|
I have now used both the low- and the high-band QDXes daily as??for a period of two weeks without destroying the set of four BS170 final transistors. Here are three tips:
- Reduced power supply voltage during tuning
- Current limited power supply
- Zener diodes?
It's described more in detail here:?
-- Sverre
LA3ZA
http://la3za.blogspot.com
|
Sverre, Have you considered also using thermal paste under the BS170s ?
JZ
On Fri, Apr 28, 2023, 12:58 PM Sverre Holm < svholm54@...> wrote: I have now used both the low- and the high-band QDXes daily as??for a period of two weeks without destroying the set of four BS170 final transistors. Here are three tips:
- Reduced power supply voltage during tuning
- Current limited power supply
- Zener diodes?
It's described more in detail here:?
--
Sverre
LA3ZA
|
No never considered thermal paste. Perhaps FT8/4 with less than 15/7 seconds transmit time helps with the heat problem, compared to almost 2 minutes of continuous transmission in WSPR? ? -- Sverre
LA3ZA
http://la3za.blogspot.com
|
Sverre, Yes, very likely FT8/4 is easier on the amplifier. Thermal paste may have a place though in your list of items that protect QDX. Another idea: Use an absorptive SWR indicator while tuning up. It limits the SWR excursions that QDX may see to very safe values.
JZ
On Fri, Apr 28, 2023 at 1:33?PM Sverre Holm < svholm54@...> wrote: No never considered thermal paste. Perhaps FT8/4 with less than 15/7 seconds transmit time helps with the heat problem, compared to almost 2 minutes of continuous transmission in WSPR?
?
--
Sverre
LA3ZA
|
I'm already using an absorbtive SWR meter - it's in the Chinese QRP tuner, as shown in the image and also described here:? The advantage of reducing the power supply voltage when tuning is that even if you forget to "turn on" the SWR meter, the finals will still survive. -- Sverre
LA3ZA
http://la3za.blogspot.com
|
The image of the Chinese QRP tuner in action for my vertical balcony antenna in EA8-land is here -- Sverre
LA3ZA
http://la3za.blogspot.com
|
On Fri, Apr 28, 2023 at 10:03 AM, John Z wrote:
Have you considered also using thermal paste under the BS170s ?
you could also sprinkle over the BS170s some Water out of the Holy Fountain of Lourdes from southwestern France. It would help as much as thermal paste, but it will have the big advantage of not doing no harm and cost less. Thermal paste is good between a radiator and a transistor designed to be mounted on a radiator. The BS170s are plastic transistors and your thermal paste will neither help nor harm, because the main thermical mismatch, the main barrier will be the plastic itself.? ?So using such a paste is just a mess, nothing more than that. Sorry to have put it so sharp, but this is an issue often debated and discussed in this forum.? What Sverre LA3ZA instructs us to do is the only way to stay out of harm's way when it comes to the lifespan and wellbeing of the transistors. And strange enough, what?Sverre writes above is much easyer to do and much closer to any common sense as smearing some paste without knowing why. It is prudence, modesty and wisdom at the same time. Following this path and increasing the efficiency of the PA instead of it's big-bang-power figure will result in transistors runing cool, smooth and remaining on the safe side both in terms of breakdown-voltage as also in what their thermical load concerns. Aiming for power levels beyond any common sense, driving currents trough those small transistors against any sanity is not the way to go. It could be, for a test, for trying beat all records. This kind of motivation might work for a short period of time and for a very well behaved load that stays put at 50Ohm resistive and Zero Ohm reactive, but it would imply that you know what you are doing and have a shelf full of spare transistors. They will blow away, sooner or later.?? Have fun but stay cool. Staying cool does not need refreshments. Yours friendly, Razvan dl2arl
|
Razvan, I think what you just implied is that folding the transistors down onto the pcb and holding them there with the screw and washer is a complete waste of time and material.?
Give it a try with the transistors standing upright, no washer, and let us know how that works out.
73. JZ
On Fri, Apr 28, 2023, 6:15 PM DL2ARL < dl2arl@...> wrote: On Fri, Apr 28, 2023 at 10:03 AM, John Z wrote:
Have you considered also using thermal paste under the BS170s ?
you could also sprinkle over the BS170s some Water out of the Holy Fountain of Lourdes from southwestern France. It would help as much as thermal paste, but it will have the big advantage of not doing no harm and cost less.
