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Vox circuit problem

 

Hi, all, I see there was some other thread about a vox problem, but no resolution posted. ?I’ve been tracing the schematic and reading the manual and taking measurements, but I’m having an issue that my vox is always on no matter what. ?This came on suddenly, one time I powered on the radio and it went straight into continuous tx and will not exit to rx. ?The big Rl-1 clicks and it goes straight to tx on power up every time.
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To use the radio, I found that I can disconnect pin 4 of Q604 on the vox card. ?This is the output of the SN 72560 level detector. ?With this disconnected, the radio receives as normal, and transmits on keying the mic.
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I replaced that Q604 chip, but still have the same problem.
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This is where I’m getting confused: if I measure the voltage to ground of the blue mic wire (it normally connects to pin 4 of Q604) without Q604 connected, it is at 13.8v on rx and 0v on tx. ?So it would appear that the 604 chip would need to hold that line at 13.8 or similar volts to not close the RL-1 relay and send the unit into tx mode. ?However, the supply voltage to Q604 is only 6 volts. ?So how does a 6 volt supply turn off the relay? ?With that chip connected, the unit goes straight to tx mode, and the voltage on the output of Q604 goes to about 3.5 volts.
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Monitoring the input to Q604, it drops when I speak in the mic and goes up again when quiet, so the mic side of the vox would seem fine.
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Wonder if anyone else has had issues like this, or can suggest a possible solution.
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Thanks in advance…
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Chaz, KC3ZPB
 

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Hi Chaz

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The problem is known to occur with the VOX-UNIT on a >40 year old device.

Q604 is only a comparator that switches the relay RL-1 (TX) when pin6 is LOW.

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This can have the following causes:

  1. the ANTI TRIP circuit has no effect on the comparator getting a LOW level on Q604/Pin6.
    It is rather the opposite if e.g. C620 (10uF) makes a short circuit.
  2. the VOX AMP circuit is more likely to be a fault candidate. For example, C613 (2.2uF) can leak,
    and pull the level to GND via R639 (1k) on Q604/Pin6.
  3. also keep an eye on the transistor Q603 (2SC373). If this has a short circuit C-E,
    then the level at Q604/Pin6 is pulled to GND.

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Good luck and best 73 de Roland, HB9GAA

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HB9GAA
Brunnhaldenstrasse 8
3510 Konolfingen
Switzerland

Tel: 41 (0)31 792 04 60
Mail: hb9gaa@...

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Von: [email protected] <[email protected]> Im Auftrag von Chaz Smith via groups.io
Gesendet: Samstag, 11. Januar 2025 03:18
An: [email protected]
Betreff: [yaesuft301d] Vox circuit problem

?

Hi, all, I see there was some other thread about a vox problem, but no resolution posted. ?I’ve been tracing the schematic and reading the manual and taking measurements, but I’m having an issue that my vox is always on no matter what. ?This came on suddenly, one time I powered on the radio and it went straight into continuous tx and will not exit to rx. ?The big Rl-1 clicks and it goes straight to tx on power up every time.

?

To use the radio, I found that I can disconnect pin 4 of Q604 on the vox card. ?This is the output of the SN 72560 level detector. ?With this disconnected, the radio receives as normal, and transmits on keying the mic.

?

I replaced that Q604 chip, but still have the same problem.

?

This is where I’m getting confused: if I measure the voltage to ground of the blue mic wire (it normally connects to pin 4 of Q604) without Q604 connected, it is at 13.8v on rx and 0v on tx. ?So it would appear that the 604 chip would need to hold that line at 13.8 or similar volts to not close the RL-1 relay and send the unit into tx mode. ?However, the supply voltage to Q604 is only 6 volts. ?So how does a 6 volt supply turn off the relay? ?With that chip connected, the unit goes straight to tx mode, and the voltage on the output of Q604 goes to about 3.5 volts.

?

Monitoring the input to Q604, it drops when I speak in the mic and goes up again when quiet, so the mic side of the vox would seem fine.

?

Wonder if anyone else has had issues like this, or can suggest a possible solution.

?

Thanks in advance…

?

Chaz, KC3ZPB

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