Starting a First Watt F3 build and need some help

[MikeW]

http://www.passdiy.com/pdf/zen-v4.pdf

Look at this one page 5.

The one for the F3 is a split rail. This is the one you want.

 

[Marsfrogie]

Ah, that makes the series wiring more apparent. I think the F3 power board is accomplishing the same thing with the two jumper wires that can be seen on the center of the board in some of the photos. Correct?

 

[MikeW]

I think he put the wrong power supply in that article. It may be for the F4.

 

[Marsfrogie]

I'm pretty sure it's the correct power supply.

http://6moons.com/audioreviews/firstwatt4/f3_2.html

Look at the 5th picture down.

 

[dave_gerecke]

Hello Marsfrogie, Check out www.firstwatt.com for manuals on the various firstwatt amps that Nelson Pass has done. Once he has completed production on a particular model, he is kind enough to post the full manual, including service information and schematics on this site. He also has other designs at www.passdiy.com

Peace,

Dave

 

[Marsfrogie]

??? I've read that site upside down and sideways 3 times through. That's why I'm pretty sure it's the right power supply board. It doesn't help that I don't understand most of it though.

 

[MikeW]

The power supply doesn't match the schematic for the amp. The 6 moons article thinks a MOSFET is a power JFET. I have built this amp. Use the power supply in the Zv4 or get the boards from Peter D.

 

[Nelson Pass]

It's the same board, but it's strapped differently for +/-23V
or +46V or two isolated +23V.

 

[CeeVee]

Thanks Nelson,

Look at the F3 service manual guys, PSU PCB, options A,B and C ( the one for F3 ).

Naturaly this means that the schematic changes....the GND line ( for F3 ) will be the ( - ) and both center taps are just connected together ...not connected to audio channels.

Right Nelson ?

 

[Marsfrogie]

Well I finally got the amp mostly together and think I either have a short in the right channel or blew something.

Looking on the First Watt F3 R0 Schematic here are the voltages I get at the measurement points. The amp does not generate sound.

Input Target 46V Left: 42.9V Right: 42.9V
Next to Q5 Target Voltage 41V Left: 30.2V Right: 34.4V
In front of C1 Target 21V Left:16V Right:0V
Next to Q2 Target 3.5V Left: 0.6V Right:0V
Jfet voltage Target 1.1V Left: 0.6V Right: 0V

Any advice on where to go from here? I am out of ideas. On the left channel I was only able to get 16V tops by adjusting the potentiometer. Any help would be greatly appreciated.

 

[Nelson Pass]

I would remove Q2 and Q3 and see if you can get some expected
voltages off the Drain (45v) and Source of Q5 (probably 42-43V).

 

[Marsfrogie]

Left Side Mosfet Top Pin:32V Middle Pin: 50.5V Bottom Pin 35.4V

Right side mosfet Top Pin: 35.4V MIddle Pin: 50.5V Bottom Pin: 32.5V

I'm not positive as to which one is gate and source so I tried a graphical top down representation. The middle pin is the source I would assume, 35.4 is the gate and 32.5V is the drain.

I pulled all of the voltages again on the board.

Input Target 46V Left: 50.5V Right: 50.5V
In front of C1 Target 21V Left:32V Right:0V
Next to Q2 (R11) Target 3.5V Left: 12.88V Right:0V
Jfet voltage Target 1.1V Left: 0V Right: 0V

I know this might be difficult to interpret but I will do my best to clear up anything that seems ambiguous. Thanks.

 

[Nelson Pass]

The question was: What are the voltages on the Drain and Source
of Q5?

 

[MikeW]

Looking at the top with the pins facing you. It is Gate, Drain, Source.

http://www.irf.com/product-info/datasheets/data/irfp140n.pdf

 

[Marsfrogie]

Left Q5

Gate: 35.4V
Drain: 50.5V
Source: 32V

Right Q5

Gate: 35.4V
Drain: 50.5V
Source: 32V

Measurements taken with Q2 and Q3 removed. The other measurements in the last post were taken with Q2 and Q3 removed as well.

Thank you,

 

[MikeW]

The gate is to low. Is D1 a 1n4001 or did you put in a zener diode?

 

[Marsfrogie]

Yes the diode is a 1n4001

 

[MikeW]

There should not be a drop across R19. It is there along with C5 to give a nice stable voltage to the Gate of Q5. R20 is a gate stopper so the MOSFET doesn't self oscillate. Check and see if something else is connected to the Gate of Q5 or R19.

 

[Marsfrogie]

I will check that as soon as possible. Unfortunately I am forced to leave town tomorrow and will probably not be able to see the amp again until Friday. Thanks for the help and I will report results as soon as I get them.

 

[Nelson Pass]

There should not be a drop across R19. It is there along with C5 to give a nice stable voltage to the Gate of Q5. R20 is a gate stopper so the MOSFET doesn't self oscillate. Check and see if something else is connected to the Gate of Q5 or R19.

Or see if there is any voltage drop across R20 - there shouldn't be
any.