ADDING A RESET SWITCH TO A 1541 (& clone)/ 1571 / 1581 11-27-04.
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.