Posted by
Don Wilhelm-4 on
Feb 11, 2011; 5:41am
URL: http://elecraft.85.s1.nabble.com/Erratic-transmit-power-ALC-oscillating-tp6013475p6014613.html
Bob,
I would not be concerned about the slightly higher current draw. Change
CAL CUR to 3.50 and you should have no problems. If you add the KPA100
you will need to increase the setting to 3.50 amps anyway, so do it
now. Despite the manual recommendations, many current K2s require more
than 2.50 amperes under some specific circumstances.
BTW - the CAL CUR setting can actually be used as a Low Battery Alarm.
The K2 tries to maintain the requested power level, and when the voltage
drops, that means more current must be drawn for any given power level,
so if you are operating on batteries, you can set the CAL CUR value to a
lower level to give you an indication that the battery voltage has
dropped to a level that requires increased current.
The RF gain will vary from K2 to K2, and since you can obtain greater
than 12 watts on 10 meters, that says your K2 has greater than average
RF Gain - and also says you did a meticulous job of construction -
congratulations.
73,
Don W3FPR
On 2/10/2011 10:51 PM, n7zf wrote:
> Don,
>
> Thank you for your suggestions. It sure is hard to debug circuits that have
> feedback loops.
>
> I was using the TUNE button for the readings that I reported, and the radio
> was connected to a 50-ohm resistive dummy load. I did not know that the
> TUNE was an approximate power level.
>
> The keying waveform matches the diagram in the mod instructions.
>
> I have retested using my cw key. I think I am only seeing the 'erratic'
> power on 80m and 40m. Output on the other bands seesm to be consistently
> the same. At '5 watts' on 80m and 40m I would occasionally get around 4.2
> watts and usually get around 6.4 watts.
>
> The power will drift for about 3 to 5 seconds then stabilize. You mentioned
> that it should take about 3 dits for the power to stabilize, so it seems to
> take much too long on my K2. Having reinstalled the power amp finals this
> week, I do not believe that there is an issue with heatsinking or mounting
> of the power transistors causing the drifting.
>
> I looked at the app note regarding R98 and made the change.
>
http://www.elecraft.com/Apps/Power_Control_Mod.html>
> With the potentiometer at 1000 ohms, I got 12.5 watts on 10m so I put in a
> 1K resistor. Then I retested at the 5 watt level. On 40m, for example, I
> now (usually) get 5.6 watts out instead of 6.x watts. But about 10% of the
> time I get about 4.3 watts out.
>
> The reason that I changed the transistors and T3 in the power amp and tested
> the LPF is because this K2 draws more current than it should when
> transmitting. At 5 watts, the current draw is 2.04 to 2.18 amps on 80m
> through 15m. On 10 and 12m the current is 2.52 to 2.56 amps. The voltage
> drops from 13.6v to 13.1v on all bands (at 5watts). At 10 watts, the
> current is 2.50 to 2.75 amps.
>
> Power amp transformer T4 was wound with the standard 3-turn winding.
>
> Should I be concerned about the higher than normal current draw?
>
> 73,
> Bob - N7ZF
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