OK about time to some more updating:
Here's my way of restoring the back-up battery on the Berzerk ZPU board. I've installed it on 3 boards so far and all have been working fine for between 1 and 3 weeks.
The idea was not mine, I first learned about it from Bob Roberts' site.
Also the suggestion to use the schottky diode instead of a regular one comes from Mark Spaeth. If, instead of doing this mod you would prefer a very comfy way to reach the same goal, you could try to get one of his NVRAM boards for your ZPU board, or even better, his ZZPU board, but both are simply unavailable at this moment (or maybe you have better luck than I).
What you need:
1) A CR2032 Lithium NON rechargeable battery, buy a brand and not some cheap (Chinese) offer, we DON'T want any more leaks. I used Panasonic (RS online part 457-4757)
2) A holder that fits the CR2032 size: 20 mm. (RS part 430-675)
3) A 1N5817 schottky diode (RS part 625-4972)
4) tools, soldering station, solder,small drill !, DVM
Bob Roberts also sells a set he calls Tron Lithium Battery Conversion Kit for $4.00 that is suitable for this mod too. I'm not sure if Bob sells a schottky or a regular diode in that kit though....
A) First find R29 on the board. It's very close to the + terminal of the original battery. Desolder that resistor and remove it from the board (On boards with acid damage this can be tricky, add a bit of new solder to get the old solder flowing. If that doesn't help some flux may be helpful (in this case, normally do not use it for electronics).
You don't need the resistor anymore.
B) In the position of R29 solder in the 1N5817 diode.
MAKE SURE THAT THE CATHODE (THE WHITE LINE ON THE BODY OF THE DIODE) IS
AWAY FROM THE PLUS TERMINAL. So the cathode is soldered near the R29 marking. See the picture:
The diode is installed to BLOCK the charging voltage (about 5 volts) that normally charged the original battery. Since we are using a non-chargeable battery it is ESSENTIAL that this charging voltage does not reach the battery. If you put the diode in the wrong way this voltage WILL reach the battery and will try to charge it with probably some devastating results !On some boards I experienced the traces between the + terminal (hole) and R29 were eaten or broken. It's easy with the length of the diode leg to bridge that gap. So, solder it to the + terminal directly in that case.
C) Lay the board flat and put the holder on the board. Put the + terminal of the holder in the hole of the + terminal on the board. This is to "measure" your holders distance between it's soldering pins, but usually it's the same for the various brands.
Now, looking sideways you can see where you will need to drill a tiny hole through the PCB:
I've used two brands of holder and the distance of the pins was the same. When you turn over the PCB, you can see that Stern (in it's wisdom) has put a "raster" of ground area right under the original battery area. This is very useful for us now.
If you look at the "raster" and count the "holes", the hole has to be drilled in the 4th "hole" from the + terminal (the + terminal is on the right in this picture) on the top line:
Drill a hole that is a little bit larger than the "hole" in the raster. This is to make sure we hit a contact area of the ground "raster".
Scrape away a bit of the protective layer around the hole so the copper becomes clear.
Now pre-tin this area and the + terminal. ALSO pre-tin the pins of the holder.
Insert the holder and solder the pins. Push the holder sideways to the left so it's fixed against the edges of both holes (the wide + hole gives you plenty of "room" to play with.
Now solder the pins to the edges. Let the solder flow and do it's work, don't smear !
This should be (about" the result (poor picture quality,sorry):
Flip over the board and there's your holder tight and snug against the board.
Insert the battery. DO NOT TOUCH IT WITH YOUR BARE FINGERS. Use a bit of paper or plastic or if you want a rubber glove, but make sure there is no fingerprint on either side when you insert it. Our skin carry it's own acid which is not good for the contact between battery and holder.
Time to check your work !
Get out your DVM, switch to voltage, DC (duh!) and measure between GND and pin 18 of the RAM chip at position 1E.
You should get a reading like this:
If so, it's working OK. If not there could be a trace break on your board. Check that with your DVM and check the solderings you made on the battery holder.
The reason why we used a schottky diode is that it has very little loss of voltage when it's in conducting situation. Here's the voltage measured directly at the battery (while installed):
As you can see hardly any difference. Initially I tried a regular diode but this dropped the voltage to about 2,75 Volt. Still plenty for the RAM. The specs of the RAM say it requires 2V minimum to keep the RAM data. You will never reach that voltage with the lithium battery unless it's REALLY old. Lithium cells hardly ever leak, but I still suggest to check every now and then, you never know.
Put a date somewhere on (or near) the board to remember when you installed the new battery.
Now re-install the ZPU board in your machine. To be 100% sure you installed it properly, you can check the voltages on both sides of the diode (to GND) to see you succesfully blocked the charging voltage. The cathode should read about 5 Volts, the other side about 3.1V.
This completes the hardware work. But you're not finished just yet....
Now the hardware part is done. There's still somethin