Hi,

No, I actually meant pins 8 & 14. These are the signal inputs to the INA2137 Amplifiers. The other pins that Trevor mentions are also input pins. The signal node on the Amplifier has 2 precision resistors on both Inverting and non-inverting inputs. With the part you can configure it for a gain of 1/2 or 2 using the correct pin resistor without external components.

I traced out the circuit path out with an Ohmmeter and the Left and Right signals originate at the SAA770H Chip. Pin 9 is the Rear-Left and Pin 16 is the Front-Left.


Rear Left Signal Path
---------------------
Pin 9 connects to the small blue 100 Ohm resistor array Pin 3 (it is next to the Chip and has 8 LCC pins).

The pins on the resistor array are counted different from a normal chip. The top Right (closest to the SAA770 Chip) is pin 1, Top Left (away from the SAA770 Chip) is Pin 2, Second Right is pin 3, Second Left is pin 4, Third Right is pin 5, Third Left is pin 6, Bottom Right is pin 7, Bottom Left is pin 8. The resistors are "flow-through" designed.

Locate a set of 6 small yellow Tantalum Capacitors near the board edge. From there, the signal at the resistor array pin 4 goes to the 5th Yellow 10uF Tantalum (-) from the board edge. From there the signal goes from the 5th yellow Tantalum Capacitor (+, the stripe) to a Zero-Ohm resistor located just below the INA2137 furthest from the board edge then to the INA2137 pin 8 as the input to the Amplifier.

From there the signal is amplifed by the INA2137 at pin 9 and goes to the large black 47uF Tantalum capacitor (+) that is located lower-left kiddy-corner of the Red Box you drew on your photo. The large black 47uF Tantalum capacitor (-) goes to the 2nd row of 100 Ohm resitors away from the INA2137 chips, 2nd resistor from the right, bottom pad. The top pad of the same resistor goes to the EMI Filter (elongated brown part) located just above it, bottom pad. The EMI Filter should be near Zero Ohms. The EMI Filter output goes to the 3rd pin from the Right of the white connector near the RCA connectors marked "Analogue".

That is the Rear-Left signal path (there are other components, but, they are not directly in the signal path).


Front Left Signal Path
----------------------
Pin 16 connects to the small blue 100 Ohm resistor array Pin 7. From there, the signal at the resistor array pin 8 goes to the 3rd Yellow 10uF Tantalum (-) from the board edge. From there the signal goes from the 3rd Yellow Tantalum Capacitor (+) to a Zero-Ohm resistor located just to the left side of the INA2137 pin 8 closest to the board edge, then to the INA2137 pin 8 as the input to the Amplifier.

From there the signal is amplifed by the INA2137 at pin 9 and goes to the large black 47uF Tantalum capacitor (+) that is located closest to the board edge upper-Left kiddy-corner of the Red Box you drew on your photo. The large black 47uF Tantalum capacitor (-) goes to the 2nd row of 100 Ohm resitors away from the INA2137 chip, 4th resistor from the right, bottom pad. The top pad of the same resistor goes to the EMI Filter (elongated brown part) just above the resistor, bottom pad. The EMI Filter should be near Zero Ohms. The EMI Filter output goes to the 6th pin from the Right of the white connector near the RCA connectors marked "Analogue".

That is the Front-Left signal path (there are other components, but, they are not directly in the signal path).

The Front-Right and Rear-Right signal path is identical with different components and pins (pins 14 input, pins 13 output on the INA2137s). The connector pins are pin 1 and pin 4).

With an Ohmmeter, you should be able to trace out the path now. You can bypass the Tantalum capacitors with an 10uF/25V Aluminum Electrolytic capacitor to see if they are open (Highly unlikely).

Is there a configuration parameter that can be set to cause this problem? Hammered CMOS config file, I just don't see that happening....

If all of the component paths look good, an oscilloscope or a signal tracer should be used to diagnose where the problem is.

We could do a little more troubleshooting still with a capacitor:

1) Another capacitor trick by connecting the capacitor from the small blue 100 Ohm resistor array pin 4 or 8 to the INA2137 pin 8 and bypass most of the path. It's very unlikely that both INA2137s could be damaged, but possible I guess.

2) We could also place the capacitor between the INA2137 pins 8 & 9 as a quick test too.

3) Another capacitor trick by connecting the capacitor from the small blue 100 Ohm resistor array pin 4 or 8 to the emi filter bottom pad.

4) Another capacitor trick by connecting the capacitor from the small blue 100 Ohm resistor array pin 4 or 8 to the emi filter top pad.

At this point, a schematic of the output section would be easy to produce with component placement too. Another day.



Let us know what you find.

Ross