K6JRF's Page formerly W6FZC 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
(3) components. |
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! 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.
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.
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Rx Mod Results To 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.
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