Advice replacing coin battery on trapdoor RAM.

[mrknight]

I would replace it with an identical one just to make sure everything will work as intended.

Tried to search for a circuit diagram but I didn't have any luck.
http://www.amiga-hardware.com/download_photos/supraram500rev1_back_big.jpg
http://www.amiga-hardware.com/download_photos/supraram500rev1_top_big.jpg

Have a look at the photo. These is a diode, D1 connected to the plus side of the battery. The other side of the diode is connected to what seems to be the supply voltage of the RT-clock. And this pin should also be connected to the voltage pin from the bus. This is all I can do now without having the proper data sheets and/or circuit diagrams close at hands. Since you know what a diode is, I guess you can easily verify this yourself;)

Good luck!

[mrknight]

Ah I didnt notice the diode - that battery must be an aftermarket fitment.

I'm sure the coin battery is part of the original design. The battery is powering the RT-clock while the computer is off, and the bus voltage is powering the clock when the computer is on. Since the battery and the bus voltage is connected to the same pin (Vcc on the RT-clock), the supplies need to be protected from each other. That's why you have D1 at the battery side and D2 at the bus voltage side.

Just grab one of these from AmigaKit (has the diode already installed)

The circuit on AmiKit is just a coin battery replacement for rechargeable batteries and is redundant for this RAM expansion since it already got a protection diode.