KX1 : low power

[luc Favre-2]

Hi all,
I got a problem building my KX1: receive is OK, but transmit power is far
too low (40 m < 2W, 20 m  <0,5 W with 13,5 V).
Of course, I checked and double checked solders, solder bridges, components
and values. BPF and LPF are OK as measured with RF generator and
oscilloscope. Output waveform good. Frequency right.
Transmit voltages are all OK, but Q7's transmit base voltage is minus 1,1 V
(minus) instead of  the requested  plus 0,7 V. Resistances and other
components Q7 circuit are OK? 6V_2T is + 5,5 V.  Q7 replaced Q7 without.
Any idea will be welcome !
73
Luc/HB9ABB/F6HJO


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[Don Wilhelm-4]

Luc,

Before offering any specifics, can you make some RF Voltage measurements
during a TUNE operation please?  With that information, the problem can
be isolated to one stage.
Measure the following points for both 40 meters and 20 meters:
Q1 gate
Q1 Source
Q4 base
Q5 base
Q5 collector
Q6 base
Q6 collector

Please also tell us whether the voltages are peak to peak, zero to peak
or RMS values.

73,
Don W3FPR

luc Favre wrote:

> Hi all,
> I got a problem building my KX1: receive is OK, but transmit power is far
> too low (40 m < 2W, 20 m  <0,5 W with 13,5 V).
> Of course, I checked and double checked solders, solder bridges, components
> and values. BPF and LPF are OK as measured with RF generator and
> oscilloscope. Output waveform good. Frequency right.
> Transmit voltages are all OK, but Q7's transmit base voltage is minus 1,1 V
> (minus) instead of  the requested  plus 0,7 V. Resistances and other
> components Q7 circuit are OK? 6V_2T is + 5,5 V.  Q7 replaced Q7 without.
> Any idea will be welcome !
> 73
> Luc/HB9ABB/F6HJO
>
>  

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[luc Favre-2]

Don,

Thanks for your quick reply. Following values are Vrms made with Elecraft RF
Probe and DDM.
(7 MHz value/14 MHz value)
Q1 gate : 0.17 / 0.16
Q1 Source : 0.11 / 0.18
Q4 base : 0.10 / 0.18
Q5 base : 0.9 / 1.1
Q5 collector : 4.7 / 6.0
Q6 base : 1.3 / 1.2
 Q6 collector : 8.9 / 10.7

I suppose these values are quite consistent.
Q5 collector values and Q6 base values seem to have the same relation
betwenn them as the impedances (1:3). Am I right ?

Thanks for your help.
73
Luc/F6HJO/HB9ABB


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[Don Wilhelm-4]

Luc,

Those RF voltages look normal until one reaches the Q5 collector.  
Changing R11 to 4.7 ohms (as in the power mod) will likely help, and you
can increase R30 to 33 ohms (the remainder of the power mod) if changing
R11 does not help enough.

I am concerned because you indicated only 0.2 watts output on 20 meters
- with 10.7 volts on the collector of Q6 you should have more output
than 0.2 watts unless you also have a problem in the low pass filter
area - check the number of turns on the toroids and check the values of
C46, C47 and C54.  Too many toroid turns can reduce the cutoff frequency
of the 20 meter lowpass filter down into the 20 meter band and absorb a
lot of the signal.  When counting toroid turns, count each pass of the
wire through the center of the core as 1 turn.

73,
Don W3FPR

luc Favre wrote:

> Don,
>
> Thanks for your quick reply. Following values are Vrms made with Elecraft RF
> Probe and DDM.
> (7 MHz value/14 MHz value)
> Q1 gate : 0.17 / 0.16
> Q1 Source : 0.11 / 0.18
> Q4 base : 0.10 / 0.18
> Q5 base : 0.9 / 1.1
> Q5 collector : 4.7 / 6.0
> Q6 base : 1.3 / 1.2
>  Q6 collector : 8.9 / 10.7
>
> I suppose these values are quite consistent.
> Q5 collector values and Q6 base values seem to have the same relation
> betwenn them as the impedances (1:3). Am I right ?
>
> Thanks for your help.
> 73
> Luc/F6HJO/HB9ABB
>
>
>
>
>  

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