Tutorial
- how to locate the basic test points of an unfamiliar HDD PCB
Introduction
This tutorial in intended to show how one may determine the test points
for the onboard DC-DC converters, spindle motor, VCM, and preamp supply
rails of an unfamiliar HDD PCB. We also identify any protection
devices. I have done as much as possible in the absence of power in
order to simulate a non-functioning PCB.
Our example will be a Samsung SP0411N IDE PCB (PANGO
REV06).
It is assumed that the reader understands how to use a digital
multimeter.
Preliminary observations
(a) Date Codes
These can be observed on the drive's label and on the various ICs and
semiconductors. The latter are most often in YWW or YYWW (Year / Week) format.
In our example the most obvious date codes are ...
0320 0324 0327 K323 Y325
This suggests that the drive was manufactured during or after the 27th
week of 2003, ie some time in early July.
(b) SDRAM and EEPROM
These ICS are two of the easiest to identify. Their datasheets should
be easy to locate, and they will tell us the supply voltages and supply
pins of each chip.
In fact the SDRAM (IC42S16100-6T) requires a 3.3V
supply on pins 1 and 25, and the EEPROM (M29F102BB) is
a 5V device.
Locating the DC-DC
converters
DC-DC converters are of two types, either linear or switchmode.
Linear supplies will either have a single regulator IC, or a pass
transistor that is driven and monitored by the motor controller IC.
Switchmode DC-DC converters will be identified by an inductor, either a coil or a 2-pin rectangular package.
In the vicinity of the inductor will be a MOSFET and Schottky diode. Sometimes the MOSFET and
diode are combined in a single FETKY
package. The MOSFET is usually controlled by pulses from the motor
controller (Pulse Width Modulation),
but some designs use a separate PWM
controller IC.
In our case the PCB has no coils, so there don't appear to be any
switchmode supplies.
This is not surprising since switchmode converters weren't regularly
used in HDD applications of that time.
Instead we need to focus on the larger discrete semiconductors,
specifically the 3-pin parts marked LT25, X2B,
DA and CF. These would be power
devices of some kind. Our initial guess would be that these are
transistors (NPN or PNP), or MOSFETs
(N-channel or P-channel), or 3-terminal LDO (low
drop-out) regulators.
In fact the marking code of the LT25 device would
suggest that it may be a 2.5V LDO regulator (we have
already determined that the K323 marking is a date code). A reasonable
assumption would be that this IC may supply one of the voltages for the
Marvell 88i6522-LGO MCU. LDOs have an input pin, an
output pin, and a ground pin. Using the 4-pin Molex power connector as
our reference, a quick continuity test with a multimeter confirms that
the uppermost pin connects to +5V and the lowermost to Ground. This
would suggest that they are input and ground pins, with the middle pin
being the +2.5V output. However, contrary to our initial guess, the
+2.5V pin does not connect to the MCU.
We now need to determine which device provides the +3.3V
for the SDRAM. This is done by testing for continuity
between the SDRAM's supply pins and each of the pins of the DA,
CF, and X2B devices. This time we
find that the leftmost pin of DA is the +3.3V supply, and its centre
pin is +5V which makes it the input. The +3.3V supply
also powers the MCU and is probably its Vio rail.
The CF device now looks like it may be a third regulator. If we assume
that its middle pin is an input, then we find that it connects to the
centre (output) pin of LT25. The lowermost pin of CF connects to a
heavy copper trace, so it would probably be an output. This means that
CF down-regulates the +2.5V supply to yet another supply rail. More
testing reveals that this lower voltage powers the MCU via numerous
pins, and is probably its Vcore rail. There appears
to be no continuity between any pin of the MCU and the +5V input.
Datasheets confirm that CF and DA are in fact NPN transistors,
and their pinouts are BCE (Base / Collector /
Emitter). My first guess would be that the bases of both CF and DA are
controlled by the HA13627, but CF is in fact driven
from the MCU. This would now suggest that CF may not be a regulator as
originally thought. Instead it may just be a switch, and +2.5V may in
fact be the Vcore supply.
