QDX - Low Output - Blown BS170s

[@Randrew]

Thanks for the video! You're definitely not the first to blow the BS170s... after a couple of iterations, I came up with a fairly robust solution for running continuously at 13.8 VDC supply on a "9 VDC" build of a Rev 3 DQX: 1 - I used new ONN Semiconductor BS170s tested to matched Vth (not sure if that last is important at all) 2 - I cut an 18x22x36 mm piece from an aluminum, fanless, CPU heatsink, filing the new edges to remove burrs 3 - I applied a very thin layer of "high temp" silicone RTV between the PCB and the transistors bodies, bolted them down per kit instructions and let the RTV set. 4 - I drilled a hole in the center of the new heatsink of a size appropriate for the kit-supplied screw to self-tap into it 5 - I removed the hold-down nut and washer from the transistors, applied a thick mix of RTV and fine aluminum filings between, around and atop the transistor bodies, to where it could contact the heatsink, being careful not to short out the transistor leads. 6 - With the RTV still gooey, I screwed the heat down firmly to the transistors and let the RTV set 7 - I cut a 20x24 mm hole in the top enclosure cover so the heatsink can stick out the top. I used this configuration at 13.8 VDC supply and FT8/FT4 duty cycles for hours at a time over at least a 2 month period with no problems. The whole enclosure does warm up noticeably with heavy usage - I think the PCB itself is still spreading a lot of the transistor heat, per design - but found that a quiet, 80mm PC fan blowing roughly in the direction of the QDX and its new heatsink was enough to keep it quite cool. I did later revise this QDX of mine to the "12 VDC" configuration so I could run it at 12 VDC and not overheat the input attenuator of the QRP-Labs 50W amplifier I built later. I regularly use this combo at home with the amplifier dialed down to about 20W output. In this new configuration the QDX still works well at 13.8 VDC (~5.5 Watts out) to 18 VDC (~8.5 W) by itself when I go portable. NOTE: I've only run WSPR continuously at 5W output because it never occurred to me to do it at any other power ;) -N5RGN PS: You're not in my logs yet, but maybe I'll see you on 20m, FT8!

[@na5y]

Thank you for the great tips there Randrew! I was using WSPR when I blew mine which I think is a little more taxing (its full transmit for 2mins) - but your heatsink solution is intriguing. Thank you for the comment!

[@sarahjrandomnumbers]

I blew one of my BS170's recently too, I just replaced all of them like you because why risk it for the pennies a new mosfet costs. Plus I knew I'd probably blow some up, so I got 20 of them in stock ready for when I need them :).

[@na5y]

I have a pile of those BS170s although the numbers are shrinking... I often find one or two on the garage floor! Thanks for the comment

[@F4LPM]

like the USDX same fet and i have already 2 time replaced

[@stevew9945]

interesting video keep up the good work

[@na5y]

Thank you Steve

[@rfdave3980]

Did you put heatsink compound on the outputs? Just wondering.

[@na5y]

I was *thinking* of doing that but never did...

[@john8520]

Great video, very informative! Can you detail how you're running the RF from the QDX into your scope? Thanks!

[@na5y]

Sure John. I have the QDX output going to a 50 ohm dummy load and I connect the oscilloscope input in parallel to that. You can see the wires I am connecting the oscilloscope to in the bottom right at 1:21 in the video. I hope this answers your question and thank you for the comment.

[@john8520]

@@na5y That makes a lot of sense, thanks for the quick reply!

[@warplanner8852]

Ian, did you do any matching of the FETs before installing them?

[@na5y]

No I didn't - I did read somewhere once that matching FETs in this configuration wasn't critical. I'll see if I can find the link...

[@johnwest7993]

I'm surprised it lasted all day. Do you know what the QDX output power was from the 14 volts that blew the FET? BTW, I just stumbled on your site, and I'm slowly watching all of your videos. Excellent stuff.

[@na5y]

Thank you for the kind comment John. I didn't measure the output power - I accidentally left the power supply at 14V from some other testing I was doing.

[@Randrew]

By my measurements, running a 9V QDX build at 14V yields: 9.8W output on 20m, 9.8W on 30m, 9.8W on 40m and 10.2W on 80m. Caution: even fairly brief transmissions at 14VDC is likely to blow a transistor or 3 unless you've taken extra steps to cool them.

[@johnwest7993]

@@Randrew, I'm not even sure you 'can' cool them enough. I suppose it depends on what output transistors it has.

[@Randrew]

@@johnwest7993 " it depends on what output transistors it has" -> Output transistors = 4 x BS170. They're kinda small. " I'm not even sure you 'can' cool them enough" -> See my other post in these comments. Very short version: I glued a decent heatsink to the transistors and run at 14 VDC with impunity.

[@R2AUK]

Generally MOSFETs are sensitive to the static electricity. Since you blow up so many maybe it worth considering using a grounding bracelet...

[@na5y]

I swear I am not blowing these through static Alex! ;)

[@Paul_VK3HN]

You're not the first to blow up the BS170s I suspect. But easy to diagnose and fix.

[@na5y]

And they are pretty cheap too! Thanks for the comment Paul

[@johnwest7993]

@@na5y, I looked over the schematics for both the QDX and the (tr)uSDX before I purchased them, and it quickly became apparent that the BS170's are a dual purpose component. In addition to operating as the RF final amplifiers they also function as the circuit's fuse. I bought a dozen of them to go with these kits. I may buy more, just in case. :)

[@na5y]

@@johnwest7993 There are Four BS170s in the PA but that mosfet across the input supply is an SMD p-channel. I have blown both types now!

[@Paul_VK3HN]

@@johnwest7993 Three legged fuses, and they open a lot faster than a wire in a glass tube.

[@johnwest7993]

@@Paul_VK3HN, very reliable. :)