Hi Peter,
The Battery voltage is not the SRAM voltage.
You really need to check the voltage at the centre terminal of D1 (or SRAM pin 18) and any earth. Even though your SRAM values were ok the voltage may be low and I have noticed sometimes the SRAM can keep values on low voltage sometimes not.
So you must check SRAM voltage is above 2.8VDC.
As for the C21 voltage...you are measuring across C21. It won't matter which side your leads are on. One way you'll get +5V (if ok) the other -5V (if OK)...aka putting the probes across the wrong way just reads the voltage in reverse potential.
But they measure it across C21 with the negative probe on the left side (power connectors on bottom, hand set cable on left) of C21. The left side ..closest to RJ22 (actually RJ11, 4P4C in Australia) is the earth side, see attached pic. The red arrow is C21 earth side.
Yep your looking for ....-5V or if leads wrong way +5V (see below).
Brendan
Just an edit:
I measured David's on bench and found 0V powered up across C21. You need to plug in a working RS232 lead (or your Gemini BT box
) and connect to the device (edit edit: actually I didn't need to connect, I just plugged in a RS232 to USB and it worked) to see the -5V. Also the earth I marked above is correct but since it's a -ve charge pump you'll see -5V DC or if you have the leads the wrong way +5V DC.