Amiga Technical Resource


A do-it-yourself guide to replacing the Amiga real time clock battery



Introduction:

Amiga computers are usually fitted with a real time clock (RTC) device, the main job of it is to retain the time and date when the system is turned off.

The "big box" Amigas such as the A2000, A3000, A4000 had these built onto the motherboard.
The "wedge" Amigas such as the A500 and A600 and A1200 did not have motherboard RTCs, but could be added in the form of a plug in expansion, usually on a memory expansion module or a 3rd party CPU card.
An exception is the A500+ which had the entire RTC on the motherboard.
RTC Battery

To keep the RTC functioning with the system powered off, a small 3.6V rechargeable battery is used.
The problem with these nickel-cadmium and nickel-metal hydride batteries is that they have a rated service life of around 6 years and if kept in service, are likely to begin leaking their corrosive inner chemicals onto the computer's printed circuit board.

The motherboard corrosion commonly causes faults such as the RTC not working (can't remember time) and mouse not working or "jumpy" pointer. Depending on the severity of the corrosion, it may cause many other problems as well.

Most Amiga models around today would have been manufactured before 1994, so battery replacement is well overdue for most of them. The exceptions are models fitted with the silver "coin cell" battery, mainly fitted to the A4000T and some A1200 CPU boards.
These coin batteries do not have as many problems with leaking corrosive chemicals.





Disclaimer and caution

While the information on this page has been checked and is correct to the best of my knowledge, there is still the possibility of unintentional errors.
Please report any errors directly to this address so they can be corrected.

As with any repair work to delicate electronic equipment, you risk causing further damage to your system or yourself. I cannot be held responsible for any equipment damage or personal injury.

It is strongly advised that you possess some good common sense and preferably have previous experience in working with electronics before undertaking any repair on your system.
As always, you should take all anti-static precautions when working with semiconductor devices.




You can do it!

This guide is to help you replace the battery and repair corrosion problems yourself.

You will need:

You will also require some of the tools listed here. Tools


A replacement battery is also required.
The correct replacement is a Varta 3/V80H, also listed as Varta part number 55608303059 which are available from most large electronic component stockists.

Sources for replacement batteries:

I also keep a small stock of them, so if you get stuck, contact me and I can probably supply one.
Alternatively, you can use almost any 3.6V cordless phone battery pack, available from most communications retailers.




Getting started:

This guide is aimed at the A4000D, but since the RTC circuit in most Amigas is similar, you should be able to apply most of this information to other models.


First up, you'll need to know what the problems are, so test the mouse port and joystick port are operating correctly. I'd recommend JoyTest 1.04 from Aminet.
To find out if the RTC is seen by the system or not, use the Workbench Time prefs program. If the save button is greyed out, the RTC is not being seen by the system.
A better test is to use Scout from Aminet. Under System, it will report if the RTC is found or not.
If the RTC is not being seen by the system, this usually indicates other hardware problems. Usually corroded PCB tracks or damaged ICs. If this is the case, you'll also need to refer to the repair section.


Begin by clearing yourself a large workspace, like a clean table covered with newspaper, or better yet, a proper static safe workbench. Also make sure you have several hours of uninterrupted time to do the job.
To avoid the risk of static damaging any components, wear a grounded anti-static strap.

Remove the computer's case and any Zorro expansion cards fitted.
Collect removed screws in a small dish to avoid loosing any.

Locate the RTC circuit which is located roughly half way between the mouse connector and the SIMM memory slots.
The image below shows the main devices in the area:

RTC overview




Next locate the battery and assess the extent of the corrosion.
Leaking batteries typically have a white "fur" or crystals on their end terminals, as shown here:

Corroded battery in A2000 Corroded battery

The picture above is inside an A2000.
Sometimes the corrosion causes the socket pins and legs of the surrounding ICs to turn green in colour which often causes intermittent problems due to the corrosion causing an intermittent contact between the socket and the IC legs. Fortunately this has not happened yet in this A2000.
Often the only way to fix this is to completely replace the IC socket and clean the IC's legs. Or alternatively remove the socket and solder the IC directly into the PCB.



Obviously the battery needs to be removed. If you simply can't be knackered fixing the computer, the least you should do is remove the battery using a small pair of side-cutters to cut the 3 legs holding it to the motherboard. This will stop further corrosion.

Of course the better way is to do it properly.
If you are experienced with soldering, the battery can be removed from the top of the motherboard, without removing the motherboard from the lower case.
It is a lot easier (but more work) if you remove the motherboard from the lower case, but I'll let you decide exactly how you want to do it.

