Posted by
Sarah K. on
Nov 23, 2007; 10:40pm
URL: http://elecraft.85.s1.nabble.com/K2-AGC-Threshold-Too-Low-tp367004p367016.html
Thanks to everyone who replied! Wow, help sure comes in a hurry on
this list!
The general consensus appears to be that I'm ok for the time being
(yay!), so I'm going to go ahead and move on.
I did test with the internal voltmeter as well, and it reports 3.5V.
Special thanks to VE3TIX who asked if my DMM batteries are fresh...
no, they weren't! I replaced the battery and the display is a lot
easier to read now, thanks! Sadly, it still reports 3.59V.
Although I am starting to wonder about the general advisability of
relying on a meter that came free with my oscilloscope. :-)
Also, in the process of fiddling with things, I discovered that my
little bench power supply is very noisy! I was using my Yaesu VX-7R
as an ad-hoc RF probe; K4DGW sent me a link to a message from Don
Wilhelm in the list archives which got me thinking about noise, so I
grabbed the Yaesu to try to use as an RF probe, and discovered lots
of noise on 7MHz, 10MHz, and probably everywhere else (!), which went
away completely when I switched off the power supply! I rigged a
bunch of D batteries for 12V and powered the K2 with that instead; it
worked fine, the noise went away, but the AGC reading still wouldn't
go over 3.59v.
But I think I'm going to need a power supply... any recommendations?
I can probably try to stop by HRO tomorrow.
Oh, and if anyone has a recommendation for a high quality but fairly
inexpensive DMM, I'd be interested to hear about that too...
Thanks again everyone,
Sarah AF6FH
> On Fri, 2007-11-23 at 12:53 -0800, Sarah K. wrote:
>> