Somewhat high transmit current K2

[Lambs, Dick & Judy]

I'd appreciate suggestions for finding the cause of somewhat elevated  
transmit current on my K2 (serial 02165).

On 7.1 MHz, with all options removed, at full output (about 15 W to a  
50 Ohm perfect dummy load) the K2 displays 2.8 A.   Other bands are  
similar.

This is with actual 13.8 V from the supply.  An independent ammeter  
in the supply line agrees closely.  The supply voltage drops to 13.5  
on transmit as measured at the K2 connector.  The internal K2 display  
shows 13.4 V and 0.22 A on receive, with voltage dropping to 12.7 on  
transmit.

I’ve checked toroid values (color and turns count through hole) and  
all other components in the bandpass, lowpass, pre-driver, and driver  
circuits.

I’ve listed below signal tracing Vrms values below for the  
transmitter stages at 5 W output.  Compared to the values in the  
signal tracing section of my manual, the Xmit mixer and buffer  
outputs are low, but output from the bandpass filter is OK. The pre-
driver output is low.  The signal voltage on the driver input is very  
low.  However, the driver output is OK.

The DC voltages on the pre-driver, driver, and PA amp transistors  
agree roughly with the manual values.

I don’t know where to go next.  Any ideas?

    *******************************

Signal tracing values Vrms (expected/actual)

Xmit Mixer Output:  .0016/.008
Buffer Output:  .200/.065
Band-pass Filter Output:  .030/.025
T-R Switch #1 Output:  .029/.023
Pre-Driver Otuput:  .120/.061
Driver Input:  .026/.006
Driver Output:  1.8/1.8
PA Input:  .38/.41
RF Detector Input:  2.0/2.0

   **********************************

Thanks
Dick, K0KK

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[Don Wilhelm-3]

If you have checked the low pass filter area carefully, then the next place
to check is T4 - you should have 2 loops (green) on one side and 3 loops
(white) on the other.  Make certain that the links are correct - they are
not full turns but go through only one side of the binocular core.

T3 is the next thing after T4 - be certain it is well soldered and the
bi-filar turns are laying next to each other.

Then check T1 and T2 - these are not usually suspect in lower than normal
gain situations, but make certain the turns are correct.

If all else fails, you may want to try replacing Q5 to see if the
replacement has higher gain than the one you have installed.

Your real problem seems to be lower than normal gain from the input to Q5 to
the output - low gain makes the driver and finals work harder to produce the
requested output and thus draw more than the usual amount of current.

Lastly, your 2.8 Amps is only about 10% higher than the norm, so if you
can't find anything that is an easy fix, you may just want to accept it as
is.

73,
Don W3FPR

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]]On Behalf Of Lambs, Dick & Judy
> Sent: Wednesday, December 07, 2005 8:04 PM
> To: Elecraft - Maling List Elecraft - Mailing List
> Subject: [Elecraft] Somewhat high transmit current K2
>
>
> I'd appreciate suggestions for finding the cause of somewhat elevated
> transmit current on my K2 (serial 02165).
>
> On 7.1 MHz, with all options removed, at full output (about 15 W to a
> 50 Ohm perfect dummy load) the K2 displays 2.8 A.   Other bands are
> similar.
>
> This is with actual 13.8 V from the supply.  An independent ammeter
> in the supply line agrees closely.  The supply voltage drops to 13.5
> on transmit as measured at the K2 connector.  The internal K2 display
> shows 13.4 V and 0.22 A on receive, with voltage dropping to 12.7 on
> transmit.
>
> I’ve checked toroid values (color and turns count through hole) and
> all other components in the bandpass, lowpass, pre-driver, and driver
> circuits.
>
> I’ve listed below signal tracing Vrms values below for the
> transmitter stages at 5 W output.  Compared to the values in the
> signal tracing section of my manual, the Xmit mixer and buffer
> outputs are low, but output from the bandpass filter is OK. The pre-
> driver output is low.  The signal voltage on the driver input is very
> low.  However, the driver output is OK.
>
> The DC voltages on the pre-driver, driver, and PA amp transistors
> agree roughly with the manual values.
>
> I don’t know where to go next.  Any ideas?
>
>     *******************************
>
> Signal tracing values Vrms (expected/actual)
>
> Xmit Mixer Output:  .0016/.008
> Buffer Output:  .200/.065
> Band-pass Filter Output:  .030/.025
> T-R Switch #1 Output:  .029/.023
> Pre-Driver Otuput:  .120/.061
> Driver Input:  .026/.006
> Driver Output:  1.8/1.8
> PA Input:  .38/.41
> RF Detector Input:  2.0/2.0
>
>    **********************************
>
> Thanks
> Dick, K0KK
>
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[Lambs, Dick & Judy]

Don,

After checking out all the leads you suggested, and still coming up  
negative, I think your final suggestion of accepting the slightly  
higher current makes good sense.  Nonetheless, I had fun learning  
about the K2 circuits, and I'm now much more adept and confident in  
troubleshooting my K2.

Thanks again for your help and the wonderful support you give on this  
reflector.

73
Dick, K0KK


On Dec 7, 2005, at 9:13 PM, Don Wilhelm wrote:

>
> Lastly, your 2.8 Amps is only about 10% higher than the norm, so if  
> you
> can't find anything that is an easy fix, you may just want to  
> accept it as
> is.
>
> 73,
> Don W3FPR

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[Don Wilhelm-3]

You are quite welcome - just enjoy that K2.

There is one thing that I failed to mention, and that is your power supply.
The nominal current is about right for a power source of 13.8 volts.  If
your power supply voltage is less than that - say 12 volts, the higher
current is to be expected.

The reason is that the K2 microprocessor attempt to control the power
output, and when the voltage is low, it still tries to produce the requested
power - Power = Volts x Amperes, so if the voltage goes down, the current
must rise if the power is to remain the same.

73,
Don W3FPR

> -----Original Message-----
>
> Don,
>
> After checking out all the leads you suggested, and still coming up
> negative, I think your final suggestion of accepting the slightly
> higher current makes good sense.  Nonetheless, I had fun learning
> about the K2 circuits, and I'm now much more adept and confident in
> troubleshooting my K2.
>
> Thanks again for your help and the wonderful support you give on this
> reflector.
>
> 73
> Dick, K0KK
>
>
> On Dec 7, 2005, at 9:13 PM, Don Wilhelm wrote:
>
> >
> > Lastly, your 2.8 Amps is only about 10% higher than the norm, so if
> > you
> > can't find anything that is an easy fix, you may just want to
> > accept it as
> > is.
> >
> > 73,
> > Don W3FPR
>

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