FT1000 Improved Rx Bandwidth Page
This Rx Page discusses the changes to the AF UNIT BOARD (AFUB) that increase the
Receive (Rx) 3dB bandwidth to better than 5Khz on the high side and to 45hz on the low side by replacement of three
Where did the bandwidth go?
For some time, I have pondered this question. After changing the two filters as described in FT1000D Mod Menu (1) and (2) and Audio Techniques Menu (1), the flat portion of the band still remained constant. Looking at the chart below, you see that it starts to roll off at 1.5Khz and at 5Khz, it's down at least 12dB or more!
So the hunt began to find out just where this happens. Although I've looked before 'casually' at the circuitry on the AFUB, I didn't really pay that much attention. The other night, I looked at it a little 'harder' and the lights went on.
Referring the PSpice schematic above, we see where the 'culprits' (boxed in RED) are. But first, here's the theory of operation for the circuit. The detailed schematic can be found in the FT1000 Service Manual on page 7-43 (AF Unit Schematic) in the upper left quadrant.
The input from the IF Unit Board (Rx In) passes through an all pass network into the emitter follower buffer transistor, Q1 (marked Q3014 in the FT1000 Service Manual). The low impedance output is AC coupled into an shelving low pass filter consisting of U1 (actually Q3016-1) and associated components R3052, R3053 (each 15K), and capacitors C3063 (0.0068u) and C3064 (0.0033u). The filter has a -3dB cutoff frequency of 1.2Khz in a stock radio; 6.6Khz after the modification.
The output of the filter is AC coupled into a controlled gain amplifier whose low frequency response is set by the value of C3082 (22uf). The cutoff is 98hz in a stock radio, and 45hz after the mod of cap C3082. The output of this circuit is AC coupled into an amplifier IC (not shown) for further amplification prior going out to drive the speaker and/or 'PHONES' output jack.
Q3016-1 is active when either LSB or USB is selected from the front panel. So in summary, R3052, R3053 and C3082 form a BANDPASS filter whose characteristics show as the GREEN chart below with a 'stock' FT1000D. Note that if this change is installed into a 'stock' FT1000, the results will be barely noticeable because of the 2.5Khz cutoff frequency of the FT1000 Yaesu filters.
The BAD part of this change is the fact that the two (2) resistors, R3052, R3053 are CHIP style and are located to the underside of the AFUB. The capacitor is a standard radial style unit and can be easily de-soldered and replaced. The parts do not have Radio Shack equivalents. A 100uf, 16V electrolytic capacitor and two 2.0K ohm, 1/8w film resistors will work just fine for this mod. They are probably in your junk box! You should be proficient at un-soldering as well as soldering to do this change. If not, please seek competent help.
Remove the two chip resistors (R3052, R3053) and replace with 2.0K ohm, 1/8w resistors as shown below. Solder to the points indicated by red circles. One end of resistor R3052 solders to the foil as shown after scraping the photo resist away. Also one end of each resistor is commoned together at the pad of R3052 as shown. Make sure to use insulation on the leads of each resistor to prevent shorting to adjacent pins. There is not too much room here.
Rx Mod ResultsTo say the results of changing three (3) parts were spectacular is a bit of an understatement!!! The chart clearly shows the before AQUA and after RED and BLUE traces. For the WB filters in the '2.0K' position, the response is still basically flat at 5Khz! Too bad the cascaded pair were not a little wider! For the '2.4K' position filters, it 'ruler' flat to 3.1Khz.
'2.0K' filters: InRad #710 (6Khz @ 8.215Mhz) and InRad #707 (CMF 6Khz @ 455Khz)
Note that the '45hz' low side response is not practically attainable since the TCP adjustment has set the low frequency slope of the filter to around 105hz or so. Thus even though the RX Mod has enabled the circuit to pass low frequencies, the filter's cutoff and TCP adjustment [see Menu Item (2)] will not pass those frequencies.
After the circuit values are changed the circuit's new response is shown in the RED and BLUE traces. The cutoff frequencies for a stock and modified circuit are summarized in the table for the WB filters ['2.0K'] only:
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