Problem connecting to external PA

alq23
 

Posts: 4
Joined: 23 May 2022, 10:13

Problem connecting to external PA

Post by alq23 »

Hello all,

I tried connecting my (MX-P50M) 50W power amplifier PTT line to the trusdx PA jack, but when the connector is plugged in the amplifiers red TX light turns on and stays on, the light turns on even as the connector touches the jack.

Does anyone have a similar PA that could help me troubleshoot?
Is there something wrong with either of my devices or does the trusdx simply not support this PA?

Thanks in advance for any feedback.
DL2MAN
 

Posts: 706
Joined: 30 Dec 2021, 19:18
Contact:

Re: Problem connecting to external PA

Post by DL2MAN »

Hi,

2 Possibilities:

Either your cable is wrong, or the PA needs active high, while (tr)uSDX gives active low.

Image

73 Manuel; DL2MAN
DL1GKC
 

Posts: 6
Joined: 28 Apr 2022, 07:52

Re: Problem connecting to external PA

Post by DL1GKC »

A87F7C0C-1CA4-439A-B74B-3F8B7ADE7D7B.png
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I can confirm the behavior. My MX-P50M plug is configured to work with the KX-2. Basically it switches the PTT when the black and red lines are shortened. If the plug is inserted in the tr(u)SDX, it immediately switches to TX.

For the tr(u)SDX a different connector may be used if you look at the scheme and compare it with the KX2/KX3 connector:
48AD2D52-97CA-49E6-8517-A863A72BA67B.png
48AD2D52-97CA-49E6-8517-A863A72BA67B.png (237.41 KiB) Viewed 5531 times
alq23
 

Posts: 4
Joined: 23 May 2022, 10:13

Re: Problem connecting to external PA

Post by alq23 »

Hello,

Thank you Manuel and DL1GKC for the feedback. Very helpful and much appreciated.

Have a good one!
wa2t
 

Posts: 180
Joined: 01 Jan 2022, 16:30

Re: Problem connecting to external PA

Post by wa2t »

FWIW, the PTT interface does work, I have connected the (tr)uSDX to my RF2K+ amplifier and it works fine.

73,

Robert, WA2T
noreply
 

Posts: 10
Joined: 14 Apr 2022, 14:01

Re: Problem connecting to external PA

Post by noreply »

Hello,

@alq23 – I can confirm that I have same problem with my (tr)uSDX and my MX-P50M

The amplifier needs the PTT red “wire” go to GND – so in effect if you “short” the PTT wires from the amplified – it will switch the PTT relay.

I have checked the behavior of the PA output on the (tr)uSDX

On the 3.5mm connector the “tip” when the radio is keyed will go LOW.

I checked this “pin” with meter and it does go to GND potential with respect to the radio when radio is keyed.

The GND potential is same as the MX-P50M GND – so in theory when the “tip” of the 3.5mm plug is connected to the amplifiers PTT “red” wire – it should activate the relay.

Unfortunately, this does not happen – DESPITE having the correct wiring on the connector(s).

I can only assume that the PA output from the (tr)uSDX is not true GND as it goes “through” the ATMega and most likely there is a resistive load inside – albeit very low.

As it stands the (tr)SDX PA – despite correct connector wiring is not compatible with the MX-P50M PTT keying.

Unfortunately, some signal conditioning must happen for this to work properly.

Perhaps Manuel could elaborate as to how the ATMega handles the PA signal within its internal circuitry – as I could not get this info from the datasheet (could have missed this if indeed in the datasheet).

Cheers
John
 

Posts: 49
Joined: 19 Feb 2022, 08:56

Re: Problem connecting to external PA

Post by John »

"Unfortunately, this does not happen – DESPITE having the correct wiring on the connector(s)."

The question is, "What is the correct wiring on the connector?" All the cable for your amplifier's transmit function needs to do is short together. Polarity doesn't matter. It will work either way. Used with the (tr)uSDX it does matter. So does the sleeve on your cable's connector go to circuit ground? Or does the tip? It looks like for the (tr)uSDX to enable your amp properly the sleeve on your cable needs to be the connection attached to ground.

Also, (after the cable wiring has been verified,) all the current the I/O pins of the microcontroller (the Power Amp control line,) can deliver is 40 mA, as per the datasheet. It is a logic control line, not a high-power switching line, while that connector on your amp is simultaneously driving the coils on 2 relays (K1, K2,) whose coil's minimum turn-on current demand is not specified. So if the Xmit relays draw more than 40 ma to turn on, then you will need to add buffer transistors. I don't think, (at least I haven't found one yet,) that there is a software function to reverse the logic on the external amp function.

