Falcon NVRam battery replacement

After over 10 years of operation, the builtin NVRAM battery needs to be replaced. There are several method available to do this, and are documented elsewhere on the web. Most of the solution involved the removal of the epoxy mold compound, exposing the internal battery and soldering a new battery across it. These method requires delicate cutting of the mold compound else risk cutting the internal circuit.

There exist a replacement method documented in the the DOITF030 of the DOIT-Archives by Robert Schaffner (http://www.doitarchive.de/index.htm, doitarchive@doitarchive.de - not verified URLs may be obseleted) under the NVRAM section. This method involves cutting one pin of the NVRAM chip, and soldering a simple diode circuit and battery on to it. 

	The circuit theory is simple. The replacement battery is placed across the NVRAM. Diodes are used so that current only flows in the correct direction. When the Falcon power is off, the battery powers the NVRAM. The diode at the +5V Falcon is reversed bias and switches off the NVRAM from the rest of the falcon. This prevents the battery from powering the entire bird. When the  Falcon power is on, the diode at the battery is in reverse bias, effectively switching off the battery. Thus the Falcon power is used to power the NVRAM.
The original circuit schematic from the DOITF030 document
	To implement, we will need to solder 2 diodes to the Vin of the NVRAM at pin 24. To have the a diode between the Vin and the +5V power, we need to cut the leg is pin 24. A sharp cutter is required, making sure that the cut leave sufficient space at the chip leg to solder the diodes on to. The other end of the battery can be soldered directly onto the ground at pin 12. There is no need to cut the legs here. I decided to put the diode assembly on a small PCB. Unfortunately for me, putting a 3.6V battery did not work, and the Falcon refuse to boot up. After sometime, I replaced the 3.6V battery with a pack of 4 x 1.5 AA batteries (similar to my TT030). With the 6V battery over the NVRAM, the Falcon was able to boot AND remember the datetime plus NVRAM setting.
Implementation of the circuit (click to enlarge)

It may be that with 6V across, it is always powering up the NVRAM even when the Falcon is powered on. However, after 6 months the NVRAM still remembers the datetime and settings.

After replacing the NVRam battery, do not forget to reset it by using one of the NVram reset programs or the NVram configuration programs.

Have Fun!!