Distortion + Problems

[jeff g]

I just finished the Distortion + based on the PCB layout from GGG

No parts substitutions (intentionally), but a had a bad trace from the Output Lug Pot 3 to R9, so I rewired with the output lug wire through its pad and through the R9 pad and soldered to the R9 lead on the other side of the board. 

Also, I used caps that are marked with 100 nS for C3 and C7


After firing up, I've got an LED that lights and weak volume output but no effect.

Voltages are as follows (using a full 9v battery)

Battery + (PCB) = 10.73
Battery -           = 0

D1 -
A = 0
K = 0

D2
A = 0
K = 0

C1
+ =  - 3.3
- =   0

C6
+ = - 5.2
-  = 0

IC (LF 351)

Pin 1 = - 0.2
Pin 2 = - 4.8
Pin 3 = - 2.0
Pin 4 = 0
Pin 5 = - 0.2
Pin 6 = - 5.11
Pin 7 = - 10.55
Pin 8 = 0


Anywhere in particular I should look for problems?

Thanks

[burnt fingers]

THe first thing to check is C3 and C7.  100n is 100nanofarads which equals .1 microfarad.  your input cap is .01 microfarad so essentially your letting more frequencies pass to ground than your letting through to the rest of the circuit.


Scott

[petemoore]

Pin 1 = - 0.2  Not Connected..ok
Pin 2 = - 4.8  Is a bit less than 1/2v...probably ok
Pin 3 = - 2.0  This ones misbiased,
Pin 4 = 0 = ground ok
Pin 5 = - 0.2  Again misbiased, however being the same as pin3, they are separate?
Pin 6 = - 5.11  real near 1/2v of V+
Pin 7 = - 10.55 =V+ = close to battery reading..
Pin 8 = 0
  I would look at and test with DMM around the bias arrangements of pins 3 and 5.

[jeff g]

Thanks for the replies

I'll replace C3 and C7 with the right values - I thought those may have been wrong, but caps sometimes confuse me with their labels

petemoore - what exactly do you mean by testing bias arrangements around the IC pins?   Is it just checking solder joints and trace continuity / bridges coming off those pins?

[petemoore]

petemoore - what exactly do you mean by testing bias arrangements around the IC pins?   Is it just checking solder joints and trace continuity / bridges coming off those pins?
  Everything you can think of, to apply the DMM in some 'useful manner...
  Close in bright lamp and a magnifying glass...
  DMM test the values of resistors.
  DMM test continuities of resistor leads to where they look connected.
  DMM test all marked ground points.
  A smaller resistance reading of a resistor CAN MEAN there is an alternate path through the circuit for the DMM's current to read, so these readings could be anomolies...you'll need to either do the math, pull one end to measure the resistor only, or figure it's what it is ['anomolous' reading] and move to the next test. You won't see this recommended because it's so 'thissy and that'sy' to use, I use it anyway alot to find if a resistor is actually it's coded value, more importantly of course the value shown on the schematic.
   

[petemoore]

http://www.generalguitargadgets.com/diagrams/dist_plus_sc.gif
  Pin 3...for starters...
  goes through a 1 meg to the divider center [the two 1megs strung between V+/V-], you should get ~1/2v of the battery at divider center node.
  What to do when it doesn't work shows how to do all the debugging except the ;funky' in circuit resistor testing technique depicted ^.
  Which translates basically into anything/everything you can do to test...visual inspections of traces, resistor color code readings, actual resistance readings [DMM], voltage readings...maybe you see a large voltage drop somewhere.
  because everything must be right, anything you can do to get there is a good test, even if all it does is eliminate another potential problem area...like the voltage readings did...the 'off' voltage pins of the OA show there is a bias problem.
  single OA gain stage voltages:
  pin 4, ground @ 0V is correct.
  Pin 7 should be just a little less than an open battery, the circuit draws current and will drop a bit of voltage...pin 7 looks ok V Wise...
  All the other pins used should be near around 1/2v, both the +/- inputs and the output need to be biased with room to swing voltage 'from the middle' toward both the + and - '*rails' [*power supply limits of 'V+ ~9v', ..and.. 'ground 0V']