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IC-746 Pro
| IC-746 PRO Heatsink for PA Driver |
| The IC-746PRO has
had a history of failed uPC1678G ICs resulting
in no RF output. Icom's position is that voltage
spikes are the source of the problem but users
have observed that this driver IC
(IC151) runs "hot" and think that failures
are from thermal stress. In normal operation,
IC151 dissipates 250 milliwatts (with 330 mW
max rating at 25 deg C ambient). The legs of
this very small MMIC chip become the heatsink
for the 1/4 watt of heat generated. Adding an
external heatsink for IC151 is
cheap insurance against potential failure.
My heatsink
requirements are as follows:
- no soldering to
the circuit board,
- no drilling of the
case,
- no epoxy on the
chip,
- a thermal path to
the aluminum case, and
- convective cooling
fins.
Fabrication.
The source of my aluminum heatsink is
from Radio Shack (PN 276-1368). You have to make
one cut with a hacksaw to reduce the length to
3/16 inch (5mm). Use a small file to remove the
aluminum burrs.
Installation.
The proper position for the heatsink is
flat on the chip and touching the case as shown
in the picture. The heatsink should be
clear of any other components.
- Unplug the gray
cable with the black band and move it out of
the way.
- Apply a small
amount of heatsink grease (as used in
your PC)to the top of the IC.
- Clean the
heatsink with a solvent (like acetone)
and apply a small amount of JB Weld epoxy to
the top half of the heatsink wall. JB
Weld has good thermal conductivity and good
high temperature characteristics.
- Place the
heatsink into position as before and
insure that it is flat on the chip.
- Let the epoxy cure
overnight. Check that the leftover epoxy has
setup hard.
- Position the loose
cable in the heatsink slot as shown
and reconnect it to the circuit board.
The heatsink may
be removed at a future date to replace IC151
(hopefully not) by twisting it sideways with
pliers. |
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| Referring IC-746 model
backlight problem i meet same think newer
model. I think that this problem occur Europe model
IC-7400 too.
First i change backlight cold cathode
lamp but no help. Second i read carefully what
it made for IC-746 repairing.
In the metal box has
few component witch make cold cathode lamp
requiring voltage. There is one bigger
transistor what is Q302 2SB1201 PNP transistor.
Because there is no enough cooling area in
printed circuit board, this transistor go
faulty.
I remove it using
normal 50W Weller solderstation and
desolderbaraid. Next i drill to the metal shield
one side (most of all emptiest room) and replace
original 2SB1201 to BD650.
Other type is ok if you check 2SB1201 current
and voltage values.
It is very important
insulate this transistor from metal shield (it
is grounded) because all transistor legs have no
ground contact !
Then shorten new tr.
legs and solder short pieces of wire tr. legs
and place where original tr. was.
Put all boards back and
front panel to the main body. If all ok there is
backlight on radio.
I no take any liability
for this repair instruction. You make all you
own risk. |
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For all those who wants a little more of driving
power with the new IC-746,here is the way to do
it easily.
Important settings pots
within the IC-746 HF/VHF radio. Remove the
twelve screws holding on the bottom cover. With
the Radio laying flat on it's top remove the
bottom cover.
Underneath the radio
there is a board named MAIN UNIT (B5083J) where
there are 4 important small pots provided to
increase or decrease the power output in
different bands as follow:
- R-989 HF Power
output (rated at 100W)
- R-991 50 Mhz band
Power Output (rated at 100W)
- R-993 144Mhz band
Power Output (rated at 100W)
- R-994 AM Power
output (rated at 40W)
- R-546 Tx gain,set
at factory for best linearity in TX IF
(better not to play with it)
Do this totally at your
own risk. Don't get more than 140 Wts in FM
because with high SWR you might damage your
final stage and void your factory warranty.
Note: Be sure to do all
the setings over a pure 50 ohms dummy load.
On my
particular radio (usa version) ser 3300++ i
found a different board # and different variable
resistors
THE BOARD # IS B5699G
R320-ADJ AM POWER
R317-ADJ VHF POWER
R311 ADJ 50 MHZ POWER
R306 ADJ HF POWER |
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| IC-746 PRO AM tx improvements |
| This rig is a lousy
performer on AM: Low modulation level (at 30w
carrier, peaks never reach 70w), with muffled
sound.
There are three serious
design flaws who affects mostly AM mod.
- The ALC circuit
loop in SSB mode keeps the RF peak at
presetted value. In AM, a capacitor is
connected as a low pass filter, to keep
average RF value constant. This is done by a
saturated bipolar transistor, who behave as
a diode, not actually connecting the
capacitor to ground.
