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Qmx in receiver mode tx voltage in red

Giovanni Ilari IT9XJJ
 

 

Giovanni,

Factory build or kit? Does TX work OK?

Assuming that the voltage reported has not been requested via settings, here are a couple of possibilities:

1) Q507 has failed, shorted. An ohmmeter check should reveal that.

2) Q507 is OK but something is drawing the gate of Q507 downwards, turning it on.

?A similar case long ago revealed a solder bridge between Q507 gate and one leg of L502. An ohmmeter check and visual inspection should find that.

73 JZ KJ4A?



On Sat, Feb 24, 2024, 2:39?AM Giovanni Ilari IT9XJJ <giov.ilari@...> wrote:
Giovanni Ilari IT9XJJ
 

Low power on all high band about 1-2 watts, kit 12v, tks for answer, i will check what you tell me. Tks, i will inform you next. 73 de it9xjj
Giovanni Ilari IT9XJJ
 

Yes! Q507 shorted! Measured drain and source and also shorted with gate! Vy difficult desolder! I have not hot air desolder, is there a fast solution??
 

Giovanni,

Before you attempt removal, please post what readings you obtained. Remember, there are other circuit elements present that must be considered.

JZ


On Sat, Feb 24, 2024, 7:29?AM Giovanni Ilari IT9XJJ <giov.ilari@...> wrote:
Yes! Q507 shorted! Measured drain and source and also shorted with gate! Vy difficult desolder! I have not hot air desolder, is there a fast solution??

 

G.,
One more thing...

If a short (e.g. solder bridge) exists between gate and L502, as was the case prior, it will read as a hard short gate-to-drain at Q507.?

You should check that as well before any attempt at surgery.

JZ

On Sat, Feb 24, 2024, 7:32?AM John Z via <jdzbrozek=[email protected]> wrote:
Giovanni,

Before you attempt removal, please post what readings you obtained. Remember, there are other circuit elements present that must be considered.

JZ

On Sat, Feb 24, 2024, 7:29?AM Giovanni Ilari IT9XJJ <giov.ilari@...> wrote:
Yes! Q507 shorted! Measured drain and source and also shorted with gate! Vy difficult desolder! I have not hot air desolder, is there a fast solution??

Giovanni Ilari IT9XJJ
 

Gate source 0.6 ohm, source gate 0.6 ohm, gate drain 0.8 ohm, drain gate 0.8, source drain 0.3 ohm? drain source 0.3
Giovanni Ilari IT9XJJ
 

Gate and l502 are far away
 

Ouch! That looks pretty bad, Giovanni!
?
That transistor is a pretty robust device. Whatever killed it must have been an energetic event. Carefully inspect around the PA circuitry and make sure there are no solder balls or wire bits lurking!

Yes, removal of Q507 is not easy. Lift-up damage to the G and S pads is possible! You might carefully cut those leads first and remove them individually.

After that, heating the drain connection and perhaps flooding it with a solder that has a lower melting point than the lead-free solder used in production might do the job.

Jeff Moore may have additional tips for you on safe removal.

JZ

On Sat, Feb 24, 2024, 7:48?AM Giovanni Ilari IT9XJJ <giov.ilari@...> wrote:
Gate source 0.6 ohm, source gate 0.6 ohm, gate drain 0.8 ohm, drain gate 0.8, source drain 0.3 ohm? drain source 0.3

Giovanni Ilari IT9XJJ
 

I remind a short cut there
 

Hello Giovanni,
John alerted me to your post.

the safest way to remove Q507 is to first carefully remove the inductor L502. ?That should be easy with flux, solder wick and a decent iron. ?Then cut the two legs of Q507 with small sharp diagonal cutters. ?That frees Q507 at that end. ?Then with a medium tip on your iron and lots of flux, heat the main pad under the drain. ?Be sure to touch the pad and the drain. ?This should free it for removal with tweezers. ?If not, your iron is not hot enough or enough wattage. ?In this case you can flood the pad with low temp solder and flux then remove.

good luck,
Jeff
W1NC
 

Thank you, Jeff!

Good Luck, Giovanni!?

BTW,? I am looking at your photo and also studying Hans' layout to see what you are flagging. Do I see a subtle BS170 gate to drain short in your photo?

JZ

On Sat, Feb 24, 2024, 8:29?AM Jeffrey W Moore via <jeffreymoore=[email protected]> wrote:
Hello Giovanni,
John alerted me to your post.

the safest way to remove Q507 is to first carefully remove the inductor L502.? That should be easy with flux, solder wick and a decent iron.? Then cut the two legs of Q507 with small sharp diagonal cutters.? That frees Q507 at that end.? Then with a medium tip on your iron and lots of flux, heat the main pad under the drain.? Be sure to touch the pad and the drain.? This should free it for removal with tweezers.? If not, your iron is not hot enough or enough wattage.? In this case you can flood the pad with low temp solder and flux then remove.

good luck,
Jeff
W1NC

 

I forgot to say…
clean up the Q507 pad after removal. ?Clean off leg pieces and all solder. ?Then apply fresh flux before soldering in a new one.

Jeff
W1NC
 

Giovanna,

I think the arrow you have drawn on your photo indicates a connection between BS170 transistor drains and ground, if I am reading the PCB layout correctly. That would not be a good condition!

On another QMX that Jeff had repaired,? one in which Q507 was destroyed, he found a short circuit from primary winding to secondary winding in T501.

JZ

On Sat, Feb 24, 2024, 8:35?AM Jeffrey W Moore via <jeffreymoore=[email protected]> wrote:
I forgot to say…
clean up the Q507 pad after removal.? Clean off leg pieces and all solder.? Then apply fresh flux before soldering in a new one.

Jeff
W1NC

Giovanni Ilari IT9XJJ
 

Many thank's..... i think use qmx only as receiver....?
Giovanni Ilari IT9XJJ
 

I will check it
 

OK Giovanni!?
Don't give up! This QMX can be restored.
JZ

On Sat, Feb 24, 2024, 8:50?AM Giovanni Ilari IT9XJJ <giov.ilari@...> wrote:
I will check it

Giovanni Ilari IT9XJJ
 

I made the short circuit on purpose because on the wiring diagram the source goes to ground, as soon as I connected the power the fuse blew, i mistake!?
 

Yes, that was a mistake! Hopefully Q507 is the only casualty. Get that corrected and check back here if there is something else to find. Good luck!? JZ


On Sat, Feb 24, 2024 at 8:59?AM Giovanni Ilari IT9XJJ <giov.ilari@...> wrote:
I made the short circuit on purpose because on the wiring diagram the source goes to ground, as soon as I connected the power the fuse blew, i mistake!?

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