But the first thing you need to do is to find out why the tip on your cable appears to act as a ground. My hunch is that it is doing so because it is wired to ground, and the sleeve wire is floating until switched to ground.
noreply
 

Posts: 10
Joined: 14 Apr 2022, 14:01

Re: Problem connecting to external PA

Post by noreply »

Thanks for your comments – yes you have some very valid points.

In theory the correct wiring for the MX-P50M amplifier is simply to take the "RED" PTT wire to GND to engage the relay.

But yes, in the MX-P50M this will have a considerable amount of current flowing through the “RED” wire – and any other device like the PA connection on the (tr)uSDX.

There is a modification you can make on the MX-P50M – as you suggested a buffer transistor switch – this has been published by KB9RLW – copy of which I attach below for those of you that have not seen it before.

I have made this mod – but despite the buffer transistor – which will reduce the current and offer some back EMF protection with the diode – it will still not operate with the (tr)uSDX – most likely because of the way the ATmega is handling the “GND” connection in its PA output.

I suspect that a similar transistor “buffer” should be applied to the (tr)uSDX’s PA output to “switch” any external load to GND.

I am waiting for some components – to implement this external buffer for the (tr)uSDX myself.

If anyone reading this – who has a better “component box” in their shack than me (my is empty at the moment) – they could try this themselves and report back here?

Cheers
Attachments
amp_mod.jpg
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John
 

Posts: 49
Joined: 19 Feb 2022, 08:56

Re: Problem connecting to external PA

Post by John »

You have said it "will not operate" with the mod, but you didn't mention what was actually happening. Does the amp do nothing when you press TRANSMIT? Does it key the transmitter the moment you push in the plug? Have you observed the voltage on the (tr)uSDX transmitter keying signal going from high to low when you key the transmitter?

I researched and found that the relays require almost exactly the maximum amount of current that the (tr)uSDX transmit control signal is capable of providing, about 40 ma. It 'might' work with just the micro-controller's current sinking ability, at least for a while. But that's not something a wise engineer would do. So no matter what the wiring situation is, it's clear that a buffer is necessary in order to properly drive amplifier's relays.

The circuit you showed looks appropriate to me, but only if the micro-controller's amplifier control signal is configured as an open collector output. If the micro-controller's output is not configurable as anything but a totem-pole output then when the micro-controller's output is high it will only reach roughly the voltage that it is being powered from, and that is far too low to keep that 2N3906 turned off, and thus the transistor will always be turned on, at least partly. So, while I'm no electrical engineer, I think a different circuit is required for the interface if the micro-controller does not have a totem-pole amplifier control signal output.
noreply
 

Posts: 10
Joined: 14 Apr 2022, 14:01

Re: Problem connecting to external PA

Post by noreply »

Hi John,

No, I did not take some hard readings since don’t have any “tools” with me – currently not at home QTH.

I agree with your comments – the (tr)uSDX needs a circuit which will take the amplifier PTT “RED” line to GND

I was thinking of using a TLX9304 – this is an opto coupled open collector device – I thought since the (tr)uSDX’s PA output is directly connected to the ATmega – its best to fully isolate this to prevent possible RF getting back to the ATmega.

This device is available as a SO6 package – so could be easily fitted to end of interface cable inside a suitable 3.5mm connector housing and some additional heat shrink if required.

I am getting some components soon – so hope to rig something together on breadboard first – to fine tune a successful operation – then make up a suitable cable with the circuitry inside.

I’m certain something can be “rigged” to operate successfully and safely.

Will post my solution when I finish

Thanks for your input, John

Cheers
noreply
 

Posts: 10
Joined: 14 Apr 2022, 14:01

Re: Problem connecting to external PA

Post by noreply »

Hello,

OK, finally had some time to do further testing and here are some observations; -

The output for the PA (as seen from schematic) is directly connected to the ATmega

The default PA output is at 5V (ATmega output high)

When PA is active (TX enabled) the PA output is 0V (ATmega output low)

There is approx. 60ma flowing in the PA output line when the PA is in default “off” mode

There is approx. 0ma flowing in the PA output line when the PA is active “on” mode.

This behavior will never work with the MX-P50M and the modification “TX buffer” circuit – because it will be always in the ON state despite the state of the PA output

For the PA function of the (tr)uSDX to work with the MX-P50M (and possibly other amplifiers which have similar problem) the LOGIC of the PA functions should be reversed; -

Default state (off) output should be 0V

Active (on) state should be 5V

Now if we use an optocoupler (4N25 – but others could be used) – then connecting PA output to pin 1 (anode) and GND of the PA to pin 2 (cathode) – and the MX-P50M PTT line to the respective pins of the collector and emitter of the 4N25 – not only will the optocoupler provide additional isolation from the direct connection to the ATmega output pin but will switch the amplifier’s PTT line correctly.