Thermal paste is good between a radiator and a transistor designed to be mounted on a radiator. The BS170s are plastic transistors and your thermal paste will neither help nor harm, because the main thermical mismatch, the main barrier will be the plastic itself.? ?So using such a paste is just a mess, nothing more than that.
Sorry to have put it so sharp, but this is an issue often debated and discussed in this forum.?
What Sverre LA3ZA instructs us to do is the only way to stay out of harm's way when it comes to the lifespan and wellbeing of the transistors. And strange enough, what?Sverre writes above is much easyer to do and much closer to any common sense as smearing some paste without knowing why. It is prudence, modesty and wisdom at the same time. Following this path and increasing the efficiency of the PA instead of it's big-bang-power figure will result in transistors runing cool, smooth and remaining on the safe side both in terms of breakdown-voltage as also in what their thermical load concerns.
Aiming for power levels beyond any common sense, driving currents trough those small transistors against any sanity is not the way to go. It could be, for a test, for trying beat all records. This kind of motivation might work for a short period of time and for a very well behaved load that stays put at 50Ohm resistive and Zero Ohm reactive, but it would imply that you know what you are doing and have a shelf full of spare transistors. They will blow away, sooner or later.??
Have fun but stay cool. Staying cool does not need refreshments.
Yours friendly, Razvan dl2arl
|
You left out #4:
Do not exceed the build voltage.
If you built for 9V then its 9V max.
If you built for 12V then, I should not have to tell you 14.2 is going to hurt.
--
Allison
------------------
Post online only, please no email.
|
On Fri, Apr 28, 2023 at 03:31 PM, John Z wrote:
?
Give it a try with the transistors standing upright, no washer, and let us know how that works out.
yes, it does work! Been there, done that. Done that on the safe, on the reasonable side of the power yield ambition. There are two types of heating. Having two different names: The one is the long term, average dissipated power of the poor overdriven transistors. This is what you might "feel" by touching those washers. This is what you might hope to get dissipated by those fancy washers. I like them because they keep the transistors in place. For nothing else. If you drive the transistors for a long time beyond any common sense, this average long term dissipated power will even start to smell and oh! wonder, the transistors might even support this ill-treatement in spite of the smell. In this case the washers might help distribute the smell over a wider area and give it even spices of burnt PCB, because the heat has nowhere else to go because the case is tight. Like letting you go inside a neoprene suit. But there is another type of heat production, coming from excessive peak power. The quick, instantaneous, the short term one, the one that will not endure being delayed by the? barrier raised by the plastic layer between the silicon substrate and whatever heat-sink-compound you provide towards the outside. This will be a quick death, not even causing a "puff", not producing any smell, not having any other expression of pain. The transistors will just die a quick and silent death. If you would measure the temperature OUTSIDE of the plastic shell, you would not even notice a sensible increase. It will be so quick, the world outside would not even notice their distress. What I am trying to say: Against overheating in the long term, it is wiser to avoid heat production by remaining on the humble side of the aimed power yield. How much power in W this will mean: your mileage might vary in accordance to the frequency. I like to think here in terms of drain current, even if this is only a part of the story. Myself I try to avoid torturing the transistors with currents higher than 200mA(DC) each. Might seem little, but aiming for nothing more will keep the transistor(s) much safer than greasing. Yess!!!! It would help (for the mean average dissipated power) if one would be able to apply that heat-sink-compound in direct thermical contact to the silicon substrate. But appliying that mumbo-jumbo to the plastic case is like bribing your mother-in-law to get better meals from your spouse.?? Against the short term overheating, against instantaneous lethal peak power levels... only common sense will help. I am persuaded (without knowing it) that many cases of "sudden QDX death" beweeped in this forum are due to spikes existing in poorly designed and carelessly used (switching) power supplies or spikes in the antenna system (which includes the ground-path!). This is where Sverre LA3ZA's Zenner diodes (understatements of even better transient supressors) would help. No need to grease anything here too. And as Allison states: you only get what you paid for: if you calculate it for 9V and drive it with 12V or if you design it for 12 and drive it to 14,2V it will be a short ride. Might be fun, but it will be sure a short one. So again: I am not trying to win the smart-guy contest just by insisting against your oppinion.? What I am trying to suggest here is: ¡ã plan the PA on the safe side following?Sverre LA3ZA's wise advice ¡ã stay on the humble side of the power and ¡ã do not to beat the saddle hoping that the horse will understand (i.e. grease the plastic case cool hoping that the silicon inside those BS will survive any abuse) Have fun! 73, de dl2arl
