ADDING A RESET SWITCH TO A 1541 (& clone)/ 1571 / 1581

             latest changes and/or corrections: 11-27-04

     There is a -drive- reset switch in the SX-64 and one in the
C128DCR. It is sometimes advantageous to be able to reset the drive
alone, and not the rest of the system. This article explains how to add
a switch to your disk drive so that you can do a drive reset without
turning off the computer or drive.
     When a 1541 is first turned on (with or without the computer
connected), an internal circuit called a "one-shot" holds the reset
line low for a half-second until all voltages stabilize. The drive then
goes through an internal start-up sequence. That's what's happening
when the activity LED comes on and goes out as the drive is powered up. 
The best way to add a reset switch to a stock disk drive is to use that
one-shot circuit as the trigger. Implimenting it is a little tricky 
because there are so many different models of 1541. I've done a bit of
"homework" to make it easier for you.
     The most common 1541 type, which is to say the most plentiful,
uses the PCB #251830 (number stamped on the PC board). It is a medium
sized board, housed in the standard brown case, and usually has the
Newtronics drive. Fortunately, the older drive with the ALPS mechanism
is similar. It uses a different PCB (#1540050), but the parts layout is
about the same, at least as far as this modification is concerned. The
mod involves wiring in a push-button switch and mounting it somewhere
on the drive. The added wires can be soldered to the bottom of the PC
board (requiring removal of the board), or to the top by soldering to
component wires.
     Locate capacitor C46. It is a 100uF 16 volt electrolytic mounted
near the front of the board, about in the middle. It is near IC UD3 
and diode CR7. The added drive reset switch will be wired across the
capacitor C46. That cap is usually mounted with enough of it's leads
sticking up that you can solder your two wires underneath it. If you
can't get to those solder connections, as an alternative you can use
the anode of CR7 as one lead of your switch, and the negative end
(which is grounded) of the very large filter capacitor C17 (left rear
of PC board) as the negative end. By the way, don't use the metal
chassis as a ground... it's isolated and will not work. To identify the
anode of CR7, examine the diode. Notice a small black band at one
end... that is the -cathode- lead. The anode is the other end... it's
closer to the front of the drive.
     When you wire in the momentary-contact pushbutton switch, add a
100 ohm 1/4 or 1/2 watt resistor in series (not across, but in-line
with one leg). This is to prevent the discharging of the capacitor into
your switch, which may eventually damage the contacts. It also serves
to protect the drive electronics in case you connect the switch to the
wrong place... it will not work, but it should do no harm.

     If you have the very old type of 1541 (white case, long board),
the capacitor will be C56, and the diode C17. They are located near the
front left side of the PCB.

     The newer white case 1541C has a short board (mine is labeled 
251854) and the capacitor to look for is C13, a 47uF 16V unit. It 
and the other components that make up the reset circuit inside the
drive are near the DOS ROM IC UA2. Since the cap is mounted close to
the PC board, it would be difficult to solder to its leads. Instead,
you can attach one end of your added switch to one end of resistor 
R13 (the lead nearest the edge of the circuit board) or the anode of
diode CR7 right next to the resistor. The other end of your switch
goes to ground. A convenient place is the negative end of the large
filter capacitor C8 which is nearby, or you can tie the wire to the 
metal case. Don't forget to install the 100 ohm series resistor for 
safety. Connecting your switch wires can be done on top of the PC
board without removing it from the drive.

     In the 1541-II, the capacitor is C3. It is mounted so close to the
board, you may not be able to solder to it. Instead, wire one lead of
the added switch to the end of resistor R2 nearest capacitor C3. The
other switch wire goes to ground, which is the large copper trace at
the edge of the PC board.  Obviously, you must remove the drive from
the chassis to get to the PC board. It is held down with four screws.
Mark the plugs in some manner to make sure you put them back correctly. 
Don't get them backwards! When you reassemble the drive, make sure the 
added switch wires do not interfere with the spindle flywheel, and are 
not pinched when you re-install the drive. A good place to mount the 
push-button switch is in the upper left corner of the drive face.

     The capacitor in the 1571 drive is C28, a 47uF 16V electrolytic.
It is located at the rear of the drive PC board near the serial ports.
To get to the board, you must remove the power supply cage... four
screws. Lift the cage up and over... and set it on top of the drive
assy. The wires are long enough that you will not need to remove any
plugs. Mark them if you do remove them to make sure they go back on the
same way. Mount the push-button under the power or drive LED on the
front panel.

     The capacitor in the 1581 drive is C20, a 47uF 10 volt. It is
located about 2" from the serial ports. You must remove the drive (four
screws) to get to the PC board. Install the push-button switch on the
front panel, just to the right of the C= logo. Use a sub-miniature
type... the spacing is very tight inside that case. Make sure the wires
don't short to any metal part of the drive, and make sure no wires are
pinched when you re-install the drive in the case.

     The capacitor in the Excelerator+Plus drive (a clone of the 1541)
is C12, a 100uF 25V electrolytic at the rear of the drive. Since the 
cap is mounted so close to the PC board, connecting to it is virtually
impossible without more disassembly. An easier connection point is one 
end of resistor R7, a 100K 1/4 Watt at the rear of the drive on the 
bottom board. Remove four screws holding the bottom plate and the drive 
in its case, unplug the power switch connector, and slide the drive out 
of the front of the case. 
     Solder one wire from your reset switch to the end of R7 nearest the 
edge of the PC board (farthest away from the power switch connector). The 
ground wire for your switch goes to pin 2 or 3 of the power connector J7. 
Don't forget to add the 100 ohm series resistor to the switch. FYI, the 
connection point is the junction of ICs U15 pin 5 and IC U17 pin 4. A 
convenient place to mount the pushbutton switch is on the front panel
just to the right of the drive activity light. Make sure your added 
wiring doesn't get tangled in the drive mechanics or get pinched when
you reinstall the drive in its case, and don't forget to reconnect the 
power switch plug before you put the cover back on. 

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