FT-897D/857D - DISTORTED
AUDIO REPORTS ON SIDEBAND
IMPORTANT NOTE: The idling current issue addressed here pertains
to older units utilizing the now discontinued 2SC5125 power
output transistors driven by the Mitsubishi RD07MVS1 FET
devices. In later production runs, it appears that Yaesu has
switched to a different power amplifier design. Reports from
users of the newer production runs indicate that the
wandering idling current problem is no longer an issue. Keep
in mind that before you attempt any adjustments, it would be
very wise to acquire from Yaesu the correct service manual
for your model radio. It is also highly advisable to log the
factory settings your radio has before you even think about
changing them. This allows you to return your rig to working
values should you run into trouble. With that in mind, if
you have determined that your rig utilizes the older design,
then proceed at your own risk! That sounds scary, but to
tell the truth, Yaesu has made it quite simple to check the
resting current of the stages involved.
A LITTLE HISTORICAL
BACKGROUND ABOUT THIS PROBLEM
More than a few owners of the older FT-857/897 series of
radios have received the bad news that their radios sound
absolutely terrible in the sideband mode. Reports of trouble are
often mistaken for RF feedback or poorly adjusted audio. Others
have blamed the trouble on the ALC clamp that engages to
foldback output when the PA is loading into a high SWR. In many
cases, however, the truth seems to be that many of these radios
rolled off the assembly lines and into their boxes with the
exciter and linear stages incorrectly adjusted. The error in
adjustment was not slight either. My FT-897D (manufactured in
June 2004 with serial number 4H370xxx) was almost completely
unreadable on 6 meter sideband. A check of the idling current
revealed the following:
Pre-Driver resting current: 25 ma (should be 35 ma)
Driver resting current: 56 ma (should be 30 ma)
Exciter stage total resting current: 80 ma
(should be a sum of 200 ma for both exciter transistors)
Exciter transistor number one: 10 ma (should be 100 ma)
Exciter transistor number two: 70 ma (should be 100 ma)
HF Power Amplifier transistors: 82 ma (should be 300 ma)
VHF/UHF Power Amplifier: 232 ma (should be 300 ma)
Those are pretty significant errors in adjustment! What is
alarming is that two other local hams with 897D radios put them
on my bench and had very similar numbers! I have received
reports from over thirty other owners who checked their units
and found the same errors in adjustment. So... if you have found
yourself reading this page, and you have received reports of
distorted audio now and then, you may find that your distortion
problems will be over with the simple adjustments below (gleaned
from the Yaesu Service Manual and amended for clarity) to get
your rig running in linear mode.
If you are a little squeamish about soldering to the tiny test
pads to check and adjust the pre-driver and driver idling
current, then consider skipping step one below and just do the
exciter and PA stages. In my experience, those two stages tend
to be the ones that stray into non-linear mode most alarmingly.
Appreciation for discovering this problem should be directed to
DG2IAQ (Jochen) who first uncovered the problem when examining
settings in his rigs.
1.
CHECKING AND ADJUSTING THE 857/897 PRE-DRIVER AND
DRIVER BIAS
Before alignment, set the mode to CW and tune the radio to
1.800 MHz. To prevent accidental keying, nothing should be
plugged into the CW jack during the alignment procedure.
Connect the transceiver to a 50 ohm dummy load.
Power down the radio.
These stages are accessed by removing the top cover
of the radio. After removing the screws, lift the cover away
and disconnect the speaker lead and the battery switch lead
from the mainboard.
Locate the little slide
switch near the front of the rig, on the right side. Note
its position (normal operating position is switched toward
the front of the radio) and then slide it toward the rear of
the rig. This opens up the tiny surface mount pads that are
used to measure the pre-driver and driver stages. The
service manual has an excellent illustration showing the
position of test points TP1017, TP1018, TP1019, and TP1020.
(At the bottom of this page view the diagram to find where
these test points are located.) Using a low power soldering
iron with a very fine tip, solder one end of any value 1/8
watt resistor onto the tiny pad of TP1017 so that it is
standing straight up. Do the same for the other three test
points. Connect your ammeter to the lead ends of the
resistors soldered to the pads (not through the resistor) as in the picture below. The 1/8
watt resistors just seemed easier for me to position and
hold steady than bare wire. It has been suggested to me that
soldering a small value capacitor (around 50pf) across the
pads for TP1017 and TP1018 and doing the same for TP1019 and
TP1020 would provide a strong two point connection that
could be left in with no ill effects. I intend to do
this the next time I go back in.
FT 857/897 Pre-Driver and Driver Test Points
(Showing connection to TP1017 and TP1018))
To check the factory setting of the idling
current of the pre-driver, connect your ammeter leads to
TP1019 and TP1020, power the radio up and check again to be
sure you are in CW mode, then press the PTT switch of the
microphone and read your ammeter. The correct value for the
pre-driver stage should be 35 ma. If your rig is reading
more than plus or minus 2 ma difference, adjust VR1004 to
get a reading between 33 ma and 37 ma. (Note: I actually set
this stage to 40 ma on my own rig.)
