Mosfet boost won't work

[madboy]

Hey folks.  The mosfet boost ( GGG ) won't work... the pcb is from GGG
The schematic and article are here

http://www.muzique.com/schem/mosfet.htm

The layout is here

http://www.generalguitargadgets.com/diagrams/ge_boost_lo_amz_mosfet.gif

No substitutions.  The voltages are as follows

D 6.25 volts
G 2.80
S 3.27

Zener
cathode 2.80
anode 0

Fresh battery... 9.59 V

Do the voltages tell you anything obvious? Other than checking for solder bridges, is there anything else I should check?
Thanks in advance...

[dmk]

transistor orientation? cap polarity?
double check everything i suppose.
do stick with it though, its a great boost and with a master volume it can get nice and crunchy too

[boogietube]

I had the tranny polarity reversed. I replaced the 62k resistor with a 100k trimmer and dialed the bias in. These two things seem to be everyone's problem with this circuit.  Works great now.
Hope this helps

[madboy]

I checked the tranny ( both ways ), tried a j201 ( and swapped the legs where I had to ).  I'll have to see if I have a trim pot kicking around.
Any idea what the voltages should be?

[PerroGrande]

Your gate voltage is too low.

My gate voltage is around 4.5 volts quiescent, with the Source around 2.5 volts.  I've got around 6 volts on the drain.

With your gate voltage below the source voltage, you're not going to get happiness from a MOSFET.  The gate has to be at least Vt (Threshold voltage) above the source for the MOSFET to be on.  Yours is off, thus the quietness.

[markm]

The trim pot does work quite well.
Maybe this'll help?

http://aronnelson.com/gallery/album76/MosfetBooster_LAYOUT

[madboy]

Thanks for all the advice so far, folks! I do appreciate it.  I tried all the permutations of the bs170 pinout to no avail.
I tried the trimpot... I managed to set it so that the source was at 2.5 volts, but the gate came down with it to 2.39 V. I replaced the 9.1 V zener that connected the two... nothing.

[johngreene]

Your BS170 may be damaged. The BS170 needs the gate to be higher than the source in order for the device to turn on (2V higher typ). Your voltages show the gate lower than the source. The device should be off in this condition. It obviously is not.

A J201 will not work in this configuration because it needs its gate to be lower than the source to turn on. A BS170 will work very similar to an NPN transistor that has a large Vbe.

--john

[madboy]

Is there any way to check the tranny? I only have the one... or are there any substitutions?

[petemoore]

  Other Mosfet, RS sells an IRF510, might not be your first choice, and will almost certainly require 'leg crossing' but should fire up in that circuit.
  Note mosfets max G/S voltage, and sensativity to static, can be damaged even from handling.

[PerroGrande]

That is a really good point about MOSFET handling precautions.  The gate barrier is only about 20v "wide", and casual static electricity can very easily run into hundreds of volts. 

MOSFETs used to ship (back in the day, lol) with a small metal ring shorting out all the leads to protect them from static.  I haven't seen that done in a while, but the principle still applies -- handle with care!

The IRF510 that RS sells will work with the circuit, but its overall gain is a bit lower.  I'll check to see if I have one banging around and see what kind of figures I get with it.

[markm]

That is a really good point about MOSFET handling precautions.  The gate barrier is only about 20v "wide", and casual static electricity can very easily run into hundreds of volts. 

MOSFETs used to ship (back in the day, lol) with a small metal ring shorting out all the leads to protect them from static.  I haven't seen that done in a while, but the principle still applies -- handle with care!

Good advice, I don't think they are overly sensitive but, I must say I have Zapped a few myself. 

[petemoore]

  It's mostly damp in the lab...it probably discharges to ground any *static there, I've worked Mosfets like other transistors often with no problems..YSMV.

[markm]

  It's mostly damp in the lab...it probably discharges to ground any *static there, I've worked Mosfets like other transistors often with no problems..YSMV.

Usually my MosFET work is trouble free too. I think it takes more to Zap them than some have lead on but, perhaps not....I don't really know for sure all I can say is every time one has gone belly-up on me....it's been a surprise and not always a pleasant one at that. 

[petemoore]

  yupp, doesn't say anything about current .. lol.
  Static can easily hit the 20V mark, if that discharges across G/S then G/S becomes suspect [like you'd know, other than known blown tranny, not that I know how easy it is first hand]. I kind of think it's not that easy too.
  When I was up here installing mosfets on the kitchen stove...I'd handle them by the body, then snip the lead length with insulated handles, and stick the Fet in the socket. Once in the socket they are more or less immune from G/Sv above the 20v mark.