Posted by
Andy Webster on
Jul 31, 2010; 1:36pm
URL: http://elecraft.85.s1.nabble.com/K2-alignment-problem-at-10-12m-tp5257291p5358531.html
After finding some time to look at the transmit signal tracing with the RF
probe it looks like I have a problem in the bandpass filter for 10/12m
although the power output on 10m looks OK at the moment... The measurements
from the xmit mixer out do indeed vary widely so I'll list them here in case
this indicates another problem (I did 40m for reference as in the manual and
12m and 10m as these are my suspect bands):
Xmit Mixer Output: 1.2mV (40m) 180mV (12m) 25mV (10m)
Buffer Output: 53mV (40m) 1600mV (12m) 450mV (10m)
BPF output: 15.3mV (40m) 0.3mV (12m) 50mV (10m)
...so the values do vary widely between bands (do these look reasonable?)
and the 12m tx path clearly falls over at the BPF. Interestingly 10m doesn't
seem to suffer the same effect so the big question is what is different
between 12m and 10m?
Although 10m and 12m use the same BPF and LPF sections, there are relay
clicks when switching between these two bands - what is happening there?
I've swapped the tuning capacitors (C44 and C46) with the other tuning caps
from the 15/17m BPF area and the fault didn't move.
I've lost count of the times I've inspected the area around the BPF for any
visible faults and I'll admit to a certain degree of frustration now...
...any ideas anyone?
Thanks again,
Andy
G7UHN
--------------------------------------------------
From: "Don Wilhelm" <
[hidden email]>
Sent: Monday, July 05, 2010 11:13 PM
To: "Andy Webster" <
[hidden email]>
Cc: <
[hidden email]>
Subject: Re: [Elecraft] K2 alignment problem at 10/12m
> Andy,
>
> Are you peaking the inductors for 10 meters before you peak the trimmers
> on 12 meters? That is the required order for the bandpass filters - of
> the band pairs, the inductors must be peaked first and then the
> capacitors.
>
> On your 10 meter problem, check all the components in the Low Pass
> Filter again. Re-count the turns on the toroids making certain you
> count the number of times the wire passes through the center of the core.
> Normally high current messages combined with low power output point to a
> problem either with T4 or the LPF.
>
> Does the power start to drop off on all the higher frequency bands?
> With a 13.8 volt supply, the normal power output from a well constructed
> K2 will be above 14 watts on all except 10 meters and possibly 12
> meters. If the power drops off for bands below 12 meters, look
> carefully at T4 - the bare link wires should go straight through one
> hole of the core (not wrapped around).
>
> When you find your 'scope probe, you can investigate the RF Voltage at
> various points along the transmit chain (build the RF Probe from the
> parts in the K2 kit if you can't find the probe, or it is not a 10X
> probe). You can compare a working band with a non-working band at each
> point.
>
> While looking at the RF voltage levels, keep in mind how the K2 does the
> power control - it samples the output, and if not up to the requested
> power, the microprocessor increases drive in an attempt to provide more
> output. That means you will have HIGHER than the expected RF Voltages
> at every point that is OK - the problem will be found just before the
> point where the RF voltage drops substantially.
> Set the power at 5 watts for that Signal Tracing, and refer to the
> Transmit Signal Tracing portion of the Troubleshooting Appendix in the
> back of the K2 manual for the expected values and test points.
>
> 73,
> Don W3FPR
>
> Andy Webster wrote:
>> Hello all,
>>
>> First up I'd like to say what a superb kit the K2 is, I've really enjoyed
>> the time spent quietly soldering away and the instructions are first
>> class with some great finishing touches (all the resistors in sequence on
>> a custom strip - what will they think of next!). It's a shame it's nearly
>> over...
>>
>> Anyway, I have a problem on the final part of alignment. All of the lower
>> bands seem fine on both pre-alignment (receive signals peaked for signal
>> strength) and transmitter alignment (filters peaked for ~2W output)
>> except on 12m and 10m bands.
>>
>> On 12m the receiver is very quiet (although I can hear a broadcast
>> station to try and peak the filter against) and there seems to be little
>> effect when adjusting C44 and C46. On transmit into a dummy load there is
>> consistently only 0.2-0.3W showing.
>>
>> On 10m the receiver seems OK (receiving some strong CW) and L12 and L13
>> can be peaked for maximum response but transmit into the dummy load is
>> hit and miss, sometimes approx. 2W, sometime it shows a High Current
>> warning and displays around 10-15W before I quickly deselect Tune.
>>
>> Obviously these two bands share filters so my attention is drawn there
>> but I thought I'd float the exact symptoms at the collective while I
>> search for my 'scope probe. I've closely inspected the band pass and LPF
>> areas for faults and resoldered all the joints just in case of a dodgy
>> one to no avail. The number of turns on the LPF toroids is correct. Does
>> the different behaviour of the two bands on tune tx indicate a more
>> specific area for attention?
>>
>> Next step for me is to run a scope along the signal path but appreciate
>> any thoughts as to schoolboy errors I have obviously made...
>>
>>
>
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>
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