Advance Technical Repair of Laptops Motherboard

Computer Repair Mastery Course

Get Instant Access

Key: x = don't care, H = High (Off), L = Low (On)

LED Control and Output Cables

Sheet 5 of the schematic shows the LED control (panel LED's connector). The LEDs are mounted on their own daughter card with control wires cabled from the A3 Motherboard PCA. A description of each LED is provided in Table 2-5.

Table 2-5. Functional Description of LED Signals


Functional Description


Turns on the STANDBY LED when asserted Low (YELLOW)


Turns on the OPERATE LED when asserted Low (GREEN)


Turns on the WARNING LED when asserted Low (RED)


Turns on an analog switch to place the rectified and filtered shunt voltage generated by the HVCOM current onto the SMUX line when asserted Low


Turns on a switch to connect the output of the temperature to SMUX.

Sheet 1 of the A3 Motherboard PCA schematic shows the connection and wiring for the output cable from the A3 Motherboard PCA to the front panel binding posts. Note that there are two guards - IGUARD and GUARD (voltage guard). These guards may be tied together through K8 when voltage is selected. The other signal leads are OUT_HI, SNS_HI, OUT_LO, and SNS_LO. The output high and low signals (OUT_HI and OUT_LO) are used for the main output for volts, current, and ohms. The output sense signals (SNS_HI and SNS_LO) are used to sense and internally adjust the output signals. The sense terminals are not used for standard (uncompensated) two wire ohms or ac and dc current.

Signal Buses

The system analog and digital buses are brought to the circuit cards through the A3 Motherboard PCA connectors J105-108 and J205-208. J105-J108 provides the in-guard (IG) digital bus signal lines while J205-208 route the in-guard analog signal lines. Guard is tied to chassis through a set of diodes and MOVs (CR57, CR58, and RV3 and CR69, CR68, and RV1) and prevents the guard from floating more than 20 V from chassis. The guard is tied to SCOM through CR55, CR56, RV2, and R86. Note that relay K5 on Sheet 1 can connect the guard trace directly to SCOM.

Low Volt Buffer

The low voltage buffer circuit is shown on Sheet 4 of the A3 Motherboard PCA schematic (along the bottom-middle of the Sheet). The V3_3 input signal to U2 comes from the A6 Digital Synthesis PCA (A6). U2, combined with Q1,2,7 and 8, act to buffer the V3_3 signal and isolate the output voltage from the A6 card. Q3 and Q4 limit the output current that may be drawn.

In-guard Power Supplies

The power supplies for the analog circuits, also referred to as in-guard supplies, are shown on Sheets 2, 3, and 4 of the A3 Motherboard PCA. On Sheet 2 of the A3 Motherboard PCA schematic, the raw transformer secondaries enter at P2. 5AC1, 5AC2, 15AC1 and 15AC2 go to Sheet 3 of the A3 Motherboard PCA schematic along with the GUARD signal. RT7-10 protects the transformer from large current draws that might occur if a diode bridge on Sheet 3 of the A3 Motherboard PCA schematic or other components short.

The 45 ac, 180 ac, and 360 ac provide the raw voltages that will be used by the A8 High Voltage and A5 Ohms PCAs. If secondary voltages become too large, TRIAC Q19 will turn on the limit voltage, open the mains fuse, and prevent damage. CR62 is the full wave rectifier for the +/-45UNR supply. The +/-45UNR are regulated to become the +/-45V supplies. MP7 and MP8 are assemblies that contain the heat sinks and the main pairs of drive transistors for the 45 V regulated supplies. U18 controls the regulation. Q12 with resistor R48 and R54 and Q13 with resistors R49 and R57 limit the output currents to ~120 mA. CR28 with VR10 and VR11 and CR 26 with VR12 and VR 13 protect against high voltages damaging the regulation circuits. Note that the HVCOM line caries the ground return currents for the high voltage supplies back to the center tap of the transformer.

On Sheet 3 of the A3 Motherboard PCA schematic, CR67 and CR51 rectify the 15 V ac and 5 V ac (left side of the Sheet), respectively. The resulting +15UNR goes through U21, a dual regulator., and becomes the guarded +15 V supply. -15UNR goes into U22 and becomes the -15 V supply. VR14 and VR15 limit short term over-voltages to 22 V or so. The 5 ac signals are regulated to be the power for the relays (+5RLH) and the logic (+5 V). The +/-15 V supplies are referenced to SCOM, while the +/-5 V supplies are referenced to DCOM. Note that SCOM and DCOM grounds are kept close to each other electrically due to the Schottky diodes, CR18 and CR19. CR29 and CR31 limit the amount that the +/-15 V and +/-5 V power supplies can float from each other. Each regulator in the design is protected against short-term over voltages occurring at the regulator output with diodes CR41, CR48, CR35, CR36, and CR34. The 6 turn beads reduce conducted noise.

