[K2] PA ALC problem on CW

[picmaster]

Hi everyone,

I have an issue with my K2 internal power amplifier. After completing the
kit assembly I went on to pass through the transmit test procedures with
a good dummy load, but I got the famous "HiCur" over-current warning
message when using the TUNE button (any band, any "POWER" setting).

Initially I thought that I had a short somewhere, but in the end I found
out that the transceiver was consuming 4-5A because it was really producing
some power on the dummy load (about 20W distorted signal!).

So the PA looks fine this time and I started to wonder how actually the radio
was controlling the output power (when I touch the front panel "POWER"
control, the display reacts properly and the power is changed in the range of
0.1-15.2W, but this doesn't change the PA output - it still puts 20W when
pressing TUNE).

// Small offtopic - the POWER know has some dead region in the most left
// position, maybe because the diode analog inputs multiplexing...

I can confirm that the "RF Output Detector" circuit (RF board) works and the
signal from this circuit reaches pin 2 of U6 (Control Board). Unfortunately I
can also confirm that the 8-bit DAC (MAX534) doesn't output any voltage
on "OUTA" when pressing TUNE. The DAC output is not shorted to ground,
so my estimation is that channel "A" is defective.

I was wondering actually whether I can disconnect pin 2 of U8 (MAX534)
and try to apply external regulated 0-5V control voltage directly to the BFO
attenuator ("V ALC" signal, RF Board), in order to observe whether this will
change the transceiver output power when TUNE.

My question to you guys is: can you share what's the actual voltage of
the "V ALC" signal in your radios for power values 1W, 5W and 10W on CW?

Thanks in advance for your help,

Nikolay (LZ1NRD)

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[Don Wilhelm-4]

Nikolay,

I believe you may be looking in the wrong place.
If you have zero volts at Control Board U8 pin 2, and it is still
producing power output, you have a problem.   I would suspect your K2
may be oscillating and trying to transmit on some frequency far removed
from the ham bands.

If it is oscillating, you may wish to remove the voltage from Q7 and Q8
to avoid damaging them, but you will have to have some way to monitor
the RF voltage at the collector of Q6 (an oscilloscope is ideal).

Check the end of R98 on the bottom of the RF Board.  If there is voltage
there but no voltage at CB U8 pin 2, you will have to find out what is
wired incorrectly.  The circuits that drive VALC are the ALC components
on the Control Board.

Use the internal counter probe in CAL FCTR to be certain you have the
proper frequencies at TP1 and TP3.  TP3 will be the BFO frequency (near
4915 kHz), and the VFO frequency at TP1 should be 4913 kHz higher than
the frequency on the display for 17 meters and lower (higher for 15
meters and above).

If you have an external frequency counter, you can check the frequency
at jumper W6 during a TUNE.

IF you need a short explanation of the workings of the K2 power control
loop, please ask.

73,
Don W3FPR

On 3/17/2012 1:34 PM, [hidden email] wrote:

> Hi everyone,
>
> I have an issue with my K2 internal power amplifier. After completing the
> kit assembly I went on to pass through the transmit test procedures with
> a good dummy load, but I got the famous "HiCur" over-current warning
> message when using the TUNE button (any band, any "POWER" setting).
>
> Initially I thought that I had a short somewhere, but in the end I found
> out that the transceiver was consuming 4-5A because it was really producing
> some power on the dummy load (about 20W distorted signal!).
>
> So the PA looks fine this time and I started to wonder how actually the radio
> was controlling the output power (when I touch the front panel "POWER"
> control, the display reacts properly and the power is changed in the range of
> 0.1-15.2W, but this doesn't change the PA output - it still puts 20W when
> pressing TUNE).
>
> // Small offtopic - the POWER know has some dead region in the most left
> // position, maybe because the diode analog inputs multiplexing...
>
> I can confirm that the "RF Output Detector" circuit (RF board) works and the
> signal from this circuit reaches pin 2 of U6 (Control Board). Unfortunately I
> can also confirm that the 8-bit DAC (MAX534) doesn't output any voltage
> on "OUTA" when pressing TUNE. The DAC output is not shorted to ground,
> so my estimation is that channel "A" is defective.
>
> I was wondering actually whether I can disconnect pin 2 of U8 (MAX534)
> and try to apply external regulated 0-5V control voltage directly to the BFO
> attenuator ("V ALC" signal, RF Board), in order to observe whether this will
> change the transceiver output power when TUNE.
>
> My question to you guys is: can you share what's the actual voltage of
> the "V ALC" signal in your radios for power values 1W, 5W and 10W on CW?
>
> Thanks in advance for your help,
>
> Nikolay (LZ1NRD)
>
> -------------------------------------
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> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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>
> This list hosted by: http://www.qsl.net
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[picmaster]

Hi Don,

Thanks for your suggestions. I disconnected pin 2 of MAX534 and
applied external control voltage there (simulating a healthy channel
"A" of the DAC). Fortunately, now I can control the power output from
the transceiver from 0 to MAX. The sine wave looks undistorted and
exactly on the desired frequency (but to be honest, initially I also
thought like you that the PA could be oscillating).

The current consumption is now much better - between 0.8 and 2.1A for
output power values 1-10W. The power indication on the display also
shows meaningful values during TUNE.

The strange thing to me is why the DAC pin was outputting 0V, but the
ALC circuit still drove the BFO attenuator gain to the max... There's a
remote chance that the damaged output was somehow in HI-Z state and
the solder flux residual on the PCB was leaking some tiny amounts of
current into the non-inverting input of this sensitive CMOS opamp
(datasheet states that the input leakage current is 2fA!).

Looks like MAX534 just made it into my shopping list for the next week...

Thanks for your comments. I'll drop a line here when I have some more
progress with the PA ALC.

73,
Nikolay (LZ1NRD)

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[Arrigo IV3CWH]

Hi Nikolay i have your exactly same problem.
How did you resolved it?

Thanks

Arrigo IV3CWH

[picmaster]

Gents,

Looks like I forgot to share the outcome of this story.
In the end, I got new MAX534 and replaced it, and everything
is now fine. I can't tell you whether the chip was deffective
from the beginning, or it was damaged by EMC during my
soldering. Anyway, things are OK for quite some time.

Kind regards,
Nikolay / LZ1NRD

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