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QCX PTT Mod


Hans Summers
 

Hi Paul

Connect the ground anywhere convenient, it won't make much difference... yes IC3 pin 7 would be fine. 

73 Hans G0UPL
http://qrp-labs.com

On Tue, Sep 15, 2020 at 9:18 PM w2eck <eckerpw@...> wrote:
One more question, before I proceed. When connecting the PTT cable to the QCX, it says:
" At the other end of the cable, connect the ground signal to QCX ground, and the “ring” PTT to IC3 pin 11 of the QCX."
-- So is QCX Ground" the  pin 7 on IC3 or the Gnd connection on the pwr plug or WHERE should this be connected?? :0)

73 Paul


w2eck
 

One more question, before I proceed. When connecting the PTT cable to the QCX, it says:
" At the other end of the cable, connect the ground signal to QCX ground, and the “ring” PTT to IC3 pin 11 of the QCX."
-- So is QCX Ground" the  pin 7 on IC3 or the Gnd connection on the pwr plug or WHERE should this be connected?? :0)

73 Paul


Hans Summers
 

Hi Paul
 
In the paper Hans put out on the PTT mod to the QCX for use with the 50W PA, he describes a hardware mod
 by adding a 10K resistor (value entirely uncritical) between the RX signal and +5V. My question is this:
- my junk box has 100, 100k and 1k resistors.
 - so Hans says value uncritical- so would any of these I have work in this application?

Even the mod itself is not particularly critical. I did find it prevented a particular glitch startup condition with a particular power supply, which once in 6 months of operation, caused me to kill the IRF510s. 

What happens is that on power-up, the microcontroller resets itself. Then starts the program. Early on in the program, the processor pin 14 is set to high, which puts the QCX into RECEIVE mode. But until this happens, pin 14 is high impedance and IC3 pins 12 and 13 are left floating; the only things on this signal are IC3 pins 12 and 13, and the gate of Q5 (Tx/Rx switch) and C34, 0.1uF to ground. Regrettably C34 will cause the voltage to be - momentarily at least - pulled to ground; thereby putting QCX into TX state and activating the PTT. 

To avoid this, a pull-up resistor from pins 12 and 13 to +5V keeps everything in a determined state (Receive) during this short interval. Ok so it still has to charge up the 0.1uF capacitor. I chose 10K. 1K would be fine too. If you go too low, then during TX the microcontroller I/O pin will have to work too hard to pull current through it, which could exceed the rating of the I/O pin. 1K will draw 5mA which doesn't harm anything but is a bit of a waste. 10K, 0.5mA. If the resistor is too high, then it will be slow to charge up the 0.1uF capacitor and we will be back to a momentary activation of PTT. 

If your junkbox only has 1K and 100K resistors then 

1) Please stock up on some more junk ;-)

2) I'd tend to use the 1K resistor, accepting the 5mA higher current draw on TX, which won't be noticeable, in preference to the unknown quantity of any risk of lengthening the momentary PTT pulse.

73 Hans G0UPL
http://qrp-labs.com


Shane Justice
 

Paul,

I wonder what value Hans meant?- Value could also point to the wattage capacity of the resistor, or the tolerance. I doubt he would specify a resistance value and then comment it doesn't matter. He may have meant any value in the ranger of 100k- say 90k to 120k.

Probably best for Hans to say what value was noncritical, and provide some additional detail.

Best regards,
Shane


On Sep 15, 2020 at 07:46, w2eck <eckerpw@...> wrote:

In the paper Hans put out on the PTT mod to the QCX for use with the 50W PA, he describes a hardware mod
 by adding a 10K resistor (value entirely uncritical) between the RX signal and +5V. My question is this:
- my junk box has 100, 100k and 1k resistors.
 - so Hans says value uncritical- so would any of these I have work in this application?

73 Paul
w2eck


w2eck
 

In the paper Hans put out on the PTT mod to the QCX for use with the 50W PA, he describes a hardware mod
 by adding a 10K resistor (value entirely uncritical) between the RX signal and +5V. My question is this:
- my junk box has 100, 100k and 1k resistors.
 - so Hans says value uncritical- so would any of these I have work in this application?

73 Paul
w2eck