Outguard Power Supplies

The connector P1 and the out-guard secondaries labeled 12GAC and 5GAC are shown on Sheet 2 of the A3 Motherboard PCA schematic. These unregulated supplies including +5VG_UNR, become the regulated supplies for the outguard controller card (A9) and include +5VG and +/-12VG. These supply power the A9 Out-Guard CPU PCA through the connector J1 (found on Sheet five). RT1, and RT3-RT5 protect the transformer from large current draws that might occur if there is a short down stream. Each regulator in the design (U20, U25, and U19) is protected against short-term over voltages occurring at the regulator output with diodes CR42, CR43, and CR59. The 6 turn beads reduce conducted noise. The regulated +/-12 V supplies also provide power to the 24 V fan located at the front of the instrument. A second fan connector was added for a future version of the instrument. GCOM is the reference and is tied through resistors to earth, and tied to DCOM (the in-guard digital ground) through a 24 V bi-directional Zener, VR17.

Miscellaneous Circuits

On Sheet five of the A3 Motherboard PCA schematic, J1 connects the digital signals from the A9 Controller card to the A3 Motherboard PCA. The serial data signals form the communication path to the A6 Digital Synthesis PCA (which also controls the in-guard digital bus). J6 is used for trouble shooting the circuitry in manufacturing and service testing. Also on that Sheet is an area marked NOT INSTALLED. These circuits may be used in a future version of the product.

Sheet 3 of the A3 Motherboard PCA schematic shows U27, a switch used to connect signals to the SMUX bus line for monitoring the A6 Digital Synthesis PCA. CKHVCUR* is driven by U7 pin 12 on Sheet 5 of the A3 Motherboard PCA schematic for connecting the output of the circuit checking the HVCOM current to SMUX. CKIT* can turn on a switch in U27 to connect the output of the temperature sensor h30 to SMUX.

Troubleshooting Test Points

When a problem occurs with the instrument operation, one likely place to look is the A3 Motherboard PCA and its power supplies. There are a number of convenient test points to monitor the supply voltages. The test points are listed in Table 2-6.

The test points can be divided into several groups: general, function unique, out-guard, and fault. General test points include V3BUF, +/-5UNR, +/-5V, +/-15UNR, and +/-15V. The V3BUF monitors the buffered signal used for ac and dc output voltages between 220 mV and 2.2 V +/-5UNR, +/-5V, +/-15UNR, and +/-15V monitor the main supplies used in all of the in-guard board circuitry.

Power supplies unique to the A7 Current PCA are monitored by MMONGO, MMCOM, IREF, ICOM, +/-15I_UNR, and 15I. Power supplies unique to the A8 High Voltage PCA are +/-45UNR, +/-45V, +/-180UNR, and +/-360UNR. The A8 High Voltage PCA uses the +/-45V, +/-180UNR and +/-360UNR to produce the high voltage outputs. The A5 Ohms PCA uses the general and +/-45Vsupplies.

Out-guard test points monitor provide access to +5VG_UNR, +5VG, GGND, +/-12VG_UNR, and +/-12VG power signals. These power supplies are only used for the controller card (A9) and for manufacturing test fixtures.

Besides monitoring the power supplies, several test points are used to monitor fault conditions. Note that the FAULT* signal is a wired "O". The main fault signal is TP3. If any fault occurs, TP3 becomes active. TP2 monitors the 15V BALANCE FAULT* signal and provides status on the +15 V and -15 V supplies. If either supply varies from the other by too much (indicating excessive load or failure), the fault becomes active. TP4 monitors the HVCOM signal trace. If the HVCOM signal moves more than 0.5 V from SCOM, the fault becomes active. TP5 monitors OVER22V. This signal indicates if SCOM differs from chassis ground by more than about 23 V. TP49 monitors 45 V BALANCE FAULT. This fault is similar to the 15 V BALANCE FAULT and monitors that +45 V and -45 V supplies stay within a fixed range of each other.

Table 2-6. A3 Motherboard PCA Test Points List







Was this article helpful?

0 0
The Ultimate Computer Repair Guide

The Ultimate Computer Repair Guide

Read how to maintain and repair any desktop and laptop computer. This Ebook has articles with photos and videos that show detailed step by step pc repair and maintenance procedures. There are many links to online videos that explain how you can build, maintain, speed up, clean, and repair your computer yourself. Put the money that you were going to pay the PC Tech in your own pocket.

Get My Free Ebook

Post a comment