Power down and repeat the process for the
driver stage while connected to TP1017 and TP1018. If your
rig is reading more than plus or minus 2 ma difference from
30 ma, adjust VR1003 for an indication between 28 ma and 32
ma. (Note: In my own rig, this stage appears to be one of
the most critical as far as getting the rig to sound clean
on sideband. My radio would still clip slightly at the 30 ma
setting, so I ended up setting mine to 50 ma. Keep in mind
that there are 2 x RD01MUS1 devices in this stage, and these
devices are rated as high as 600 ma CCS current each.
Setting 2 of these devices at 50 ma for amateur ICAS service
is not even remotely a strain.)
Power down the radio and unsolder
your leads from the solder pads at the test points.
2. CHECKING AND ADJUSTING THE 857/897 EXCITER
STAGE BIAS
Before alignment, set the mode to CW and tune the radio to 1.800
MHz. To prevent accidental keying, nothing should be plugged
into the CW jack during the alignment procedure. Connect the
transceiver to a 50 ohm dummy load.
Power down
the radio.
Remove the
jumper at J1004 (it may be somewhat hidden by the
cable that terminates into J1003) and connect the
leads of your ammeter to the two pins at J1004. This is the same
type of jumper that is used on computer boards. An easy way to attach the test leads to your ammeter
is to grab a 2 pin header connector from an old computer case
and strip the ends so you can connect to your meter with a
pair of alligator clip leads. During the alignment procedure
your meter will not be conducting more than a few hundred
milliamps.
Power up the radio.
To check the factory setting
of the idling current, check again to be sure you are in CW
mode, then press the PTT switch of the microphone and read
your ammeter. The correct setting is 200 ma. Even if the
setting is correct, you may wish to proceed with the alignment
to assure that each driver is set correctly so that the total
current load is shared equally.
To adjust the idling current,
turn both VR1001 and VR1002 fully counterclockwise.
In CW mode, press the
microphone PTT and adjust VR1002 for an indication of 100ma
(plus or minus 10ma).
In CW mode, press the
microphone PTT and adjust VR1001 for an indication of 200ma
(plus or minus 10ma).
(Note: In
my radio I set this stage to 2 x 105 ma for a total value of
210 ma.)
Power down the radio.
Remove your connection to
J1004 and place the factory jumper back onto the test points.
FT 857/897 Receiver
Board (Accessible by removing the top cover)
3. CHECKING AND ADJUSTING THE 857/897 POWER
AMPLIFIER BIAS
Before alignment, set the mode
to CW and tune the radio to 50 MHz. To prevent accidental keying, nothing
should be plugged into the CW jack during the alignment
procedure. Connect the transceiver to a 50 ohm dummy load.
Power down the
radio.
This stage is accessed by removing the bottom cover of the
radio. After removing the screws, lift the bottom cover away. If
the internal power supply is installed, remove it. If batteries
are installed, remove them. The bottom sheet metal protective
plate now may be removed. Reconnect the power supply to
the radio. If you are using the internal supply, the power lead
is long enough to allow connection as long as you keep the
supply in close proximity to the radio.
Unsolder the jumper that is across TP3021 and TP3022. This may
be somewhat obstructed by one of the cables that connects to
T3005 so it may be necessary to temporarily route the cable away
from where you need to work. Be careful not to allow drops of
solder to stray onto the PA board as you unsolder the jumper
strap.
Connect your ammeter to TP3021 and TP3022 with a pair of
alligator clip leads. Your meter will not have to carry more
than a few hundred milliamps of current during the procedure. Power up the radio.
To check the factory setting of the idling current, check again
to be sure you are in CW mode, then press the PTT switch of the
microphone and read your ammeter. Note that when you first press
the PTT switch, the idling current rises slowly for several
seconds to a maximum value then stabilizes. Wait until the
maximum value is reached before reading the value. The correct
setting is 300 ma.
Should adjustment be
necessary, while in CW mode, press the PTT switch of the
microphone and adjust VR3002 for a reading of 300 ma (plus or
minus 10 ma). Note that when you first press the PTT switch, the
idling current rises slowly for several seconds to a maximum value
then stabilizes. Wait until it has stabilized before doing the
adjustment.
(Note: I found that the PA stage setting has a tendency to
wander from day to day. This is probably due to thermal
variances in the bias components. At any rate, I ended up
setting this to 400 ma for the HF/6 meter final. This value has
worked very well for me in daily service.)
Before you power down, while you are in there, you may also want
to check the resting current on the VHF/UHF power amplifier.
Set the radio to the 440 MHz band and set the mode to CW. While in CW mode, press the microphone PTT switch and note
the resting current. The correct setting is 300 ma.
Should adjustment be necessary, adjust VR3001 for an indication
of 300 ma (plus or minus 10 ma).
(Note: This stage seems to stay stable at whatever I set it to.
I keep it at 300 ma and it performs well at that setting.)
