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QCX+ PTT output

 

I'm getting way ahead of myself because my QCX+ 40M kit doesn't arrive until tomorrow but with it having a PTT jack I am wondering if it will work with my Elecraft KPA500 amplifier.

The KPA500 requires:

The driving transmitter must ground the PA KEY line circuit during transmit while sinking up to 1 mA.
If the transmitter Key Output is not a contact closure or equivalent, it must be at a level between +4 and
+16 VDC during receive. (Compatible with all known transmitters. No key line buffer required.)

The QCX+ PTT provides:

The PTT output is at the tip of the 5V/PTT connector jack. This signal is 0V when the QCX+ is in
Receive mode, and +5V when the QCX+ is in Transmit mode.

Am I correct in that the QCX+ will key the KPA500 with no modification?? ?I think so.

73, Bill NZ0T
 

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Though it looks like it will work based on the signal level requirements, it would be wise to get a scrap of copper clad board and wire up an optocoupler ugly style for your PTT output.? That will protect the circuitry inside the QCX should any mishaps occur with the PTT cable and if you use a jack that is isolated from the chassis it also prevents a ground loop from forming between the amp and exciter when both cables are connected.


---------- Original Message ----------
From: nz0tham@...
Date: November 15, 2020 at 9:32 AM

I'm getting way ahead of myself because my QCX+ 40M kit doesn't arrive until tomorrow but with it having a PTT jack I am wondering if it will work with my Elecraft KPA500 amplifier.

 

Sounds complicated!
 

Bill

that advice in the last message is good - you have quite an investment in that KPA500.? i wrecked my original QCX troubleshooting its PA.? most important is probably the voltage of the TR interface in both key up and key down states - you can measure it before connecting to the PA to make sure it is below its maximum safe level.? that buffer suggested seems reasonable.?

I am also suspicious that big PA may need about 30 watts drive (?) to achieve a reasonable level of output?? check that, or tickle it with your normal exciter with about 3 or 4 watts to see how much RF comes out.? this may save you a lot of trouble by first seeing if the QCX has enough RF output to exercise it.?

perhaps enjoy the nice QCX with its QRP output - then decide if you 'need' any more power.? its the price of a cup of coffee for the QCX PA versus a mortgage payment for that 500 watt box.? [yes the fear or complex toroids may be a square - but winding the T1 in the QCX qualifies you to address the transformers in that little PA!]?

enjoy your radiations whatever devices exit the RF into the ether.?

73 Curt
 

5 watts from my K3 drives the KPA500 to 90 watts on 40 meters.? Does anyone have info on how to build the opto interface?? I'm not savvy enough tech wise to design one myself.
 

>> Am I correct in that the QCX+ will key the KPA500 with no modification?

No.?? The KPA500 requires a low to key it and the QCX provides a high signal during transmit. You need an inverter.
You can use a BS170 to key the amp.? It will easily sink 1 ma.
Connect the key line of the QCX to the gate, connect the drain to the KPA500 key input and connect the source to? ground.

I don't see how an opto isolator will help, you will need to connect the grounds of the QCX and KPA500 together which is going to happen when you connect the coax between them.? They won't be isolated.

