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I seem to have dodged this bullet, or something very similar.

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While powering up my QMX+ for the first time noticed that although it powered up the supply was current limiting.? Tried increasing the limit and ended up at 380 mA, then found that the limit was increasing and one bank of BS170s were getting hot.

I was also finding that the QMX+ would shut down spontaneously. just touching things seemed to cause this and sometimes I was getting high current and sometime not.

I began to suspect the power supply, maybe the connector or my plug.? After a lot of fiddling around - one way to check the power connectors is to use a meter in diode mode with the power reversed so I can see the 0.6V drop across D110 the surge protection diode.? Something definitely wrong, sometime 0.6V, sometimes nothing, as in no connection.? The solder joints looked perfect but reflowed them anyway and everything started looking normal.? Now everything seems fine, no high current when in receive and after reflowing a few more joints everything seems fine. I'm getting just over 1W output running at 7.5V.

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I've seen all sorts of bad behaviour when a processor gets short power drops - negative spikes - and wonder if the TX line to IC503 became active with CLK2 not running.? This would have turned one bank of BS170s on and as they heat up the current increases but my current limited supply protected then and everything else.

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Initially I was seeing low power output, 0.4 to 0.6W seemed low, even for 7.5V and the output waveforms were different on the two banks of BS170s.? Reflowing their solder joins, after replacing T501, fixed that.

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What saved my QMX+ from damage was running with a low voltage and with a current limited supply.? Keeping the current and voltage down prevented damage while I sorted out the problems.

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Chris, G5CTH

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Han, fine if I could get a little help. And you made good investigations to find the problem, well done! Thank you also for this information.

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73 Ludwig

Chris,

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Not clear what you were doing there.

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Strikes me as a very bad idea to apply reverse power to the QMX from your power supply, if that is what you did.

You could wind up with a whole bunch more problems to diagnose.

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There are far better ways to find an open solder joint.?

Use your ohmmeter, or power up properly and use your voltmeter.

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And if I want to see the volts across a diode, I'd just use the voltmeter function of my DVM.

Diode Test mode is meant for use when you have a diode sitting loose on the bench (no power supply on it)

and want to have some rough idea if it functions.

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This got me curious though, I had never used Diode Test mode with this meter.

On my DVM in Diode Test mode, it seems to apply 3.3v to the device under test through a 1.9k restance,

and reports the voltage it sees across the diode.? The red probe is positive.

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Jerry, KE7ER

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On Sun, Mar 30, 2025 at 11:23 AM, Chris wrote:

On Sun, Mar 30, 2025 at 02:25 PM, Jerry Gaffke wrote:

Strikes me as a very bad idea to apply reverse power to the QMX from your power supply, if that is what you did.

You could wind up with a whole bunch more problems to diagnose.

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I don't buy this, sounds like FUD to me.

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My meter in diode mode generates 3.9V and, crucially 1.5mA.? This into a circuit that's protected from reverse voltage by the TVS diode, Q103 and Q105.? A lot needs to be broken before this gets anywhere where harm could be done and the 1.5mA max current will also help.? In continuity check it's more like 2V and 0.25mA.

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The point I was making in the rest of the post was that by being careful with the power supplied, using low voltage and current limits my QMX was preserved.

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I have replaced ic503 and bs170s. Everything is working great now!

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73 N6HAN