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50w PA oscillations

 

Hi All,

I just finished the build of the PA, and after adjusting bias tested it connected to dummy load, driven by QMX+. The QMX goes into protection mode.
Looking at the circuit with oscilloscope I can see that there are oscillations in the area (junction D3 - D5 - Q3 - R8)

Unmounted Q3 for now and replaced Q5 & Q6. Still oscillation.
Nothing on RF in - RF out, so ignoring the “RF amp” portion for now.

Inspected solder point etc but everything looks good.

Anyone had similar experiences or ideas for troubleshooting?

Regards
Jonas
 

You’re going to have to be clearer about your set up?
?
if you’re saying an unpowered or powered 50WPA connected to the QMX+ makes it oscillate (in TX) when an antenna doesn’t the first thing to check (with a VNA) is the amp input impedance
?
But then you say nothing on RF in it RF out (do you mean it’s oscillating when PTT active and otherwise disconnected (not entirely surprising - Put a 3dB attenuator on the open input)?

Please set out the exact layout and conditions that trigger your problem -?
 

Hi,
if this is in the situation when the PTT is activated it is possible that the T/R switching is not working correctly.
?
--
73 Bojan S53DZ
 

Mistakes I made that had similar results while using qmx:
?
I forgot to use putty to change band settings to allow a + 5 volt signal on middle ring (Stereo 3.5mm Mini Jack?).? By default it will not set middle ring to +5volt on transmit.
I would not enable receive on 3.5mm jack.? If you did it would always be in transmit mode and would not receive.
?
I do not know if your qmx has the modification for the PTT circuit that is described in modification? section.
?
And the most evil problem is the 47 uh rf? inductors.? They break if you look at them.? You can not tell that they are broken unless you use an ohmmeter.
If it is broken? it will not supply +5 volt to 1n4007 "pin diode".? No power out?
?
The receive will not work unless 50 watt amp has power and all 47uh are good.
The transmit will not work unless 50 watt amp has power and all 47uh are good.
?
Do not disable swr protection on qmx.? If you have high swr it is because the pin diode is not forward biased.??
It is always the axial inductor is broken.
?
 


Hi,
I have the QMX+ connected to the RF input of PA and the PA RF output is connected to dummy load.
QMX+ & PA is powered up.?
When QMX+ goes into transmit severe oscillations occur at junction of D3, D5, R8 (something like 10v RMS here)
The QMX+ is in practice mode so no RF out. Also?+5v PTT on ring is configured in the band setup (80m)

I was under the impression that no PTT mod is required on the QMX+??

Best Regards
Jonas

Den tis 24 sep. 2024 kl 10:45 skrev Bruce Akhurst via <bruce=[email protected]>:

You’re going to have to be clearer about your set up?
?
if you’re saying an unpowered or powered 50WPA connected to the QMX+ makes it oscillate (in TX) when an antenna doesn’t the first thing to check (with a VNA) is the amp input impedance
But then you say nothing on RF in it RF out (do you mean it’s oscillating when PTT active and otherwise disconnected (not entirely surprising - Put a 3dB attenuator on the open input)
Please set out the exact layout and conditions that trigger your problem?
 

Hi,
what is the PTT +5V line voltage when active? Does the red LED lights up bright?
I had to lower the value of the R210?in the QMX+ (just added 100R in parallel) to stabilize the PA RX/TX switching.
--
73 Bojan S53DZ
 

Hi Bojan,

Thanks for the hint, PTT voltage when in TX is only 2.5V. And Red LED is not as bright as green one.
Now I need to find the culprit. I measured R14 in-circuit to be 9.96kohm, which looks good and should mean that there are no solder bridges causing the low PTT voltage. Guess I have to remove Q5 and Q6 again…


Jonas

26 sep. 2024 kl. 15:31 skrev Bojan Naglic via groups.io <bojan.naglic@...>:

?
Hi,
what is the PTT +5V line voltage when active? Does the red LED lights up bright?
I had to lower the value of the R210 in the QMX+ (just added 100R in parallel) to stabilize the PA RX/TX switching.
--
73 Bojan S53DZ
 

Went on to remove Q5 and Q6 again from amp board. Still only 2.45v on junction or R14 & R15 when TXing.

Then I decided to see how much current the QMX+ could deliver on PTT port. The answer is 1.67mA.
Perhaps this is not enough to power the LED without voltage dropping too low on my amp?

Regards
Jonas

PTT voltage when in TX is only 2.5V. And Red LED is not as bright as green one.
Now I need to find the culprit. I measured R14 in-circuit to be 9.96kohm, which looks good and should mean that there are no solder bridges causing the low PTT voltage. Guess I have to remove Q5 and Q6 again…


Jonas

26 sep. 2024 kl. 15:31 skrev Bojan Naglic:
Hi,
what is the PTT +5V line voltage when active? Does the red LED lights up bright?
I had to lower the value of the R210 in the QMX+ (just added 100R in parallel) to stabilize the PA RX/TX switching.
--
73 Bojan S53DZ
 

Hi Jonas,
the red LED works at about 1.8 to 2.2V. So the current from the QMX+ should be around 4 mA when active: (5V-2V)/(220R + 470R).
The Q204 is made for the current up to 130mA. So there is quite a margin.
If everything else is OK, I would lower the serial protective resistor R210 in the QMX to increase the PTT voltage at the PA PTT input.
The BS170s in the PA need solid G-S voltage over 2.1V typical (0.8 to 3V!) to do the switch. In my case they both needed around 2.8V to make a solid ON state.
Also the PTT cable with 3.5mm stereo connectors should be good (with the ferrite rings).
--
73 Bojan S53DZ
 

Thanks Bojan,

I started by reinstalling all the FETs I had removed for testing, and then lifted the?+ leg of the LED. Result: stable. Amp works now.??
Guess I have to find schematic?+ component placements for the QMX+ next to follow Your advice.?

Regards, Jonas
 

On Sun, Sep 29, 2024 at 02:33 PM, Jonas Sanamon wrote:
Guess I have to find schematic?+ component placements for the QMX+ next to follow Your advice.?
Or you could build a small buffer to go between the QMX+ and amp keying input.? Then there would be no need to dig into your radio, plus it would afford a measure of protection for the radio.
73, Don N2VGU

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