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QMX #12v

 

Morning all.
I recently received my QMX (hi bands) and I¡¯m very impressed. I have a QCX Mini on 40m which is a fine radio but the QMX feels more smooth somehow; probably psychosomatic!
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Anyway, with all the discussion around the sensitivity of the finals to heat when running at 12V I was wondering if anyone had tried improving the heatsink on those transistors. I am pretty sure a small heatsink could be fitted in place of the washer and might help with avoiding issues when working that POTA DX at 12V.
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Thoughts anyone?
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73?
de
G7RUX
k
 

A note about fanless heatsinks¡­ these are better referred to as heat spreaders or heat delay devices because they slow down heat buildup by spreading it over a large surface area. Finned heatsinks can provide a larger surface area and are better at radiating heat away. But¡­ in a closed container, the heat buildup will eventually reach the temperature of the devices being protected when there is no mechanism for mechanically conducting the heat away from those devices and no airflow across the heatsink. A small heatsink with a fan is superior to just adding a lot of metal. Further, there needs to be a way to remove heated air out of the container by having air holes to allow fresh air to come in underneath and go out on top.

Tony AC9QY


On Sat, Apr 19, 2025 at 5:19?PM Wiggle Pig via <wigglepig=[email protected]> wrote:
Morning all.
I recently received my QMX (hi bands) and I¡¯m very impressed. I have a QCX Mini on 40m which is a fine radio but the QMX feels more smooth somehow; probably psychosomatic!
?
Anyway, with all the discussion around the sensitivity of the finals to heat when running at 12V I was wondering if anyone had tried improving the heatsink on those transistors. I am pretty sure a small heatsink could be fitted in place of the washer and might help with avoiding issues when working that POTA DX at 12V.
?
Thoughts anyone?
?
73?
de
G7RUX
k
 

I can't speak for the QMX as I haven't built one yet, but with previous designs the washer is not intended to be a heatsink. It's purpose is to hold the finals down flush with the copper pad on the PCB, which is the actual heatsink. The transistors also transfer heat via their leads. I doubt that replacing the washer would accomplish anything significant.
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If I'm wrong, anyone here is more than welcome to correct me. I'm not yet too old to learn, but I am getting there!?
 

Lawrence,

While the board and washer both absorb heat, there¡¯s no place for it to go other than spread around inside the enclosure. The longer the transmitter is on, the more the heat builds up, elevating the temperature inside the enclosure.

A solution I used for many years in portable radios was to solder metal tabs to the heat-sunk area of the board (at ground potential of course) and either bolt or solder those tabs to the radio¡¯s metal enclosure. This direct-conduction method piped heat rapidly to the enclosure which then radiated away from its large surface area. This was the most effective heat removal strategy in my hi-rel career where fans were not allowed due to their high failure rates rates.

Tony AC9QY

On Sat, Apr 19, 2025 at 5:44?PM Lawrence KC6WOG via <lsgoodwin=[email protected]> wrote:
I can't speak for the QMX as I haven't built one yet, but with previous designs the washer is not intended to be a heatsink. It's purpose is to hold the finals down flush with the copper pad on the PCB, which is the actual heatsink. The transistors also transfer heat via their leads. I doubt that replacing the washer would accomplish anything significant.
?
If I'm wrong, anyone here is more than welcome to correct me. I'm not yet too old to learn, but I am getting there!?
 

The Assembly manual clearly states that the PCB board is the heatsink, not the washer.
I've been using a self-built high band, 12v, QMX, in our Florida heat, for many months and have never felt it hot, or even warm to touch. NOT a problem!
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OTOH, I don't do .
--
72 de N4REE, Bob Easton - QRP CW forever
 

So, as a bit more information (not all of it, obviously as you need to be security cleared for that!) I was thinking more of mainly increasing the thermal mass a little and perhaps trying to couple it to the case with a thermal pad or similar.
I have no real intention of using this unit for high duty cycle modes at the moment.
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I¡¯m aware of the intention that the tinned pad on the board is the majority of the heatsinking but having done a fair bit of design and dev work on small RF units myself I¡¯m all too aware that the mounting washer here provides the majority of the sink mass, with the pad and board structure also providing a good chunk. Given that the usual failure in the finals is through overheating, as a number of examples have shown, I intend to try to reduce that heat by sinking it a little better; replacing the washer with a better heatsinking arrangement will likely improve things.
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tra
Piggly
 

On 20/04/2025 18:54, Wiggle Pig via groups.io wrote:
Given that the usual failure in the finals is through overheating, as a number of examples have shown,
A number of examples have shown BS170s blown apart.
But is that due to overheating in service or a result of a high voltage causing one to short THEN blow?

73 Alan G4ZFQ
 

The other factor in all of this is the plastic package design. The heat sink practice for those other styles won't necessarily work here. The qmx design takes all this into account. Many cw qrp transmitters use 2 or 3 bs170 devices. Running digital waveforms provides a bit greater stress. Keeping the voltage at 12 volts or lower helps.?
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Curt
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On Sat, Apr 19, 2025 at 03:19 PM, Wiggle Pig wrote:
I have a QCX Mini on 40m which is a fine radio but the QMX feels more smooth somehow; probably psychosomatic!
That's interesting. I feel the opposite about my QCX Mini compared to the QMX. The QMX Mini receiver sounds better to my ears. Maybe once Hans gets around to eliminating all the SMPS-induced birdies, I will change my mind.?
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The heatsink question has been discussed to death here. I trust Hans' design and don't see any need to improve it. I've never blown the finals.?
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73, Mike KK7ER?
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