Raymarine Smart Pilot
Feb 2025
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I had this Smart Pilot S3 in the workshop some
time ago and now I've got it back with the message that it wouldn't
turn on.
Years ago, after it
failed (in Turkey) a local repairer had failed to fix it so the
owner brought it to me. At the time I wasn't too happy with what
I'd done (I'd noticed a botched job on part of the input circuit
at the upper right below but wasn't convinced that had been the
original fault). I replaced some parts near the heatsink but
I recall finding at least a clue to the original fault after
finding a group of short-circuited and burnt components in the
centre of the board and had fitted a new ES2J diode, a low value
ballast resistor and an IRF7406 MOSFET chip Anyway the thing
still didn't work hence its arrival back on my work bench.
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The first thing
was to recollect what I'd found a couple of years or so ago,
but one puzzle is the power requirement. The customer reckoned
it was 24 volts but info on the Web tells me it could be 12 volts
or 24 volts ... The labelling doesn't have any indication of
two different supply voltages but the power input circuit is
a bit odd and it was this that seems to have been a problem in
the past because of visibly damaged parts and a sort of messy
"repair"carried out in Turkey, which I'd spotted the
first time I'd seen it. I'd replaced a voltage regulator, an
LT1270A and a couple of nearby diodes which had apparently failed
again? after the first repair.
From what circuit
I'd managed to trace previously, this LT1270A area connects to
a relay whose task seems to be to control power to the main part
of the circuitry. I'd also found a short-circuit diode and a
bad surface-mount transistor near the middle of the board (below
left). That was a couple of year ago.
Before tackling the
repair this time I searched high and low on the Internet for
a service manual for the S3 Smart Pilot but failed to find one
however, in my searches, I discovered the details between different
versions of the equipment. This particular model is the S3 and
includes a DC-DC converter for powering the board and its ancillaries
from either a 12 volt or 24 volt supply. The upgrade to 24 volts
I believe is to enable the unit to drive more powerful 24 volt
ancillary equipment. The extra parts for the DC-DC converter
can be seen in the picture below right. Apparently the input
circuit must be able to automatically handle operation from either
12 volt or 24 volt batteries.
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Below, the parts
replaced in the centre of the board when the Smart Pilot first
arrived here |
FIG1
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FIG2
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I
guess the first step is to see if the DC-DC converter is working,
then see what drives the small relay which is common to both
S2 and S3 versions. Well, applying power to the battery terminals
resulted in no voltages at all at the pins of the LT1270A so
I attempted to trace the circuit. This is difficult for a few
reasons.. firstly three large toroids mask a lot of the tracks,
secondly the circuit board is multi-layer and tracks are buried
in the sandwich (and I don't have a circuit diagram) and thirdly,
none of the parts carries a code on the circuit board, neither
are there polarization markings for semiconductors. I thought
initially that I'd perhaps fitted one of the two diodes, when
I'd first seen the Smart Pilot, back-to-front. The reason being
the lack of markings and my limited knowledge, at the time, of
the function of the LT1270A not to mention burnt unrecognisable
parts and a previous scruffy repair. When you look at the data
sheet for the LT1270A (links to the semiconductor parts are provided
further down the page) it seems the tech author or his boss decided
to limit the amount of information on it. When I'd replaced the
chip way back I recall discovering a note telling readers that
more information was in another data sheet.. that of the LT1072.
I read this again and checked the circuit for the nth time. The
positive and negative feeds from the 24 volt battery connectors
together go through a dual choke. The negative feed emerges and
passes via a Schottky diode to the LT1270A centre pin. What happens
to the positive feed? Not an easy thing to determine... It goes
to the 86 ohm coil of the relay and one of its contacts but then
seems to disappear into the middle layers of the circuit board.
It emerges at the other side of the relay coil from where it
passes via a diode (one of those I'd swapped previously) to the
centre pin (=collector) of the NPN TIP31A transistor next to
the LT1270A. Picture on the right.
Those two diodes below
the TIP31A are right, an FES2J (V6) 600V 1A and left, an MURS120
(U1D) 200V 1A
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FIG3 |
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Maybe the fault tackled
by the chap in Turkey wasn't just a bad diode or two? Maybe he'd
missed something or made an error and introduced further damage?
A strong possibility is that there's a fused track in the board's
multi-layer sandwich. To prove that this might be a possibility
I added a wire from the Vcc pin on the LT1270A (picture above)
to the battery positive terminal....
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With the battery positive
input terminal connected to the Vcc pin on the LT1270A, I initially
powered the board from 12 volts with a small current limit. Nothing
untoward happened so I slowly increased the voltage and current
limit. As the voltage rose from 12 volts the current settled
at around 200mA so I increased the power supply to 24 volts and
found the current draw didn't increase much which is indicative
of a working or stable circuit (ie. a short would have resulted
in ever-increasing current)
Next I checked the
board with a thermal imager, (see the large picture below).
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FIG4
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The
results looked OK with only one of the tiny surface mount components
getting unduly hot (right). It's a 15 volt zener diode and it's
running at over 130C.
The microprocessor
(that large square chip above) is obvious and is running warm
but perhaps not abnormally so?
