Copyright 2000 R.G. Keen. All rights reserved. No permission for serving from web sites other than http://www.geofex.com
The hard thing about new effects is that since they've never worked, there are often several mistakes or flaws, and worse yet, the more of a beginner you are, the more flaws you're likely to have, and consequently the more debugging skill this may need. It's a frustrating circle. At least with an older effect you're fixing, it's likely to only have one major flaw, since it used to work.
If you follow this set of steps, you will be able to filter out almost all of the bugs you can run into. If it seems a lot of work, remember that you're making up in doing individual steps what you may not have in experience. As your experience grows, you'll start knowing what to do to skip steps that your new-found experience tells you is not necessary. Remember that you may have to fix more than one problem. Just do it in order, getting one problem fixed and going to the next. At the end, it will work.
Pick out your symptom:
Works fine before mounting in box, then dies.
Something about putting it into the box is interrupting the signal, or a causing a short that radically shifts the bias. Check carefully for ANY place metal on the effects board or a wire end touches the box.
Incorrectly wired jacks. If you get the wires reversed on the input or output jacks, it may sound fine before you put it into the box because the grounds on the jacks may not be connected to each other. When you put them in the (presumably) metallic box, the ground on the jacks are shorted together, and the signal is shorted. The fix - check your wiring and the jack lugs again!
Doesn't work at all, or something is clearly wrong with the effected signal
Do this in order.
There are two major uses for all devices in effects circuits - linear and switching. Linear devices amplify and process signals. Switching operation is sometimes used to re-route signals, or to operated a Low Frequency Oscillator for modulation or something similar. If you can't tell from the schematic which devices are used for amplifying and which are not, ask someone. If you got your effect layout or board from GEO, it will have a description of how the circuit operates so you can tell.
If you have:
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