Dr. Boogey debug help

[Tantalum7]

I just finished putting together a DrB, and while I get some distorted signal through, as soon as the input level rises (such as hard strumming a chord), the signal cuts out. Voltages are below.  The trim pots for Q1 and Q2 seem to have almost no effect on the drain values (a few tenths of a volt at best).  Audio probing yields a grainy weak distortion through Q1 and Q2 that never seems to improve much along the rest of the audio path.  V+ is 9.37V from a regulated power supply.  I'm thinking there's something funky happening at Q1 and Q2 that's pushing the jfets overvoltage? and not allowing them pass signal, but I can't see anything obvious in looking at the circuit board.  Any suggestions would be great.

Q1: 2N5457
D 9.22
G 1.23
S 0.00

Q2: J201
D 9.32
G 0.95
S 0.00

Q3: J201
D 5.1
G 0.5
S 0.00

Q4: J201
D 3.72
G 0.36
S 0.00

Q5: J201
D 9.38
G 3.91
S 3.72

Link to schematic: http://gaussmarkov.net/layouts/drboo/drboo-schem.png

Thanks,

Scott

[Ripthorn]

A hard strum that makes it so that there is no sound is indicative of clipping one of the stages so bad that it can't handle it (overloading).  Audio probe through to find which stage and then you can put in a voltage divider or other method to reduce the output of the previous stage.

[Tantalum7]

Yes, that sounds right.  I'll have to get more precise with the audio probe and work a little more at it.  Thanks for the help.

[Ripthorn]

Q5 doesn't look right, the only time I have seen a jfet with the same voltage on G and S is when it was pretty well toasted.  Maybe swap that out.

[Tantalum7]

I saw this as well, which is why I swapped another J201 in and got virtually the same readings.  I've probably got a short or a solder bridge.  This is a pretty cramped board so keeping things clean and neat is a bit of a challenge.

[Ripthorn]

Your Q1 and Q2 drain readings are too high, I also seem to remember that allowing full supply voltage gave very weak performance.  Bias both to 4.5V and don't stray too much when tweaking (you'll see what I mean if you do).

[Tantalum7]

I have tracked down every voltage from V+ through VA out to Q1 and for some reason I can't seem to get trim1 to bias Q1 at all.  I replaced the trim with another trim, then I even replaced that with with a standard pot thinking that there might be some oddity to the pinout of the trim pots.  The voltage across the 100K only varies by .02v even though the pots measured externally check out as 100k.  I'm sure that the problem is that Q1 and probably Q2 are biased too high, but I can't figure out what could be causing the bias trims not to bias and for the j201s to be getting full V+.  Is anyone who is quite familiar with the DRB willing to make a suggestion?  I couldn't find anything like this searching on the forum.

Thanks

[MicFarlow77]

Hey Tantalum7,

Q5's drain is connected straight to the 9V... so that looks good.

Just taking a stab at it here, but did you put in the two jumpers at the bottom of the board for the ground returns?

Thanks,

Mick

[Tantalum7]

Thanks for your input.  Yes, those two jumpers as well as the third required one are in place, and the solder connections seem clean.  I've also checked component values of just about everything connected to Q1 and Q2, but nothing seems to be out of place.  Something is preventing the jfets from biasing, and it's got me stumped.  I may disconnect all the outboard connections and simplify things as much as possible to rule out bad connections within the switches, pots and jacks before going over every component on the board.  You learn a lot more when things don't work, but it gets frustrating at times.

[MicFarlow77]

Before pulling off all the outboard stuff, try a couple other easier things....

1 make sure all pots are at half thier travel... I've been nabbed by this a couple of times...
2 take out the trimmer for just Q1 and check the voltages on both sides of it. The right side should be right at 9 volts and the left at near zero. If you are getting 9v on both sides, then there is certianly a short somewhere. If your meter has an audible beep function for checking continuity, set that up and start ringing out parts of the power supply lines and ground planes checking for shorts.
3 take a sharp edge (razor blade or xacto blade) and run it along as many traces as you can around that area of the board to catch any possible little itty bitty bit of solder that might be causing you grief.

Hope to hear positive results soon.... you'll get it going soon, I'm sure!

Can you post of pic of the solderside?

Thanks,

Mick

[Tantalum7]

Thanks for sticking with me.  It's a bit late to start probing the innards tonight, but I'll have at it again tomorrow.  I did do a cursory X-acto run through everything that looked like it might be close, but I will be more thorough tomorrow and also clean off any remaining flux.  I will try to post pics as well.

take care,

Scott

[MohiZ]

Ok, there's a problem with Q1 - Q4. The gate should read zero on each one and the source should be mildly positive. Check the pinouts. J201 is D S G from left to right looking at the flat face with legs pointing down. Check that the traces from source to ground are intact, preferably with a continuity meter. Also check that the drain trimmer is of the correct value.

For some reason the jfet is not conducting, and that leads me to think the source-to-ground connection is problematic. Either that, or the drain is shorted to the power supply voltage.

[Tantalum7]

Found the Problem!  After checking and resoldering every connection without luck, I decided to trace all the voltages in from V+ through each jfet.  It seemed to me that the way to keep the voltage on the gates high was to prevent a proper path to ground.  I decided to double check all the resister values between the gates to ground and low and behold, 1.8K is not in fact equal to 1.8M!  The trouble was that the board is so tightly packed (like building a ship in a bottle), that it was impossible to measure the value of the resistors on the board.  With 1% metal film resistors the .1 multiplier on the fourth band is pale yellow and the 10^4 multiplier is yellow.  I assumed the resistors were 1.8K when they were actually much higher.  I found all the 1.8K resistor spots and replaced them with verified 1.8K resistors and I was screaming away in boogie madness.  Thanks to everyone for their help.

[MohiZ]

It's great that you got your problem solved, but I think you're mixing up gate and source, because source is the one with the 1.8k resistors, not the gate. The gate is where the signal goes, and the source is the one that's usually connected to ground. So when you're talking about gate, I think you actually mean source 

[Tantalum7]

I'm still relatively new to electronics and effects building (only a few months and a few builds under my belt).  I saw that the center pin of most jfets is the gate and it appeared that the resistor was between this and ground on the circuit board layout, so that's why I said it.  The schematic makes it more clear.  Had I looked at the schematic more first, it might have taken me longer to find the real problem .  But now I know more than I did.