|
Humour aside Razvan does have some valid points. I have not yet put my QDX into transmit mode but already have concerns about the design of its PA stage. The requirements for not exceeding 12V (in a 12V HB version) or daring to transmit into anything other than the religious 50 ohm load is an indication that the design is maybe just a tad close to the limits for comfort. It almost made me laugh when I attached the little washer to the rounded side of the BF170s asking myself if I had mounted them upside down. Surely you should put the heatsink on their flat side I asked. In the desire to make the thing as small and cheap as possible it seems common sense has been overlooked. A couple of TO220 case devices on a slighly bigger board and a bit more metal to heatsink them would have made a far more robust PA and removed the need for all these discussions on this reflector. No criticism of Hans of course, the QDX is an incredible little rig, but as an engineer in my professional life I would have not done it that way. 73 Dave G3YMC On 28 Apr 2023 at 15:15, DL2ARL wrote: you could also sprinkle over the BS170s some Water out of the Holy
Fountain of Lourdes from
southwestern France. It would help as much as thermal paste, but it
will have the big advantage of
not doing no harm and cost less.
|
On 29/04/2023 05:45, Dave Sergeant G3YMC wrote: already have concerns about the design of
its PA stage Dave, I agree, there is something about this simple circuit that sometimes causes failure. I suspect that the vast majority of users have no problems. Hans has said he has abused his test units and not had any such failures. Obviously most satisfied constructors forget this group, only a few carry on reading. Most who return to the group with PA failure are not really certain of what actually triggered it. If anyone is worried I suggest the QDX is run conservatively. I never intended to run at 5 Watts, I use WSPR on 60m and aimed for 1 Watt. How about building for 12 volts and using 9 or less? A little while ago there were tests of winding T1 with different ratios for less power although I favour lower voltage. My way was to build for 12 volts, use 7.5 and also have 2 or 3 diodes in series with L14 to get just over 1 watt. 73 Alan G4ZFQ
|
Hi Dave, all
The washer isn't the heatsink. The heatsink is the PCB. The washer and bolt are just the mechanical means of bolting the flat side of the transistors to the PCB.
As others have said... Thermal paste doesn't harm but doesn't make a lot of difference, either. Be careful too because some pastes are electrically conductive and that won't go well. Make sure you don't use that.
Personally I have found the QDX PA reliable and durable as have many others. A lot of failures have posthumously been attributed to sustained chronic SWR mismatch or acute mismatch during auto tuner use, supply spikes etc., over-voltage, including from fully charged 12V-nominal batteries.
Remember also that on any forum such as this one, you will tend to hear far more frequently from people with a problem, than those without. You tend to get the mis-impression consisting of the proportion of people with problems, out of the collection of people with problems. The actual proportion of people with problems, out of the total of all users, is a lot lot lower (and difficult to quantify).
73 Hans G0UPL
On Apr 29, 2023, at 8:45 AM, Dave Sergeant G3YMC < dave@...> wrote:
Humour aside Razvan does have some valid points. I have not yet put my
QDX into transmit mode but already have concerns about the design of
its PA stage. The requirements for not exceeding 12V (in a 12V HB
version) or daring to transmit into anything other than the religious
50 ohm load is an indication that the design is maybe just a tad close
to the limits for comfort. It almost made me laugh when I attached the
little washer to the rounded side of the BF170s asking myself if I had
mounted them upside down. Surely you should put the heatsink on their
flat side I asked.
In the desire to make the thing as small and cheap as possible it seems
common sense has been overlooked. A couple of TO220 case devices on a
slighly bigger board and a bit more metal to heatsink them would have
made a far more robust PA and removed the need for all these
discussions on this reflector.
No criticism of Hans of course, the QDX is an incredible little rig,
but as an engineer in my professional life I would have not done it
that way.
