K6JRF's Page
formerly W6FZC
FT1000D VOX Board Page
(Updated: Oct 15, 2001)

This VOX Page details the required modifications needed to the FT1000D Switch A Board make it more responsive to the 'male' voice and less susceptible to false triggering and lock up. Note 6 details all of this.

NOTE 6

This note details changes to the FT1000 VOX circuitry resulting in improved VOX operation and much less false triggering/lockup. The note includes the effects of both the VOX and ANTIVOX ckts in detail.

Problem:
I hate VOX and you probably do too! Maybe it's because it always false triggers when you drop a pencil on the desk or open a coke can or make any sort of high frequency noise, including background noise from the speaker! You get the idea. But it also doesn't fire when you have the typical male voice, or if it does, as your volumes drops, so does the vox in the middle of a word. Very frustrating!!! We've all been there. The FT1000 is no better and maybe worse than any other radio.

Analysis:
You can best follow this if you have the schematic from the FT1000 SERVICE MANUAL from page 7-131. Attached to this msg is a ckt simulation of the FT1000 VOX amplifier. It's located on the SWITCH (SW) A UNIT BOARD located under the top of the lift-out panel on the radio. It houses the vox GAIN, DELAY and ANTIVOX adjusting pots. Analysis of this ckt shows that the VOX and ANTIVOX peak at around 1Khz, just where the male voice doesn't and where the 'pop' from opening a soda can does!! That's why it false triggers and also drops out easily.

Referring to the attached file, FTVOX2.GIF, you see the VOX and ANTIVOX BEFORE frequency response. Note that it peaks around 1Khz for both ckts. By changing two (2) capacitor values, the ckt appears as in FTVOX3.GIF. The schematic is included in the file, FTVOX1.GIF showing the first stage of the VOX and ANTIVOX amplifier. Note the use of two (2) 0.047u caps in series. Replacing one with a 'short', adding 0.22uf in parallel to the other and rolling off the amplifier with a 220pf cap shifts the response down to the region inhabited by most male voices. Note that these charts are in LINEAR not LOG plots.

It works like a charm! You also can now reduce the vox DELAY control because it now responds instantly to your voice, so a long delay time is not needed to maintain transmitter action! If a nearby S9 +20db signal occurs, your VOX will not lock up!!

Vox Board Parts Placement

Instructions:
1) Remove top cover of unit exposing the SW A UNIT Board. Remove the four (4) screws holding the board down. Lift the board up, carefully undressing the cables, to expose the bottom of the board.
2) There you find C8319 (0.047u), tacked from the J8305 connector and C8318 (22p) mounted across R8325 (3.3Meg). Remove C8319 and replace with a short jumper wire. Replace C8318 with 220p cap. The 220p cap should be a ceramic low voltage such as Radio Shack 272-124.
3) Next solder a 0.22uf cap across C8311 existing cap (0.047u) to increase the value to 0.267uf. I used monolithic ceramics which were in the junk box. A Radio Shack part number is 272-1070, a 0.22uf PC Mount cap. Any cap that's small will do. This changes the VOX portion of the circuit.
4) Replace the board and the four (4) screws. Next adjust the GAIN, ANTIVOX and DELAY levels to suit your taste and you're all set for great vox operation.

Vox Board Backside

COMMENTS:
This change does not require 'hifi' audio changes as described in my previous reports (1 through 9). However, if there are installed, you'll find very, very smooth vox operation since the radio's response covers the 100hz to 3Khz range in a very flat manner. Thus it will respond uniformly. If the FT1000 is 'STOCK', which implies a 300hz to 2.5Khz bw, it still works quite nicely since the -3db response is about 500hz.

VOX Mod Results
The results of the capacitor changes are shown here. Note that un-soldering is required to do this enhancement.

PSpice Simulation Schematic

Due to a problem with the op amps, my original pcb was replaced with a new board from Yaesu. The problem manifested itself as gradual increase in the input bias current that caused an excessive drift problem. As it got worse, the VOX would lock up when first triggered. This, in turn, would keep the transmitter on until the VOX pushbutton was de-selected.

After trying to fix it by changing the bias resistors and op amps, the board finally became too unstable, so it was replaced which fixed the problem permanently! While replacing the VOX board, I found a way to improve and simplify it!

FT1000D Stock Radio Vox Operation

FT1000D Mod Radio Vox Operation

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