Gristlelizer: 8; Me: 0

Weeeeellll, the Gristlelizer still isn’t working, but some progress is being made.  All the symptoms remain the same, i.e. the LED blinks, and sound gets through in bypass mode, but not in effects mode.

I replaced all of the integrated circuits, and also some of the resistors.  Replacing the resistors was actually a dumbass mistake — as I was testing the resistors with the multimeter, I was thrown by the off reading I was getting for many of them.  Of course, the problem is that I forgot that resistors in a circuit will be affected by the other components around them, so the “inaccurate” performance I thought I was seeing was, in fact, exactly what I should have been seeing.

Kerry made an extremely helpful suggestion, involving using the Level pot as an improvised audio probe to trace the signal through the circuit to discover where it stops.  I’ve done this, and now I’m really stumped.  Here’s the deal:  The signal seems to be fine when it reaches IC4, which is the first IC it hits.  But there’s no output from the IC on terminal 6.  So that’s where the signal is getting stuck.

You’d think simply putting it a new IC would fix the issue, right?  Wrong.  So my electrons are evidently getting stuck in IC 4 and not progressing through the rest of the circuit.  I’m busy eddycatin’ myself about what might be going on here — if nothing else, this project has certainly been a great learning experience — but I feel like I’m a long way away from figuring this out.

If anyone has any ideas about what the problem might be, please chime in…

5 Comments

  1. Wow, that’s strange. What’s the voltage on pin 6? If it’s +9V then I’d guess that it’s shorted to pin 7. And are the voltages of the power pins okay?

    (And I have to ask: you know the direction of pins 5-8 on these ICs? Pin 5 is on the same end as pin 4, but in the other row, and so 5-8 run the other way.)

  2. Voltage on pin 6 is right at 9V. For ALL of the iC’s, I’m getting readings around -13 for the V- pin, and right under 9 for the V+ pin.

  3. -13 V is unexpected, but at least it’s negative! Ignoring that for now . . .

    I just realized that 9 V on the output can be caused simply by V(pin 3) > V(pin 2). The feedback resistor R21 is supposed to keep V(pin 2) about equal to V(pin 3).

    Here’s how IC4 is supposed to work:

    Standing alone, it’s a high-gain amplifier that pegs the output at one of the supplies depending on the relative voltages on pins 2 and 3. But with the feedback resistors R20 and R21, V(pin 2) becomes nearly the same as V(pin 3) for small input voltages (about a volt). So when the IC is hooked up correctly and operating with an input less than about 1V, V(pin 2) should be nearly the same as V(pin 3), and V(pin 6) should be about 11 times V(pin 2).

    (I won’t be able to compare the supply voltages with mine because I reworked the power supply with linear regulators instead of Zener-diode regulators.)

  4. We got the latest manual in an email today. I didn’t think to ask about connecting CVOUT to CVIN.

  5. I discovered a resistor with one end still unsoldered, but it’s lead was nicely trimmed as if it had been. I also hadn’t soldered a corner pin of one of the ICs. I need to examine this board carefully!

Leave a Reply