1541 drv8to9

     CHANGING A 1541 FROM DEVICE 8 TO DEVICE 9, 10, or 11

(also included: SX-64, 1570/71, C128DCR internal drive, and MSD-SD2)

         Latest additions & corrections: 10-31-02

 There were at least three versions of the 1541, each with a

different PC board, and two types of drive mechanisms. The following is the procedure for modifying a 1541 to enable a different device number. All drives were shipped from the factory as device #8, with no jumpers cut. The jumpers, by the way, are two tiny silver half-circles with a "dash" in the center. They resemble a fat letter H. To open a jumper, cut the dash with a sharp knife. If you accidently cut the wrong one, you can apply a blob of solder to re-connect the two half-circles, or just lay a small bit of wire across it and solder it. Don't heat it for more than a few seconds or you will lift the trace off the board. Jumpers are usually marked 1 (or A) and 2 (or B). To change the device number from the factory setting of #8 to device #9, cut the jumper marked 1 or A. For device 10, cut the one marked 2 or B. If you cut both jumpers, you have device 11. These two jumpers represent grounds going to a 6522 VIA chip. Jumper 1 (or A) is pin 15 of the chip, and 2 (or B) is pin 16. You can remove that IC, lift one or both pins, then re-insert it into it's socket to change the device number (for example, lift pin 15 for device #9) but it is best to cut the proper jumper... the chip pins will break off if bent more than a few times. By installing a device 8/9 switch (a SPST toggle switch that opens and closes jumper #1), you can easily switch back and forth between drive 8 and 9 without opening the drive. Each terminal of the added switch is wired to one side of the cut jumper. The solder pads are so tiny that, unless you are proficient with a soldering iron, it is best to leave that work to a technician. Obviously, the modification should only be done with the drive turned off and all plugs disconnected. The most convenient place to mount the switch is on the front right-hand side of the drive. The spacing is tight behind the front panel... make sure you drill the hole with enough room behind it for the body of the switch. Solder the wires to the switch before you mount it... it's easier. Facing the front of the drive with the case top and shield removed, the drive select jumpers are located as follows for the various models: The early VIC-1541 (sometimes called the 1540) in the white case had the long board and used the ALPS drive mechanism with the push-down door. The drive select jumpers, marked 1 and 2, were near the rear of the board, right next to the 6522 chip UAB1. Cut jumper 1 for device #9, jumper 2 for device #10, and both for 11. Next came the shorter board 1541 in the standard brown case. It used the ALPS (push-down) mechanism and a silver colored metal shield. The PC board number was 1540050. Some of those board's drive select jumpers were marked A and B, some 1 and 2, and some were unmarked. The jumpers were located 2" from the front of the board and 2 1/4" from the left side. They are positioned between electrolytic capacitor C46 and transistor Q3. Jumper 1 (or A) would be closest to the transistor. Again, cut 1 or A for device #9, etc. as above. A later version 1541 (sometimes called the 1542) had a shorter board (PCB #251830 or 251777), and used the Newtronics drive exclusively. The metal shield was gold in color. This drive was apparently the most common. Drive select jumpers are 1.5" from the front of the board and 2" from the left side. The jumper pads marked 1 and 2 are between transistors Q4 and Q5. Jumper #1 is closest to transistor Q5. The latest version was the 1541C (PCB# 251854) and used the Newtronics mechanism in a off-white case. That's the only one with the photo-sensor to detect track zero, but it was disabled by Commodore, so the drive head still bangs like the earlier drives. The drive select jumpers are called J1 and J2, and are located 1 3/4" from the front of the board and 1/2" from the left side (near plug P1).

SX-64 INTERNAL 1541: The built-in drive in the SX-64 is a 1541 with the ALPS mechanism.
(That is the only one I've seen installed.) The controller board (called the FDD) for that drive is PCB# 251109. It has two jumpers, like the stand-alone drives, for drive selection. They are located between plug P19 and plug P22 (which goes to the drive reset switch) on the FDD board. Jumper #1 is the one closest to P19. I wired my own SX with a four position slide switch on the left rear side of the unit to be able to set the number from 8 to 11. The SX is very compact and hard to work on. The modification to add the switch should really be done by a tech. As with the stand-alone 1541, device # changes are made by cutting the bar between the half-circle jumpers.

THE 1541-II, 1571 and 1581: All of these Commodore drives were factory equipped with two tiny paddle switches on the rear of the drive to select the device number. They were shipped with both switches up for default device 8. If you want to change it, use a small screwdriver, toothpick or other small object to push one or both paddles down. Don't use a pencil... the lead may break off and get into the drive causing a short. To set for drive 9, push down the left switch, for 10, the right switch, and for 11, both switches. The new device number is "memorized" by the drive every time it is powered up or the computer and connected periferals are reset.

C128DCR INTERNAL DRIVE: The internal 1571 in the metal case USA version C128DCR has no external switches for drive select. It is factory set for device #8. There are drive select "jumpers" inside the computer however, on the motherboard near the drive. They are the two tiny silver half-circles with a bar between them. Cutting the bar opens the jumper. If you look at the PC board just under the left side of the drive, you will see them. It is not necessary to remove the drive unless you plan to solder in there. The jumpers are located near ICs U106 (a 6522), U114 and U112. DS (drive select) jumper #1 is nearest to U112, towards the front of the computer. Electrically, the jumpers connect two pins of IC U106 VIA chip (65C22 or 6522 in some models) to chassis ground. Jumper 1 grounds pin 15 and jumper 2 grounds pin 16. That information is helpful just in case the foil pad(s) jumpers on the PC board are accidently damaged when you are installing a drive select switch. If the pads lift off the board, you can still make connections directly to the IC pins for your switch. The 1570 drive is not as common but is similar to the 1571 in that it uses the same board, will do burst-mode data transfers with the C128, and will do MFM as well as GCR disk encoding like the 1571. Since it has only one head, it uses SSDD disks like a 1541. It has no drive select switches but instead has internal jumpers. They are connected to VIA U9 pins 15 and 16. I have never seen a 1570, so my information about it is minimal.

MSD-SD2: Turn off the drive and remove all cables. Remove six screws from the drive case and remove the top cover. Locate the jumper block JB1 on the top of the PC board near the rear of the drive. It is located between ICs U10 and U11. At that location, there are four solder pads with two bare wire "jumpers" soldered in horizontally. This is the factory configuration for device 8. If JB1-1 is cut (or simply unplugged if a "header" with a removable jumper is installed), the drive will respond as device 9 when powered up or reset by the computer. If the jumper at JB1-2 is removed, the drive will be device 10, and with both jumpers removed, device 11. A SPST (single pole, single throw) switch can be installed instead of the jumper to allow the drive to be changed later without opening the case. Note: there is room on the rear of the case for added switches. Electrically, the two jumpers ground two pins of an IC, namely U22 buffer (74LS240) pins 11 and 17. With a jumper at JB1-1, pin 17 of U22 is grounded, and with a jumper at UB1-2, pin 11 is grounded. This info is helpful in case the solder pads or board traces become damaged and cannot be used, wires can be soldered to the chip pins directly.

Ray Carlsen CARLSEN ELECTRONICS... a leader in trailing-edge technology.