Voice changer PC board problem

Discussion in 'The Projects Forum' started by misterconsister, Sep 25, 2011.

  1. misterconsister

    Thread Starter New Member

    Sep 23, 2011
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    I have just completed my first printed circuit board kit, a voice changer kit from Velleman. It looked pretty good all told as I studied-up on soldering techniques for PC boards beforehand. However, as a newbie, I had to make a mistake somewhere.

    The on/off LED light comes on when the 9V power is switched on but there is no modulated output going to the speaker. There is output at the speakers, a fairly constant 6V or so, but it doesn't change no matter how I adjust the volume, the mic sensitivity or the vibrato\pitch up\pitch down\robot buttons.

    Shouldn't I be getting a modulated voltage at the speaker outputs?

    Although all the other connections seem to be adequate, I certainly botched an LED connection on the board by manhandling it too much when it didn't sit in the correct position and I damaged the trace and the film over the trace. The LED is supposed to indicate "modulation" and seems to be isolated but I can't really say that for sure. A nube issue no doubt.

    What would cause the constant output voltage and would the LED affect the whole PC board's performance? The diagram for can be found at:
    http://www.vellemanusa.com/downloads/0/minikits/manuals/manual_mk171.pdf. I don't think I damaged any of the copper or film near any other connections. Also, if it matters, I cleaned it with isopropyl alcohol and a toothbrush applying the treatment about 5 times. Then I blew it clean with compressed air and held it uner a hot light for a few minutes.

    What did I do wrong (if possible to determine cause(s)) and can I do anything about it now with this board?

    Eric
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    The lamp output doesn't provide any feedback or control to the IC so a failure in the LED circuit shouldn't disable it. Most errors on Velleman kits are in part placement and polarity so double check them, especially the diodes, electrolytic capacitors, microphone element and IC. Put up clear pictures of both sides of the PCB so we can see what you've done wrong. :rolleyes:

    http://www.holtek.com/pdf/consumer/8950v110.pdf
     
    Last edited: Sep 25, 2011
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you're getting 6v across the speaker terminals, C8 is bad (shorted) or you've installed it backwards, which will kill it.
     
  4. misterconsister

    Thread Starter New Member

    Sep 23, 2011
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    Thanks for your quick replies!

    I looked into your responses. There's no short across the C8 terminals and to the best of my ability (I'm new at this), I can't tell if anything has been installed backwards.

    What's that abomination in the middle? When it didn't work I tried to circumvent the fact that the modulation LED's posts didn't seem to be making contact with the board. So I, ahem, soldered two jumpers from the LED posts to two places directly 'downstream' and on the same trace as the bad post terminations. Doesn't look pretty at all but there are no shorts and there is full connectivity now between the LED posts and the trace they were supposed to connect to. I'm a tad embarrassed to post the photos though.

    Still, same problem. Steady voltage at the speaker out. I may have mispoken, it is 7.7V now, not 6V.

    Is there any hope?

    Eric


    I've attached
     
  5. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Hi misterconsister,

    The soldering looks good. There are three types of parts on this circuit that must be installed in a specific direction: the Zener diode, the LEDs and the electrolytic capacitors.

    The Zener is installed correctly.

    I'm not sure on the LEDs - check this by matching the bottom shape of the LED to the drawing on the circuit board. Notice the board shows a half-moon where the LED goes. The bottom of the LED (rather side) has a flat contour whereas the rest is round. This flat contour must be placed on the board to match the flat half of the half-moon drawing on the board for each LED. LD1 looks like it is installed backwards, but I can't see enough detail from the picture to be sure. Check both LEDs.

    The electrolytic capacitors are round cylinders with the silver tops. On the board you saw two holes to place the capacitor leads into. One of those holes either had a + or - sign next to it. Notice the capacitor has a - sign printed on one side of it close to one of its leads. That lead is the negative one. It should be placed in the hole with - sign next to it. If only a + sign is next to one of the holes, then the negative lead on the capacitor goes into the opposite hole. This is very important for all capacitors. If you're not sure you did this correctly, you'll need to desolder each electrolytic capacitor and check OR look at the Velleman circuit diagram and verify the negative leads are going to the right places.

    The Velleman instructions with circuit diagram:
    http://www.vellemanusa.com/downloads/0/minikits/manuals/manual_mk171.pdf
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I don't see anything obviously wrong either.

    However, you say that you are not reading a short across the C8 terminals - yet at the same time you say you're reading 6v-7.7v across the speaker terminals. Something isn't adding up here. If you have a 4 to 8 Ohm speaker connected across the speaker + and - board terminals, you should see nearly 0v DC there.

    Maybe your speaker is no good (open)? Try measuring the resistance of the speaker when it is not in the circuit.
     
  7. misterconsister

    Thread Starter New Member

    Sep 23, 2011
    10
    0
    I had a number of problems. One is my multimeter wasn't reading correctly for volts or amps. As for the board itself, my father in-law saved the day as he is an antique raido buff and had a look at my circuit. He did some tests, discovered where there was a circuit break and repaired the break with a much nicer soldering job than I had done. The problem was caused by my nube error of manhandling the board and tearing the copper in the area where I botched an LED connection.

    But I learned a lot and am working on my second project kit now - a voice echo PCB kit.

    Thanks for all your help.

    Eric
     
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