Blatty or Gated sound -

This is one of those "dead giveaway" symptoms for distortion devices and in fact for most effects. When an effect makes no sound at all until the input is very strong or very weak, something inside is biased improperly. The signal itself is pushing something into the range where it can pass sounds, but only when the signal level is right. There are two exceptions to this - Noise Gates and Compressors. These devices are supposed to gate or modify the amplitudes of the signal  as part of their normal operation. Debugging them is best done by other means.

This happens two ways:

• Sound only when you really whang on the strings loudly
• Sound only when soft notes are supplied, and no sound when loud notes are hit.

In both cases, the procedure is the same - measure the DC voltages on the transistors and opamps in the effect, and figure out which stage or part is biased wrong.

• NPN transistors
  • For linear amplifying, the collector must be more positive than the base; the base must be more positive than the emitter by about 0.4 to 0.7V for silicon, and 0.0 to 0.3 for germanium; the emitter should be the most negative pin. The voltage difference between the collector and emitter is the biggest voltage that the transistor can swing linearly.  If the collector is not a volt or more above the emitter, or the base is not a base emitter drop above the emitter as noted above, the transistor cannot be acting as an amplifier. This is a very good pointer to what can be wrong. See the debugging example below. If the collector is at the same voltage as the base, or even closer to the emitter than the base is, the transistor is saturated and simply can't be amplifying. Likewise, if the base-emitter is below the cutoff voltage for the transistor, no current can be flowing.
  • If no sound happens until you really hit it with a lot of signal, the signal is having to supply the bias, so look for a transistor with its base too close to its emitter - less than 0.4V or so.
  • If no sound happens until the note is very quiet, look for a transistor that's almost saturated, with its collector voltage too close to its base or emitter voltages.

  • Debugging example: A fellow had built a replica of the Fender Blender circuit. It was not working, and I asked him to list the  voltages on the transistor pins. He found:

    Q1 E-0.03 B-0.56 C-2.69
    Q2 E-2.10 B-2.67 C-3.93
    Q3 E-1.89 B-2.53 C-1.96
    Q4 E-0.00 B-0.18 C-3.80
    Q5 E-0.00 B-0.21 C-3.70
    
    Here's what the voltages say about each transistor.

    For an NPN silicon transistor (these all are) to be working as a linear amplifier, it MUST have its base higher than its emitter by about one silicon diode drop, 0.5 to 0.7V, and the collector must be higher than either base or collector. The collector-to-emitter voltage is the size of the negative half signal swing, and the power-to-collector voltage is the size of the positive half signal swing.

    >Q1 E-0.15 B-0.70 C-3.22
    Base is 0.55 higher than emitter, collector a few volts higher than either one, and room to swing up and down on the collector - this one looks OK. We could tell how much current is flowing because the emitter to ground voltage must flow through the 15K emitter resistor, so the current is 0.15V/15K or 10uA. Looks all right.

    >Q2 E-2.66 B-3.24 C-7.75
    This one has its collector tied to the + supply, so its collector is equal to the battery voltage (and your second battery is getting near end of life, too ;-).
    The base is 3.24-2.66= 0.58V more positive than the emitter, the collector to emitter is 7.75-2.66V = 5.09V so there's room for a signal to swing. It can swing -2.66V before it clips, and more than that positive, so this one is OK, too.

    >Q3 E-3.95 B-4.63 C-3.99
    Hmmmm.... base is 0.68V higher than the emitter, within the range of operation, but higher than its brothers under similar conditions. That indicates a lot of current through the base-emitter. Further, we see that the collector is only 40mv higher than the emitter - not good! This one is probably saturated, so no signal will come through. I'd bet that either the R11 150K resistor is open (bad solder joint, open PCB trace, broken or wrong value resistor) or there is DC coming in through what should be an open circuit at C5 0.1uF - either a solder short, shorted capacitor or some other such condition. Temporarily unsolder one of C5's leads and bend it up out of the board. If this fixes it, at least c5 is bad. If not, you have a problem with either R11 or a bad transistor.

    >Q4 E-0.00 B-0.27 C-7.73
    Ack! Another problem. Base is not at least 0.5V higher than the emitter. That means no current flows, and we see that this is true, because the emitter and collector are at ground and power supply respectively , indicating no current flow through the resistors. This one's not getting enough bias on its base. I'd guess that one of the diodes or associated resistors (R15, R16) has a solder short to ground as a first try.

    >Q5 E-0.00 B-0.44 C-7.29
    And another one. Base only 0.44V, no voltage drop across the emitter or collector resistors, so no current is flowing. Something is not letting enough bias get to the base. There is a solder short/wrong value resistor/something else with R28/R26/C13

• PNP transistors
  • The debugging procedure for PNP's is exactly the same as for NPN's, but with the voltage directions reversed. A PNP needs its base biased a bit negative from its emitter to be a linear amplifier where the NPN needed positive. In both cases, what matters is that there is a bit of current flowing through this junction. To have some room to swing (hey, we all need that!) the collector must be at least a volt or two lower than its emitter (the NPN's collector needed to be more positive). Like the NPN, if the collector is at the same voltage as the base or even closer to the emitter than the base, the transistor is saturated, and no audio signal amplification can happen.
• N-channel JFETs
  • Best to debug this with the Audio Probe. 
• P-channel JFETs
  • Best debugged by Audio Probe.
• Operational amplifiers (op-amps)
  • For an opamp to be working as a linear amplfier, both the inverting and noninverting inputs and the output pin must be within a few millivolts of the voltage on the noninverting input. This is a consequence of the negative feedback.
  • Measure the +input, -input and output pins of a single opamp. If they're not within 10mv of the voltage on the (+) pin, either the opamp is dead, the biasing is seriously wrong, or it's being used as a comparator and switching. If you know it's supposed to be used as a linear amplifier, there's a problem in this stage.
  • Even if all three pins are close together, they may still be too close to the + or - power supply to the opamp. If the + input is not sitting more than 1.5-2V away from the  + or - supplies, many opamps will act oddly, and could give the blatting sound.
  • Be aware of the pinouts of the chips you use. Get on the net and find them and print them out. Many opamp IC's have more than one opamp inside. Dual opamps are very common, usually more common than single opamps, and have the advantage of always having the same pinout. Quad (4!) opamps are fairly common, but there are several pinouts for these, so be sure you know which pin is which.
  • Measure the voltages directly on the pins of the IC, not at a socket pin. Sometimes socket contacts are bad. As are solder joints.
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