Correction:
- Replace C305
(10uF) by a 100uF one.
- Replace Q306
(DTC114EU) by a FDN339AN mosfet
- The first tx mixer
(IC400) has gain in excess leading for IMD
distortion in SSB and low peak level in AM.
Correction:
- Replace R407
(220ohm) by 680 ohm. Re-adjust R331 to a
higher RF level.
- There is a thick
felt muffler in front of microphone electret
unit. Remove it.
After modification my
rig presents excellent AM, and the worst case
IMD distortion in SSB 2-tone test dropped from
-24 to -32 dB. |
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| IC-746 PRO Transmit Problem |
| A significant number
of hams have reported failures of IC-746Pro
Transmit functions within a few hours to a few
months of otherwise satisfactory operation. The
remaining receive and menu functions are normal,
but TX is dead on all bands, HF, 6M and 2M.
Several hams(see eham.net product reviews)
report multiple TX failures, with the rigs going
back to Icom in Washington state more than once.
Indications are that:
IC151 on the RF Unit;
and
D24 and Q25 on the CTRL unit are all bad.
These three components
are all tied to the ANT jack(s) via the HRX
line, between the RF Unit and the CTRL Unit. The
problem is ESD (Electro-Static Discharge)
related, and results from a discharge to the ANT
connector that will damage these components even
when the radio is powered OFF. The source of the
ESD could be applied when you change antennas,
and might take a step with the cable in hand to
connect to a bandswitch or the amplifier,
etc...or picked up on the antenna as an
environmental electro-static charge, common
especially during the dry winter months
(especially the winter of 2002-2003). NOTE:
THERE IS NO ESD PROTECTION on the HRX line.
I wrote to Icom Japan
regarding this problem, and it is my
understanding that there has since been an
"internal" memo referred to as Internal Service
Note #920 to addresses this issue.
In the memo, a pair of
1SV252 PIN Attenuator Switch Diodes are placed
back-to-back on the HRX line to prevent ESD from
damaging the aforementioned components. The
diodes are mounted on the RF Unit at the
junction of R1 and J1 (HRX) to GND.
These are very small,
surface mounted components, so be forewarned
that unless you are trained and equipped to
mount these in an ESD safe environment, with
proper surface mounting equipment, eye
magnification, etc (i.e., no coffee w/in
24-hrs), then you are better off sending the rig
to Icom and waiting for them to install the ESD
fix.
The parts are available
from Icom Parts for $1.34ea, Part No. 91213606,
PIN Diode 1SV252, Qty 2 required.
The diodes are packaged
as a 3-terminal device, two diodes in series,
with the common junction (cathode-anode) not
used. One package is connected with Cathode to
HRX, Anode to GND; and the other Anode to HRX,
Cathode to GND.
While you're attending
to this fix, see my IC-746Pro SSB Monitor Level
fix, also found on this site. |
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| IC-746 PRO SSB Monitor Level Fix |
CAUTION: Be certain to
observe Anti-Stat precautions for an ESD-Safe
work area.
- Remove the bottom
cover of the IC-746Pro.
- Remove all cables
and connectors and from the Main Unit PC bd.
- Remove all screws
from the Main Unit PC Board, while noting in
particular the following three screws:
- Remove the
machine screw on IC1240, which bonds the
heat sink tab to the cast metal chassis;
- Remove the
screw from the rear panel to the plastic
ALC and SEND connector housing; and
- Pay attention
to the center PC bd screw which holds a
grounding spring contact against the
underside of the metal shielded
sub-assembly.
- Change R1080 from
100K ohm to 22K ohm. R1080 is located on the
bottom side of the Main Unit PC Board, next
to IC1080, from pin 3 to C1080 (+).
See Service Manual
Figure 9-9, Main Unit Bottom View, Page 9-8
See Service Manual Schematic Main Unit(1)
Page/Figure 12-3
Alternatively, place a
30K in parallel with R1080, ~22K net value.
Preferably use a surface mount resistor of the
same size, or a 1/16 watt discrete resistor,
using a surface mount tip, ESD protected
soldering iron at approximately 650 Deg F.
This modification
raises the SSB Monitor level only, and provides
approximately +9dB of additional gain. Adequate
SSB Monitor volume can now be achieved when the
Monitor Menu is set to ~90% without boosting the
AF Gain setting. |
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| TX modification for IC-746PRO (version
#02 USA) |
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- Remove the bottom cover of IC-746PRO.
- Remove DSP subbblock.
- Carefully remove two small SMD 0 ohm
resistors (jumpers) W1603 and W1604, on the pictures both
jumpers already removed, reset of IC-746PRO is not necessary.
- Enjoy.`````````````````````.
