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BITX40: Delay of PTT, RX amp keeps active too long

 

Hi,

Experience a long PTT switch over time, resulting in a extended RX audio.

Ashhar mentioned this probably is caused by the large uF in the audio amp circuit in yesterdays Amateur Round table interview at W5KUB (interview is on youtube):

?



Anybody has a simple solution to resolve the RX audio to remain (<1 sec) when already in TX?

Ron - PA3FAT
 

A few possibilities:

Swap ?pins 3 and 5 of relay K2, add a 100k resistor from K2-1 to ground. ? The resistor slows down the falling edge there to avoid a pop in the speaker, the 100k should discharge the 1uF at C115 with an RC of about 1/10 second.

Reduce the size of the 470uF cap at C110. ?Perhaps 47uF is sufficient if you use headphones with the volume set fairly low.

Add an NFET or NPN transistor with an appropriate load resistor to quickly discharge C110 when the TX rail is high. ?If you have some spare IRF510's lying about to replace the final when it blows, add one in there with the gate tied to the TX 12v rail, the drain tied to the top of C110, and the source to ground through a 100 ohm resistor.

Haven't tried any of these, as I don't yet have a Bitx40.

Jerry, KE7ER

---In BITX20@..., <ronvandoremalen@...> wrote :
Experience a long PTT switch over time, resulting in a extended RX audio.
Ashhar mentioned this probably is caused by the large uF in the audio amp circuit ?
 

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Hi Jerry,

?

Assume RX is pulled to ground by K1 relay. Would it be a solution to discharge via diode/resistance combination just in front of D18. What¡¯s the actual use of D18 btw?

?

Ron ¨C PA3FAT

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Good question regarding why D18 is in there.

The diodes are needed on the bidi RF amps, as Ashhar notes in his circuit description:

? "The diodes in the collectors prevent the switched-off transistor's collector resistor (220 ohms) from loading the input of the other transistor."

If D18 were not there, then C110 will supply power to the rx side of those bidi amps for a second or so after you hit the PTT switch. ?With that rx side diode conducting we are back to the problem of the rx side 220 ohms loading the input of the tx amp.

You could use a relay instead of my fet or npn to discharge C110 ?to ground through a 100 ohm resistor.
And you could use that relay (or my fet or npn) to discharge the entire rx 12v rail as you suggest, though you would either have to remove D18 or put a resistor across it. ?Think about what happens if some sort of fault causes whatever npn/fet/relay to turn on while the rx rail is powered up (hence the 100 ohm resistor).

I only suggested the IRF510 because it is so monstrous and easy to hack with. ?Could just as well be a surface mount 2n3904, since the current involved is only 12v/100 = 120ma, and only happens for roughly 470uF*100ohms ?= 0.047 seconds. ?If using an npn there, you would want a resistor from base to the tx rail of around one to 10 kohms to limit the base current.

Jerry, KE7ER


---In BITX20@..., <ronvandoremalen@...> wrote :

Assume RX is pulled to ground by K1 relay. Would it be a solution to discharge via diode/resistance combination just in front of D18. What¡¯s the actual use of D18 btw?

?

Ron ¨C PA3FAT

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I doubt my previous suggestion of "Swap ?pins 3 and 5 of relay K2, add a 100k resistor from K2-1 to ground" would do anything but make the transition noisier.?

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