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FIG5
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Now that the DC-DC
converter can be made to run by feeding 24 volts to the LT1270A
there are two more things to check. That diode (right) with the
confusing markings is an MBR1645.. it's in the negative feed
to the 24 to 12 volt converter but what's its purpose? Possibly
protection against reverse connection of the battery?
What's the function
of the TIP31A next to the LT1270A? Also what's the purpose of
its FES2J emitter diode (the smaller of the ones I replaced)
and is it correctly polarized? How is the transistor switched
on and off as connections to it are hidden in the board multi-layer
sandwich?
There's a clue to
one puzzle. On the terminal strip label there's "OFF"
marked against two terminals and looking in the user manual it
seems that connecting these two pins turns off the computer.
Maybe the TIP31A is wired to the relay and controlled by a signal
from the "OFF" switch? That would make sense as the
relay is present in both the S2 (for solely 12 volt so no DC-DC
converter) and the S3.
Oddly Pin 1 of the
TIP31A measures 6 ohm to the battery negative terminal (certainly
not close to zero compared with similarly located ground connections),
very odd so I'll need to figure out if this is due to a short
or a low value resistor, inductor or what? It turned out to be
a 6 ohm resistor.
See the following
data sheets
Note: The circuit
used by Seamarine S3 seems to be similar to the one depicted
on page 4-240 "Flyback converter" in the LT1072 spec
but running 24 volt input and 12 volts output (FIG7 below).
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FIG6  |
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FIG7 |
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I then attempted
to discover the reason for the 6ohm to ground reading at the
TIP31A emitter. Now that I have a thermal camera this was quite
easy. I applied a low positive voltage to the TIP31A emitter
with respect to ground and gradually increased this together
with the current limit.
I immediately located
a hot part in the centre of the board. This was a tiny 6 legged
surface mounted part labelled "X1" and is a dual transistor
type IXM1 running at about 170C. Close by are two grounded resistors
in parallel, 8.2ohms and 22ohms which together equal 6ohms. But
surely the current should be passing through the resistors, so
I need to remove and test the chip. The chip tested OK.. both
its transistors measured normally so I need to trace the circuit.
The answer was obvious
after checking the circuit around the IMX1 (X1 below left). The
transistor pairs in the IMX1 are connected as a long-tailed pair
with emitters connected to directly to ground and the one with
its base directly wired to the 6 ohm resistor to ground plus
the emitter of TIP31A was being biased ON by the test voltage.
Tracing the circuit, which relies on lots of hidden track buried
in the inner planes of the multi-layer board, was difficult,
but I found the voltage into the opposite IMX1 transistor originates
from a couple of tiny chips further up the board (oddly right
next to the ES2J diode and IRF7406 I'd replaced... see below).
My guess is that the output of the long-tailed pair, which drives
the base of TIP31A governs whether the relay is turned on or
off. The input to the second IMX1 transistor pair might be derived
either from the "OFF" switch or perhaps a fault detection
circuit or even both? Could the relay switch power to the LT1270A..
that might account for the missing voltage to the input of the
DC-DC converter?
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FIG8 |
These
pictures show (above) the long-tailed pair chip X1 and the emitter
load of the TIP31A (= 8R2 + 220) and (right) the input circuit
for the long-tailed pair, along the board 6DW (= BC817) + X1
(=IMX1)
Also seen is ES2J
which is a replacement for the original short-circuit diode.
Is the proximity to this a coincidence or is that diode related
to the absence of the supply to the LT1270A??
Below, the area of the board
giving trouble.
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FIG9  |
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FIG10
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The next step
is to refit the IMX1 which tested OK and carry out some voltage
measurements. I could also test whether the relay contacts connect
to the LT1270A. I fitted the IMX1 and decided to trace the circuitry
around the LT1270A. As this mostly used tracks inside the multi-layer
sandwich I removed firstly the 22 ohm resistor adjacent to the
end MBR1645 and this enabled me to determine the local connections
but there was nothing really of interest. Next I looked for a
link between the Vin pin of the LT1270A and to my surprise I
found a low resistance between this pin and the adjacent Vsw
pin. The only way to figure out the reason for this was to remove
the LT1270A and after doing this I found a low resistance of
18 ohms between the chip pins, but whether this is a short or
part of its internal circuitry I can't say. A puzzle is that
when I tested the chip it was correctly turning out 12 volts
(but this was a false error condition). Anyway I've ordered a
replacement from China which will take around 3 weeks. I then
looked for a connection between the plated-through hole for the
Vin pin but found none.
Below left, the back of the
board in the area giving trouble
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FIG11
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You can clearly see
the effect of (historic) excessive current having been drawn
in the 24 volt feed to the standby and run circuitry. This is
governed by the state of the Off switch. The Off function is
still working but the run condition which turns on the relay
is inoperative. The run circuit is in the centre of the board
and itself has a couple of problems. Namely absence of the 24
volt (or thereabouts) feed plus faulty parts.
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Here's what I believe
must be the background to this problem. Something happened which
resulted in excessive current being drawn by the LT1270A circuit.
