Amiga Technical Resource

A do-it-yourself A3000 INT_2 modification guide



Introduction
A4000D and T models have an interrupt line called _INT2 on the CPU (FAST) slot. However, this pin was not connected in the A3000D and A3000T, causing PPC and other 68k CPU boards which use this line to not function correctly. Most CPU boards which have onboard SCSI will require this modification to operate in the A3000D and A3000T.

The modification simply involves connecting the INT_2 line to pin 82 of the CPU slot, CN606. Electrically, the INT_2 signal is physically available in several places, but the most convenient connection point is pin 19 of the A3000D Zorro riser slot, CN600.

The details below apply specifically to the A3000D, though the same can be applied to the A3000T, though the physical layout is different to that of the A3000D. Electrically, this modification is the same on both D and T models.



 Parts and tools required



IMPORTANT!

Ensure you take all static electricity precautions while working inside the machine!
Ideally wear an anti-static wrist strap at a static safe workstation.


Disassembling the A3000D
  1. Remove the 4 beige screws around the outside of the case and the silver screw in the top centre of the back panel.
    The case slides forward to remove it.

  2. Remove any expansion cards if fitted and any blanking plates covering up the expansion slot holes.

  3. Remove the Zorro riser card by gently rocking it left and right while pulling directly upwards.

  4. Remove the centre plate assembly. There is a screw by the power supply, one by the expansion slots, one at the top front of the case, one under the front of each floppy drive bay and one at the back of the inside plate under the hard drive.

  5. Lift out the centre plate assembly, taking care to unplug the main board power connector, ribbon cables for the SCSI hard drive and the floppy drive.

  6. Remove the 2 Philips screws holding down the main board and the 4 x 1/4" hex mounting posts.

  7. Remove the 2 countersink Philips screws either side of the side connector plate.

  8. Remove the 12 x 3/16" hex retaining nuts from the rear D-range connectors.
    Note that the nuts on the 15 pin 30kHz monitor port are a different thread type, so don't mix them up.

  9. Unplug the front LED panel connector.

  10. Lift the front edge of the main board up and out of the lower case.

IMPORTANT!

Also check the real time clock battery for corrosion, or replace it if in doubt!
For more information see the RTC battery replacement pages.




Performing the modification
  1. Turn the main board upside-down and rotate it so the CPU slot CN606 is nearest you. Heat the end of the enamel wire to "tin" it with solder and connect it to pin 82 of CN606 as detailed below.
    CN606-Where to solder

    Click on the image for a larger version.


    Soldering to CPU slot

    This photo shows the wire soldered to the underside of the CPU slot.


  2. Cut the wire to length so it reaches CN600 pin 19. Tin the other end of the enamel wire and solder it to CN600 pin 19.
    CN600-Where to solder

    Click on the image for a larger version.


    Soldering to CN600

    This photo shows the wire soldered to the underside of the Zorro riser slot, CN600.


  3. Carefully inspect both solder joints to make sure the joint is secure and there are no solder bridges/shorts.
    Completed modification

    It should look like this when finished.


  4. Turn the main board right side up and check your work as detailed below.
    Testing the completed job

    Click on the image for a larger version.


  5. Once checked, turn the board upside down and apply small drops of adhesive along the length of the wire to hold it securely in place.

  6. Reverse the disassembly instructions to reassemble the machine.
    Take care to correctly fit the mylar insulator behind the Zorro riser card.

  7. Fit CPU board and other expansions and test machine as required.




 Questions and answers
Q: Is the INT_2 modification necessary for all CPU boards?
A: No, some will work without it. Anything with a SCSI controller or a PPC will require it. Some others may require it as well.

Q: Will it damage the computer/CPU board if the modification is done and the CPU board doesn't require it?
A: No.

Q: I've seen a modification similar to this elsewhere, but the wire connects to a different point, is this the same thing?
A: Yes, it does exactly the same thing. The connection points are electrically the same. I've used these different points because it makes the job slightly tidier.

Q: The A3000 won't boot when turned on, I just get a yellow screen.
A: Fit the Zorro riser card, the machine won't operate correctly without it.

Q: It won't run with my new CPU board.
A: Does the card in fact work at all? Test it in an A4000 or known good machine.
Make sure the two main board "clock source" jumpers are set correctly for your particular CPU board.
Check if the card requires OS 3.1 in ROM to operate.
Remove the CPU board, all expansions and drives. Set both clock source jumpers to "INT". Make sure the A3000 powers up into the "insert disk" pre-boot screen.

Q: The CPU board runs OK, but its SCSI controller doesn't seem to work, or it works very slowly.
A: Did you properly check your job after the modification? Check it again.
Check the SCSI termination and setup is correct.

Q: The whole machine seems dead! Only the power supply fan operates.
A: Did you plug in the main board power connector?

Q: I see signs of fuzzy corrosion on the RTC battery, what should I do?
A: Replace the battery immediately, or remove it from the main board at least.
For more information see the RTC battery replacement pages.

Q: Where can I get additional technical information on A3000s?
A: Here.

Q: These instructions suck!!! Where can I get something else?
A: Just after writing these, I found instructions online here.



 Disclaimer
Please note that I hold no responsibility for any damage caused to you or your equipment.
This information is provided in good faith, though may contain unnoticed errors.
Any work you do to the computer system is completely at your own risk.


"Amiga" and its logos are copyrighted trademarks of Amiga Inc.



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