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:
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.
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