Ron
 

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It's not complicated.? An optocoupler has an LED on the input side and a light activated transistor or FET on the output side.? They are electrically isolated and the transistor is turned on solely by the light from the internal LED.?Some typical optocouple candidates are (there are many options) TIL-111, CNY17 etc.? Looking at the QCX+ schematic the PTT line comes from the IC3D pin 11. If you connect to that signal at JP22 run a wire to the cathode of the LED in the optocoupler and then you want to limit the current through the LED so you need a series resistor.? The LED is going to take about 1.6v check the datasheet from you optocoupler for Vf and also not the current they show under the test conditions but it will usually be around 10 to 20mA.? Let's assume 10mA for this discussion.? If you run the resistor from the anode of the LED to +5v to get the needed resistor value it's only ohm's law.? Subtract the Vf of the diode and you've got 5v-1.6v = 3.4v on the resistor.? So for 10 mA you would need 3.4v / 0.01A = 340 ohms.? Use 330 ohm standard value and call it good.? The output side of your optocoupler if it is a bipolar transistor use the collector to the PTT on your amp and the emitter to the GND on your amp.? If FET output it doesn't really matter but put drain to PTT and source to GND and you are done.? If you have the experimenter's mezanine board in your QCX+ it would be an ideal place to put the optocoupler.? One note, if you add a RCA jack on the QCX+ for this purpose get the kind where the outer shell is not connecting the chassis. there are some jacks that have a plastic mounting and the shell is isolated otherwise you could drill a larger hole to clear the connector, mount the connector on a scrap of plastic and rivet the plastic to the back of the chassis so the RCA jack protrudes through the hole which will keep both of the output side connections of the optocoupler isolated from the QCX+ chassis.?? Does this make sense?.....Joe


---------- Original Message ----------
From: nz0tham@...
Date: November 15, 2020 at 11:17 AM

Sounds complicated!


?
 

Hi Bill,
you need to invert that signal from the QCX+ to key the KPA500
Like most linears, the KPA500 needs 0V to put it into TX, and the QCX+ puts out 5V for TX, so just the opposite.
So you need to insert either an optocoupler as suggested (see drawing)
or a transistor (e.g. 2N2222) with 10k resistor to its base would also work, but then there is no galvanic separation of the QCX and the amplifier.

73,
Luc ON7DQ
 

Ron,

Thanks.? So tip of the QCX+ plug (into PTT socket) to gate.? Source to the ground side of the cable and center of the RCA plug to the KPA500 to drain?? Sounds easy enough to make up a cable like that.

I don't want to endanger the QCX+ of course and the last thing I want to so is endanger my $2K+ KPA500.? I must say I'm curious as to why the QCX+ PTT line would not have been designed to be compatible with the accepted norm for HF amplifiers.? To be clear I am NOT being critical of the design because Lord knows I am knowledgeable enough to be so but I'm just curious.

73, Bill NZ0T
 

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Oh sorry I steered you wrong there, I looked at the schematic, saw the NAND gate and thought active low.? Then I saw the comment from Ron Carr.? Still, you can do it, no problem just reverse the connections to the LED ie from JP22 to ANODE and then CATHODE through the serial resistor and to ground.?

---------- Original Message ----------
From: VE3VXO <ve3vxo@...>
Date: November 15, 2020 at 12:30 PM

It's not complicated.? An optocoupler has an LED on the input side and a light activated transistor or FET on the output side.? They are electrically isolated and the transistor is turned on solely by the light from the internal LED.?Some typical optocouple candidates are (there are many options) TIL-111, CNY17 etc.? Looking at the QCX+ schematic the PTT line comes from the IC3D pin 11. If you connect to that signal at JP22 run a wire to the cathode of the LED in the optocoupler and then you want to limit the current through the LED so you need a series resistor.? The LED is going to take about 1.6v check the datasheet from you optocoupler for Vf and also not the current they show under the test conditions but it will usually be around 10 to 20mA.? Let's assume 10mA for this discussion.? If you run the resistor from the anode of the LED to +5v to get the needed resistor value it's only ohm's law.? Subtract the Vf of the diode and you've got 5v-1.6v = 3.4v on the resistor.? So for 10 mA you would need 3.4v / 0.01A = 340 ohms.? Use 330 ohm standard value and call it good.? The output side of your optocoupler if it is a bipolar transistor use the collector to the PTT on your amp and the emitter to the GND on your amp.? If FET output it doesn't really matter but put drain to PTT and source to GND and you are done.? If you have the experimenter's mezanine board in your QCX+ it would be an ideal place to put the optocoupler.? One note, if you add a RCA jack on the QCX+ for this purpose get the kind where the outer shell is not connecting the chassis. there are some jacks that have a plastic mounting and the shell is isolated otherwise you could drill a larger hole to clear the connector, mount the connector on a scrap of plastic and rivet the plastic to the back of the chassis so the RCA jack protrudes through the hole which will keep both of the output side connections of the optocoupler isolated from the QCX+ chassis.?? Does this make sense?.....Joe


---------- Original Message ----------
From: nz0tham@...
Date: November 15, 2020 at 11:17 AM

Sounds complicated!


