ADDING A RESET SWITCH TO A 1541 / 1571 / 1581

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The Project 64 etext of ~Adding a Reset Switch to a 1541/1571/1581~,
by Ray Carlsen .

DRESET10.TXT, July 1997, etext #252#

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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.

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End of the Project 64 etext of Adding a Reset Switch to a 1541/1571/
1581.