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I recently had a breakdown in my QDX - after a prolonged operating period, the unit failed, the fuse blew and a nasty burning smell emerged.
I replaced the BF170s in the PA, and all was well. AFAICT, only one was faulty - cracked, reading less than 20 ohms, and pulling down the supply rail. The others seemed OK, though I replaced all four with new, from the same batch.
Then I noticed something (see photo): an irregularity in the tinning. In the vicinity of one of the transistors, I think the failed one, was a slight lump, and another smaller bump. I wondered if that was just coincidence, or might that have prevented the flat surface of the transistor making good thermal contact with the ground plane, reducing the heatsinking effect?
Anyway, before replacing the transistors, I carefully smoothed off the irregularity with some fine glasspaper to ensure better contact. Perhaps something to check before assembling other kits.
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I doubt that any such imperfection is terribly important, but smoothing it out can't hurt.?
However, I believe the imperfection is most likely secondary to the failure of the BS170, rather than the other way around.?
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73, Dan? NM3A
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That blob may have been a stress concentrator which caused the molded epoxy case of the BS170 to crack. If the washer, nut and screw were fairly tight to begin with, heat would expand the transistor's epoxy case and make things even worse.
JZ KJ4A
I doubt that any such imperfection is terribly important, but smoothing it out can't hurt.?
However, I believe the imperfection is most likely secondary to the failure of the BS170, rather than the other way around.?
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73, Dan? NM3A
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I think that bending the device lead without supporting it between the bend and the body was the cause of the cracked encapsulation.
Hold the lead close to the body with tweezers or fine needle-nosed pliers and use another tool to bend the lead.? Don't exert any force on the FET body.?
If you break the device open you will see that the lead does not protrude very far into the encapsulation, thus has limited strength.
Those solder blobs on the ground pad were caused by someone's tinned soldering iron touching the pad.
73, Don N2VGU
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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.
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Tisha, AA4HA
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Tisha AA4HA,
The QDX was my first introduction to use a bolt and a washer for heat transfer.? It was so foreign that I just stared at the picture in wonder of what the heck it was!
I really like your ideas too. Rather than copper and copper crush washers alone, maybe there are more ideas to include...? I saw the copper pads below the transistors of the PC Board and wondered why not use a few layers of common softer heavy-duty aluminum foil.? I really like your copper hardware ideas though.? Don't bother with more than pin punctures in the foil, because whatever foil pushes through holes in the board along with bolts through washers would help conduct heat by adding to contact areas.? So before adding the washer, why not add four or more layers of aluminum foil because it also conforms to contours of the transistor cases.? To add perfection to the mix by?dramatic numbers of small?wrinkles in the foil (so as to crush under that first layer on the bottom but only when there's a blob of solder under a case) but always wrinkle the layers that rest between the cases and the washer??
Keep imagining,
73? Paul KY4XJ
On Thursday, April 4th, 2024 at 00:55, Tisha Hayes, AA4HA <Tisha.Hayes@...> wrote:
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.
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Tisha, AA4HA
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I have yet to start my builds (I bought one of everything), but I've been reading the posts here for a while and slowly getting prepared.? I have even bought a pack of BS170s in anticipation of possibly needing them - hopefully I don't!? I've also grabbed various inexpensive heat sinks made for rasp pi, etc, considering a possible modification to the bolt on steel washer design.
Tisha and Paul, if you have any links or pics of implementation of M2.5 copper screws/nuts, copper crush gaskets, or other heat dissipation ideas, I'd appreciate it.? A cool radio is a happy radio!
Peace,
Todd K4MSW
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A bit different, but I used TN0110s for my last QDX build instead of the BS170s as has been discussed in other threads here, and then used a Wakefield LTN20069 heatsink drilled and tapped.? (pics below)
-Nate
N8BTR
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Nate,
Do you still have plans to re-do all your QDX versions? You also used zener diode protection as well as those substitute MOSFETs? Looks like you're on a path to be all set. I only have the one QDX low bands version. Mine's wired for 12V along with mostly solar-charged/powered gear. One day I'm going in to revise wite Han's RWTSW modification. I am banking ideas to add while I am in there. I'm going to need new enameled wire for a RWTSW revision and I am thinking to order a new binocular form with it.
Tisha,
I wonder where their thermal pad in RTL-SDR blog dongles is sourced. Those pads are very soft and provide contour contact to carry the heat inside right into the aluminum cases. That would certainly solve for our contour contact and so sink heat even better. My one reservation: the RTL-SDR gets very very warm but it doesn't reach a self-destructive temperature. Their dongle has a TCXO. Thus adding a thermal pad they get higher frequencies by way of increased range.