73 Dave G3YMC
On 28 Apr 2023 at 15:15, DL2ARL wrote:
you could also sprinkle over the BS170s some Water out of the Holy
Fountain of Lourdes from
southwestern France. It would help as much as thermal paste, but it
will have the big advantage of
not doing no harm and cost less.
|
Dave et Al,
I have used the first prototype High Band since I designed the filters last year. I used at the home QTH and then took it to QRP Labs HQ for assessment and ultimate implementation. I have used that and another HB almost continuously. I¡¯ve operated with it portable in Prague, Rome and Istanbul with various wire antennas out hotel windows on fishing rods and wires on lead sinkers slingshotted into local trees. I used to use a small 12V PS with a variable buck down and meters but now, due to noise reduction measures, I run all my QDXes and RaspberryPIs on Lithiums, 2 in series that start at 8.4V and I usually change them and recharge when somewhere at 7.3V. RasPis I use a 30,000mAH power bank.? The one thing I am very careful about is antenna matching. I used to use LC matches with plastic printed spherical variometers as the variable L. I now use FRI-matches with 3D printed large toroids. I also have a switch that puts a LED swr bridge in line and a switched LED current monitor antenna wire.?
The first and original HB QDX is still going strong! It was built for 9V but usually operates at an average of 7.8V.?
Careful management of the antenna match is the most critical to long QDX life!
73
toggle quoted message
Show quoted text
On 29 Apr 2023, at 07:25, Hans Summers <hans.summers@...> wrote:
? Hi Dave, all
The washer isn't the heatsink. The heatsink is the PCB. The washer and bolt are just the mechanical means of bolting the flat side of the transistors to the PCB.
As others have said... Thermal paste doesn't harm but doesn't make a lot of difference, either. Be careful too because some pastes are electrically conductive and that won't go well. Make sure you don't use that.
Personally I have found the QDX PA reliable and durable as have many others. A lot of failures have posthumously been attributed to sustained chronic SWR mismatch or acute mismatch during auto tuner use, supply spikes etc., over-voltage, including from fully charged 12V-nominal batteries.
Remember also that on any forum such as this one, you will tend to hear far more frequently from people with a problem, than those without. You tend to get the mis-impression consisting of the proportion of people with problems, out of the collection of people with problems. The actual proportion of people with problems, out of the total of all users, is a lot lot lower (and difficult to quantify).
73 Hans G0UPL
On Apr 29, 2023, at 8:45 AM, Dave Sergeant G3YMC < dave@...> wrote:
Humour aside Razvan does have some valid points. I have not yet put my
QDX into transmit mode but already have concerns about the design of
its PA stage. The requirements for not exceeding 12V (in a 12V HB
version) or daring to transmit into anything other than the religious
50 ohm load is an indication that the design is maybe just a tad close
to the limits for comfort. It almost made me laugh when I attached the
little washer to the rounded side of the BF170s asking myself if I had
mounted them upside down. Surely you should put the heatsink on their
flat side I asked.
In the desire to make the thing as small and cheap as possible it seems
common sense has been overlooked. A couple of TO220 case devices on a
slighly bigger board and a bit more metal to heatsink them would have
made a far more robust PA and removed the need for all these
discussions on this reflector.
No criticism of Hans of course, the QDX is an incredible little rig,
but as an engineer in my professional life I would have not done it
that way.
73 Dave G3YMC
On 28 Apr 2023 at 15:15, DL2ARL wrote:
you could also sprinkle over the BS170s some Water out of the Holy
Fountain of Lourdes from
southwestern France. It would help as much as thermal paste, but it
will have the big advantage of
not doing no harm and cost less.
|
I have only had my HB for about a month and I have blown a couple of PAs due to matching issues. That aside, this QDX is FANTASTIC! The design is remarkable and well executed. I love the electronic aspects of the hobby so I always end up fiddling with things
but I would be proud of this design if it were mine. Just amazing what was done for the price. This thing makes me happy! I have ordered another for the low bands.
Tony
AD0VC
toggle quoted message
Show quoted text
Dave et Al,
I have used the first prototype High Band since I designed the filters last year. I used at the home QTH and then took it to QRP Labs HQ for assessment and ultimate implementation. I have used that and another HB almost continuously. I¡¯ve operated with
it portable in Prague, Rome and Istanbul with various wire antennas out hotel windows on fishing rods and wires on lead sinkers slingshotted into local trees. I used to use a small 12V PS with a variable buck down and meters but now, due to noise reduction
measures, I run all my QDXes and RaspberryPIs on Lithiums, 2 in series that start at 8.4V and I usually change them and recharge when somewhere at 7.3V. RasPis I use a 30,000mAH power bank.?
