IRF840 heating up when coil attached

[Oldretired]

I recently purchased 2 Mirage Pi boards (unpopulated) on a garage sale. One is the Mirage Pi ultra Jan 2016 and the other one March 2018. After populating the Jan board I checked the voltages on the 7805, 7905 and all the Ic points and all are correct. No parts heat up, (even with speaker and/or headphones attached) even after 20 minutes but as soon as the coil is attached the IRF heats up within a couple of seconds. The 180 mm(I?d) coil has 297uH inductance and 1.1 ohm resistance.
any help or advice will be greatly appreciated.

[Eddy71]

Were the transistors bought on Aliexpress? Of the twenty lots I bought there, eighteen were fakes.

[kt315]

i had told my case as i bought many times XR2206 on aliexpress. it was a long story. every chip was fake.
after two received i wrote to sellers to send me ONLY working, but got just fake again. then i began to read the feedbacks more
attentionally and went on working one.
main rule i opened reading the feedbacks. russians, ukrainians and germans gives sure diagnosis sometimes.

[SVEN1]

I have
found that using a 68k resistor in the Pulse generator and the 390 ohm dampening resistor
along with a 65 us pulse width cause the 840 FET to run pretty hot. I would suggest
increasing the 68K to 82k and the 390 ohm to a 470 ohm if you are going to use a 65us pulse
width. If you are using a 50us pulse width like we did in the past, the values that you are
using might keep it cool enough. However, I would still change the 390 ohm to a 470 ohm.
You can still try a 100K instead of the 68K or 82K.
You can still try a 390 ohm resistor.
Could be a bad the IRF 840 is bad.
Did you attach the coil correctly to the PCB?

See what happens.

March 2018 READ ME FIRST.pdf



Mono DD Coil parts.pdf

[Oldretired]

Logged in again and now attachments are fine

[Oldretired]

Also now found the 68k resistor on the PDF. Many thanks for the help. I’ll let you know how it goes

[Dean Sarelius]

I have
found that using a 68k resistor in the Pulse generator and the 390 ohm dampening resistor
along with a 65 us pulse width cause the 840 FET to run pretty hot. I would suggest
increasing the 68K to 82k and the 390 ohm to a 470 ohm if you are going to use a 65us pulse
width. If you are using a 50us pulse width like we did in the past, the values that you are
using might keep it cool enough. However, I would still change the 390 ohm to a 470 ohm.
You can still try a 100K instead of the 68K or 82K.
You can still try a 390 ohm resistor.
Could be a bad the IRF 840 is bad.
Did you attach the coil correctly to the PCB?

See what happens.

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[ATTACH]56488[/ATTACH]

Hi Sven,
Just wondering as I can not see any ground balance on the Mirage is this a bi-polar design or can you guide me as to the recomended coils...cheers