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slideman82 9th January 2013 07:38 PM

Nmos 200 problem
Well, sometimes it blows 4A fuses when turned on... I think it has to be with the voltage multiplier, its transistor probalby can't turn on as fast as it should, so as it's opened current through power transistors (IRF640) is too high, could be that?

Any ideas will help, of course, and I'm thinking on removing voltage multiplier and leave a single preset.

slideman82 9th January 2013 07:41 PM

Btw, voltage multiplier transistor is a BD139

Samuel Jayaraj 10th January 2013 03:06 AM

But does not the Nmos200 use BC546 as Vbe multiplier?

slideman82 10th January 2013 11:45 AM

Should be the same... but maybe not.

slideman82 10th January 2013 08:01 PM

Change to BC546, even with bias at minimum, still blows both fuses (used to be only the -40V rail). Measured output mosfets with diode tester and are not shorted, all of them measured 502 mV. also measured other transistors and none of them are shorted. Don't know really what the problem is...

AKSA 10th January 2013 09:05 PM

Could you give us the schematic, if possible?

Idefixes 10th January 2013 09:26 PM

Mine is running perfectly with MJE340 instead of BC546 and no other changes.

Hugh here is Quasi Nmos200 Quasi's web page :


AKSA 10th January 2013 11:19 PM

Thanks Marc,

I will examine it, might be able to help.


JMFahey 11th January 2013 03:33 PM


all of them measured 502 mV.
That's shorted :( .
They should measure open with red to drain, black to source.
The opposite way you will find a parasite diode (built-in) which will give around 650mV.
To be certain, pull them from the PCB and measure them "outside".
The Vbe multiplier is never "too slow".
Before pulling them, check that drains are not shorted to the heatsink.

PS: you *did* use nipples on those exposed IRF640 metal tabs, didn't you?

AKSA 11th January 2013 10:14 PM

1 Attachment(s)
Attachment 323264


Here is the schematic (I understand this is OK to publish since it's on the public domain).

If the Vbe multiplier BC546 is substituted BD139, the bias voltage at the output gates will be slightly increased because the BD139 has MUCH less beta than the BC device. This would be usually 80 compared with 300, increasing the voltage dropped across the base and collector. This increases the bias voltage considerably which drives the output very hard.

The reason is that the bias generator has around 5.3 volts as designed, and the source resistors are very small at 0.22R. If a small increase from 5.3V to say 5.5V the additional bias will be impressed - anything up to 10A - upon the source resistors, hugely increasing the quiescent, in your case, blowing the fuses.

Vbe multipliers are extremely tetchy, and should has be built as designed to deliver the intended quiescent. You should be able to correct this using a BD139 by reducing the base/collector resistor from 470R to 390R. This is a guestimate, you will need to experiment with the pot at half set, ie 100R.



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