article from Dave's Amiga Hardware Page
Although I no longer use any of my A500s regularly, I had a few spare hours during a recent rainy day and I decided to try out an idea I had. The intention was to power an A500 SCSI controller and hard drive off the same power supply that powers the A500, eliminating the 2nd power supply and it's attendant complexity and snarls of cables. I tried this idea on both a GVP A500 HD+ and a Commodore A590. Both modifications worked very well. Normally this extra power supply powers the hard drive, so it's a simple matter to provide power from the expansion bus, provided your hard drive is not too greedy for power. I would not recommend this modification if you are still using an old XT clunker in your A590 or if you are using a new 10,000 RPM screamer - they would probably use too much power. Ordinary 5,400 RPM drives work well. Incidentally, depending on your drive, you may have to do a warm reboot on first powering up if it doesn't boot.
As always, do this modification at your own risk. I will not be responsible if you damage yourself or your Amiga by attempting this modification. I would also advise not connecting the hard drive power supply once this mod is applied. I don't know what will happen, and it's best to be safe rather than sorry!
As always, do this modification at your own risk. I will not be responsible if you damage yourself or your Amiga by attempting this modification. I would also advise not connecting the hard drive power supply once this mod is applied. I don't know what will happen, and it's best to be safe rather than sorry!
Modifying the GVP A500HD+
This was the easiest of the two to modify. You only need to remove the lid (after disconnecting the drive from it's power supply and from the Amiga). Then all you need to do is to locate CN 5 and CN 6. They are on the PCB, right at the back next to the hard drive power connector. Using a soldering iron, simply bridge the two pads of CN 5 together and then likewise for CN 6. Refer to the picture above. In the enlarged view you can also see two other jumpers - CN15 & CN16. Do not solder those. Put the lid back on and you are finished.
While not strictly relevant to the above mod, I found that the GVP will not function correctly if there is an Adspeed CPU accelerator present in the A500 running at 14 MHz. I eventually solved the problem by switching the Adspeed temporarily to 7MHz and then using the GVP Expert Pref program to adjust the settings in the hard drive RDB. You need to set the buffer to point to Chip RAM. Then set the Mask value for Chip RAM by entering the value of 0x001fffff into the mask field.
Modifying the Commodore A590
The A590 unfortunately does not have the convenient jumpers like in the GVP, so the work is a little more difficult. The A590 needs to be completely disassembled and the PCB removed. Referring to the picture above, two wires need to be run between the hard drive power connector and the interface connector to the A500. The first wire goes from the terminal marked "yellow" on CN7 (the hard drive power connector) and goes to pin 6 on the expansion connector. This is the +5V power line. The second wire goes from the terminal marked "Orange" on CN7 and goes to pin 10 on the expansion connector. This is the +12V power line. Disregard the thin blue wire in the picture. Reassemble the A590, making sure a sharp soldered joint does not pierce the insulation of the wires you added. Make sure you don't connect a power supply to the power socket on the A590 after doing this mod. I don't know what will happen if you do.
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