Low outout power on higher bands, K2

[Beth Wenzel]

Hi guys,

Been working on this problem for some time now,with no luck. 30 meters and below, I get 12-13+ watts out. 20m about 12, then a rapid drop off until I get about 1.5 watts on 10 meters.

I have checked all of the relays for proper switching in the bandpass and low pass filters, Have replaced the finals. Have rewound all of the transformers in the PA, and have cross checked all of the components in the RF chain.

10 meters peaks ok at 1.5 watts, but the ALC is in full drive to get 1.5 out.
The gain ratios in the PA look ok relative to the lower frequencies, however the bandpass filters on 15 through 10 seem to exhibit a higher insertion loss than the lower frequencies, (using a sig generator and an o-scope to check) in fact if I jumper the filters out, the PA puts out full power in 15m but only 5 watts on ten (I'm guessing there is an impedance mismatch problem).

Any ideas? this is getting to be very frustrating.


73 Mark AB9LZ.
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[Leigh L. Klotz Jr WA5ZNU]

Check the bottom of the RF board for a missing capacitor.  You can
search for my callsign and low power for a message trail including part
numbers.
73,
Leigh/WA5ZNU
On Sun, 2 Sep 2007 1:35 pm, Beth Wenzel wrote:
13+ watts out. 20m about 12, then a rapid drop off until I get about 1.5
watts on 10 meters.
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[Beth Wenzel]

Thanks for the tip Lee, it got me to looking at the TX mixer / buffer part of the chain. The ALC / BFO line into the mixer is consistent across bands, (however the micro controller quickly drives the ALC to max at 28 mhz). The mixer output looks ok, as well as the output on pin 6 of the buffer. However when I measure of the cathode end of D6 the relative output is down, as if the bandpass filter is sinking the energy.

"Leigh L Klotz, Jr." <[hidden email]> wrote: Check the bottom of the RF board for a missing capacitor.  You can
search for my callsign and low power for a message trail including part
numbers.
73,
Leigh/WA5ZNU
On Sun, 2 Sep 2007 1:35 pm, Beth Wenzel wrote:
13+ watts out. 20m about 12, then a rapid drop off until I get about 1.5
watts on 10 meters.

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[Don Wilhelm-4]

Beth,

If the bandpass previously was tuned properly and the behavior is as you
stated, check the VCO frequency at TP1.  Compare the frequency obtained
with the internal counter probe with the chart on the Schematic Key
sheet in the manual.

If the frequency of the VCO is not correct, you can obtain RF out of the
mixer and buffer stages, but when it gets to the bandpass filter, the
filter will 'swallow it up'.

You can also use the counter probe to check the frequency at other
places in the K2 other than the Test Points designated - you may have to
invent some kind of short attachment device, but it should work - the
counter itself shoud be good up to 40 MHz.

73,
Don W3FPR

Beth Wenzel wrote:
> Thanks for the tip Lee, it got me to looking at the TX mixer / buffer part of the chain. The ALC / BFO line into the mixer is consistent across bands, (however the micro controller quickly drives the ALC to max at 28 mhz). The mixer output looks ok, as well as the output on pin 6 of the buffer. However when I measure of the cathode end of D6 the relative output is down, as if the bandpass filter is sinking the energy.
>
>  
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