4N25.jpg
4N25.jpg (149.17 KiB) Viewed 5264 times

For this to function properly – the LOGIC of the PA must be reversed.

If not, then when PA is OFF the PTT on the amplifier will be ON until the PA is ON and the amplifier will be OFF

If the LOGIC function of the PA on the (tr)uSDX could be reversed in the firmware – or an option for it to be enabled either way (how about it, if possible, Guido? ;) ) – then this will be a great fix for everyone and a much safer CCT for the PA output by using an optocoupler.

Cheers
N6ALT
 

Posts: 21
Joined: 02 Jan 2022, 20:02

Re: Problem connecting to external PA

Post by N6ALT »

I have to agree with noreply, I could use a relay to make it work with all of my homebrew and HardRock 50 amps but it would be so much easier to be able to just select a logic low output in the menu instead of high. Maybe there is no program room left to implement this feature :| .

Joel
N6ALT
Ohwenzelph
 

Posts: 207
Joined: 01 Jan 2022, 03:47

Re: Problem connecting to external PA

Post by Ohwenzelph »

Would a 2N7002 work better?
noreply
 

Posts: 10
Joined: 14 Apr 2022, 14:01

Re: Problem connecting to external PA

Post by noreply »

Ohwenzelph wrote: 18 Jul 2022, 20:14 Would a 2N7002 work better?
As a buffered PA output from the ATmega?

YES - but this is additional HW and real-estate on the PCB - keeping in mind that one of the objectives of the (tr)uSDX is to keep it simple and cheep.

A firmware "toggle" for reverse logic would solve the problem.
noreply
 

Posts: 10
Joined: 14 Apr 2022, 14:01

Re: Problem connecting to external PA

Post by noreply »

Below is a WORKING circuit to allow proper operation of the MX-50M amplifier and the (tr)uSDX with the additional safety of opto-isolation of the ATmega PA output pin.

(tr)uSDX PA to MX-50M interface.jpg
(tr)uSDX PA to MX-50M interface.jpg (167.07 KiB) Viewed 5129 times

Ideally, you should get SMD versions of the components and can mount inside the radio if this is going to be permanent set-up.

If the firmware can be changed to invert the PA output from the (tr)uSDX - we can remove the 74HC04 which is used as a buffer / level converter

Or at least you can wire-up the opto-coupler inside the (tr)uSDX

Hope this is helpful to everyone having problems interfacing the MX-50M with the PA output of the (tr)uSDX

Cheers
BrumasterM01
 

Posts: 15
Joined: 12 Jul 2022, 17:00

Re: Problem connecting to external PA

Post by BrumasterM01 »

Great solution Noreply. Thank you.

Unfortunately, for me you’re about 2 days late. I had performed the KB9RLW mod to my mx-p50m. and had been watching the thread waiting for you guys to figure this out. I’m not an electronics wizard and rely heavily on community forums like this.

So this weekend I cut up my case and glued in a switch button next to the Tx button. Not an elegant solution, but works. At 45 watts, I can be heard better than I can hear. Maybe that’s not a good thing and violates the entire reason for a QRP rig. But I’m getting through POTA pile ups, where I was missing out before.

KA6VPJ
Attachments
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BrumasterM01
 

Posts: 15
Joined: 12 Jul 2022, 17:00

Re: Problem connecting to external PA

Post by BrumasterM01 »

Hello Noreply,

Where are you tapping the "+3.3V - + 5V from the (tr) uSDX" to drive the 74HC04?

If you have already performed the mod, any pictures would be appreciated.

I'm ordering the 74HC04 and 4N25 from Digikey today and plan to do the modification this weekend.

I've built a MiniPa 70 amplifier and getting about 50W output with 2W in on 20 M with a low pass filter. Not bad for a $20 amplifier. Seems to perform as well as the much more expensive MX-P50M and smaller foot print.

My external switch is working fine, but I would prefer to use the PTT feature.

I should also clarify, that I don't use the USB port. I drive the 2 watts out of the USDX with 2x 18650(7.4-8.4V) batteries for the MiniPA. I use 3X 18650 for the MX-P50M. With 3X 18650, I was getting 70+ watts on the MiniPA, but feared the transistors were getting too hot and worried about splatter.

