开云体育

ctrl + shift + ? for shortcuts
© 2025 开云体育
  1. QRPLabs
  2. Topics

Short circuit in buck converter QMX

 

Hello all,

After finishing my QMX I tried to switch it on and upload the firmware, starting with? voltage at 7V and current control on about 250 mA, voltage dropped to 1V. So my conclusion was the must be a short circuit somewhere. And yes, I found a short circuit between two drains of Q104 and Q105 on the 5V buck converter. It is very difficult to see the short with naked eyes, so I used mij microscope to see it.? Ofcourse you can measure it with a multimeter. See attachments. So change the position of Q105 a bit to the right and everything works fine.

Succes.

73's Jacques PA3EVZ
g4edg
 

Hi Jacques

Similar problem here. First time powering up I set the supply to 8v and limited current to 250ma. Power supply shut down upon pressing power button. Problem seemed to be with Q103 as there was short between G-D. I tried to lift the leads from the PCB to test out of circuit but they fractured from the body :( The drain of Q104 is as you say very near the Drain tab of Q103, I think a small globule of solder was bridging the two.

Better order a replacement.?

Thanks for posting this Jacques

Steve G4EDG
 

Same problem here with the Q103/Q104 short.? Also damaged Q103 same as Steve.? Replacement xstrs on order from Digikey.? Hope this is extent of power problem.
Bob/WB0AUQ
 

Jacques,
Thanks, I had the same problem, I was able to unsolder Q103 and move it a small distance. Problem solved. I hope Hans will make some minor modifications to the board so this will not occur in future boards.

Dennis
KK5DB
 

> I hope Hans will make some
> minor modifications to the
> board so this will not occur in
> future boards.

He will...

73 Hans G0UPL


On Thu, Aug 3, 2023, 11:08 PM Dennis Rieger <kk5db@...> wrote:
Jacques,
Thanks, I had the same problem, I was able to unsolder Q103 and move it a small distance. Problem solved. I hope Hans will make some minor modifications to the board so this will not occur in future boards.

Dennis
KK5DB

 

Hi Steve,

No thanks at all.
You're welkom.
I'm glad that my post was helpfull.

73, Jacques
Op 3 aug. 2023, om 17:07, "g4edg via " <aol.com@groups.io target=_blank>[email protected]> schreef:

Hi Jacques

Similar problem here. First time powering up I set the supply to 8v and limited current to 250ma. Power supply shut down upon pressing power button. Problem seemed to be with Q103 as there was short between G-D. I tried to lift the leads from the PCB to test out of circuit but they fractured from the body :( The drain of Q104 is as you say very near the Drain tab of Q103, I think a small globule of solder was bridging the two.

Better order a replacement.?

Thanks for posting this Jacques

Steve G4EDG

 

Hi Dennis,

No thanks at all.

You're welkom.

I'm glad you found the problem.

Up til now, as far as I know,? there are 4

buck converter with this problem.


73, Jacques PA3EVZ


Op 3 aug. 2023, om 22:08, Dennis Rieger <kk5db@...> schreef:

Jacques,
Thanks, I had the same problem, I was able to unsolder Q103 and move it a small distance. Problem solved. I hope Hans will make some minor modifications to the board so this will not occur in future boards.

Dennis
KK5DB

 

Pretty sure mine has the disease also. ?Tomorrow’s task to fix.?


Aug
AG5AT
 

Guess I better check my PS boards over very carefully before I install them!

73, Bill NZ0T
 


I've had gotten the kit several days ago - have broken the pieces apart, but haven't populated the boards. Good to hear about this now.

I had gone to check - this is tough. Q103 overlaps the tab from Q104 enough that a visual inspection isn't enough - and the clearance doesn't allow the probe of my multimeter to get in there. I had to cheat and tape a sewing needle to a probe to get in there. Fortunately, no short found in my instance.

Thanks for the find.

John KI3J
 

I recently received two kits. One had the short, the other did not. I'll try to move Q104 slightly once I get a hot air desoldering tool on order. 73, Mitch NK3H
 

Where are the attachements? I would really like to see a picture of this area...
--
Dan Reynolds -- KB9JLO
<><
 

Here is a picture of the 5V board showing the problem area between the Drain of Q103 (large part) and Drain of Q104. You can even make out the start of a solder ball between the two. You can easily heat up the Q103 Drain and push it over a little to fix any accidental contact exposure.

73 Kees K5BCQ
 

This is probably a good thing to check and fix even if your particular board does not have a short (.....today.....). Looks like the Mfg placement equipment put a slight counterclockwise twist on the Q103 location which does not help.?

73 Kees K5BCQ
 

Hello all,

I reported this problem on july 6.
But here with again.

73, Jacques PA3EVZ
Op 6 jul. 2023, om 11:20, Jacques PA3EVZ <jacques.pa3evz@...> schreef:

Hello all,

After finishing my QMX I tried to switch it on and upload the firmware, starting with? voltage at 7V and current control on about 250 mA, voltage dropped to 1V. So my conclusion was the must be a short circuit somewhere. And yes, I found a short circuit between two drains of Q104 and Q105 on the 5V buck converter. It is very difficult to see the short with naked eyes, so I used mij microscope to see it.? Ofcourse you can measure it with a multimeter. See attachments. So change the position of Q105 a bit to the right and everything works fine.

Succes.

73's Jacques PA3EVZ
 

Thanks for posting, I checked mine and it has the short circuit too!? Luckily I haven't powered it up yet.

Glenn VK3YY.
 

On Mon, Aug 7, 2023 at 02:44 AM, Jacques PA3EVZ wrote:
Thanks to Jacques PA3EVZ for originally finding the problem, it's easy to see the solder bridge on the picture I took and it definitely warrants checking your QMX before you power it up or even if it appears to be operating fine now. The short is from the large Q103 Drain Tab to the small Q104 Drain.

73 Kees K5BCQ
 

John,

Thanks for the sewing needle tip.? Checked and I have no short!


73, Bill NZ0T
 

Don't even need sewing needles! Armed with the cameo schematic, normal probes on gate and drain of Q105 should read about 100k if no short between drain of Q105 and that BSS123 so close by.

Mine reads 95 and 99 point something either polarity convincing me I don't hafta slide Q105 to the right and maybe my QMX won't catch fire on first power up!

Thanks, boys, for providing this topic and solution!

73
Karl
KI4ZUQ
 

Hi all

Just to support what Karl's saying and clarify, please refer to the attached photo. The green arrow shows the location of the potential short. The drain pin of the small transistor (Q104) is clearly connected via a visible top-layer trace to the nearby Q105 gate (also see schematic). So no needles are necessary, only normal DVM probes between the points with yellow arrows.?

73 Hans G0UPL

1 - 20 of 44