Thanks for your reply Ken. I’ve triple checked the orientation of the diodes against the PCB and against the working module. Ditto for the other components. I swapped the LM3900 with the same results. I can’t see any obvious error although it’s obvious I’ve made one.
The values in black are Volts. Blue are Millivolts. The value in Red is the suspect output. I tested the jack signal tabs and the pins on the LM3900 both with nothing plugged into the modules and then with a 4.2 Volt square wave coming out of an MFOS oscillator (C3). The Jumping values seemed to be in the millivolt range and appeared the same on each module.
I’m not sure if this info sheds light on where the problem may be. I’m guessing it’s a short somewhere as the value of the input drops from 4.2V to .37V as soon as the jack is plugged in.
I took some measurements;
4.2 vDC (C3) IN
No Input
4.2V Input
#001
#004
#001
#004
IN Top
0.088
0.100
4.2
4.2
OUT DC Top
0.792
0.786
4.2
4.2
IN Middle
0.100
0.040
4.2
4.2
OUT DC Middle
0.803
0.794
4.2
4.2
IN Bottom
0.060
0.006
4.2
0.37
OUT DC Bottom
0.786
0.780
4.2
1.44
LM3900
No Input
4.2V Input
Pin
Function
#001
#004
#001
#004
1
1IN +
0.409
0.402
0.403
0.402
2
2IN +
0.408
0.403
0.403
0.400
3
2IN -
0.558
0.549
0.554
0.543
4
2OUT
0.768
0.790
0.775
0.789
5
1OUT
0.772
0.801
0.781
0.798
6
1IN -
0.554
0.549
0.551
0.544
7
GND
0.228
0.218
0.234
0.231
8
3IN -
Jumping
Jumping
Jumping
Jumping
9
3OUT
14.202
14.200
14.200
14.169
10
4OUT
0.763
0.787
4.160
1.456
11
4IN -
0.545
0.545
0.541
0.538
12
4IN +
0.395
0.402
0.474
0.414
13
3IN+
Jumping
Jumping
Jumping
Jumping
14
Vcc
15.000
15.000
15.000
15.000
#001 is the working module. #004 is the non-functional module.
I’m not sure if this info sheds light on where the problem may be. I’m guessing it’s a short somewhere as the value of the input drops from 4.2V to .37V as soon as the jack is plugged in.
Rich