TX/RX after modification: 0.1 - 0 MHz,
108 - 174 MHz.
Now you can do SWR spot & plot
measurement even out of ham bands! Be carefully with transmitting
out of 1,8 - 29,7 and 144-148 MHz.
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User comment |
Get your bifocals on and sharpen your
fingernails, let the mod begin |
From:
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| I
just completed the modification to my 746pro (US version)
using the steps outlined and it worked perfectly. One
suggestion, have two things handy when you do the work, one is
a good sized magnifying glass as it very difficult to see the
two resistor jumpers that needed to be removed. I have very
good eyesight and without the magnifying glass it would have
very difficult to distinguish W1603-W1604 from the others that
are so, so, so close by. Second, I found that it was easier to
just hook & scrape them off with my fingernail while observing
thru the magnifying glass rather than trying to use any other
type of instrument that I had lying around. They are so small
that even attempting to use tweezers to crush the resistors I
found not possible as the ends of the tweezers were too large
and would not just grab one resistor but would come in contact
with the adjacent resistor(s)as well. When that happened you
ran the risk of clipping something that you did not intend
too. This may sound crude and a rather non-technical method of
removing these resistors however it worked. By all means,
don't try this without the magnifying glass. I thought the
ones in the 706 were small, those are giants in comparison to
these. Good luck! |
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As you see below,
IC-746PRO (European version) will be exactly the same as his USA
brother, after mods, sure :-). Enjoy!
IC-746PRO(#02USA)
W1602 1 1
W1601 1 1
W1609 0 0
W1608 0 0
W1607 0 0
W1606 0 0
W1605 1 1
W1604 1 0 Removed
W1603 1 0 Removed
W1610 1 1
IC-746PRO(#03 EUR)
W1602 1 1
W1601 1 1
W1609 1 0 Removed
W1608 1 0 Removed
W1607 0 0
W1606 1 0 Removed
W1605 1 1
W1604 1 0 Removed
W1603 0 0
W1610 1 1
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| Icom 746Pro SSB Monitor
Level Fix |
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CAUTION: Be certain to observe
Anti-Stat precautions for an ESD-Safe work area.
- Remove the bottom cover of the
IC-746Pro.
- Remove all cables and connectors and
from the Main Unit PC bd.
- Remove all screws from the Main Unit
PC Board, while noting in particular the following three screws:
- Remove the machine screw on IC1240,
which bonds the heat sink tab to the cast metal chassis;
- Remove the screw from the rear panel
to the plastic ALC and SEND connector housing; and
- Pay attention to the center PC bd
screw which holds a grounding spring contact against the
underside of the metal shielded sub-assembly.
- Change R1080 from 100K ohm to 22K ohm.
R1080 is located on the bottom side of the Main Unit PC Board,
next to IC1080, from pin 3 to C1080 (+).
See Service Manual Figure 9-9, Main Unit
Bottom View, Page 9-8
See Service Manual Schematic Main Unit(1) Page/Figure 12-3
Alternatively, place a 30K in parallel
with R1080, ~22K net value.
Preferably use a surface mount resistor of the same size, or a
1/16 watt discrete resistor, using a surface mount tip, ESD
protected soldering iron at approximately 650 Deg F.
This modification raises the SSB Monitor
level only, and provides approximately +9dB of additional gain.
Adequate SSB Monitor volume can now be achieved when the Monitor
Menu is set to ~90% without boosting the AF Gain setting.
Vince, K1VF
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| Icom IC-746Pro Transmit
Problem |
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A significant number of hams
have reported failures of IC-746Pro Transmit functions within a
few hours to a few months of otherwise satisfactory operation. The
remaining receive and menu functions are normal, but TX is dead on
all bands, HF, 6M and 2M. Several hams(see eham.net product
reviews) report multiple TX failures, with the rigs going back to
Icom in Washington state more than once.
Indications are that:
These three components are all tied to
the ANT jack(s) via the HRX line, between the RF Unit and the CTRL
Unit. The problem is ESD (Electro-Static Discharge) related, and
results from a discharge to the ANT connector that will damage
these components even when the radio is powered OFF. The source of
the ESD could be applied when you change antennas, and might take
a step with the cable in hand to connect to a bandswitch or the
amplifier, etc...or picked up on the antenna as an environmental
electro-static charge, common especially during the dry winter
months (especially the winter of 2002-2003). NOTE: THERE IS NO
ESD PROTECTION on the HRX line.
I wrote to Icom Japan regarding this
problem, and it is my understanding that there has since been an
"internal" memo referred to as Internal Service Note #920 to
addresses this issue.
