Please help with debugging of Runoffgroove's Professor Tweed

[cat]

1.What does it do, not do, and sound like?

No sound when on only when in bypass mode (off )

2.Name of the circuit =

Professor Tweed by Runoffgroove

3.Source of the circuit (URL of schematic or project) =

http://www.runoffgroove.com/professor.html

4.Any modifications to the circuit? Y or N

No


5.Any parts substitutions? If yes, list them.

No

6.Positive ground to negative ground conversion? Y or N

No

7.Turn your meter on, set it to the 10V or 20V scale. Remove the battery from the battery clip. Probe the battery terminals with the meter leads before putting it in the clip. What is the out of circuit battery voltage? =>

9.93

Now insert the battery into the clip. If your effect is wired so that a plug must be in the input or output jack to turn the battery power on, insert one end of a cord into that jack. Connect the negative/black meter lead to signal ground by clipping the negative/black lead to the outer sleeve of the input or output jack, whichever does not have a plug in it. With the negative lead on signal ground, measure the following:
Voltage at the circuit board end of the red battery lead =

9.74

Voltage at the circuit board end of the black battery lead =

Black battery lead goes to jack not to circuit board.

Q1
C= 9.73
B= 2.83
E= 0.00

Q2
C= 9.71
B= 2.17
E= 0.00

Q3
C= 9.69
B= 1.88
E= 0.00


Im not getting even a hint of sound when the effect is switched on, only through the bypass. I used all MPF102 JFETs just like suggested. and have them orientated as instructed in the schematic.

Any help would be much appreciated as always.

Cat

[cat]

I have hooked it up like so;



Cat

[burnt fingers]

Verify the pinouts of your fets.  Double check all your connections.  It is really easy to miss something when you're eager to get it working.  I spent two nights trouble shooting a DOD 250 and was absolutly positive I wired everything right.  I ended up not grounding the IC. 

[B Tremblay]

In addition to checking your FET pinout and orientation, have you tried adjusting a trimpot while measuring the adjacent FET's drain voltage (red probe to FET drain, black probe to ground)?

[ildar]

Your JFETs should be set at 1/2 supply voltage afaik. Adjustthe trimmers and check your voltages again. These can be finicky and will give you no signal if misbiased.

[cat]

OK, thanks guys, Ive adjusted the trimpots but I cant get the drain down on Q1. Im still getting no sound when the effect is on, only when in bypass.

These are my results for the drains.

Q1.

D= 8.78

Q2.

D= 4.60

Q3

D= 4.49

Any ideas?

Thanks guys,

Cat

[petemoore]

 These are Jfet's with Sources Gates Drains 
{For these readings
  C=D
  B=G
  E=S
  Looking at the MPF 102 the pinout is DSG, source in the middle, so I put gate on bottom S in the middle and D on top, then the Q's face input.
Q1
C= 9.73 [Drains should be 1/2v, or just under 5v ITCase.
B= 2.83 [Gate voltage should be less than source
E= 0.00 [source should be above gate voltage

Q2
C= 9.71   "                  "same as Q1 about
B= 2.17
E= 0.00

Q3
C= 9.69            ....."                 "
B= 1.88
E= 0.00
  I would guess pinout is a likely problem, find the MPF102 data sheet, then with Q in hand, pinout sheet and schematic on table with board, determine the pin assignment on the board.
  Sometimes I've had to face the transistor # toward input [that puts the drain 'on top'], then invert the source and gate pins by twisting 'em "real careful' around each other [flat jaw pliers? then trim to length] so you don't have to reroute the board. If the socket pins follow the schematic...that'd put the gates in the middle.
  Adjust the trims up from 0k, like in the middle or higher, putting more resistive wafer [higher R value] between the drains and V+ for starting point.
  After correct pinout is made, and re-double-checked, try re-adjusting the drains to about 1/2v.
  I look for the small resistor on the source to find it's socket pin, large one on the gate, then drain goes through Trim to V+.
 

[cat]

Thanks pete, according to the website www.runoffgroove.com the pinouts should read like so:

Q1 MPF102

D = 4.60
S = 1.14 (368mV)

Q2 MPF102

D = 4.56
S = 1.16

Q3 MPF102

D = 4.61
S =   .891

Zero on all gates

Here is the data sheet for my FET's:



So Im pretty sure Ive got the FET's orientated correctly (flat side towards input).

Now my problem seems to be with the trim over Q1, I cant get the voltage down on Q1 no matter how I adjust the trimpot.

Cat

[petemoore]

  ..small resistor on the source, large one on the gate? that's one EZ-Cheezy way to ~figure pinout.
  What size trimpot for the Drain?
  Measure/test the trimpot with the DMM..connect one lead to the battery clip or V+ Rail, then other probe to the drain, adjust pot, see resistances...
  0k-100k...'should' provide a value for correct bias adjust...I don't know...you have a different Jfet to try in that position?

[cat]

All the trimpots are 100k.

I might have to get myself another FET and try that out.

Cat

[cat]

Oh god, Ive left out the 1k5 resistor going to the source of Q1. 

What an idiot! Thanks for all the help guys, I reckon if I have all the parts in I might have a better chance of making it work. I feel like such a noob. 

Cat

[cat]

Ok so Ive got the sucker pretty much working I just had one more question for those of you who know your stuff.

I am experiencing a volume drop when the pedal is in use. It is not so dramatic that it alters the sound quality of the pedal but I would like it to boost the volume slightly rather than drop it as it is an overdrive pedal.

Would adjusting the trimpots to allow a little more current through help me in this regard?

These are the drain readings I have now:

Q1 - 4.61

Q2 - 5.28 ( I think I need to drop this a little more )

Q3 - 4.31

Thanks guys, Ive really appreciated the help Ive gotten here.

Cat