TEMPERATURE COMPENSATION PCBA .1 TEMPERATURE COMPENSATION CIRCUITRY

The Temperature Compensation PCBA is located adjacent to the ReadlWrite PCBA and is approximately 203.2 mm (8 inches) long by 101.6 mm (4 inches) high. The Temperature Compensation PCBA operates in conjunction with two externally mounted thermistors to provide thermal compensation. Control circuitry for the Emergency Unload mode is also included in this PCBA to protect the heads and media from possible damage during an emergency unload condition. Refer to Schematic No. 108175 and Assembly No. 108176.

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The switched current resistors (R24, R26, R28) receive their commands from the Current Address Counter on the Logic PCBA via J503 (zone 0-C8). The three most significant bits are employed to provide off-track compensation and are comprised of LCA E1, LCA 71, and LCA 61 (zone C,08), in order of declining bit weight.

Figure 5-9 illustrates the placement of the five connectors on the Temperature Compensation PCBA. These connectors provide power to the PCBA in addition to providing input and output signal paths between this PCBA and the Logic PCBA, the Servo PCBA, and the two externally mounted thermistors.

There are two temperature sensing thermistors, one for the removable cartridge, the other for the fixed disk(s) ambient surface temperature. On top load drives, RT2 is mounted on the rear right side of the adapter bowl and protrudes through a port into the interior of the bowl. On front load drives, RT2 is mounted so that it extends through the rear flapped opening of the cartridge and extends above the surface of the disk when the cartridge is fully seated in position.

The fixed disk(s) sensor, RT1, for both top and front load models, is mounted on the positioner base as part of the head ramp. The thermistor extends into the fixed disk area immediately adjacent to the heads and is positioned above the outer surface of the disk.

RT1 is connected to the input of amplifier U1 (zone F7) via pin 1 of J505. RT2 is connected to the input of amplifier U3 via pin 6 of J505. The outputs of U1 and U3 are applied simultaneously to pins 2 and 6 of U7.

The output of U7 pin 7 (zone F5) drives a multiplexer digital-to-analog converter which consists of three FETs in U6 pins 3, 11, and 14 (zone G,E6). The fourth FET (at U6 pin 6) is driven by the output of U7 pin 1 when the Transient Compensation signal (NLTPCG) is active.

The NLTPCG signal from the Transient Compensation circuitry (sheet 3, zone 01) drives FET U6 pin 6 (zone 06) whose output is applied to the summing junction of U4 pin 6 along with three outputs of FET U6 at pins 1,9 and 16. The output of the summing amplifier is the Servo Compensation signal which is sent to the Servo PCBA via J504 pin 2 (zone C3).

• • • •

TPl TP2 TP3 TP4

Rl PROVIDED BUT NOT USED

TP5 TP6 TP7 TP8 TP9 TP10 TPll TP12 TP13

. . . . . . . . .

J506 UPPER AND LOWER PLATltR SELECT, AND INDEX/SECTOR MARK

J504 ltMPERA TURE COMPENSA nON

J502 POWER

J505 TI-lERMISTOR INPUTS AND REFERENCE VOL TAGE

1 ' - - J 5 0 3 CYLINDER DEMAND ADDRESSES

I

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Figure 5-9. Temperature Compensation PCBA Test Point and Connector Placement

116E

Sheet 4 shows the Emergency Unload circuitry which, although physically located on the Temperature Compensation PCBA, is electrically associated with the Servo PCBA.

The voltage drop across R164 (sheet 3, zone F5) on the Servo PCBA (drawing 108130) appears on J504 pin 4 which is the input to U16 (zone F5). This input is compared with the voltage divider network consisting of R56 and R55.

The output of U16 drives the base of 01 which uses VR5 in its emitter circuit to limit the voltage at the base of 02. When 02 is on, current is applied to the positioner coil via J504 pin 1, and Relay K1 on the Servo PCBA.

Addressing of the removable cartridge is detected in U12 from the Lower Platter Select (N LLPSG) and Upper Platter Select (N LUPSG) signals. Immediately after the removable cartridge is either selected or deselected, U13 (sheet 3, zone F5) serves a a one shot to mask index and sector signals and also read signals (for soft-sectored formats) for approximately 10 msec. This allows the positioner to recover from the position transient caused by the change in the temperature compensation signals.

The Delayed Index And Sensor (NLDISG) signal (sheet 3, zone F1) is sent to the Logic PCBA to mask the Index and Sector Count signals.

The Temperature Compensation Read Enable (NTTREG) signal (sheet 3, zone E1) is sent to the ReadlWrite PCBA to disable the Read Enable circuitry.

In addition to the Temperature Compensation electronics, the Temperature Compensation PCBA contains a time delay circuit. This circuit consists of a monolithic timer, U8, a 4-bit binary counter, U10, and associated components. A low on U8 pin 4 (zone C4) resets the timer. The timer will free run, generating one pulse per 2 minutes as determined by R52, R53, and C7. The binary counter is clocked by each pulse and counts eight of these pulses before an output is delivered at J506 pin 10. This 16-minute timer circuit allows the internal temperature of the drive to stabilize before disabling the differential temperature compensation and the index and sector mask of U13~ The timer circuit operates after the RUN indicator illuminates on the drive control panel.

Jumper W2 (zone C4) is used for test purposes only.

5.7.2 EMERGENCY UNLOAD CONTROL CIRCUITRY

During an unload condition, the sudden acceleration of the pOSitioner assembly sometimes causes the heads to torque and thereby gouge the media. Sheet 4 of the Temperature Compensation schematic, 108175, shows the circuitry that controls the speed of the positioner during the unload mode.

Controlling positioner speed means applying sufficient current to the voice coil to limit the speed of the positioner as it reaches the unload ramp and then to apply a higher level of current to the voice coil to allow the heads to fully unload.

During an unload condition, pOSitioner current is sensed at U16-3 via the line on J504 pin 4 (zone F7). A current threshold is established by the divider network, R55 and R56. When the pOSitioner current is below threshold, U16-6 will be positive, 01 will turn on and VR5 will conduct. 02 will open and the limit current will be transmitted to the emergency unload driver via J504 pin 1 (zone E2).

If the positioner current reaches the threshold current, U1 will be negative at U1~6, VR5 will cease to conduct and R59 will bias 04 to saturation and establish the high limit of the current to the emergency unload driver via J504 pin 1 (zone E2).

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SECTION VI