It's usually easier to start on the end further away from the SIMM slots, the positive end.


Positive end of battery



Use the old toothbrush to clean away the excess white crystals from the solder joints and begin by heating the two legs on this tab and APPLYING a bit of fresh solder.
You'll find that you can heat one leg until the solder completely melts, rock the battery slightly so it lifts, heat the other leg, rock the battery slightly, etc, going between one and the other several times, until the whole end lifts up as shown below.
Careful not to use too much force while rocking the battery, or you may damage the PCB's pads and tracks!

Positive end removed




Remove the other (negative) end of the battery next. It only has one leg on this side, but it is soldered into a large copper track which acts like a heatsink, making the joint difficult to heat up.
This one is a lot easier to unsolder from underneath, but if you have the right shaped soldering iron and you're experienced, then it's possible to remove it as shown here:

Removing last battery terminal




Once the battery is removed, use solder wick to remove the solder filling the terminal holes in the motherboard:

Clear holes using solder wick



Using combinations of a toothbrush and cleaning cloth, clean the visible corrosion damage from the board using the methylated spirits or isopropyl alcohol.
You can scrub quite hard with the toothbrush with little risk of damage. The components and soldering are quite robust.

Don't worry if some of the solder resist (the green coloured lacquer covering the copper tracks) comes off when scrubbing the effected areas. It if begins flaking off, it indicates the battery's chemicals are between the copper layer and solder resist.
You can see where this has occurred as the solder resist has a pale, blotchy look to it.
It's best to carefully scrape off all the solder resist that looks like this so you get down to the bare copper.
If you have a fibreglass brush/pen, it makes this job very easy.
Thoroughly clean the bare copper tracks using the alcohol.

The exposed copper can be coated with a solder resist repair/overcoat pen to prevent the copper from oxidising and to help protect it from corrosion. Nail varnish can also be used for this job.

Applying solder resist repair


It's a good idea to do this last, after you have finished testing as there may be additional soldering and repair work to be done.
At this stage you can apply the new solder resist beside and underneath the battery, as it gets fitted soon.


When you did the tests before beginning the job, you'll know if there are further problems with the RTC or the mouse/joystick port which need to be repaired.
If this is the case, please refer to the repair section.

Now is a good time to partially reassemble the system. Temporarily connect the power supply, floppy drive and hard drive to make the system boot.
You should test the mouse and joystick ports again and make sure that Scout reports "BattClock found"
Using the digital multimeter, check for approximately 5V DC across the battery holes in the motherboard with the system still powered up. This checks the battery charging and RTC power supply circuits are working.
Measure from one of the two positive holes, to the single negative hole beside the SIMM slot U850.

There should also be around 5V between pin 9 (ground) and 18 (VCC) of U178, the real time clock IC.
If any of these voltages are missing, go to the repair section for more information.

If it all checks out OK, you can disassemble the system again, finish applying the rest of the solder resist, then fit the battery.




Fitting the replacement battery:

If using the correct Varta 3/V80H style replacement battery, it will fit in exactly the same position as the old one.
If the motherboard is removed from the lower case, it's best to solder the battery's legs from underneath, or otherwise just solder it from the top side.


If using a 3.6V cordless phone battery as a replacement, here's what to do:

  1. Cut off the two pin plug, leaving the two wires as long as possible.
  2. Strip approx 3mm of insulation from both wires and "tin" them with the soldering iron and solder.
  3. Solder the black wire to the single negative hole nearest the SIMM socket U850.
  4. Solder the red wire to one of the 3 holes nearest U177. You'll note there is a row of three holes, even though the previous battery only used two of them. All 3 are electrically joined together, so you can use any one of them.
    Double check the polarity before making the final connection as reverse polarity can damage the RTC!
  5. Use some double sided foam tape to stick the battery pack down on the motherboard.


Once all this is finished and you have correctly done the previous checks, you can reassemble the system and set the clock, remembering to select save from the time preference window.
Completely switch off the system for 30 seconds and power it up again. You should find the time and date will now be correct.


A special note for A3000 users: The RTC has a small amount of memory also backed up by the battery which is used to store settings for the motherboard SCSI controller. After replacing the clock battery, you will need to use SetBatt-1.2 from Aminet.

Usage:
SetBatt A

This clears the "amnesia" bit. The SCSI controller settings may also have been lost due to the power interruption to the RTC and may need to be altered depending on your SCSI devices.






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