In the memo, a pair of 1SV252 PIN
Attenuator Switch Diodes are placed back-to-back on the HRX line
to prevent ESD from damaging the aforementioned components. The
diodes are mounted on the RF Unit at the junction of R1 and J1 (HRX)
to GND.
These are very small, surface mounted
components, so be forewarned that unless you are trained and
equipped to mount these in an ESD safe environment, with proper
surface mounting equipment, eye magnification, etc (i.e., no
coffee w/in 24-hrs), then you are better off sending the rig to
Icom and waiting for them to install the ESD fix.
The parts are available from Icom Parts
for $1.34ea, Part No. 91213606, PIN Diode 1SV252, Qty 2 required.
The diodes are packaged as a 3-terminal
device, two diodes in series, with the common junction
(cathode-anode) not used. One package is connected with Cathode to
HRX, Anode to GND; and the other Anode to HRX, Cathode to GND.
While you're attending to this fix, see
my IC-746Pro SSB Monitor Level fix, also found on this site.
Good luck, K1VF.
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User comment |
Icom IC-746Pro Transmit Problem |
From:
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The photo shows the botton side of the
RF Unit, prior to installing the surface mounted pin diodes,
with the pad locations circled.
You must unsolder the two highlighted tabs, and bend the
shield up slightly in order to access the lower position.
 |
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User comment |
NO Transmit Issue |
From:
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I
have been following the NO TRANSMIT issue of the 746PRO since
I have received serial #32xx that suffer the failure. Icom in
Canada states the cutoff S/N is 3351 where the Internal
Service #920 modification has been applied at the assembly
line.
Although I have studied this failure from information from
many, many users and have seen the same identical failure in
current models with the S/N #37xx and higher. Fluke or same
issue?
I have ordered a new 746PRO with the S/N #4xxx and I will keep
everyone posted on it's status.
Icom in Japan states there is an issue, Icom in Canada also
acknowledges the issue, why Icom U.S.A. plays games and states
no problem exists is beyond me. Sounds like a "Class Action
Suit" in the future, unless they fess up and put out a service
bulletin and repair radios in warranty and out of warranty
with this issue.
Questions for Icom:
Why do all previous Icom 746/756 Series radios have the ESD
back-to-back diodes installed, and there are NO SUCH reposts
of DEAD TX. Ob the other hand, the 746Pro has HUNDREDS of
reports of the problem, and in fact the PC Bd is laid out for
the parts, but they were depopulated, perhaps in error.
Ask why Icom Cananda and Japan acknowledge there is an
issue...
Finally, ask about Internal Memo #920 which describes the
problem and fix.....
Lastly, ask them why such cock-pit error causes problems only
with the 746Pro, and occurs w/o any amplifier w/in 100miles of
the rig....'
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User comment |
A problem with driver PA (uPC1678). |
From:
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Very common failure of device IC 746
746PRO, 756, 756PRO and 7400 is destruction IC uPC1678 (driver
for PA).
This is draft mistake, which ICOM company don’t want to
accept.
ICOM company stays on idea, that this driver IC failed only by
static electricity from HF antenna (in this case other two
capacitors and two diodes must be destroyed, but these
components are after destruction of this driver IC still
heath) and even they released service note, where they
recommend add diodes to input of HF receiver.
And on IC 746PRO from S/N 3300, they add this diodes in
factory.
BUT
I had on my table IC746PRO S/N 3422 with factory installed
these diodes and driver IC was DEAD. And components on static
electricity path are still healthy.
Construction problem is somewhere else.
Author leaves this IC under power also in receive state, which
main mistake is.
IC has idling current about 50mA and produce heat in receive
regime.
In radio transit regime „eat“ higher current and also higher
temperature.
A now we are in construction problem.
Producer of this IC uPC1678 in application note describe, how
big must be minimal area of copper on PCB for good cooling.
This IC is cooled only through legs!!!!!
And this was by ICOM absolutely underestimated, beside minimal
area of copper with dense area of connecting hole about 6cm
sq. In IC74X device are only about 1cm sq. of flat and thereby
also cooler. This is reason why this IC heats till overheat
deal.
From IC 74X production start I replaced about 20 this IC, and
this is reason why I start to search where is problem.
Problem can be solved by adding some cool wing connected to
device chassis, but in possible mounting place body is not
flat, but rounded. Or is possible to think about switching off
this IC in receive mode.
If I talk about underestimating of cooling areas, there is
another place in IC 74X, regulation transistor of display
light converter. So lower display light intensity user set, so
higher heat is produced by regulating transistor. Due
absolutely minimal cooling area, on which is this SMD power
transistor soldered, very soon came to “baking” of
this transistor.
73 Karel OK1DNH
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KB2LJJ