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QMX+ bias current + output power fluctuating

 

My QMX+ went together really nicely, along with my battery and speaker mods.
However, the bias current is varying wildly on TX, also transmitter voltage is indicating low and output power varies.? All other tests and sweeps look ok. I've attached a short video showing the diagnostics test on TX.
?
I've replaced C504 as has been suggested and reflowed all joints that looked like they might be dry.?
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I'm a bit stuck now on what the issue might be - any thoughts from the hive mind?
 

Just tried PTT test in diagnostics and get the same bias current fluctuation.
I've tried various firmware versions - now on 0.26
 

Just bumping this as I still haven't had any luck sorting it out.? Hans has been helping, but is a bit stumped as well!
?
Wondering if anyone else has seen this issue?
 

I have the same problem on QMX+....
--
73
Karl
KI4ZUQ
 

Bis current fluctuates wildly high, occasionally flashing green about 28 to 30 ma but wildly high and red flag around the value. The 12 volt supply may indicate 12.0 to 12.1 with a slow variation. That's on a cheapo mini meter digital display but mimics the putty indication.
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D502 and C504 look like the only two regulating components in the TX BIAS circuit.?
--
73
Karl
KI4ZUQ
 

D502 is the zener regulating the 12 volt. I must look up the data sheet but it looks the likely culprit in wild current swings. Will advise...
--
73
Karl
KI4ZUQ
 

Diode D502 is a Schottky commutation diode. It plays no role in regulating the bias voltage/current. Neither does C504 when functioning properly. Regulation is done under MCU control by monitoring ADC_BIAS and adjusting PWM_BIAS as needed.

Loss of regulation or erratic behavior might be due to:
- erratic, failed C504,
- an erratic short in the PIN diode array in the LPFs,
- a damaged Q502 (BSS84),
- a damaged MCU input (ADC_BIAS).



On Sun, Sep 8, 2024 at 8:00?AM Karl via <karlshumaker=[email protected]> wrote:
D502 is the zener regulating the 12 volt. I must look up the data sheet but it looks the likely culprit in wild current swings. Will advise...
--
73
Karl
KI4ZUQ
 

One more thing to check: A failing Q501 could also lead to erratic behavior.

On Sun, Sep 8, 2024 at 8:28?AM John Z via <jdzbrozek=[email protected]> wrote:
Diode D502 is a Schottky commutation diode. It plays no role in regulating the bias voltage/current. Neither does C504 when functioning properly. Regulation is done under MCU control by monitoring ADC_BIAS and adjusting PWM_BIAS as needed.

Loss of regulation or erratic behavior might be due to:
- erratic, failed C504,
- an erratic short in the PIN diode array in the LPFs,
- a damaged Q502 (BSS84),
- a damaged MCU input (ADC_BIAS).



On Sun, Sep 8, 2024 at 8:00?AM Karl via <karlshumaker=[email protected]> wrote:
D502 is the zener regulating the 12 volt. I must look up the data sheet but it looks the likely culprit in wild current swings. Will advise...
--
73
Karl
KI4ZUQ
 

Diode D502 is a Schottky commutation diode. It plays no role in regulating the bias voltage/current. Neither does C504 when functioning properly. Regulation is done under MCU control by monitoring ADC_BIAS and adjusting PWM_BIAS as needed.
?
Loss of regulation or erratic behavior might be due to:
- erratic, failed C504,
- an erratic short in the PIN diode array in the LPFs,
- a damaged Q502 (BSS84),
- a damaged MCU input (ADC_BIAS).
?
You are right! Symbol on Hans' schematic shows as Zener. Anyway, the QMX+ goes on the 'scope this afternoon to see what I can see. SOMETHING is driving the bias nuts and I dearly want to find it!
?
--
73
Karl
KI4ZUQ
 

The symbols for Schottky and for Zener are very similar, easy to mix them up!

Jeff Moore suggests that the first thing you check are the solder joints on C504. He says that this is the most frequent failure he sees in the PIN bias SMPS.
Good luck!

On Sun, Sep 8, 2024 at 2:37?PM Karl via <karlshumaker=[email protected]> wrote:
Diode D502 is a Schottky commutation diode. It plays no role in regulating the bias voltage/current. Neither does C504 when functioning properly. Regulation is done under MCU control by monitoring ADC_BIAS and adjusting PWM_BIAS as needed.
?
Loss of regulation or erratic behavior might be due to:
- erratic, failed C504,
- an erratic short in the PIN diode array in the LPFs,
- a damaged Q502 (BSS84),
- a damaged MCU input (ADC_BIAS).
?
You are right! Symbol on Hans' schematic shows as Zener. Anyway, the QMX+ goes on the 'scope this afternoon to see what I can see. SOMETHING is driving the bias nuts and I dearly want to find it!
?
--
73
Karl
KI4ZUQ
 

Jeff Moore suggests that the first thing you check are the solder joints on C504. He says that this is the most frequent failure he sees in the PIN bias SMPS.
?
I will do that first!
?
Just finished testing my QMX (80-20) and bias was steady at 30.5 ma all bands...
--
73
Karl
KI4ZUQ
 

Solder joints are perfect C504...
--
73
Karl
KI4ZUQ
 

Problem was two of three connections on Q516 were open! Touched solder iron to them and now bias is nominally 30.5 ma all bands!
?
Next is to bring up power out on a few of these bands as none are 4 watts.
?
Thanks to all for inputs and suggestions!
--
73
Karl
KI4ZUQ
 

Excellent news!


On Sun, Sep 8, 2024, 4:54?PM Karl via <karlshumaker=[email protected]> wrote:
Problem was two of three connections on Q516 were open! Touched solder iron to them and now bias is nominally 30.5 ma all bands!
?
Next is to bring up power out on a few of these bands as none are 4 watts.
?
Thanks to all for inputs and suggestions!
--
73
Karl
KI4ZUQ
 

Interesting! Now I want to rush home from work and check mine!
 

Nope - touched up all the solder joints in that area, but no joy. I'm wondering if its worth replacing Q502 or any of the others ?(not easily sourced here in NZ - probably a Digikey order). However,? I do suspect the MCU, given that it doesn't look as though I've got any waveform at ?PWM_BIAS and from what Hans has said, the waveform at ADC_BIAS is wrong as well. I don't think I can replace the MCU - even using ChipQuik - so might need a new mainboard.
 

I have a hard time believing there are that many bad parts in these boards. I had C509 on my first QRP Labs kit (QMX) missing with evidence it was turned up on end and thisQMX+ where the feet of the transistor were bent out of shape and did not get soldered properly. But that is just one part misapplied on each board out of over a hundred.
--
73
Karl
KI4ZUQ
  

I too had a problem with output fluctuation. 
I found that with a 9V supply the problem went away. 
I must have made a mistake when wrapping the T501.
 

Still no joy with mine.? I replaced Q502 and no difference.?
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Tried 9v and 12v (wound for 12).? I'm stumped!
 

Just realised that I never posted the resolution to this. After a lot of testing and investigation, we thought it was the processor causing the problems and Hans kindly sent a replacement main board which worked perfectly.

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