Amiga : Unstable A500, Memory issues

(one of) my Amiga test machine(s)
Introduction : Commodore A500 memory issues

The Amiga on which I test all the hardware that I deliver in addition to my prototypes, had significant stability problems from one moment to the next.
Hanging while moving the mouse, freezing the mouse pointer when clicking and more of those perils.
Unfortunately that also means that hardware testing doesn't go well anymore, and even though I always have 3-4 machines open to test hardware, I can't miss one (type / revision / configuration is different with every system)
So it is important that the system functions reliably again as soon as possible.

That means troubleshooting : one of the things you can use for this if the machine doesn't boot is the Diagnostic Bootrom , which does a lot of hardware testing where most of the amiga hardware is not needed to run.
Even Amigas that no longer provide a picture can still be read via a serial connection.

Fortunately, my machine was still able to start things from the shell, so Keirf 's Amiga Test Kit was enough for me to put my system through its paces.

 

Testing the Amiga 500 for Memory Issues
Testing the Amiga 500 for Chip Memory Issues

It soon became apparent that the problem was in the memory, first a scan of the total memory and later specific parts to pinpoint where memory issues were exactly.
Selection of Chipmem and running the test again still gave a memory error, the other segments could all be run through without memory issues.
So apparently there is something wrong with the chipmem, the memory scan shows 2 memory issues, in the 32 bit overview and in the 16 bit overview.
In this case, I can ignore the 32-bit message as I don't have 32-bit memory installed.

The 16 bit part indicates that D15 (16th bit) gives an error, so we are a bit further with our memory issues.
The schematic of the Amiga 500 in revision 5 as I have it indicates that the 15 bit is at location U31.
That is the ram building block that is on the side of the trapdoor lock.
So now it is important to test my findings in practice, and that can be done in different ways:
Replacing the IC, but that is preferable if you are sure that it is also the correct IC.

Amiga 500 Memory layout schematics
Pggybacking a RAM Chip to fix Memory Issues

Another option is Piggybacking, you place a working RAM over the suspected faulty RAM (Make sure all pins make contact, and make sure they are not shifted).
Bending inward slightly causes them to clamp onto the legs of the faulty RAM, so to speak, placing the new RAM over the supposedly faulty RAM.
Checked that there are no bad connections and start with that bite.
The memory test is working, and the machine is no longer reporting memory issues, the system is running stable.
This is not a permanent fix, there is a chance that the RAM will shift and the pins will contact things where they shouldn't.

To rule out coincidence, the piggyback was undone and the test started again.
Immediately the memory problems come back in exactly the same location, so it is a very strong suspicion that we have the right chip.
I did the piggyback again and let the machine run memory tests all night, but didn't see a single error message after all night.
So now it is important to make the fix permanent, again this can be done in at least 2 ways:

Testing the Amiga 500 Chip Mem for Memory Issues
Result of the memory test after a succesful piggybacking attempt

If you want to do it neatly, you desolder the IC, if you have good material you can remove the chip in its entirety.
If you only have a soldering iron and desoldering wire, you can choose to cut all the legs of the RAM as high as possible above the motherboard, desoldering loose legs is a lot easier than a whole IC if you only have a soldering iron and desoldering wire.
Once you've removed all the legs and the holes in the motherboard are clear of tin, check carefully for damage to and around the islets, they are easier to repair now than when the new IC is already in place.
Then solder an IC socket at the location of the IC and you can easily change another RAM at that location in the future if necessary.

If you don't really like the desoldering, you can also make the piggybacking permanent.
It is advisable to cut off the VCC pin at the IC so that the old ram no longer receives voltage, then solder all pins on the old IC, except the VCC pin, which should not be on the IC, but on the motherboard. stinging piece to be soldered.
With this you create a new RAM that is fed by the motherboard and further all contacts are passed on via the old RAM.
This should permanently fix the memory problems, and your machine can last for a while.

Close up of a piggybacked Ram Chip in the Amiga 500

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