The one thing I am very careful about is antenna matching. I used to use LC matches with plastic printed spherical variometers as the variable L. I now use FRI-matches with 3D printed large toroids. I also have a switch that puts a LED swr bridge in line
and a switched LED current monitor antenna wire.?
The first and original HB QDX is still going strong! It was built for 9V but usually operates at an average of 7.8V.?
Careful management of the antenna match is the most critical to long QDX life!
73
Ross
6
On 29 Apr 2023, at 07:25, Hans Summers <hans.summers@...> wrote:
?
Hi Dave, all
The washer isn't the heatsink. The heatsink is the PCB. The washer and bolt are just the mechanical means of bolting the flat side of the transistors to the PCB.
As others have said... Thermal paste doesn't harm but doesn't make a lot of difference, either. Be careful too because some pastes are electrically conductive and that won't go well. Make sure you don't use that.
Personally I have found the QDX PA reliable and durable as have many others. A lot of failures have posthumously been attributed to sustained chronic SWR mismatch or acute mismatch during auto tuner use, supply spikes etc., over-voltage, including
from fully charged 12V-nominal batteries.
Remember also that on any forum such as this one, you will tend to hear far more frequently from people with a problem, than those without. You tend to get the mis-impression consisting of the proportion of people with problems, out of the collection
of people with problems. The actual proportion of people with problems, out of the total of all users, is a lot lot lower (and difficult to quantify).
73 Hans G0UPL
On Apr 29, 2023, at 8:45 AM, Dave Sergeant G3YMC < dave@...> wrote:
Humour aside Razvan does have some valid points. I have not yet put my
QDX into transmit mode but already have concerns about the design of
its PA stage. The requirements for not exceeding 12V (in a 12V HB
version) or daring to transmit into anything other than the religious
50 ohm load is an indication that the design is maybe just a tad close
to the limits for comfort. It almost made me laugh when I attached the
little washer to the rounded side of the BF170s asking myself if I had
mounted them upside down. Surely you should put the heatsink on their
flat side I asked.
In the desire to make the thing as small and cheap as possible it seems
common sense has been overlooked. A couple of TO220 case devices on a
slighly bigger board and a bit more metal to heatsink them would have
made a far more robust PA and removed the need for all these
discussions on this reflector.
No criticism of Hans of course, the QDX is an incredible little rig,
but as an engineer in my professional life I would have not done it
that way.
73 Dave G3YMC
On 28 Apr 2023 at 15:15, DL2ARL wrote:
you could also sprinkle over the BS170s some Water out of the Holy
Fountain of Lourdes from
southwestern France. It would help as much as thermal paste, but it
will have the big advantage of
not doing no harm and cost less.
|
For those of you with matching issues blowing your finals, Elecraft made a lovely QRP tuner good for 20 watts, model T-1. I have used one to match the antennas for my QDX's. all running 13.5 volts, with no problems of any kind.? Sure, it costs more than 5 QDX's, and it does work extremely well with that multiband end fed you like to use.... Might be worth a try. 73 N9AEP
On Sat, Apr 29, 2023 at 8:11?AM mux_folder2001 < canthony15@...> wrote:
I have only had my HB for about a month and I have blown a couple of PAs due to matching issues. That aside, this QDX is FANTASTIC! The design is remarkable and well executed. I love the electronic aspects of the hobby so I always end up fiddling with things
but I would be proud of this design if it were mine. Just amazing what was done for the price. This thing makes me happy! I have ordered another for the low bands.
Tony
AD0VC
Dave et Al,
I have used the first prototype High Band since I designed the filters last year. I used at the home QTH and then took it to QRP Labs HQ for assessment and ultimate implementation. I have used that and another HB almost continuously. I¡¯ve operated with
it portable in Prague, Rome and Istanbul with various wire antennas out hotel windows on fishing rods and wires on lead sinkers slingshotted into local trees. I used to use a small 12V PS with a variable buck down and meters but now, due to noise reduction
measures, I run all my QDXes and RaspberryPIs on Lithiums, 2 in series that start at 8.4V and I usually change them and recharge when somewhere at 7.3V. RasPis I use a 30,000mAH power bank.?
