Velleman Voice Changer Does Not Work

Thread Starter

Spikechan

Joined May 21, 2017
10
Just completed a Velleman Voice Changer board, but it is not working. The Zenerdiode, LEDs and electrolytic capacitors are all installed correctly (so, not a polarity issue). Soldering joints are all good. The LEDs light up when the battery is connected. Changing the microphone sensitivity and/or the volume has no effect. The output voltage (at the speaker terminals) is a few millivolts (hence, no sound from the speaker). Any ideas? Bad microphone?
 

AlbertHall

Joined Jun 4, 2014
10,586
Does LD2 light up and does its brightness change with how loud the sound is?
If you have a voltmeter, measure the voltage across ZD1 - it should be around 3V.
 

Thread Starter

Spikechan

Joined May 21, 2017
10
Does LD2 light up and does its brightness change with how loud the sound is?
If you have a voltmeter, measure the voltage across ZD1 - it should be around 3V.
LD2 lights up briefly when the switch is closed, but then goes out. It does not change with sound. The voltage across ZD1 is 3.2 V.
 

Thread Starter

Spikechan

Joined May 21, 2017
10
Can you measure and list here the voltages on all the pins of IC1 please.
pin #1 1.24 V
#2 3.17 V
#3 3.16 V
#4 3.16 V
#5 3.16 V
#6 0 V
#7 0 V
#8 1.1 V (measuring this pin makes LD2 light up, connected to mic sensitivity?)
#9 1.16 V
#10 3.19 V
#11 1.9 V
#12 1.5 V
#13 1.16 V
#14 3.19 V
#15 1.33 V
#16 1.31 V
 

AlbertHall

Joined Jun 4, 2014
10,586
All those voltages seem reasonable.
Touching pin 8 flashes LD2. Can you confirm that touching pin 9 does not flash LD2 please.
Measure the voltage across the microphone (expecting something more than 0V and less than 3V).
Short the microphone terminals (won't harm anything) and see if LD2 flashes.
 

Thread Starter

Spikechan

Joined May 21, 2017
10
All those voltages seem reasonable.
Touching pin 8 flashes LD2. Can you confirm that touching pin 9 does not flash LD2 please.
Measure the voltage across the microphone (expecting something more than 0V and less than 3V).
Short the microphone terminals (won't harm anything) and see if LD2 flashes.
Touching pin 9 does not flash LD2. The voltage across the microphone is 0V. LD2 does not flash when the microphone terminals are shorted.
 

Thread Starter

Spikechan

Joined May 21, 2017
10
Update:
So, looks like there was a solder joint that wasn't so ok :D The voltage across the microphone is now 2.1 V. There is a 'hum' from the speaker, but no voice.
 

AlbertHall

Joined Jun 4, 2014
10,586
With it switched off, can you measure the resistance from both ends of R7 to ground (negative battery connection). I suspect that one of those readings will be very low (less than 10Ω).
 

Thread Starter

Spikechan

Joined May 21, 2017
10
With it switched off, can you measure the resistance from both ends of R7 to ground (negative battery connection). I suspect that one of those readings will be very low (less than 10Ω).
On the side of R7 closest to the microphone, the resistance is 1.4 kΩ, and on the other side of the resistor it says overload.
 

AlbertHall

Joined Jun 4, 2014
10,586
On the side of R7 closest to the microphone, the resistance is 1.4 kΩ, and on the other side of the resistor it says overload.
Well, that's definitely not right - but I don't know exactly what is wrong.
Can you post pictures of that area of the board from both sides.
 

AlbertHall

Joined Jun 4, 2014
10,586
I can't see any problem there.
Please re-check the resistances from R7 to ground.
Also, are you sure that the microphone is the right way round (that is also a polarised component).
I am off for my beauty sleep now, back in the morning.
Night, night.
 

Thread Starter

Spikechan

Joined May 21, 2017
10
I can't see any problem there.
Please re-check the resistances from R7 to ground.
Also, are you sure that the microphone is the right way round (that is also a polarised component).
I am off for my beauty sleep now, back in the morning.
Night, night.
The resistance is the same as before. The microphone can only be put into the board one way (so, yes, it is correct).
Thank you for your help; have a good night :)
 

AlbertHall

Joined Jun 4, 2014
10,586
Good morning,
In that case it would seem that R7 is open circuit. In my experience it is rare for new resistors to be faulty but you do seem to have one. Remove it from the board and check its resistance to confirm the problem.
 

Thread Starter

Spikechan

Joined May 21, 2017
10
Update:
Checked every soldering joint on the board and made each as perfect as possible - everything works! :) Looks like the culprit may have been the joint on one side of R7 (seemed fine, but just a little tweak made all the difference). Thank you again for all of your help!
 
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