My Blue Magic ain't working. :(

[Chris Goodson]

I got it all put together and it's dead as a doornail.  All the part's are the proper values except for the drive pot which is a 1meg with a 1meg resistor across it.  
I've been trying to send a signal thru it with an audio probe(working from the output backwards) and it makes it thru in most of the circuit, except things start going wrong in the area before Q1.
One thing I've noticed is when I touch the probe right after the input cap the sound cuts on briefly and then shuts off..?

Here's some voltages-

9volt battery=9.84V

Q1(BS170)
S=202mV
G=2.68V
D=203mV

Q2(J201)
G=.3 mV
S=.48V
D=5.9V  (i need to readjust the trimpot)

Any suggestions would be much appreciated.

[Paul Marossy]

What about your board? Is it perf, DIY etch job, RTS board? Some people were having some problems with their RTS boards, although I think one of those people had a bad pot.

I built mine per the .PDF file at GGG, and had no problems. http://www.generalguitargadgets.com/v2/diagrams/bmagic.pdf

I know that one person had the same type of problem in the exact same area as you, right before Q1. Hopefully, they can chime in here...

[Chris Goodson]

It's a GGG board.

[R.G.]

Q1(BS170)
S=202mV
G=2.68V
D=203mV

Hmmm.... let's think a minute. From reading "Designing MOSFET Boosters" at GEO, we know that the gate is high enough for a BS170 to be turning on. We can also tell that I = .203v/1000ohm = 203uA is flowing in the source resistor. What ought to be happening is that the same 203mv should be flowing in the drain resistor and therefore the drain should be sitting at 9.84V - 4700*203uA = 9.84V - .954V = 8.88V. It's not, since it's down at the same voltage as the source. If the drain and source were shorted and both drain and source resistors were soldered in properly, then the voltage they're both at would be 9.84V* 1000/(5700) = 1.578V. That's not happening, so one or both of those resistors is not connected right. It's likely that the drain resistor is not connected properly to the 9V source, since if the source resistor were open, the drain/source would be up near 9V.

The gate's a little low, since 9.84V* 2.2M/(2.2M+4.7M) = 3.13V, but there are obviously other problems first.

Ohm's law is a powerful tool. It's possible to have a successful career as an EE with nothing more than a good command of it.

[Chris Goodson]

R.G., I think you hit the nail on the head.  After taking a close look at the drain's resistor I realised that red stripe was really orange.  I havn't changed it yet but I've got a feeling.....
Your point about Ohms law was also well heard.  I had it down pretty well in college but I guess I let all that stuff float out the window in favor of merely sniffing solder fumes.  Maybe it's time to dust off the old calculator as I feel you will eventually grow tired of pulling lazy people like me out of these jams.

[cove]

Everyone was so helpful with my blue magic ..here are the postings for my problem.

http://diystompboxes.com/sboxforum/viewtopic.php?t=22345&postdays=0&postorder=asc&start=0

looks very close to mine with regards to the transistor voltages.. i started at the input and worked to the output and found a bad tone pot....

if the sound is dropping out at the input cap theres not much choice of were the problems lies..it has to be at the input jack, cap or around that area....if the sound cuts out at the input cap...then you should not get and signal farther down the line from there...

look closely at the circuit board from GGG (if your using one) and the parts layout i believe there is a difference between the two..there was and extra solder pad or something like that... look close

good luck...

[Chris Goodson]

It's working now; the problem was in fact Q1's drain resistor being the wrong value.  I'm painting a box for it right now, heh purple. :twisted:

[Paul Marossy]

Cool! I hope you like yours as much as I like mine.  

[Boofhead]

Just for the record, don't lose sleep about your slightly low gate voltage.  It's acutally the 10M input impedance of the multimeter loadings the circuit and that causes the expected 3.1V to measure 2.7V - in operation the circuit is actually seeing 3.1V.  You can calculate the expected measure value by replacing the 2.2M in RG's equation with 2.2M in parallel with 10M  (ie. 1.8M).  Usually you can ignore the input impedance of DMM's but with MOSFET gate bias circuits it can still have a small effect.