Is this a FAKE MosFET 2SJ78?

[dicky96]

Hi guys
This is the second time I came across this situation when repairing Amplifiers

I Have this InterM amp and someone has worked on it before by another repair shop who couldn't fix it. I replaced a couple of short circuit output devices and found I still had a problem with one bank of output transistors on one channel being hard on when the amp is powered up.

Fortunately the light bulb limiter i was using saved any more damage being done

I traced the problem to a 2SJ78 MosFET that drives the output, it had been replaced already with a 2SJ200 'equivalent' device but I quickly spotted from the datasheet that this 'equivalent' has the Source and Drain pins transposed.

So I ordered a 2SJ78 from ebay and fitted that. I found I still had had exactly the same problem with one output bank on that channel being hard on so I unsoldered the replacement and checked it for polarity

Once again I have Drain and Source wrong way round!!

Here is the datasheet
http://pdf1.alldatasheet.com/datashe...CHI/2SJ78.html
1-Gate
2-Source
3-Drain

From the attached photos you can see my Atlas DCA55 shows the 2SJ78 I bought...
1 - Gate
2 - Drain
3 - Source

I then wondered if the datasheet was wrong so I tested the same device from the working channel and that pinout matched the datasheet.

As I said this is the second time i have found a power MosFET with reversed Source and Drain pins. The first time it caused a lot of head scratching before I fixed the amp (I actually soldered short wires to the device pins and crossed them over to the correct pads on the PCB and that did the trick)

It's only because I came across this exact problem before that I found out what is going on quite quickly this time. Still IMHO it's hard enough to diagnose and fix these amplifiers without this sort of additional headache!

So what's going on here? Is it a fake I have been sold? Has the official pinout of this device been changed at some time over the years since this amp was manufactured?

Anyone else ever had the same problem, is it commonplace?

Rich

[budm]

Re: Is this a FAKE MosFET 2SJ78?

Look at the print on the body of the original and the new one you bought, you may be able to tell that the one you bought is not the real part. Does the original have Green body?

[dicky96]

Re: Is this a FAKE MosFET 2SJ78?

TBH budm - I just did what I did last time I came across this same issue with a replacement MosFET - I bent the outer Pins on the device to fit into the left and centre hole in the PCB, then soldered a short wire from the (cut short) center pin, applied a bit of heatshrink so it could not short the other pins and soldered the device with the Source and Drain reversed.

That fixed the problem with one output bank on Channel 2 being hard on, and the amp now works just fine (apart from now i spotted one of the cooling fans isn't running - sodding typical!)

So I was right, same as last time I had a similar problem. The replacement from ebay is electrically an equivalent but physically it isn't.

OK I'll freely admit there is a certain amount of kudos in fixing these sort of stupid faults another repair shop either couldn't fix, or worse still even caused.... but I would prefer not to have the hassle TBH

Funny enough the same customer has just brought me another Amp that another repair shop can't fix......... hmmm

[stj]

Re: Is this a FAKE MosFET 2SJ78?

most chinese stuff is manufacturing surplus,
the part could be original but part of a special order.
a lot of companys use custom parts with pins switched or in the case of memory devices, custom i.d. codes.