开云体育

After assembling a QDX rig for a local ham I received a couple of the QDX PA mod PC boards made by WB2CBA which have the FDT86256 mosfets. They have a max voltage of 150V compared to the 60V of the BS170. Since I like to experiment I decided to try and adapt one of the boards to my QMX+. I filed off the two "tabs" opposite the mosfets being careful not to file off? any of the traces. It fit perfectly where the BS170's were. After some testing it seems to work well. My QMX+ is presently wired for 9V and I have a Buck-Boost converter stuck on the back set at 10V so I can use any battery I like from 5V-30V. On 80m-30m I can easily get 5w with the 10v input. Since I like to "play with fire" I plugged in my 12.6V battery and got 6.5w-8W on the lower bands. The mosfets stayed very cool....into a dummy load. While playing on FT8 with my Hexbeam and low SWR sometimes the finals got warm and actually got pretty hot on some of the higher bands but kept work fine.?

? ?If anyone else out there can or wants to give it a try, especially if you have the proper test equipment please let me/everyone know what you found.

The info for the PC boards are here... ?

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Tim - K5DEZ

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And as you can obviously see, I added a couple small heat sinks using a thin coat of contact cement to hopefully add a bit more metal mass. Just make sure they do not touch.

Tim - K5DEZ

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The usual problem with (not entirely sure why) beefed up ?FETs is poor 10 metre performance

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Do you have any measurements ?for this Mod ??

My output wattage on the original finals was a bit low anyway. With the larger FETs it was still about the same but with the ability to run at a higher input voltage if desired, it was easy to make up any differences plus a bit more. I did some preliminary checks and overall the output is better except like you said the higher bands suffer a bit. For me, it is good enough and gives me some leeway for output. The lower bands 80m, 40m 30m do extremely well. They are up to 7W-9W range. I believe they are 10W components so there is a lot more overhead and with the 150V max voltage they are less likely to burn out. For now that's my story an I'm sticking to it until I break something else. Experimenting for me is part of the hobby. YMMV.

Tim - K5DEZ

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Tim, great idea re. the tab cutting. I have a couple of those upgrades lying around and will upgrade my QMX+ too.

Will post pics and results when done.

It will be nice to have a unit that isn't afraid of a little extra VIN. Hopefully I'll be able to run directly off of my LiFePo battery or a standard Ham supply.

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Randy, N4OPI

No joy here, but I promised to post my results.

After "upgrading", the performance was noticeably worse in my case. Especially and not unexpectedly on the higher bands. See below.

So I personally will continue using an inline regulator to protect from overvoltage but am switching from 7812s to the Low Dropout (.5v vs. 2v) LM2940C-12.

MMDV...

And thanks to Hans for providing such high quality circuit boards that we experimenters can solder, unsolder, solder, unsolder, and solder again and have no noticeable impact to the circuit traces.?

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Radio	Band										Avg.	?
?	160	80	40	30	20	17	15	12	10	6	?	?
QMX+	5	5.2	4.6	4.3	3.7	2.5	4.5	3.3	3.5	0.9	3.8	Original BS170s
QMX+	4.5	5.9	4.4	4	2.3	1.7	2.5	1.5	1.5	0.5	2.9	With FDT86256 PA Mod
QMX+	5.4	6	4.7	5	2.8	2.2	4.7	3.6	2.6	0.8	3.8	Back to BS170s (new set)

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Randy, N4OPI

Randy,

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I am not surprised by your results.? If you look at the datasheet from Onsemi, the device needs 6 volts gate to source to turn on fully:

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You could feed the 78ACT08 6 volts to see if that helps.? You could also try a buffer MOSFET (maybe a BS170) to increase the drive voltage.? Both possible solutions require a lot of board surgery.? Feeding 6 volts to the ACT08 is pushing that device to its limit and is over the limit for the TI part.? The Onsemi part can go to 6.5 volts.

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Better to stay with the BS170s.

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This is the exact scenario where TN0110 shines. 100V breakdown and its lower threshold voltage precludes the need for additional drive capability. Why not try these Randy?

Tony AC9QY?

Hey Randy, after doing some more testing I pretty much have the same results as you. On 80m 40m and 30m the difference was remarkable. 20m was mediocre. 17m and 15m were not great either. 10m for some reason was about the same. Although I was "cheating" a bit since I have my QMX+ wired for 9v and I was using 12v input. On the lower bands it was fantastic, but like stated by others the upper bands really suffer. It made the upper bands better but not by very much so in the end not really worth it, unless you like 80-40-30 only.? I just ordered a dozen TN0110 mosfets from Mouser to give them a try. Like I said, experimenting is part of the fun and learning.

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Tim - K5DEZ

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If you guys supply 74ACT08 with 6 Volts higher bands will improve

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I got couple of emails how to power 7408 with 6V in QMX+.?

This is how I modified mine, lift pin 14 of 7408 from pcb carefully. Add a 7806 voltage regulator to 12V input and connect lifted up pin14 to that 6V regulator. That’s all. This will improve your high bands dramatically.

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Gate capacitance usually limits performance at HF.

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Brent DeWitt, AB1LF
Milford, MA

Barb,

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Read the 7806 datasheet, you definitely need a cap at the output to ground.

You may already have the required cap for the input pin close enough, but also check that.

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Jerry

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On Sat, Apr 12, 2025 at 07:55 AM, Barb WB2CBA wrote:

Brent,

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in this case gate capacitance is not the issue as 2xBS170 parallel and one FDT86256 has almost equal capacitance.?

The problem is gate driving voltage is low.

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I have tried many sets of BS170, TN0110 and now one set of the FDT86256. The TN0110 works the best for me on all bands. I have never blown up a set of TN0110 and I get the best power out across the bands with them. I have not tried 6 volt supply. maybe I will some time.

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I currently have a second ACT08 stacked on top of the original. I noticed better output on 6 meters when I did that but YMMV.?

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One thing that bothers me is that some people post that they are getting 5 watts out across all of the bands. I have never managed anything like that. On 80 meters sure. But upwards from there no. I use a Boonton 92BD RF voltmeter with the DBM option and a 10 watt 20db attenuator. I have a Boonton calibrator and I trust the measurements I get. I have a low band and a high band QDX which, power wise, are consistent with my QMX+. I would really like to know how people manage to get full power out on the upper bands.

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Tony

AD0VC

The fact is the QMX+ has lower output as you increase in frequency.

Fortunately, you don't need as much power as you increase in frequency.

Anyone who reports otherwise probably has errors in their test equipment.?

I use the QRP Lab radios for WSPR propagation studies. It would be so

beneficial if behind every power output graph, one could imagine an image

of the propagation state of the band at the time. It dwarfs the importance of the output.?

This is the sort of power I get:

I'm measuring the power in 3 ways, PWR is what my QMX+ reports, PWR(pk) uses the peak voltage from my dummy load and PWR(pp) is the peak to peak voltage reported by my DSO2D10 oscilloscope. They are about what I would expect, although it's a puzzle why the 12m and 17m powers are so much lower, it doesn't seem to be anything obvious such as being off the edge of the LPF.

This is all using the standard BS170s, no messing around (so far).? It's a 12V QMX+, running on 12V.

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Chris,

Your 3 measurement systems agree pretty well but the discrepancy at 6m is fairly large. Any idea why?

Tony AC9QY