                ADDING A RESET SWITCH TO A 1541 / 1571 / 1581

     There is a -drive- reset switch in the SX-64 and one in the C128D. 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 red 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 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. 
     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 underneith 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.
     I don't have a layout for the newest type of drive, the 1541C, so I can't advise you
how to connect the switch. They may use different part ID numbers since the board is
completely different (short board) from earlier drives.
     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. Mount the
push-button 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.

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


