Pls assist : MOSFET BOOSTER debug

[bonaventura]

after several days of sleep deprivation, i must humbly request for your assistance to debug my circuit.

the circuit is the Jack Orman's mosfet booster http://www.muzique.com/schem/mosfet.htm and as my second project, i challenged myself to try to layout the components myself:


Pls note that the mosfet were soldered on the board, DGS on the 5th, 3rd, 1st row from the bottom respectively. all values were as prescribed, except for the pot which was 5K linear.

the components were soldered into place accordingly, except for the biasing trimpot which was soldered on the first column instead of the fifth for ease of access before i replaced it with a resistor. quite ironically, this is the first time, after 3 jfet preamps, i got those silvery solder joints  

the circuit was biased to get 5.48V at the output node and battery was a healthy 9.22V.

this is the problem, i get slightly less than unity with the gain pot at one end and no output at all at the other end.

these were the measurements:
Rdrain 5.48V
Rsource 3.7V
Zener 0.48V (cathode) and 3.7V (anode)

Qdrain 5.48V
Qsource 3.75V
Qgate 0.48V

VRbias1 (trimpot, 70Kohm) = VRbias2 = 5.20V

so i figured that at one end, the source bypass cap is disconnected fm ground, which is (i think) is expected when we put the gain at the minimum. but as i turned the pot to the other end, the signal was attenuated until completely gone.

i traced the circuit board for shorts and found none. right now, since the pot is acting like a volume ctrl (plus, if im not mistaken, the gate voltage is rather low), im inclined to think that this is a layout error but i traced and traced my layout and still dont know whats wrong with it.

sooo, im really stumped.

appreciate your assistance guys....

or else, we can call it a mosfet attenuator and box it  

[PRR]

The measure Gate voltage sure is wrong, but that is a 10Meg point. What kind of meter? Digital or a needle-meter? Any specs on input loading?

The trimmer actually varied the bias? Can you trim the Drain up to 7V? Report Drain, Source, and Vr voltages.

[bonaventura]

The measure Gate voltage sure is wrong, but that is a 10Meg point. What kind of meter? Digital or a needle-meter? Any specs on input loading?

The trimmer actually varied the bias? Can you trim the Drain up to 7V? Report Drain, Source, and Vr voltages.

its a dmm alright, but i cant find info on the input loading. with the trimpot maxed (110k ohm) this is what i get:
Qdrain 6.35v
Qsource 2.67v
Vr 4.12v

[PRR]

I'm stumped also. Aside from an undiscovered bad-joint, all I can think is a funny-bad (or mis-pinned) MOSFET.

[bonaventura]

mispinned mosfet? you mean it might be Source-Gate-Drain instead of Drain-Gate-Source?

[R.G.]

Not all BS170s have the same pinout. You need to look up the pinout for the manufacturer that made that specific BS170 or get one whose manufacturer is known.

You can sometimes tell the pinout with a multimeter, because the drain and source are connected by a diode in one polarity, open in the other.

[Mike Burgundy]

Also, if you have an idea of what the pinout *should* be, you can verify it like this:
clip (tiny alligator leads, a must-have!) your DMM's - lead to the suspected drain. Don't touch the MOSfet  leads unless you intend to, from now on.
Short out all leads (or at least suspected source and gate) with something conductive (I often use a resistor lead, even a finger should work). This makes sure source and gate are at the same voltage (0) and the MOS is OFF.
Verify with your DMM that Rsource-drain is HIGH (resistance test) - MegaOhms.
Switch DMM to diode (a lot of DMMs measure resistance with a voltage so low it doesn't turn on semiconductors, diode test has a high enough voltage) and touch the + lead to the gate.
This charges the gate to the DMM's measuring voltage and turns the MOS ON.
Switch DMM to resistance, touch + to source and verify Rsource-drain is LOW.
If you short out source/gate it should go high again.
If this works, both your MOSfet and its pinout have been verified. Hope that's somewhat clear....
*edit* RG's test is just as good and a lot less work, but I like the fact you see the thing *working*. And there's no actual need to take it off diode setting either. Both ON (resistance very low) and OFF (very high) will show as a reading.
hih

[R.G.]

I confess: I indulged in a bit of social engineering. Unless you're careful, handling a BS170 or other discrete MOSFET to test it has a fair chance of puncturing the gate oxide, and then causing you to buy another, ideally from a known manufacturer whose datasheet would then tell you the right pin out.

I hereby admit my sin.

[bonaventura]

update : i was bummed to learn that i dont have any spare fets. so as RG had predicted, i snuck out to get some. if my wife had known that i went to the city to get .5 dollar transistor she would have been pissed  

i desoldered the fet and tested, turned out that it was a bad fet. the gate is passing current to the drain and source (punctured?). i measured ard 2.5k ohm gate-drain, 3.7k ohm gate-source and 2.5k ohm drain-source. i shorted all leads and remeasured with same result. and to think, that i spent all those night debugging while dismissing the idea that the fet was bad, jeez!

popped in the new fet and now its working  

so im guessing that the less than unity output that i got was actually from the input reverse biasing the output node. and since the fet was actually acting like a node, the pot got access to the signal so as i lowered the resistance, all the signal went to ground. does this make any sense?

anyhow, learned some valuable lessons here, especially re: testing mosfets (gonna do that from now on) all thanks to you guys!

now, one last question before i edit my enclosure transfer again, can we do the mosfet test when it is already soldered to the board (i dont want to try now, too scared   ).

[Mike Burgundy]

Nah - with all the components attached the charge on the gate that turns the device ON will bleed away as soon as you move your DMM lead and turn it off again.
Like RG said, you have to be careful when handling MOSfets - they get damaged all too easily by static electricity.
Glad it's solved!