Blue Magic from general guitar gadgets

[cove]

Has anyone built the Blue Magic? I got it done and the bypass works but not the effect. I have voltage at the obvious spots according the schematic but need some help to trouble shoot and get it going. Do you have any transitor voltages any ideas?


http://www.generalguitargadgets.com/v2/diagrams/bmagic.pdf

http://www.generalguitargadgets.com/v2/diagrams/bmagic.pdf

thanks

[Alpha579]

what are the voltages at the drain of the mosfet and fet?

[cove]

i tested the voltages on Q2 and got 4.8 volts at the drain

Q1 is at battery voltage about 7.8 volts at the drain

thanks for your help

[petemoore]

I don't know how critical this circuit is about supply, but the battery your'e using is getting down there.
 I also don't know but from vague memories of exp./ whether the bias %ages [what the voltage 'fractions' are...example 1/2V at drain], change as voltage drops below a certain level/IIRC it does, just on the safe side I like to final bias with battery at around 9.V.

[george]

something to try would be to disconnect the lead to the output jack and connect it progressively farther back in the circuit until you get a sound (I assume you're getting nothing at all?) - that will narrow down the areas you need to inspect for dry solder joints etc and/or (unlikely) faulty components ...

hth

[cove]

were do i start to connect  the output lead in the circuit to test for sound?
how do i work backwards?

[cove]

If i turn the guitar amp up there is a hiss that comes out of the effects side of the pedal. the volume knob changes the volume of the hiss and so does the drive and tone knobs change the sound of the hiss.

but no guitar sound coming out.. i put a fresh 9 volts battery and readjusted the voltage to the transistor..

[Samuel]

Poke around the FAQ, debugging page, etc. and look for "audio probe". Basically its a 1/4" jack with a capacitor and a wire lead, which you can use to poke around the circuit (with a signal coming in to the input jack) and figure out how far the audio signal is making it into the circuit, and in that way identify the source of the problem...

[Ge_Whiz]

Did you build it on the PCB or lay it out on perfboard? Have you checked the pin-outs of the MOSFET and JFET very carefully indeed to make sure that they are connected up the right way? Did you substitute either FET?

[Cecil]

I'm having the same problem. Purchased the board from GGG an I also switched out the BS170 and the J201 and installed sockets to besure not to over heat them during install, still no sound just a slight hiss. Any suggestions. Thanks

[Paul Marossy]

I built it and had no problems. I designed my own PCB for it, though. Maybe the GGG board has a mistake in it somewhere? I would check that board against the schematic and see if everything matches up. If there is a mistake, it might be a really easy fix...

[george]

were do i start to connect  the output lead in the circuit to test for sound?
how do i work backwards?

Grab hold of the schematic so you can follow where the signal is going - it comes in to the gate of BS170, and the output goes from the drain thru a capacitor then through the drive control, through some resistors and capacitors (there are some capacitors connected in parallel on the way but these wouldn't stop the sound if they were disconnected) and arrives at the gate of the JFET

How I would do it is touch the output lead to the JFET gate and see if you are getting anything, then work back through the abovementioned signal chain until you get a sound. - if you get a sound at the gate of the JFET then you know that's where the problem is - if you don't you need to work back component by component along the path the signal is taking until you get a sound.

This is also what you would be doing with the abovementioned audio probe, but I'm a cheapskate

Might be a bit hard doing this while also trying to play your guitar so some kind of signal generator would be handy for connecting to the input.  (although I'm too cheap to but one, I sit my guitar on a stand and strum with one hand while probing with the other, awkward or what?!  :? )

Paul's advice sounds good

[petemoore]

On a transistor with the signal chain coming out of a drain or collector, I like to touch the output [Drain or collector] then the input [gate or base] to see that the base sounds more gainey and louder than the collector.
 I believe the Blue Magic achieves the OD with a Mosfet driving a jfet, or vice versa...this type of transistor stage should definitely have the input of these transistors sound louder [get amplified] than the output.

[george]

yeah if the circuit is working probably at some point along the signal chain you might get blasted because you're not going going thru the level control - so be careful to turn your amp right down!

[Paul Marossy]

Just for the record, I actually did use the GGG PCB layout now that I think about it. Double check all of your components and make sure everything is correct, especially the JFETs. If they are in backwards or have a different pinout than the ones specified, then the circuit probably won't work.

If you get it working, I'm sure you'll like it.  

[george]

it might be an idea to also check the orientation of any polarised electrolytic caps in the circuit - C2, C7 and C8

[cove]

can anyone tell me how to read my schematic to trace a signal for debugging with the audio probe. once the signal leaves the input do i just follow the tracing and keep testing for the signal. at what points should there be a signal at the transistors etc.

http://www.generalguitargadgets.com/v2/diagrams/bmagic.pdf

[Paul Marossy]

Signal path is as follows:

Input --> gate of of Q1 --> drain of Q1 --> drive section --> gate of Q2 --> --> drain of Q2 --> tone control --> volume control --> out

The audio probe will allow you to see if you have signal at any of these points. Where the audio stops is probably where the problem(s) is/are.

[Cecil]

Well I have tried it all and I only get sound to the gate of Q1. I get nothing at Q1 drain it just goes silent. I tried a new BS170 an I still get the same thing, even went back through and resoldered every thing still the same thing. Please help! thanks Cecil

[petemoore]

Sounds like a mis-pinout.
 Verify that the transistor pins S,G,D are connected to the proper connections. I've had to bend Q leads around each other and stick 'em in the socket that way, not bad, gets the pins connected right.
 If it's fixed bias on that Q, it's probably not the problem, especially if you've tried a couple transistors and nothing gets through them in that position.