OK you Elecraft gurus, I've got one for you:
K2/100 #3664 with all mods to date, all options installed, using KAT-100
tuner. Normal assembly, amplifier in the radio box, tuner in the skinny
package underneath the radio. Antenna is a random vertical loop, about
125 feet or so around, peak at about 50 feet. Feeding with coax, and
have a ferrite choke at the feedpoint. I'm not sure that has anything
to do with it, since my previous antenna was a 100 foot wire fed against
radials, and problem was also occurring. At first I suspected RF into
the power supply or radio, but now I'm not sure. The main reason I
suspect RF is that the problem goes away if I use a dummy load. It also
occurs on two different power supplies.
Occurs most notably on 40 meters. Power set to max will make it occur
every time, reducing the power will reduce the occurrences until at
about 25 watts it does not occur. On RTTY, any transmission longer than
about 7 or 8 seconds causes the K2 to demand an approximately 35 amp
current spike from the power supply just after the K2 returns to receive
mode. This causes the supply voltage to drop to about 8 volts, so the
K2 does a sort of power up reset (Elecraft in the display, frequency
changes to the remembered power up frequency, etc.). I'm monitoring
the supply current with a sense resistor (0.03667 ohms) using a scope in
differential mode to monitor the voltage across the resistor. If the
transmission is less than the 7 or 8 seconds, no problem. The current
spike starts about 1.5 to 2 ms after the K2 returns to receive mode, and
the high current lasts about 2 ms, tapering off a bit before it returns
to normal receive current. The power supply recovers pretty quickly,
the whole glitch is over in 3 ms. I think the only reason the glitch
stops is that the K2 reset puts it back in receive mode and shuts off
whatever is pulling the current. This supply is an Astron RS-35A with
all the recommended RF bypassing done. I have an MFJ 25 amp switching
supply, and the problem also occurs, although the current spike is
obviously big enough to drive that supply into total shutdown. Regular
key down transmit current is about 15 amps.
This will naturally also occur with the radio in SSB mode putting in the
RTTY tones. I also duplicated it using just a plain tone into the
microphone. This problem does not occur in CW mode. I can leave a full
power carrier on as long as I want and no problem on return to receive.
Some other observations: if I disconnect the KAT-100 and use an
external tuner on the antenna, the problem goes away. When the radio
returns to receive mode, the bargraph seems to take about a half second
to go down to zero. On CW I notice that the bargraph instantly falls to
zero when you come out of tune mode. Also, sometimes when I activate
the RTTY tones, it seems that the power control has lost control, since
I get about 170 watts out (read on external wattmeter). This doesn't
happen every time, and I haven't figured out exactly what triggers it.
Sometimes it settles out at 110 watts just fine.
This really smells like RF getting in somewhere, but I can't figure what
it's doing to the radio to begin to try and choke it off. All the audio
cables have chokes/shielding/beads/bypasses. Looking at the KPA-100
schematic, it would seem that the only devices capable of that kind of a
current spike are the finals or the TR switch diodes, and I can't figure
how they would be turning on on the return-to-receive cycle. It seems
like some sort of stored energy being dissipated as the transmitter is
shut off. I'm leaning toward the SSB module since there is no problem
in CW mode ( I know, just stay on CW!!), but I can't make the connection
between the SSB module and the PA..
Any ideas you folks can toss my way I'd appreciate. Since I didn't have
any problems in the initial construction, I didn't get to dig into the
schematics the way some of you have. I'm hoping someone can take all
these pieces and complete the puzzle.
tnx,
Ken K6MR
_______________________________________________
Elecraft mailing list
Post to: [email protected]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft Help:
http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
