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.

     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. 

     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.