The one thing I am very careful about is antenna matching. I used to use LC matches with plastic printed spherical variometers as the variable L. I now use FRI-matches with 3D printed large toroids. I also have a switch that puts a LED swr bridge in line
and a switched LED current monitor antenna wire.?
The first and original HB QDX is still going strong! It was built for 9V but usually operates at an average of 7.8V.?
Careful management of the antenna match is the most critical to long QDX life!
73
Ross
6
?
Hi Dave, all
The washer isn't the heatsink. The heatsink is the PCB. The washer and bolt are just the mechanical means of bolting the flat side of the transistors to the PCB.
As others have said... Thermal paste doesn't harm but doesn't make a lot of difference, either. Be careful too because some pastes are electrically conductive and that won't go well. Make sure you don't use that.
Personally I have found the QDX PA reliable and durable as have many others. A lot of failures have posthumously been attributed to sustained chronic SWR mismatch or acute mismatch during auto tuner use, supply spikes etc., over-voltage, including
from fully charged 12V-nominal batteries.
Remember also that on any forum such as this one, you will tend to hear far more frequently from people with a problem, than those without. You tend to get the mis-impression consisting of the proportion of people with problems, out of the collection
of people with problems. The actual proportion of people with problems, out of the total of all users, is a lot lot lower (and difficult to quantify).
73 Hans G0UPL
On Apr 29, 2023, at 8:45 AM, Dave Sergeant G3YMC < dave@...> wrote:
Humour aside Razvan does have some valid points. I have not yet put my
QDX into transmit mode but already have concerns about the design of
its PA stage. The requirements for not exceeding 12V (in a 12V HB
version) or daring to transmit into anything other than the religious
50 ohm load is an indication that the design is maybe just a tad close
to the limits for comfort. It almost made me laugh when I attached the
little washer to the rounded side of the BF170s asking myself if I had
mounted them upside down. Surely you should put the heatsink on their
flat side I asked.
In the desire to make the thing as small and cheap as possible it seems
common sense has been overlooked. A couple of TO220 case devices on a
slighly bigger board and a bit more metal to heatsink them would have
made a far more robust PA and removed the need for all these
discussions on this reflector.
No criticism of Hans of course, the QDX is an incredible little rig,
but as an engineer in my professional life I would have not done it
that way.
73 Dave G3YMC
On 28 Apr 2023 at 15:15, DL2ARL wrote:
you could also sprinkle over the BS170s some Water out of the Holy
Fountain of Lourdes from
southwestern France. It would help as much as thermal paste, but it
will have the big advantage of
not doing no harm and cost less.
|
On Sat, Apr 29, 2023 at 03:17 AM, Alan G4ZFQ wrote:
Most who return to the group with PA failure are not really certain of what actually triggered it.
I wonder how many units have been damaged by connecting a charged cable to them.? Coaxial cable makes an excellent capacitor, and if not grounded (bleeder resistor, transformer winding, etc.) the center conductor can build up an impressive charge.? Even if the device does not fail immediately it can be damaged and fail prematurely. I always short the center conductor to outer conductor momentarily before connecting a cable to anything, in the shack or the lab.?? 73, Don N2VGU
|
I have three low-band QDXs (and a high-band one on the way), built for 12V operation, and I did manage to burn up the PA? on one (literally, with scorched FETs, driver IC, and PCB) when running at 12.5V into a poor antenna.? This was FST4W-120 (like WSPR).? The QDX ran fine for a few weeks, but then went kablooey.? My fault.??
Since then I have been running the QDXs from a 9V supply, and into a reasonably good load.? Zero problems, and these are running at close to a 50% duty-cycle, with two-minute transmit sequences.
--
Paul Elliott - WB6CXC
|
Have a look at? I¡¯ve built 7 and they work very well. Coax input and outputs are grounded through the coil. I¡¯ve printed 45-60mm diam toroids and also have used a T130-0 you can get quite a good L value.? I¡¯ve converted a few people to them. I only use Airwound toroids so no losses in iron powder.?
?I¡¯ve used in my travels quite a bit and all but one use polyvaricons as caps.?
73
toggle quoted message
Show quoted text
On 29 Apr 2023, at 13:11, mux_folder2001 <canthony15@...> wrote:
?