?


?


?
 

Oops yes, now read the other comments fully,
an optocoupler will not help separating the grounds ... sorry, I was not thinking , hi
Maybe it will avoid a ground loop ...

and indeed, 5W input will not get you the full 500W out

Luc ON7DQ
 

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Hi Ron

If you connect the ground on the coax shield (I think you must) and the ground on the PTT you have created a ground loop which might inject local noise into your receiver. Ideally you only want one ground connection between the two chassis.? This works if the QCX is running from batteries but if another power source is used there may be a larger ground loop created through the common power system....Joe

---------- Original Message ----------
From: Ron Carr <rcarr@...>
Date: November 15, 2020 at 12:26 PM

>> Am I correct in that the QCX+ will key the KPA500 with no modification?

No.?? The KPA500 requires a low to key it and the QCX provides a high signal during transmit. You need an inverter.
You can use a BS170 to key the amp.? It will easily sink 1 ma.
Connect the key line of the QCX to the gate, connect the drain to the KPA500 key input and connect the source to? ground.

I don't see how an opto isolator will help, you will need to connect the grounds of the QCX and KPA500 together which is going to happen when you connect the coax between them.? They won't be isolated.

Ron


?
 

Hi Bill

I will add a configurable milliseconds delay, defaulting to zero, for starting the RF after key-down. That should account for people using amplifiers with slow relay switches.?

Active-low PTT could be taken from IC3 line 12, if it isn't going to sink a lot of current. Remember the QCX+ is designed for easy modification so you could easily cut a track at the PTT connector and add a jumper wire.?

A BS170 would work fine as an inverter too and an optical isolator is neither desirable nor necessary.?

As to why QCX+ PTT was designed this way rather than the more standard way - this was perhaps an error or oversight on my part. The 50W PA was designed before the QCX+, and was designed for the QCX. It wouldn't have been difficult to make it take the signal at pin 12. But I probably just didn't think of it that way. My bad. But hard to change now :-/

73 Hans G0UPL?
?
 

Thanks to all but I'm confused enough now that I think I'll just stay away from trying to use the KPA00 with the QCX.? I'm good at following directions and I have successfully built a lot of kits including? an Elecraft K2/100 with most of the accessory modules but I am not good at all with figuring out circuits and modifying them.? Maybe I'll build the 50 watt amp sometime.? Meanwhile I have plenty of QRO HF rigs here that I can play with.

73, Bill NZ0T
 

Bill

Sounds like there's a path. I think the last step is consulting elecraft tech support on their end of the interface. Glad you are enthused with the upcoming build.

Curious if you have used your K2/100 to drive the kpa500, now that I realize it needs this keying interface? Much of my cw operating is withbthe k2 or one the qrp rigs.

Curt
 

I no longer have the K2 and did not have the KPA500 when I had it. ?My K3 uses the Elecraft cable to interface with the KPA but my Icom IC-7300 only needs a simple 2 conductor cable with RCA plugs on each end to run the amp.
 

Edit: fixing my mistake...
?
Apologies for necroing this post, but it didn't feel confirmed. So wanted to make the instructions clearer and add in the 100W amp to make it easier for people to find in a search.
?
Just breadboarded this with my Elecraft KXPA100 amp and it works as expected.?
?
QCX-mini PTT PTT/ring -> BS170 gate
QCX-mini PTT ground/sleeve ->? BS170 source
BS170 drain -> KXPA100 RCA positive
?
?
Thanks,
Andre, WT9X

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