This open-circuited the track, inside the multi-layer sandwich,
connecting the Vin plated-through hole and the copper area connecting
to the relay normally open contact (=relay output). At this point
the circuitry connecting to LT1270A input pin failed resulting
in a short of 18 ohms. A couple of diodes burnt up, These are
connected to the TIP31A and the LT1270A described earlier. Also
damaged were a diode (from excessive current) connecting to the
relay output and a MOSFET (both decribed earlier). Confusingly
during testing, applying a 24 volt supply to the LT1270A did
result in a 12 volt output, but this was from a bad LT1270A.
At present I think
the MBR1645 is used for reverse protection (but that is debateable
after testing further). The TIP31A seems to be used for energising
the relay and it does this via the output of a long-tailed pair
which must compare two voltages, one for turning the supply off
(the OFF connection) and the other perhaps a "Good"
signal from the DC-DC converter (or the raw 24 volt battery suppy).
Clearly something
went wrong after the Turkish repairer carried out work on the
board. The LT1270A is difficult to remove from the board and
it's not unknown for a plated through hole to be damaged in the
process. Fitting a new device would appear satisfactory but did
he miss the shorted diode and MOSFET.
I need to review what
happened before I received the thing for repair.
It was reported that
it worked for a very short time before failing again. My revised
guess is an initial (primary) fault had developed which resulted
in the LT1270A circuit getting damaged. That secondary fault
had been fixed but on return to the customer the original fault
had recurred resulting in the same secondary damage (the LT1270A
circuit) but in addition had resulted in more extensive damage
to the original faulty area (ie. multi-layer sandwich track damage).
It's something akin to fitting a new fuse when an old one failed..
extra damage. Read on to see the results of my investigations.
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FIG12
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I fitted
a new LT1270A and worked out that its supply voltage was fed
by the relay. I then traced the relay drive circuit, quite difficult
as it uses lots of short tracks on both sides of the board. Basically
the relay coil is driven from the TIP31A transistor.
Just to recap on what
I believe should be the course of events.
Applying power with
the system switch turned to OFF results in a set of components
getting warm (FIG13)
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FIG13
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FIG 14 |
Here you can
see the three resistors (Off circuit) from the thermal camera
picture above. At this point I'd removed the ES2J diode so I
could trace the circuit. Its getting more obvious that this area
of circuit is responsible for operating the relay. Clearly there's
a power supply operating in order to draw current through the
resistors (it's the battery) |
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Turning the
Off switch to the On setting will disconnect the supply voltage
to those three resistors and should allow a postive voltage to
turn on the TIP31A transistor which will activate the relay.
This doesn't happen. I soldered together a couple of resistors
fed from the 24 volt power supply to produce 6 volts and touched
this to the base of the TIP31A. The relay turned on. Something
in the meandering circuit must be missing a positive supply voltage.
A clue to what's going wrong might be in the pictures below.
With the circuit in its On state the device below gets extremely
hot (FIG15). I looked up the code "Y4W" and it's a zener diode type BZX84C15. Maybe
there's a bad capacitor or maybe the device itself is bad? That
thin wire you can see is helping to identify the plated through
holed involved in the circuit. The zener is connected to
a "60W" chip which I couldn't identify but closer scrutiny
revealed "6DW", an SOT23
BC817 NPN transistor weirdly connected with a 100 ohm resistor
from base to collector and a low resistance to the emitter (FIG16).
A second clue to the
origin of the fault is absence of the green solder resist on
a couple of lengths of thinnish track (FIG11 above). These tracks
feed the 24 volt supply to the main area of the board and is
responsible for the Off circuit where it feeds the three resistors
in FIG14, and surely also for turning on the relay?
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FIG15
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FIG16 |
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Here's a block
diagram of what I believe to be the circuitry of the autopilot.
I'd guess that the battery might be optionally 12 volts or 24
volts. Between the Internal 24 volt supply and the main circuit
board power supplies (electronic switch) is an IRF7406 MOSFET
(see FIG1) which is turned on perhaps shortly after the relay
is activated. Both this and the nearby ES2J diode where both
short circuit when I was given the autopilot. Also, the half
ohm resistor was badly burnt. |
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Well, I've given
up on the autopilot. I could fool the circuit and trigger the
gate of the relay driver but there wasn't a way to do this otherwise.
I traced the 24 volt "standby" supply that isn't switched
by the relay and it managed to get the Off switch turn off the
relay but the standby voltage seemed to disappear into the multi-layer
sandwich for the On circuitry. I also found a 15 volt zener was
getting too hot. I also discovered a bad BC817 in this circuit
but swapping it for a new one failed to change matters. The operating
voltage appeared to be around 600mV maximum in the On circuits
rather than something like 12 to 24 volts so there's either an
open circuit or a leak or both within the multi-layer sandwich.
I'm not too happy with the temperature of the microprocessor
either when the relay is triggered on. In addition I can't figure
out why the Schottky diode in the negative feed from the battery
to the LT1270A develops 20 volts across itself when the latter
is receiving 24 volts. That seems to defy logic. Anyway, all's
that's left is to screw it back together and hand it back with
my regrets. |
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