An actual measurement confirms that the output of the LT25 device is
indeed +2.48V, and Vcore is +1.37V. Therefore CF is a pass
transistor under the control of the MCU.
____
| |
+5V o----|LT25|----+------o +2.5V
|____| C| +2.5V regulator & Vcore
| \| B
| CF |------o from MCU pin #24
| <|
=== E|
GND |
o
Vcore
+5V
o
|
| +3.3V Vio regulator
C|
\|B
DA |------o from HA13627 pin #26
<|
E|
|
|
o
+3.3V
Protection Devices
Connected directly in series with the PCB's +5V input is a 2-pin device
with a marking that looks like "1X1", the second "1"
being upside down. Its resistance measures around 0.15 ohms. This would
suggest that it is some kind of protection device, possibly a 1A fuse.
In fact it is actually a PolySwitch, a device which
behaves like a resettable fuse.
The centre pin of the X2B device connects to +5V and
the leftmost pin to Ground. One's first impression might be that it is
an LDO. In this case the rightmost pin would be the output, but then
one would have to ask why there is a 102 (= 1K) resistor between output
and ground. In fact X2B is not an LDO, but an SCR.
It's function is to crowbar the +5V input to ground in the event of an
overvoltage. That is, when the input voltage exceeds the breakdown
voltage of the zener diode (C2
marking code), the gate of the SCR is triggered on, causing the SCR to
latch into conduction, and resulting in a short-circuit across the +5V
input. The polyswitch then goes open circuit and effectively
disconnects the supply. When the overvoltage condition is removed and
the drive is power cycled, the polyswitch should recover and the drive
should continue working.
+5V
o
|
|
.-.
PolySwitch| |
| |
'-'
|
+----+
A| |
| z Zener
X2B V A Diode
SCR - G | Overvoltage crowbar circuit
|\---+
K| |
| +---+
| | |
| .-. |C
| | | ---
| R| | ---
| '-' |
| | |
| +---+
| |
+----+
|
===
GND
Spindle Motor test points
The spindle motor has 4 terminals -- 3 phases plus a common. The
phase-to-phase resistance is 3.8 ohms, and phase-to-common is 1.9 ohms.
A simple continuity test identifies the motor test points adjacent to
the HA13627 motor controller IC.
Pin #64 = common
Pin #63 = phase A
Pin #61 = phase B
Pin #59 = phase C
Common (Pin #64)
o
|
|
+-----+-----+
| | |
| | |
C| C| C|
1.9 ohms C| C| C| Spindle Motor windings
C| C| C|
| | |
| | |
o o o
A B C
Pin #63 Pin #61 Pin #59
+12V
o
|
|
+---------+----------+
| |
| |
||-+ +-||
||<- Q1 Q3 ->||
---||-+ +-||---
| |
| | VCM H-bridge
HA13627 pin#51 o----)-------+ | (internal to HA13627)
| ___ | |
HA13627 pin#48 o---+--|___|-+--o VCM o--+---o HA13627 pin#46
| current |
| sense |
||-+ +-||
||<- Q2 Q4 ->||
---||-+ +-||---
| |
| |
+---------+----------+
|
|
===
GND
HDA Connector test points
The VCM pins have already been identified. I have
chosen to number them as 19 and 20.
Most ground pins are obvious by inspection, but pin 4
needs a continuity test.
The Read and Write differential
pairs are also fairly obvious. However, I can't tell which is
which.
I believe pins 10, 11, and 12
are the serial data, clock, and enable
pins, although not necessarily in that order. I say this because pins
10 and 11 both connect to the same 82 ohm resistor (820) and another
8-pin device, probably a capacitor array.
The preamp's +5V supply is cleaned up by an
LC filter (serial inductor plus parallel capacitor). A similar
low pass filter connects to pin #2. It is for this reason that I
suspect pin #2 to be a second supply rail, and I've called it Vcc2.