73s
Paul KY4XJ
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On Thu, Apr 4, 2024 at 01:23 PM, xyroto wrote:
Nate,
Do you still have plans to re-do all your QDX versions? You also used zener diode protection as well as those substitute MOSFETs? Looks like you're on a path to be all set.
Yes, that's the plan! I have a Rev 5 low band QDX I started with and use now. Blew the BS170s once, and replaced them with the L14 diode added then.?
I bought and built a HB version next with the L14 diode and TN0110s.?
Then I bought 3 more. :-)?
Long term, I'll have a low band, high band, and thinking a QDX-M built for 160M all together at my remote QTH, and then a mid-band (40-15M) that I'll keep for travel/POTA/home use. The 40-15 is next in line to get built so I can hopefully have it ready for a week's trip in June, and will be paired with my QRP-Guys 40-15 EFHW w/ tuner antenna.?
-Nate
N8BTR
?
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My QDX also has TN0110 finals and this is the configuration I've been operating with happily for a while now:?
Note, I believe there should be no need for active cooling in most cases. For my QDX the efficiency is much worse only on 10m for reasons I haven't debugged yet, and the finals get quite hot running FT8 only on that band. So for now I plug in the 5V side-blower fan when operating 10m and that keeps things well under control. Just in case you find you need one, the 30x30x7mm side-blower model on AliExpress turned out to be ideal for this. The screws in the fan are nylon so they keep it positioned a bit above the board without damaging anything. I run the fan with the QDX cover removed, but you could put a hole saw through your case if you really wanted to.
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I wondered that, too. The combination of overheating and mechanical stress. We'll never know! ;-)
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Ah! I hadn't thought of copper! I DID use plenty of heat sink compound (I'm surprised the manual doesn't suggest this) and I also added an extra (steel) washer that will hopefully improve things a tiny bit. ?
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Just as a brainstorm, given two facts: 1. BS170 is cheap 2. Electronic devices have a higher failure rate in their earlier life. I'm thinking maybe we can "bake" them first. we can build a circuit to start to "bake" 6 BS170 until 2 of them fail, the rest 4 should be able to handle the stress better.
73, Wei, AG6AQ
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Hi Peter,
Does the fan cause any noticeable?interference to the receiver since it's very close by?
73,Wei,AG6AQ
My QDX also has TN0110 finals and this is the configuration I've been operating with happily for a while now:?
Note, I believe there should be no need for active cooling in most cases. For my QDX the efficiency is much worse only on 10m for reasons I haven't debugged yet, and the finals get quite hot running FT8 only on that band. So for now I plug in the 5V side-blower fan when operating 10m and that keeps things well under control. Just in case you find you need one, the 30x30x7mm side-blower model on AliExpress turned out to be ideal for this. The screws in the fan are nylon so they keep it positioned a bit above the board without damaging anything. I run the fan with the QDX cover removed, but you could put a hole saw through your case if you really wanted to.
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I've seen no interference from the fan when operating 10m.
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I'm barely starting up with amateur radio.? I intend to use JS8-Call which reportedly almost creates an oven.? Reading everyone's take, I like your added heatsink and Zener diode best and I'm wondering whether to add a small fan that I will power with a PowerAdd USB charger.? ?
It looked like you rewound that transformer too??? That aimed to prevent burning the finals too.? WoW!? Feels like this might have a chance.
I am so thankful for your ideas and practices. -Paul??
On Thursday, April 4th, 2024 at 15:58, natereik@... <natereik@...> wrote:
On Thu, Apr 4, 2024 at 01:23 PM, xyroto wrote:
Nate,
Do you still have plans to re-do all your QDX versions? You also used zener diode protection as well as those substitute MOSFETs? Looks like you're on a path to be all set.
Yes, that's the plan! I have a Rev 5 low band QDX I started with and use now. Blew the BS170s once, and replaced them with the L14 diode added then.?
I bought and built a HB version next with the L14 diode and TN0110s.?
Then I bought 3 more. :-)?
Long term, I'll have a low band, high band, and thinking a QDX-M built for 160M all together at my remote QTH, and then a mid-band (40-15M) that I'll keep for travel/POTA/home use. The 40-15 is next in line to get built so I can hopefully have it ready for a week's trip in June, and will be paired with my QRP-Guys 40-15 EFHW w/ tuner antenna.?
-Nate
N8BTR
?
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Mark Palmer
Donald S Brant Jr
Tisha Hayes, AA4HA