When all adjustments are completed, power down the radio, remove
your test leads, and resolder the strap across TP3021 and
TP3022.
FT 857/897 Power Amplifier Board
(Accessible by removing the bottom covers)
Addendum 1 - June
4, 2006: A little more than a year after
making my initial adjustments, I rechecked all the resting
current settings. The pre-driver, driver, and exciter stages
were right on the mark. The VHF/UHF PA stage was also right
on the mark, however, the PA stage for the HF/6 meter bands
had dropped from 310 ma down to 260 ma. I re-adjusted that
stage to higher than spec and will check it again after
several months. Either the bias voltage is unstable, or
components are changing value slightly as they age. It
definitely is a touchy adjustment.
Addendum 2 - June 4, 2006: Consider taking the time to check
the tightness of all the machine screws used to fasten the
upper and lower main boards to the chassis. Also check the
machine screws fastening the HF/6 and VHF/UHF final
transistors to the heat sink. On the advice of earlier
owners of the 857/897 rigs, I had checked mine when it was
new and found that many of them needed at least 1/4 turn to
reach a moderate level of tightness. At one point, my 897D
suddenly began exhibiting the full output oscillation
syndrome on 6 meters that has been reported by many others.
Keying the rig at any power level, in any mode, resulted in
full output on some unknown frequency. Within a few days it
was taking off on all the HF bands as well. As it turned
out, many of my board to chassis mounting screws had once
again loosened up. This obviously will cause many ground
path changes throughout the rig which can lead to
instability. Tightening them down again did resolve the
oscillation issue nicely on all bands. I suspect the machine
screws slowly "walk out" due to the thermal cycles they are
subjected to during normal use. Loose heatsink screws would
certainly lead to undesirable thermal stress of the final
transistors. Eventually, I installed lock washers under all
the machine screws and the problem has not returned.
Addendum 3 – June 7, 2006: The 857/897 D versions have been
blessed with some upgraded components in the driver and
exciter circuits. The driver stage has been upgraded from a
pair of 2SK2973 drivers to a pair of Mitsubishi RD01MUS1
devices. Yaesu wisely decided to stop using a pair of
2SK2975 FET drivers (infamous for failure when used as
finals in the FT-817 transceiver) in the exciter stage and
replaced them with the far more rugged Mitsubishi RD07MVS1
FET devices. A review of the Mitsubishi ratings of these
devices indicates that the idling current values stated in
the Yaesu Service Manual are extremely conservative. If you
are the tinkering type, you may wish to set your rig up to
idle a bit higher than the YAESU service menu suggested
values. A slightly higher idling current places the devices
a bit higher up into their linear region of conduction
resulting in a cleaner sounding signal. These settings do
not even come close to being risky and a review of the
tolerance values of the devices on the manufacturer spec
sheets should more than reassure you that there is plenty of
headroom to allow this. Note also that the ICOM 756 Pro
series of transmitters, which also use a pair of 2SC5125
devices in the PA stage, are set at the factory to idle at
500 ma for the sideband mode. (Reference page 30 of the ICOM
756 Service Manual.) In fact, the ICOM 746 series of
transmitters also use a pair of 2SC5125 transistors in the
PA stage and they are factory set to idle at 600 ma!
(Reference page 24 of the ICOM 746 Service Manual.) After
reviewing the parameters of other radios using the same PA
devices and the suggestions of Jochen, DG2IAQ, I ended up
going with the following setup:
|
Device(s)
|
Service Manual
|
My Factory Settings
|
My Adjusted Settings
|
|
2SK2596 Pre-Driver
|
35 ma
|
25 ma
|
40 ma
|
|
RD01MUS1 Driver (2)
|
30 ma
|
56 ma
|
50 ma
|
|
RD07MVS1 Exciter (2)
|
200 ma (2 x 100 ma)
|
80 ma
(10ma & 70ma)
|
210 ma (2 x 105)
|
|
2SC5125 Final (2)
|
300 ma
|
82 ma
|
400 ma
|
Addendum 4 – June 8, 2006: Test point diagrams for the
857/897 from the Yaesu Technical Supplement
Addendum 5 - July 1, 2008: During the summer of 2007 I
inserted lock washers under all the upper and lower
mainboard screws. This has done an excellent job of
preventing them from loosening up. If you are
experiencing repeated loosening of these fasteners due
to thermal cycling of the rig, lock washers may keep the
problem from happpening as often.
Reference 1: Yaesu FT897
Technical Supplement (2002)
Reference 2:
http://www.mitsubishichips.com/Global/index.html
Reference 3:
http://www.datasheetcatalog.com/datasheets_pdf/2/S/K/2/2SK2596.shtml
Reference 4:
http://www.datasheetcatalog.com/datasheets_pdf/2/S/C/5/2SC5125.shtml
Note: The above information is provided in the spirit of Ham
Radio helpfulness. Should you choose to make the above
adjustments, you are fully responsible for the outcome and any
damage you may do to your equipment!
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