I removed the 220k resistor on the op-amp and measured the output at the headphone jack.
I find 0V to 6.75V on both channels.
I'm powering 9V, one channel at a time.
6.75 volts sounds about right as a maximum the output can swing to for DC conditions like this.
If you set a lower voltage on the pot wiper, lets say 3 volts , then you should see the output also go to 3 volts. Reverse the battery and the reading should change to negative 3 volts on the pot and th amp output.
If you set a lower voltage on the pot wiper, lets say 3 volts , then you should see the output also go to 3 volts. Reverse the battery and the reading should change to negative 3 volts on the pot and th amp output.
This really is bizarre because the amp seems to be working... the DC voltages that it handles correctly are really little different to an audio signal.
We're not done yet though
there are lots of other easy tests we can do. Can you rig up some headphones or even an old (small) speaker with leads that we can use to see if signal is present? Headphones are best though.
An idea... you could unsolder the headphone socket wires at the socket (so nothing disturbed on the board) and just add two lengths of lead to the socket that we would use as probes. We only need work on one channel.
We're not done yet though
there are lots of other easy tests we can do. Can you rig up some headphones or even an old (small) speaker with leads that we can use to see if signal is present? Headphones are best though.An idea... you could unsolder the headphone socket wires at the socket (so nothing disturbed on the board) and just add two lengths of lead to the socket that we would use as probes. We only need work on one channel.
That's normal. For lower voltages below around 6 volt the two readings should be similar.
Do not have headphones connected doing DC tests. You would not hear DC anyway as it is non changing.
I want you to test using headphones as a probe next... give me a few minutes to explain it.
This is what you do.
1/ Unsolder these 3 wires and make sure they are not touching anything:
2/ Solder 2 lengths of wire here using the end tabs of the socket:
You now have the headphone socket wired up to use as a probe and it is no longer connected to the board. Plug the headphones in.
3/ Switch the amp on and connect it to your playing music source.
4/ I would always recommend you begin without wearing the headphones. Connect the two wires, one to ground and one to the pot wiper using the correct side for the channel in use. You should hear music if you turn the pot to full. It will be low volume but you should hear it.
Can you get as far as that?
1/ Unsolder these 3 wires and make sure they are not touching anything:
2/ Solder 2 lengths of wire here using the end tabs of the socket:
You now have the headphone socket wired up to use as a probe and it is no longer connected to the board. Plug the headphones in.
3/ Switch the amp on and connect it to your playing music source.
4/ I would always recommend you begin without wearing the headphones. Connect the two wires, one to ground and one to the pot wiper using the correct side for the channel in use. You should hear music if you turn the pot to full. It will be low volume but you should hear it.
Can you get as far as that?
Excellent 
Now, and keeping everything else the same, remove only the headphone wire that is going to the wiper of the pot and transfer that wire to the junction of the two 10 ohm resistors R29 and R32 which are the main output. Leave the ground wire exactly where it is now.
You are moving only that one wire.
Do NOT wear the headphones. The music should blast out. Does it?
Now, and keeping everything else the same, remove only the headphone wire that is going to the wiper of the pot and transfer that wire to the junction of the two 10 ohm resistors R29 and R32 which are the main output. Leave the ground wire exactly where it is now.
You are moving only that one wire.
Do NOT wear the headphones. The music should blast out. Does it?
I'll look in later... this has got to work
....but if it doesn't then we're still not out of ideas.
If it doesn't work then try adding a series cap (any small value like a 1uf to 47uf value which will block DC) in series with your test wire to the headphones and this time see if there is any audio here:
....but if it doesn't
then we're still not out of ideas. If it doesn't work then try adding a series cap (any small value like a 1uf to 47uf value which will block DC) in series with your test wire to the headphones and this time see if there is any audio here:
This evening I'll tryI'll look in later... this has got to work
....but if it doesn't
If it doesn't work then try adding a series cap (any small value like a 1uf to 47uf value which will block DC) in series with your test wire to the headphones and this time see if there is any audio here:
View attachment 1306504
There is a little circuit ( resistor, diode, small capacitor) for the power led .
I dont' now if it can cause this problem
