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QMX rev.1 3rd time SMPS issues

 

After I already had twice issues with the SMPS, which were caused by overloaded Z-diodes:
  1. /g/QRPLabs/message/118216 --> D108 in SMPS PCB#1 had to be replaced and the QMX was working again
  2. after a loose contact in the power connector (with 2.5mm inner diameter) I had again high current consumption and finally identified Z-diode D109 on SMPS PCB#2 as the root cause. After replacement QMX was working fine again.
Recently I had a different issue, again after accidentally using the power connjector with 2.5mm inner diameter.
The QMX could still be switched on, but the current consumption was significantly higher than before:
Normally my QMX required during RX 83mA @ 12V. With this fault the current increased to 311mA @12V!?
When I lowered the supply voltage to 8V, the current went down to 229mA. This is an opposite behaviour to an OK QMX, which increases current, when lowering the supply voltage, as the total power remains constant (benefit of the SMPS's).
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Looking into the HW diagnostics menu, I saw that the 5V voltage was too high and the duty cycle unreasonably low:
? ?
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Looking into the schematics
I was assuming, that Q108 (AO3415A) was damaged and would not really switch off.
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I replaced this MOSFET, but the situation did not improve, but current consumption and output voltage on the VCC 5V SMPS connector even went up to 5.75V with 8V supply voltage:
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I connected the SMPS#1 though eight 10cm jumper cables to the main board for easy access and went through several possible hypothesis', replaced components and measured:

Hypothesis

Action

Result

The CPU PWM_5V signal does not go down to 0, i.e. Q108 not switched off

Measured PWM_5V with oscilloscope: signal is switching between 0 and 3.3V

Not the problem

?

To be on the safe side, I disconnected the PWM_5V signal from the SMPS input

Current consumption down, not the problem

Components fed by the 5V from SMPS#1 create the high current consumption

Disconnected VCC output of SMPS#1 from the QMX

High current consumption remained ¨¤ not the problem

?

Replaced Q106 and Q112

No change, current consumption even went up slightly again

D106 SS14 not OK

Measured forward voltage

0.2V, OK for a Schottky diode

D104 1N4148 not OK

Measured forward voltage

0.6V, OK
?
I have now run out of ideas and I am hoping, that somebody in the forum has the right idea, what could be broken.
--

73, DB2OO -Joerg
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