A LITTLE HISTORICAL BACKGROUND ABOUT THIS PROBLEM
897

More than a few owners of the FT857/897 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 are rolling off the assembly lines and into their boxes with the exciter and linear stages incorrectly adjusted. The error in adjustment is 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)

 Exciter

 


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)

PA Adjust


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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