the size of the original.
When it got up to dissipating about 30 watts it blew up where as the original was well over 100 watts.

Having said that modern versions of older transistors can sometimes vary from the original datasheet by quite a bit.
I bought an amp to repair and replaced some 2n3055's.
The amp then oscillated badly on the output.
I had to adjust the VAS capacitor to stop it.
When I put the new transistor on my semiconductor analyser it had huge amounts of gain.
This was causing the instability.