First Build: Ruby debug help

[SouthTexEng]

I just finished a Ruby and need some help debugging. I am a newbie, so be gentle!

I am following the schematic given here (the 9V version):

Ruby

Currently it functions as a bad FM receiver. Plugging in a guitar does not produce any sound (no pops, crackles, or hum even). I can hear slight differences in the faint FM signal when I turn the volume and gain knobs (very faint volume and hiss changes). I have checked several times and am pretty sure I have everything hooked up correctly. I checked continuity and all solder joints seem to be good.

I discovered that I had reversed terminals 1 and 3 on the volume pot, but fixing it did not change my results. I also double checked that my S-G-D pins were hooked up correctly, and they appear to be.

Here are the voltages I measure with the volume and gain at max. I only have an analog meter, so the numbers are not very precise (0.2V graduation):

S: 7.4 and a tone when I touch it with the probe
G: 0.0
D: 9.0
P1: 1.4
P2: 0.0
P3: 0.0
P4: 0.0
P5: 4.0
P6: 9.0
P7: 3.0
P8: 1.2
Out: 0.0

When rotating the gain know I get a barely perceptible change in voltage on pin 8, but no other changes when rotating either knob.

I would appreciate any ideas as to why I am not getting any source amplification!

[loudthud]

The voltages on the MPF102 don't look right. Did you get the correct part or did you try a substitute? The voltages on the LM386 look good.

Do you have another JFET you can try? The voltage on the Source should be something between 1 to 3V. The resistor on the Gate terminal should be 1.5 Megohms. Try to verify this by measuring across the resistor in circuit with your guitar disconnected. If your meter won't measure that high, it should indicate a very high resistance. If you get a strange reading, try reversing the black and red leads from your meter.

[J M Fahey]

Hi.
Fully agree and add: for troubleshooting divide the Ruby in its components:
it's an LM386 amplifier, which can work on its own, with a FET "preamp" added, more precisely a buffer with a high input impedance (good for your pickup) and low output impedance (to drive that 10K volume pot).
Momentarily disconnect them by pulling the 47n coupling cap, and solder your input jack straight to the pot.
It *will* work. Not world class tone, but listenable.
Play some chords and enjoy your first amp.
Thus fortified, rise your shirtsleeves up and go deal with that FET, as Loudthud said.
Agree with the 1.5V (very good preamp Fet) to 4V (more suitable for a chopper or switch) source voltage range, but the way it's used there is very forgiving, any one will work.
Good luck and don't worry, we all started as beginners.
How did you build it? Perf board/protoboard/PCB?
Post some picture.

[SouthTexEng]

Great feedback! Thanks! I finally found some free time to follow up. Here is the latest:

1) Connecting the input jack straight to the volume pot worked. Not the best tone as you warned, but amplification at least! Volume and tone seem to work as designed.
2) The 1.5 M ohm appears to be correct, using my super cheap multimeter.
3) I am 99% sure the clerk at the store made a sub on a part or two. Naturally, I no longer have the packages or remember which ones it was! With my limited knowledge I am wondering if he subbed in a different transistor that had the SGD pins in a different location then the one called for in the schematic. Sound plausible? I searched the net for the markings on mine to locate a schematic, but no luck (a W with 457 under it, green-silver-purple stripes on front).
4) Built on perf board. Don't have a camera handy, sorry!

Thanks again! I know a lot more about which area of the circuit to tinker with. I will probably just pick up another transistor. I do have some 2N3904's here. Any chance they would work? Might as well try....

[J M Fahey]

Use *any* N channel FET (the most plentiful and cheapest around) of which you can be sure of its pinout.
Or:
source to drain: low ohms (50 r to 1 K), usually around 200r , almost symmetrical both ways.
Gate to source or drain: a diode .
In theory source and drain are fully interchangeable; in practical use I found better performance using them as factory marked, specially with higher rail voltages. (maybe with 9V it's the same).
You are very close to full success.
PS: "r" means "ohms"