Thank you,

KA6VPJ
dl6sez
 

Posts: 399
Joined: 30 Dec 2021, 22:54

Re: Problem connecting to external PA

Post by dl6sez »

Splatter....
Maybe not but is there a good working low pass filter added at the output of your MiniPA 70 PA?
When not, this would explain the 50W i can't believe with filter from this very simple design....
You will have nice 28MHz output too :)

73 de Chris DL6SEZ
Chris DL6SEZ, JN48XL near Ulm, Southern Germany
BrumasterM01
 

Posts: 15
Joined: 12 Jul 2022, 17:00

Re: Problem connecting to external PA

Post by BrumasterM01 »

Yes. I have working 20M low pass filter for the MiniPA 70.

In the picture below, you can see the filter hanging off the side.

I've got some work to do on cable management. I plan on putting it in a small enclosure and adding 40 meter filter.
Attachments
USDX 3.jpeg.jpg
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hari
 

Posts: 17
Joined: 14 Feb 2022, 09:25

Re: Problem connecting to external PA

Post by hari »

N6ALT wrote: 18 Jul 2022, 16:00 I could use a relay to make it work with all of my homebrew and HardRock 50 amps but it would be so much easier to be able to just select a logic low output in the menu instead of high.
Works fine for me with a 3,5mm tip to RCA connector and the Hardrock 50. No relay required.
73
DK2CW
 

Posts: 16
Joined: 06 Jan 2022, 21:50

Re: Problem connecting to external PA

Post by DK2CW »

Hey guys,

I analyzed the underlying issue and also have an easy solution for you!

The problem is, that the PA is keyed using an internal 12V relay. The tip of the truSDX PTT socket is directly connected to the MCU which is running at 5V. That means that if PTT is off truSDX will output 5V, while 12V are coming from the PA on the PTT line (just measure the PTT line from the PA with a multimeter to see the 12 V). That means that the truSDX can't 'block' the 12V with its 5V output (the 12V could also potentially destroy that MCU Port permanently!). This could be fixed by adding an MOSFET inside the truSDX that is switched by the MCU, so the MCU is isolated and only enables or disables the MOSFET (this is acutally done be the QDX REV 3 https://qrp-labs.com/images/qdx/schem3.png).

The keying circuit by KB9RLW is insufficient in this case because it does not protect the truSDX from the 12V by the PA.

But I found another solution. Actually only a single N-channel mosfet would be required with the gate connected to the truSDX and the drain and source to the PA. However, the output from the MCU is inverted so I added an inverter in front using another MOSFET. The final circuit is shown in the attached picture. I also added the LTSpice simulation as a ZIP file.

The circuit can be built using almost any N-channel MOSFETs. The pullup resistor R2 should be at least 1M ohm. Otherwise the current through the pullup will already key the PA (took me a while to notice that problem :D ). The resistor R1 only limits the current through the MOSFET.


The MX-P50M is working great. It gives me about 10dB more SNR on CW at the RBN and also on pskreporter when using FT8 with my QDX. So it really makes a difference for DX.
Attachments
ptt_driver.zip
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PTT_driver.png
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BrumasterM01
 

Posts: 15
Joined: 12 Jul 2022, 17:00

Re: Problem connecting to external PA

Post by BrumasterM01 »

Hey DK2CW,

Do you have any pictures of the actual circuit built?

Also if you could share the MFG PN for the MOSFETs used that be great. I've limited electronics knowledge and a quick search Digikey on N-Channel MOSFET brought up 33K parts.

I had purchased 74HC04 and 4N25 components recommended by Noreply, but never made the modification. The 74HC04 component packages were tiny and couldn't find a safe place to pull 3.3 or 5V out of the truSDX.

Thanks.
Ohwenzelph
 

Posts: 207
Joined: 01 Jan 2022, 03:47

Re: Problem connecting to external PA

Post by Ohwenzelph »

So, would two 2N7002's work?
DK2CW
 

Posts: 16
Joined: 06 Jan 2022, 21:50

Re: Problem connecting to external PA

Post by DK2CW »

Hey,

I just built the circuit on a prototype board. I used 2N7000 MOSFETs from Aliexpress. But 2N7002 also works or even BS170 if you already have them. Just use any N channel MOSFET with a drain voltage of at least 20V, drain current of at least 100mA and Gate threshold of 5V or less. Almost any will work.

To build the circuit just replace the voltage sources in my schematic with the connections on truSDX and the PA. - goes to the ground connection and + to the PTT connection on each side.

Also be carefull that this circuit will key the PA if the truSDX is not connected so connect both sides and enable the truSDX before the PA to have a stable PTT control.

I think the solution from noreply would also work if you have the parts.
BrumasterM01
 

Posts: 15
Joined: 12 Jul 2022, 17:00

Re: Problem connecting to external PA

Post by BrumasterM01 »

Thanks DK2CW.

I have a bunch of spare IRF530s and FDT86256s. The circuit seems simple enough. I'll give it a try.
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