K6JRF's Page
formerly W6FZC
ESSB Audio Techniques Page
(Updated: Jan 11, 2010)

This section analyzes some popular "myths" about the FT-2000 to find out if they are true. By the use of "Spice" circuit analysis, these will be shown to be valid or just myths!

I also switched from MSim's "PSpice" to a newer, more compact analysis program called "5Spice" written by Andresen Software. It features almost intuitive operation, has many powerful features and allows the import of various Spice models so that the ckt can be simulated using the actual real world parts. Click here to see more information on this fine circuit analysis product.

50hz Tx Audio via Rear Panel Only and Mic Gain Control

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Feed the audio into the back AF input (Patch) to get 50hz Tx response If you take the time to checkout the schematic tracing the connection from the front panel "mic" input and rear panel "patch" input, you will find that the mic input goes through a "preamp" stage while the "patch" input is directly summed into the 2nd stage opamp. Based on that, you'd conclude that the "patch" input is 'better' b/c the time constant of the resistor/capacitor combination is larger so it should give less rolloff than the Mic Input. Also the Mic gain adjustment is too sensitive indicating that there's too much gain in the preamp stage. The 5Spice results are shown in the sections below. Checkout the charts the Tx sweep simulation showing each ckt's low frequency response.

FT-2000 5Spice Mic and Patch Analysis
The picture here shows a section of the actual FT-2000 Main Board schematic centering on the Mic and Patch input ckt. The yellow line serves to highlight the main signal paths in the ckt.
FT2000 Mic & Patch Ckts from Service Manual

The picture below shows the "capture" of the ckt values in a format that is acceptable for 5Spice. The analysis will accurately show the ckt's response as a function of 'frequency'. Here the front panel input ("Mic" = V1) and the rear panel input ("Patch" = V7) are shown as the ckt exists in the current FT-2000. The reference designators (Ref Des) have been preserved and are as shown in the schematic if you wish to verify.

Mic & Patch Ckt captured for 5Spice Simulation

FT-2000 5Spice Mic & Patch Ckt Analysis
5Spice Results: Mic Input
The output of the "Mic Input" is shown on the RIGHT and below that, the "Patch Input".
For the MIC IN, the -3dB point is 56Hz. For the PATCH IN, the -3dB point is below 3hz. A large difference, indeed! The two inputs can be seen to have a large difference in the resistor/capacitor time-constants.
For the PATCH IN, it's determined by R1149 and C1194, very straightforward. For the MIC IN, it's a combination of C1222, R1184, C1207 plus the transistor's (Q1026) emitter bypass capacitor, C1204 and series resistor, R1170.

5Spice Results: Patch Input The inputs are combined ("summed") together in the opamp (U1102). The 'faster' time constant of the "patch" input is separate from the MIC IN ckt and they do act separately. So the PATCH INPUT does have better frequency response. For more Tx BW, use the back panel, PATCH INPUT. You will achieve lower bass response but you will NOT get any improvement in the hi-frequency cutoff of 3Khz. This is determined by the DSP settings.
Mic Input Ckt BW Improvement:
5Spice Ckt Results So now that we know the MIC INPUT BW is 56hz, we need to improve so that it passes 50hz (lowest typical male voice) but not too low so that is causes VOX false pickup. It does not need to have the same BW as the PATCH INPUT. And only one (1) component needs to be changed to make this happen. The following analysis shows the ckt parts that need changing.

Running 5Spice again but now looking for the minimum parts change that will effect the largest bw, the following chart shows what's needed. As you see, changing one (1) c-cap, C1222 will materially lower the frequency response of the Mic Input to 40hz. Please note that you do NOT have to change the c-resistor, R1177 to get the improved audio bw when using the front panel mic connector.

Tx Audio Gain Change:
While you're "inside" the radio, I suggest that you change the value of the Mic Preamp gain resistor, R1177 from 2.2K to 1K. As you know, there's too much gain and usually the "Mic" gain is set to 8 o'clock. Changing the resistor will cut the gain in half and allow you more adjustment range for any microphone. An easy and worthwhile change.

Main Board Access:
The following is a brief explanation of the required steps. If you have the prerequiste skills and the tools to complete this project, then no further explanation is required. Your de-soldering skills should be just as proficient as your soldering skills since you will have to remove some of the c-caps and then re-solder new values. If you're not proficient, then get somebody to do this for you.

FT2000 Mic & Patch Parts Layout

The Mic and Patch circuitry can be found in the Main Unit schematic in area C2.
1) Remove the TOP and BOTTOM covers of the FT2000.
2) Turn the radio over so that the back side (I/O connectors) are facing you.
3) Then remove the bottom "shield" cover to gain acess to the Main Unit Board.
4) Next, disconnect the cable harness that passes directly connected to J1025 so you have access to the c-caps around Q1026.
5) Using the schematic guide, remove the existing c-resistor, R1177 and replace with the indicated value.
6) Reinstall the connectors from the removed harness.
7) When complete, reinstall the shield cover first, then the BOTTOM and TOP covers in that order.

Detailed Chip Cap Info:
All of the c-caps and c-resistors are mounted on the top side of the PCB so you can access them without having to remove the Main Unit Board. The following parts are available from Yaesu/Vertex Parts Department at 714-827-7600.

Ref Des ----- Value ---------- Part Number ---------- Yaesu PN ------ Price/Ea
C1222 --------- 4.7uf ------------- TEMSVA1A475M-8R --------- K78100022 ---------- $0.60
R1177 --------- 1k ----------------- RMC1/16 102JATP ----------- J24185102 ----------- $0.02

The Mic and Patch Ckt changes dramatically open the Mic Input TX audio path which will give you more room for low-bass experimentation. My personal comment is that there's really NO AUDIO information below 40hz other than "rumble" but you are now not limited by the radio's hardware. Audio at 50hz and above will come through w/o loss.

No other mods need be done prior to this modification.

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