K1 stopped working on 15m and 20m

[John Reed, W8DE]

My K1-4 was working well when I first built it.  Now I'm having some problems on 20m and maybe also 15m.  Here is what it is doing:

40m and 30m:  receives well and transmits 7 watts as calculated with voltage measurements on the DL1 dummy load.

20m :  no reception and no signal heard from the XG2 signal generator; no voltage measurable on the DL1 dummy load indicating no power out

15m: no reception (I don't have a signal generator to test on 15m); no voltage measurable on output, but I can receive a signal from the K1 on my general coverage receiver.

What I've done so far:
1.  checked that the filter board crystals are oscillating by putting my general coverage receiver antenna near the crystals and tuned to the crystal frequency

RF probe measurements:

2.  VFO U7 pin 4/5:
40m: 0.064v; 30m: 0.034v; 20m: 0.034v; 15m: 0.132v

3.  Pre-mix bandpass filter FIL-P1 pin 6:
40m: 0.081v;  30m: 0.046v; 20m: 0.000v; 15m: 0.024v

4.  Transmit PRE (out set at 3.0W; but drops to 0.1W in tune mode on 20m and 15m)
40m: 0.080v; 30m: 0.057v; 20m: 0.001v; 15m: 0.29v

The only test equipment I have is multimeter, an RF probe, an XG2 signal generator, and a general coverage receiver.

Any suggestions for what I should look at ?

73,
John, W8DE

[Don Wilhelm-4]

John,

The most common cause of 20 meter failure on the K1 is the 22 MHz
crystal on the filter board is not active enough to provide a sufficient
LO for the Pre-Mixer.
It is possible that the 15 meter crystal (29 MHz) is not sufficiently
active too, but I don't have enough information to draw a firm conclusion.

To confirm that the crystal(s) are the problem, put the RF Probe on the
filter board P1 pin 4.  If the RF voltage is less than 70 mV, there is
not enough energy to adequately drive the mixer.

If you do have sufficient RF at P1 pin 4, then you attention should
focus on the tuning of the Pre-Mixer bandpass filter for 15 and 20
meters.  Remember that you must tune 15 meters before tuning 20 meters.

73,
Don W3FPR

On 10/24/2013 10:35 PM, John Reed, W8DE wrote:

> My K1-4 was working well when I first built it.  Now I'm having some problems
> on 20m and maybe also 15m.  Here is what it is doing:
>
> 40m and 30m:  receives well and transmits 7 watts as calculated with voltage
> measurements on the DL1 dummy load.
>
> 20m :  no reception and no signal heard from the XG2 signal generator; no
> voltage measurable on the DL1 dummy load indicating no power out
>
> 15m: no reception (I don't have a signal generator to test on 15m); no
> voltage measurable on output, but I can receive a signal from the K1 on my
> general coverage receiver.
>
> What I've done so far:
> 1.  checked that the filter board crystals are oscillating by putting my
> general coverage receiver antenna near the crystals and tuned to the crystal
> frequency
>

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[John Reed, W8DE]

Don,
thanks for the tip.  The problem was the band pass filter adjustment.

I don't have a signal generator that will cover K1's 15m reception.  So when I built the radio I only tuned it for operation on 20m.  Recently I tried to tuning the filters on 15m by tuning in W1AW code practice frequency, but had no luck.  I forgot that in doing that I had totally messed up the tuning on 20m.  

I guess I'll try to locate a signal generator.

thanks again
73,
john

[Don Wilhelm-4]

John,

If you cannot find a signal generator, do you (or a friend) have a
transmitter that can produce a signal on 15 meters?  If so (that is a
signal generator), run it into a dummy load and connect a wire "antenna"
to the K1 placed near enough to the dummy load to produce a signal that
you can hear.
Adjust the 15 meter bandpass filters for maximum response on receive,
and then further refine that adjustment using the K1 transmit, peaking
for maximum power (set the menu OUT parameter for 1.5 watts and exit
TUNE and reenter TUNE if the actual power exceeds 4 watts).
Once you have 15 meters working, then adjust the 20 meter bandpass.

73,
Don W3FPR

On 10/25/2013 5:52 PM, John Reed, W8DE wrote:

> Don,
> thanks for the tip.  The problem was the band pass filter adjustment.
>
> I don't have a signal generator that will cover K1's 15m reception.  So when
> I built the radio I only tuned it for operation on 20m.  Recently I tried to
> tuning the filters on 15m by tuning in W1AW code practice frequency, but had
> no luck.  I forgot that in doing that I had totally messed up the tuning on
> 20m.
>
>

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[John Reed, W8DE]

I don't have ready access to a transmitter or signal generator.  I can try to come up with something.

I did find that I can hear the third harmonic of XG2 7040Mhz at 21120 Mhz with my general coverage receiver when I connect the output to the receiver antenna.  I pushed some of the windings on K1's L1, the VFO inductor, so I could tune in 21120 on my K1 that I have set up with just the 80Mhz tuning range.  I had no luck adjusting C1,C2,CA,CB to hear any signal on the K1 at 21120 Mhz.  

Is it possible to swap out the 3579.5 Mhz crystal on the XG2 for one in the 15m band ?  What is the purpose of capacitors C5 and C6 on the XG2 ?  Are the values of these set based on the frequency ?  Just wondering if any capacitor values would need to change if I tried to make the XG2 work on 15 m.