Oh No... My KX1 is to young to smoke...

[Denny Bowman]

Hi to all Elecraft reflector readers….. I have been monitoring this
particular reflector for about 3 years now and this will be my first posting
to it…… Elecraft Gary (AB7MY) recommended I tell my story so those of you
that own KX1’s (particularly) or other Elecraft rigs can learn by my recent
experience….. I will now present the e-mail dialog I had with Gary which
will tell my story…. FYI these e-mails took place in a 24 hour period (30
March 05) which shows The QUICK Response Dedication Elecraft prides itself
on…..

So, here we go:

=============================================

Hi Elecraft…

Sad day in Edmonds, WA…… My KX1 # 841 started smoking and it’s not even a
teenager yet…… I purchased and built the kit in August 2004 and the rig has
worked flawlessly since then…… This morning I heard FT5XO booming into the
Seattle area on 40 meters and wanted to work him QRP with the KX1….. I
turned the little rig on just like always but this time there was no LED
display and then I saw smoke coming out the LAMP opening followed by a harsh
smell….. I immediately power the unit off and opened the bottom cover and
discovered a severe circuit trace melt down…. I looks like something failed
causing the V+ (12VDC from an external gel cell battery) circuit trace
around RP5 and under R16 and R17 to burn open (disintegrate)…… I don't have
a PWB trace layout drawing for the KX1 board so it hard for me to tell how
much board damage incurred...... I was wondering if this was a warranty
issue since the rig isn’t a year old yet….

Thanks

Denny  W7SNH

==============================================
Response from Elecraft Gary....

Hi Denny,

Such damage can occur if the thermal pad on the PA transistor is worn, torn,
not in place well, or if there are metal bits or burrs under the thermal pad
or around the PA transistor mounting hole. It can also occur if the PA
transistor mounting screw is too tight.

Later KX1 kits have a nylon flat head screw to prevent over tightening of
the PA mounting screw, and to prevent a short if the counter sink hole is
worn or is cut too deep so the tapered part of a flat head metal screw could
make contact with the inside of the PA transistor tab's hole. You can obtain
a nylon screw from the local Ace Hardware, or by emailing
[hidden email].

The damage is usually limited to the burning of the large PCB trace that
runs from the power switch over to the two PCB crossovers (or visa) near the
middle of the board. The trace can be repaired by cleaning off the burned
residue with Q-Tips moistened with acetone or isopropyl alcohol, and by
scraping and cleaning the area gently back to a sound portion of the copper
trace. R16 and R17 may need to be removed temporarily so you can splice a
short piece of #22 to #20 awg bare or insulated solid wire to complete the
circuit if the trace is burned in two. Or, route an insulated #22 to #20 awg
wire over R16, R17.

Solder the wire to a good part of the copper trace which has been scraped
and cleaned to bright copper. Tin the copper and then place the wire on the
tinned portion and hold it down with a small screwdriver as you solder it in
place. A little bit of instant glue and nail polish can be used to hold the
repair in place and protect the copper from oxidation.

Inspect PA choke L3 for burned wires, and rewind it if necessary. No other
damage is usually found.

The radio is then "good as new", other than the damage to your pride and
joy.... ;-)

This damage is not covered under warranty, since if you are not using the
internal batteries - the power cable should be fused with not larger than a
1 or 2 amp fuse. This is especially important if you are using an external
gel cell battery or a large DC power supply of 5-35 amps (like the big power
supply for your 100w transceiver).

73, Gary AB7MY

=================================================
My response....

Hi Gary.....

Thanks for the quick response and comments on my meltdown condition..... I
did some checks since it was really bugging me why this condition happened
since I have had been using my KX1 almost daily since August 2004..... I
removed the circuit card assembly from its enclosure and performed a
visual/ohm check on L3 and it was ok.... I then went to PA Q6 and did a
quick resistance measurement from collector to ground and it was a dead
short not the >500 ohms as called out in the manual.... I removed Q6 and the
short was still there..... The only other answer I could come up with is
Zener diode D7 (1N4756A) failed to a short condition.... Sure enough, dead
short in both directions.... Removed D7 and with Q6 still removed I did a
nice splice (if I don't say so myself) bridging the disintegrated circuit
trace then powered the KX1 up using my bench current limiting power
supply... All receiving functions came to life just like normal.....

Now, what made D7 (47v Zener) fail..... I sat down in my easy chair and
recreated (in my mind) all the events up to meltdown..... I was trying to
work FT5XO on 160 meters and was using my TS-940S and TL-922A linear at
about 1Kw out.... The KX1 was powered OFF and had a separate 40 meter
antenna hooked up to it...... Well, no luck with the FT5 on 160 but I then
heard him on 40 meters with a solid S7 to 8 signal on the TS-940 so that is
when I tried to work him QRP on the KX1.... The rest is smoke history.....
Question..... Do you think using the linear on 160 some how coupled RF back
into the KX1 via the KX1 antenna and took out D7.... Or, do you have any
other ideas in what caused this failure..... Note, my K2 sits near the
linear and was powered on and it hasn't shown any ill effects... Inquiring
minds would like to know…..

I will now order another D7 and also a spare Q6 (just in case) and follow
your suggestion on that Nylon Flat Head 4-40 screw..... I have also fused
the external KX1 power cord WHICH I SHOULD HAVE DONE IN THE FIRST
PLACE......

Thanks again Gary for your time and as usual your greatly appreciated
technical assistance....

Denny  W7SNH

=============================================
Elecraft Gary's final response.....

Hi Denny,

Yes, I suspect enough RF got into the KX1 to destroy the Zener diode if you
were transmitting on an antenna with a kilowatt and the KX1 was still
connected to another antenna. The Zener diode is intended to protect the PA
transistor against open antenna loads, or high SWR. So it sounds like it
sacrificed itself in this case.

You can order the replacement 1N4756A 47V 1w Zener diode and nylon PA screw
from [hidden email] and we'll send them right away.

Sorry to hear this happened to you. But thanks for the information and it
might be a good idea to mention how this happened on the Elecraft reflector
so someone else doesn't have the same thing happen. And, I am glad to hear
you were able to repair the damaged 12V trace as well. Good work! :-)

I hope your luck improves too on working the FT5. ;-)

Remember to disconnect the antenna from your Elecraft rig(s) when not using
the KX1. I have seen or heard about a lot of K2s damaged by nearby
thunderstorms if the antenna was not disconnected.

--
73, Gary AB7MY

=============================================

I received the required replacement parts today (4 April) from Elecraft and
am happy to say KX1 # 841 is back on the air and working just like new
again....... Ok, that’s my story….

To summarize what I have learned:

- If you haven’t replaced Q6 (RF Power Transistor) metal mounting screw with
the Nylon version please DO SO……
    o You can order them from Elecraft or find them at LOWES or Home Depot
for 10 cents…

- When using an external power source on the KX1 make sure the power cord is
fused (1 to 2 Amp no higher)…

- When operating in High RF fields or Electrostatic (lightning) fields make
sure the antenna is disconnected from the KX1 (and as Gary said your other
Elecraft rigs also)

Hope this lengthy posting helps others out there in Elecraft Land and may No
smoke/smell be with you….

de

Denny  W7SNH

sk






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