Dave,
That 'artifact' you heard when you had the filter set to hear zero
frequency was most likely some BFO feedthru.
Good that you have put C31 in the KPA100 - it is to avoid a possible
oscillation - it has nothing to do with the power control level.
You can tame the power variation a bit by increasing the value of R98 on
the bottom of the RF board. Increase it until you begin to see a
dropoff in power on 10 meters (check this on low power (power only the
base K2 and not the KPA100). Use the largest value that allows normal
power output on 10 meters. Normally 1k is a maximum value, but if your
base K2 has higher than normal gain, you can go a bit higher.
73,
Don W3FPR
David Lankshear wrote:
Today I decided to recalibrate my K2 using a very accurate 10MHz source, as
well as replacing C31 in the KPA100 (Rev. D) to bring it up to the latest Rev.
E.
After leaving both the 10MHz source and the K2 turned on all day to stabilise,
I made a start but the first problem encountered was that the filters were just
too darned good and wouldn't let me find zero-beat. Plenty 10MHz signal was
available and the S-meter lit up with a full line of li'l green apples, but as
zero beat was approached, both the S-meter reading and audio dropped out of
sight.
To get round this problem, I altered the LSB widest filter (2.4kHz - OP 1) down in frequency and found a healthy S-meter reading could be maintained. The low beat note could both be seen (highest LED flickering in time with the beat) and heard as a slow rise and fall in background noise.
Unfortunately, I was distracted by an artefact that appeared only when an SSB
filter was selected with no antenna connected. There was a crackling/crunching
sort of noise that was not present on any CW setting or filter. During the
hunt for the culprit, I found that if a finger was dabbed on the cathode of D4
(DSB output from BM to xtal filter) the problem cleared up. Also, if a finger
was dabbed on T2, the problem went away. Further finger-dabbing along the xtal
filter was unhelpful as I was acting as an antenna. Soldering was checked and
re-done but the problem remained, so D4 bit the dust and was replaced with a
new 1N4148. No improvement.
Ah well, time was passing, so I thought I'd return to the calibration task. No
problems were encountered and the task was completed successfully. CAL PLL was
followed by a CAL FIL where the adjusted LSB filter was returned to its
previous setting and all other filter settings were moved around before
restoring them to their original settings. I noted the last digit of the BFO
frequency had decremented by 1 in every case.
After this, I was pleased to note that the nasty background noise previously
heard when SSB was selected had disappeared, so I'm thinking it was an artefact
caused by everything warming up and drifting. Now I shall check tomorrow to
see if the artefact has moved and is present when the rig is cold.
Changing C31 in the KPA doesn't appear to have made any difference to its
operation, so what was it for? The PA's power output reading still tends to
jitter around by +/-10% or so when running into a high quality 50 ohm dummy
load.
73 DaveL G3TJP
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