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QMX+ High Current and Self Start
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Initial fault: First power up at 12 and popped a 3A fuse. Visual inspection: Shorted SMPS Q104/Q103.
After multiple parts replacements, I can now power up the QMX+ with the current limited to 250mA. After applying power for about 5 seconds, the radio will self start up. Alternatively, I can touch Q105 on the SMPS and also make it boot up. The supply drops from 12v to 7.4V and the current limit is reached. The LCD is flashing a 6V low input voltage.? I have replaced both AOD403’s on the 5V board and Q507 (shorted), c213 + r214 (shipped damaged/missing), two shorted BS170’s (changed the full set), diodes checked for proper operation.? No solder bridges and all connections and solder joints verified.?
My suspicion is the 5v SMPS is still defective but what would cause the power on condition?
When I press the encoder to power off, the display shuts down but the voltage drop is still latched with current railing the set limit.?
Please advise any more items to check. I will order a replacement SMPS just in case but I would like to troubleshoot further if possible.?
TNX, Damon kc5cqw |
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When I apply power, the input voltage stays at 12v for a second during boot up and then it latches. I’m under the impression a transistor is switching on and dumping current OR this has to do with the 5v SMPS startup voltage ramp hand-off from the 3.3v board.?
If this is true, does this sound like a bad shorted cap?? |
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Damon,
it looks like your contoller is running. So please establish a terminal session and check the Diagnostics screen. Maybe there is an overload at one voltage rail. You will find information about the 5 V and the 3.3 V SMPS in this Diagnostics screen. You may take a screen shot and show it here for some discussion.
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QMX QMX_plus Startup? from the QRP Labs Wiki contains a description for the switch on process. Following this you may find the wrong part. I guess there could be something wrong around Q103...Q105. I would check Plug-in PCB #1 off line first (see QMX QMX_Plus Power supply test Process ).
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73, Ludwig |
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These two posts above have been helpful, thank you Ludwig!
Considering the symptoms and when I touch Q104, the radio turns on, I’m leaning that direction as a point of failure.? Pressing the encoder to turn the radio off only pulls the voltage down further and that makes me want to suspect Q202 has a role but it is difficult to reach and test.?
Am I headed in a better direction or is there something else I need to focus toward?
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On Mon, Sep 9, 2024 at 11:18 PM, Damon wrote:
Running diagnostics causes the voltage/current to normalize.Strange. Which FW do you use?
How did you get the first two Pictures?
What supply voltage was shown in the Diagnostics screen (top left, dark in the picture) and what was the external voltage?
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73 Ludwig |
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I’m fairly confident in the repairs I made to the 5V SMPS and it seems to be working fine. To validate the LCD, I removed it to rule out any issues. No performance change noted.?
With a departure from that, I remembered some QDX troubleshooting which resulted in finding sorted PA mosfets. I did originally have to replace Q507 and the PA’s due to shorted failures.?
I’ve been probing the board with my DMM and decided to remove the PA’s. Sure enough, the high current draw and 4.6V drop cleared up without them in circuit. Early this morning, I started fine tuning the inductors and did some basic RX tests etc. Despite the PA’s dumping power, RX seemed fine. So, What in the world would cause the PA’s to be gated and dump all that power to overload? Why would this happen in RX? There shouldn’t be current flow if I’m not mistaken.?
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Additionally, pressing the encoder to turn the radio on does not work. I actually have to press my fingertip to Q105 to latch it on. Pressing the encoder to power down the radio “works” but there is a current draw of 0.065A until I remove the plug. Trying to restart the radio does not work until it is unplugged.?
I’m starting suspect IC503 and the MCU. I’m not wanting to reinvent any wheels so any and all advice to move forward is graciously appreciated! ?
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Further tests show poor 80/60m performance and there is a significant voltage drop compared with the other LPF’s. I set the band to 6m and found 10.47V at the switching diodes and their respective nodes except for 80/60m which measured at .557V No solder shorts were visible and no hot components found. Input RX current is around .100mA @12.0V When 80/60m is active, the voltage goes to 4.73V instead of the expected 33mV as the others measure. L507 appears open line, I will need to replace it but I fail to see if this could be related.?
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On Mon, Sep 16, 2024 at 07:02 PM, Damon wrote:
So, What in the world would cause the PA’s to be gated and dump all that power to overload?Damon, with default settings the PA voltage is high in RX mode (see here). With one problem in the PA stage a current may flow also in RX mode. ?
I guess something is wrong related to QMX QMX_plus Startup section 2 #1 and section 3 #3.
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Thats not my first place.
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Read QMX LPF section for debugging the LPF section.
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All together it looks to me like more than one failure. So a remote debugging is very difficult. It would be good to sort out one of the problems to start. I think the best is to start with the switch on / off problem.
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Like I wrote before
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Additional remove SMPS and check resistance from PWR_ON (mainboard) to GND with and without left encoder depressed.
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Which contact of Q105?
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73 Ludwig
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The 80/60m LPF has a temporary 10uH in place for testing until replacements are ordered.?
Q105 source (left leg) or Q104, I did initiate a power on by probing D101 with my DMM as well.?
JP102-7 is open line and short with encoder press. JP102-1 remains open line.?
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I will report back on the startup findings… |
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Observation:
V+ takes a couple of minutes to build up from Vin upon first application of input power. Once 5.15V is reached at V+, push button power on is possible.? If the radio is powered down, a 6.5mA draw is maintained. ?Pressing the push button will not power up again until Vin is reset by removing power and the jack terminals shorted then waiting for V+ to rise above 5.15V again.? |
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The new SMPS boards arrived an hour ago and fixed the power on issues. RX is good but the short in the PA circuit remains. I placed one BS170 in circuit to verify. I have my power supply set for 12.0V and 0.300mA CC.?
In RX without the PA’s, I get a smooth 0.1A draw. With one or more BS170’s it goes to the rails. Overload and blue smoke release if not limited.?
I have performed multiple magnified inspections of all solder joints. If it is one, it is well hidden.? |
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On Wed, Sep 18, 2024 at 07:38 PM, Damon wrote:
With one or more BS170’s it goes to the rails. Overload and blue smoke release if not limited.?Damon, ?
to do some investigation without damage of the BS170 you may feed the PA from a little AA cell in series with a 10 ohm resistor (see this thread 2nd post). Please install all BS170. Then check voltage at G and D of the BS170 and voltage at the pins of IC503. An oscilloscope would be perfect but values get by an DMM are also helpful.
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73 Ludwig |
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