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Topic: Instrument amplifier circuit fails to work when put on PCB (Read 2916 times) previous topic - next topic
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Instrument amplifier circuit fails to work when put on PCB

Im very new to electronics and I built this guitar amp circuit on a breadboard:



and it was working really nicely. Now Ive decided to put the circuit on a universal pcb which i'll use in the actual amp. I soldered all the components on but no sound comes out when i play. Because of the different layout of the universal pcb the circuit's grounds are connected differently than they were on the breadboard, so maybe thats the potential problem, since its the only difference i think there is. Sadly I dont remember what it looked like on the breadboard and I didnt take any pics. Heres the layout of the circuit on the universal pcb:



Heres something I dont understand, when I measure the potential between the output and the ground with a voltmeter, it corresponds nicely to the signal from the guitar, it seems like it should work. But when I measure the potential between the wires going into the speaker (which are connected to the output and the ground) i get zero volts between them always. Why is that and how can it be fixed?

Also sometimes when I poke around with the voltmeter at different spots on the ground and output it will randomly start working and sound will come out. I cant really notice any pattern when doing so though.

Heres the pin layout for the LM386 operational amplifier:



And this is LM386 datasheet: http://www.kynix.com/uploadfiles/pdf2286/LM386M-1.pdf

Once again Im very new so Im sure Im making a ton of embarrassing mistakes and that my soldering is terrible lol

Re: Instrument amplifier circuit fails to work when put on PCB

Reply #1 –
Sounds to me like you have a short forming somewhere between the output and ground, which disconnects at times. If you have a multimeter that can do this - and all of them should I think -  set it to short circuit detection and check for that.

Re: Instrument amplifier circuit fails to work when put on PCB

Reply #2 –
Quote
But when I measure the potential between the wires going into the speaker (which are connected to the output and the ground) i get zero volts between them always.
The output should be biased at half the battery voltage.

Quote
Also sometimes when I poke around with the voltmeter at different spots on the ground and output it will randomly start working and sound will come out.
That's probably an intermittent short or an intermittent open connection.  You may be able to find it with your Ohmmeter..  Use the Ohmmeter to make sure your connections actually match the schematic.  Do that from the top-side of the board so you are checking the connections to the component leads.

Check all of the pins on the chip relative to all of the other pins to see if any are shorted together  (I believe only pins 2 & 4 should be connected together.)  If your meter has a beeper to indicate zero-Ohms, that can be super handy for finding shorts...  I put one probe on pin 1 and drag the other probe across all of the other pins, then do the same with pin 2, etc.     

In case you don't know this, disconnect power before measuring resistance (Ohms).

Quote
Im very new to electronics and I built this guitar amp circuit on a breadboard:
I've built quite a few permanent projects on a breadboard.  It costs a little more are and it takes-up more space but it's easier & "cleaner" than soldering to a perfboard.  I had a car alarm built on a plug-in breadboard and it worked for about 20 years.


BTW - If this is for a guitar, the LM386 has an input impedance of 50K Ohms and most guitar amps are around 1M Ohm.   The lower impedance may knock-down your signal, which may be OK since I think you've got plenty of gain to make-up for it.    And, it may affect your tone (less treble).

Re: Instrument amplifier circuit fails to work when put on PCB

Reply #3 –
What is connected to pin 8? Your schematic sort of indicates pin 8 with the gain boost components but appears to not be connected on the board. It's customary to to always include pin numbers on schematics so as to avoid confusion. Pin 2 of the amp ('-' input) is supposed to be connected to pin 4 (ground) but appears to be unconnected.

You say the PCB layout and the breadboard don't look the same and BTW they never do. The way to think of a schematic is a list of interconnects. In your case R3 terminal 1 connects to IC 1 pin 8 and R3 pin 2 connects to C3 pin 1 and C3 pin 2 connects to IC pin 1. The actual physical layout can be done in different ways as long as the connections match up. Also be aware that it is very common to NOT see errors because you're sure it's already correct when in fact it is wrong. Having someone else check it over is a big help. Explaining it to someone else even if they don't understand will help YOU to understand it better.

Your soldering could use some work but you have an OK start. It looks like you're trying to treat the solder as paint. It isn't. The tool might not be hot enough OR you got told not to overheat the parts. Many years ago I was curious about how long it would take to ruin a transistor with a soldering iron. After several minutes I got bored because the transistor had not failed. Except for melting plastic or rubber parts, don't be concerned about damaging parts. This assumes you have a temperature controlled iron that is running 600-700° F. Personally I use a Metcal but a Weller Magnastat or Hakko would be good choices.

De-soldering braid and a rosin flux pen are inexpensive and very useful. Your solder should be 63/37 or at least 60/40 tin/lead and NOT plumbers materials.

Check it again and then again. It worked before and unless you damaged a part, it will again.

You can find lots of help in Google groups and DIY audio.

G²