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QMX worked briefly then died when adjusting... Help?

 

My QMX high band rev2 kit worked right away! Until I went to go do some minor adjustments to the 10m LPF and the SWR bridge (was reading 1.5-1.7 when into a dummy load) when it suddenly died.

Now, I have a strange set of symptoms. The main one is that I see 0.75V (stable on varying input voltage) on the lin_reg_en pin from the SMPS board #2, but nothing ever seems to bring up 3V3.

Current draw is consistently 0 even when pressing the encoder. The encoders seem to be making contact correctly as far as I can tell from testing on the controls board and the QMX main board.

It sure seems like something might have fried my linear regulator, but I'm not sure quite how to confirm and or if there's anything I can really do about it other than trying to order some replacement SMPS boards?

I can handle a bit of SMT rework, but I also don't really want to put the maybe still working (?) main board at risk if I don't have to?

Thanks for any advice!

Devin KN6PHZ?
 

Is there a good SMPS board testing procedure that can be done without the QMX main board? I'm guessing no, but maybe the linear side of the 3V3 board can be tested in isolation reasonably well...


On Mon, Jan 15, 2024, 2:03?PM Devin Grady <kn6phz@...> wrote:
My QMX high band rev2 kit worked right away! Until I went to go do some minor adjustments to the 10m LPF and the SWR bridge (was reading 1.5-1.7 when into a dummy load) when it suddenly died.

Now, I have a strange set of symptoms. The main one is that I see 0.75V (stable on varying input voltage) on the lin_reg_en pin from the SMPS board #2, but nothing ever seems to bring up 3V3.

Current draw is consistently 0 even when pressing the encoder. The encoders seem to be making contact correctly as far as I can tell from testing on the controls board and the QMX main board.

It sure seems like something might have fried my linear regulator, but I'm not sure quite how to confirm and or if there's anything I can really do about it other than trying to order some replacement SMPS boards?

I can handle a bit of SMT rework, but I also don't really want to put the maybe still working (?) main board at risk if I don't have to?

Thanks for any advice!

Devin KN6PHZ?
 

Confirmed at least that the linear portion of the SMPS board works as I would expect. Just a little under 3V3 being produced when enabled with a momentary contact to ground on lin_reg_en. Looks right.

However I'm very suspicious of two things now:
1) the lin_reg_en, which is pulled up on the SMPS board, in isolation reads a nice 7V (my test bring-up voltage). When the SMPS board is connected to the main board it reads 0.75 but I don't see how/why that would happen without a CPU fault.
2) the SMPS with only 7V and GND connected draw 5mA from my supply and I'm not sure why. Maybe someone with more expertise on this SMPS/linear design understands??
3) the output of the linear regulator reads 0.84, about a diode drop away from the lin_reg_en pin, and VDD reads 0.

When connected to the main QMX board, the current draw continues to be exactly 0 to the best of my ability to measure.

When pressing the encoder, none of the voltages change at all. I have thoroughly confirmed continuity exists between the encoder pin while pressed, and ground.

So my current suspicion is a short somewhere that is pulling the 3V3 SMPS board permanently into an off state. The VDD pin shows a short to ground without the SMPS installed!

Checking every point along the VDD paths I can find, the most curious part is that the VDD and VREF junction at the CPU and C214 show 2.2ohms to GND.?

Is it possible there is a near-short inside my CPU?

And if so, any ideas how that could have happened?

The only part of the instructions I (knowingly) didn't 100% follow was using a BAT41 I happened to have on hand instead of the 1N4148 recommended but I can't see how that would be a problem. And it was working fine until I went to adjust the LPFs and the SWR bridge... so those are the key suspects but I don't know what I could have done.


On Sat, Jan 20, 2024, 11:14?AM Devin Grady <kn6phz@...> wrote:
Is there a good SMPS board testing procedure that can be done without the QMX main board? I'm guessing no, but maybe the linear side of the 3V3 board can be tested in isolation reasonably well...

On Mon, Jan 15, 2024, 2:03?PM Devin Grady <kn6phz@...> wrote:
My QMX high band rev2 kit worked right away! Until I went to go do some minor adjustments to the 10m LPF and the SWR bridge (was reading 1.5-1.7 when into a dummy load) when it suddenly died.

Now, I have a strange set of symptoms. The main one is that I see 0.75V (stable on varying input voltage) on the lin_reg_en pin from the SMPS board #2, but nothing ever seems to bring up 3V3.

Current draw is consistently 0 even when pressing the encoder. The encoders seem to be making contact correctly as far as I can tell from testing on the controls board and the QMX main board.

It sure seems like something might have fried my linear regulator, but I'm not sure quite how to confirm and or if there's anything I can really do about it other than trying to order some replacement SMPS boards?

I can handle a bit of SMT rework, but I also don't really want to put the maybe still working (?) main board at risk if I don't have to?

Thanks for any advice!

Devin KN6PHZ?

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