I have only had my HB for about a month and I have blown a couple of PAs due to matching issues. That aside, this QDX is FANTASTIC! The design is remarkable and well executed. I love the electronic aspects of the hobby so I always end up fiddling with things
but I would be proud of this design if it were mine. Just amazing what was done for the price. This thing makes me happy! I have ordered another for the low bands.
Tony
AD0VC
Dave et Al,
I have used the first prototype High Band since I designed the filters last year. I used at the home QTH and then took it to QRP Labs HQ for assessment and ultimate implementation. I have used that and another HB almost continuously. I¡¯ve operated with
it portable in Prague, Rome and Istanbul with various wire antennas out hotel windows on fishing rods and wires on lead sinkers slingshotted into local trees. I used to use a small 12V PS with a variable buck down and meters but now, due to noise reduction
measures, I run all my QDXes and RaspberryPIs on Lithiums, 2 in series that start at 8.4V and I usually change them and recharge when somewhere at 7.3V. RasPis I use a 30,000mAH power bank.?
The one thing I am very careful about is antenna matching. I used to use LC matches with plastic printed spherical variometers as the variable L. I now use FRI-matches with 3D printed large toroids. I also have a switch that puts a LED swr bridge in line
and a switched LED current monitor antenna wire.?
The first and original HB QDX is still going strong! It was built for 9V but usually operates at an average of 7.8V.?
Careful management of the antenna match is the most critical to long QDX life!
73
Ross
6
On 29 Apr 2023, at 07:25, Hans Summers <hans.summers@...> wrote:
?
Hi Dave, all
The washer isn't the heatsink. The heatsink is the PCB. The washer and bolt are just the mechanical means of bolting the flat side of the transistors to the PCB.
As others have said... Thermal paste doesn't harm but doesn't make a lot of difference, either. Be careful too because some pastes are electrically conductive and that won't go well. Make sure you don't use that.
Personally I have found the QDX PA reliable and durable as have many others. A lot of failures have posthumously been attributed to sustained chronic SWR mismatch or acute mismatch during auto tuner use, supply spikes etc., over-voltage, including
from fully charged 12V-nominal batteries.
Remember also that on any forum such as this one, you will tend to hear far more frequently from people with a problem, than those without. You tend to get the mis-impression consisting of the proportion of people with problems, out of the collection
of people with problems. The actual proportion of people with problems, out of the total of all users, is a lot lot lower (and difficult to quantify).
73 Hans G0UPL
On Apr 29, 2023, at 8:45 AM, Dave Sergeant G3YMC < dave@...> wrote:
Humour aside Razvan does have some valid points. I have not yet put my
QDX into transmit mode but already have concerns about the design of
its PA stage. The requirements for not exceeding 12V (in a 12V HB
version) or daring to transmit into anything other than the religious
50 ohm load is an indication that the design is maybe just a tad close
to the limits for comfort. It almost made me laugh when I attached the
little washer to the rounded side of the BF170s asking myself if I had
mounted them upside down. Surely you should put the heatsink on their
flat side I asked.
In the desire to make the thing as small and cheap as possible it seems
common sense has been overlooked. A couple of TO220 case devices on a
slighly bigger board and a bit more metal to heatsink them would have
made a far more robust PA and removed the need for all these
discussions on this reflector.
No criticism of Hans of course, the QDX is an incredible little rig,
but as an engineer in my professional life I would have not done it
that way.
73 Dave G3YMC
On 28 Apr 2023 at 15:15, DL2ARL wrote:
you could also sprinkle over the BS170s some Water out of the Holy
Fountain of Lourdes from
southwestern France. It would help as much as thermal paste, but it
will have the big advantage of
not doing no harm and cost less.
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On 28/04/2023 23:15, DL2ARL wrote: you could also sprinkle over the BS170s some Water out of the Holy Fountain of Lourdes from southwestern France. It would help as much as thermal paste, but it will have the big advantage of not doing no harm and cost less. Can we have some warning next time please that there's a wee bit of humour. It's taken me 45mins to get the coffee out of the keyboard and off the screen :-) I've lost count and basically given up trying to persuade people that transistors like the BS170 dissipate the heat through the leads and not the plastic body. People just don't seem to want to accept this fact. Andy
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DL2ARL
Dave Sergeant G3YMC
Donald S Brant Jr