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Jeff Moore QMX repair--5stars!!!
#qmx
Good morning all, just want to give a month-overdue shoutout to Jeff Moore, to add to the accolades he already has here.
After experimenting with JS8Call with my QMX, and finding out that a 2.5 minute transmission at full power will, in fact, fry the transistors, I was about to replace those pieces myself when I decided it was a miracle I had built it correctly with step by step instructions in the first place; winging a repair job didn't seem like a good idea. I remembered seeing someone else's comment about a golden screwdriver in the QRPLabs community, found Jeff's email, and asked what it would take to get him involved. Sent it to him within the week, and waited. Now, I figured someone who's doing ham radio repair for a hobby and doesn't try to turn a dime doing it would send me a cookie cutter "Hey, I got it, I fixed it, it's coming back to you, please don't screw this thing up again...it's a qrp transmitter, not a TV dinner microwave." Instead, I received detailed, obviously individually written emails providing step by step observations of my build, observable soldering issues and how they were fixed, and...he didn't just fix what I asked for. He also observed my power out on 20m was only 3.5 watts, which I had decided was sufficient enough to not fool with (after contacting Australia from Louisiana), and got it up to 4.2 watts, just because it was doable. No extra charge. My QMX is purring along happily ever since, and I cannot compliment this golden screwdriver's handiwork enough!! 73 de AD5MO |
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Roy,
please check also the voltage at each connection of T501 in RX mode and let me know. Reference to the transformer document at page 10 all the connections marked "P" should be on the same value below 1 V (0.6 V or so). Both "S" should show 0 V. If not, T501 is wrong or the voltage source of the PA. If the PA transistors are hot only from 100 mA additional current a PA voltage of 0.6 V is not enough I think (only 60 mW of heat power). If I would search for the transistors producing the heat I would do the following: Let the PA cool down (QMX off) and remove the Display and the steel screw, nut and washer. Switch the QMX on for a few seconds (don't need the Display) and after switch off check temperature of each transistor carefully with the finger tip (it could be hot!!!). Maybe some more seconds ON are necessary for enough temperature. 73, Ludwig |
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Roy, Check the gate signals at the BS170 PA transistors. You should see zero volts in RX, and a 5V 50% duty cycle RF square wave in TX.? If you are using a DMM to measure, that TX signal will register as 2.5 volts, an averaged value. If you see voltages substantially different, something is wrong for sure. Either the driver circuit has a problem or an internal short has occurred at a PA transistor(s). GL JZ KJ4A? On Wed, Apr 3, 2024, 10:08?PM Roy - KI0ER <ki0er@...> wrote: Another clue .... I found where all the excess current is going ... the PA transistors are very hot while the QMX has only been in receive (idle) mode!? ? |
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Hi gals and guys! |
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Re: Something to look out for: possible cause of PA failure (QDX etc).
A big factor in thermal failure of the BS170 in any of the QRPLabs kits is that the TO-92 package is not very efficient at transferring heat away from the die.
The tinned pad, the washer and the screw do allow for 'some' contact but it is not a machined surface and even the multi-layer board can only do so much. Steel (the washer and the screw) are not great thermal conductors in their own right. What I have been doing is to use M2.5 copper screws, a copper crush-gasket (meant to deform), copper nut and some thermal compound to improve the heat transfer characteristics. -- Tisha, AA4HA |
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Another clue .... I found where all the excess current is going ... the PA transistors are very hot while the QMX has only been in receive (idle) mode!? ?
I've re-checked all my solder joints under strong magnification while also looking for some stray short, but nothing is popping out at me as problematic. Does this clue help narrow things down? I'm open to suggestions! -- 73 de Roy - KI0ER Littleton, Colorado USA |
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Hello Roy,
It's drawing 212 mA powered up and idle, which is high! Indeed. Around 100 mA here. |
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RF Sweeps Problem QMX R1
I just finished may QMX.? Everything checks out except the RF sweeps.?
The radio seems to work fine in all modes and passed all the diagnostics except the rf sweeps all look like noise and the level is low compared to the? published sweeps in the manual. Did I miss something running the test? I'm considering re-winding T407 just as some path forward.?? I don't have the case installed in case that makes a difference. Your thoughts are much appreciated. Traces: Kindest regards, Tim W8TWF |
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I just completed my 3rd QMX build, this being my first High-Band QMX. It is a 12 Volt 2023 Rev 2 board with 3 modifications done by me, the addition of the diode (surface mount) for potential parasitic oscillations, and the jumper for the PTT Out grounded functionality, and 3) replaced the power socket with a power cable with power-poles. It is running the _17 firmware with AGC enabled.
Everything appears to work correctly and it appears to be receiving OK ... except that I'm getting about half the power out that I should be expecting. What should I be looking at that I may have done wrong, or that should be adjusted? What other tests can I perform to determine the issue? Here's my data while the QMX is connected to a QRP-Labs dummy load: It's drawing 212 mA powered up and idle, which is high! And it feels warm (I'll look at it w/ infra-red camera next). My low-band QMX draws around 93 mA at idle. When transmitting (CW key down) on: 20M drawing 716 mA 17M drawing 775 mA 15M drawing 833 mA 12M drawing 740 mA 11M drawing 760 mA 10M drawing 755 mA Diagnostics Power Out into Dummy Load 12V power input BIAS SMPS (during transmit) 30.4 mA Duty Cycle 14% ? 20M 2.4W 10.1V SWR 1.09 17M 2.5W 10.1V SWR 1.22 15M 2.2W 10.1V SWR 1.40 12M 1.7W 10.1V SWR 1.60 11M 1.6W 10.1V SWR 1.70 10M 1.6W 10.1V SWR 1.75 ? ? Audio Filter Sweep ? ? ? ? ? ? ? ? ? RF Filter Sweep ? ? ? ? ? ? ? ? ? ? Low Pass Filter Sweep ? ? ? ? ? ? ? ? Image Sweep ? ? ? ? ? ? ? ? ? SWR Sweep (into QRP-Labs Dummy Load) ? ? ? ? ? ? ? ? ? 73 de Roy - KI0ER Littleton, Colorado USA |
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[QMX] Keying CW via CAT
Hi,
Is there any way to key the CW transmitter via CAT? I experimented with sending "TQ1;" but it doesn't output any RF. I simultaneously sent 50 Hz tone via the sound card interface and it does output RF, but only in digi mode. In CW mode is doesn't seem to do anything (but the mode, queried with "TQ;", is switched to 1 = TX). What worked was: - sending MD6; to switch to digi mode - sending TQ1; to enable transmitter - sending tone via sound card interface - sending TQ0; to disable transmitter - sending MD3; to switch to CW mode It works, but: - it looks very hacky to me (and I'm worried about timing stability, but I didn't test it) - frequency is different from the one emitted when I press the paddle by the frequency I send via the sound card interface - there's no audible sidetone - probably there's also no envelope (I didn't test it -- does the QMX apply envelope for signals transmitted in the digital mode? I doubt, but maybe?) Second point could probably be solved by sending "FA;" to read the VFO A frequency, sending another FA to lower it by the frequency emitted via the sound card interface (for example 1 kHz less, and emit 1 kHz to the sound card), and then send another FA to increase it again when keying up, but it's even more hacky this way. Is there any other way to programatically send CW with QMX? Alternatively: is there any way to use the paddle port for a paddle, with QMX's keyer, and any other port (PTT port) as a straight key input? This way I could do some hardware to just toggle PTT via another port. Basically, what I need is to be able to transmit CW both from a PC and from a paddle, at the same time. Before Saturday, as SP DX Contest is on Saturday... |
Tisha Hayes, AA4HA