That said, I confess that I'm having difficulty understanding this
circuit. A diode test on my DMM suggests that the two 3-pin devices
marked "P" and "N" may be
dual Schottky diodes. The "P" device has a common anode, and
the "N" device has a common cathode. The cathode of N is grounded.
An actual measurement (-5.0V)
confirms that "Vcc2" is the second supply rail.
inductor L
___ filtered
+5V o----UUU---+----o +5V supply Low Pass LC filter
| to preamp
---
capacitor C ---
|
|
===
GND
VCC
+
|
+-------------------+-------------------+
| |
| |
| -5.0V |
| o |
| | |
||-+ +----+----+ +-||
Q1 ||<- | | | ->|| Q2
--||-+ P1 | | | P2 +-||--
| V | V |
| C1 - | - C2 | -5V DC-DC converter
| R1 | |C | R2 |
| ___ || | --- | || ___ |
PWM1 o---+--|___|--||---+ --- +---||--|___|--+---o PWM2 (HA13627 pin#40)
(HA13627 pin#41) | 3R3 || | | | || 3R3 |
| V | V |
| - | - |
||-+ N1 | | | N2 +-||
Q3 ||<- | | | ->|| Q4
--||-+ +----+----+ +-||--
| | |
| | |
| | |
| | |
| | |
+-------------------+-------------------+
|
===
GND
The -5V negative supply for the preamp appears to be a "regulated charge-pump inverter" topology. I don't have a datasheet for the HA13627, but I'm guessing that its internal circuit could be represented by an H-bridge. Contrary to my initial guess, this circuit is actually a switchmode power supply. It works by charging capacitors C1 and C2 via Q1 and Q2, respectively, during alternate half cycles. The charge on each capacitor is then transferred to capacitor C on opposing half cycles via Q3 and Q4.
References
IC42S16100-6T, 512K x 16 Bit x 2 Banks (16-MBIT) SDRAM, 3.3V,
Integrated Circuit Solution Inc.:
http://www.datasheetarchive.com/pdf/getfile.php?dir=Datasheets-28&file=DSA-559073.pdf&scan=
http://www.eanbowman.com/blog/wp-content/uploads/211-01364-0-IC42S16100.pdf
M29F102BB, EEPROM, 1 Mbit (64Kb x16), 5V, ST Microelectronics:
http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXruwqu.pdf
http://www.datasheetcatalog.org/datasheet/stmicroelectronics/6387.pdf
PC peripheral power connector pinout:
http://pinouts.ru/Power/BigPower_pinout.shtml
2SD1664, NPN transistor, marking DA, Rohm, 32V, 1A, 0.15V
VCE(sat), Q marking = 120-270 hfe, R = 180-390:
http://www.datasheetcatalog.org/datasheet/rohm/2sd1664.pdf
2SD2150, Rohm, NPN Low Frequency Transistor, 20V, 3A, 0.2V
VCE(sat), marking CF, R = 180-390 hfe:
http://www.rohm.com/products/databook/tr/pdf/2sd2150.pdf
http://www.s-manuals.com/smd-files/pdf/2/2sd2150_r.pdf
X0202BN, ST Microelectronics, marking X2B, SENSITIVE GATE SCR,
1.4A:
http://www.datasheetarchive.com/pdf/getfile.php?dir=Datasheets-17&file=DSA-332583.pdf&scan=
http://www.mouser.com/catalog/specsheets/X02---N_Datasheet.pdf
PolySwitch Resettable Devices, Raychem Circuit Protection:
http://www.datasheetarchive.com/indexdl/Datasheet-078/DSAE0070002.pdf
ASCII circuit diagrams created by AACircuit v1.28.6 beta
04/19/05:
http://www.tech-chat.de
HDD IC Database:
http://www.users.on.net/~fzabkar/HDD/HDD_ICs.txt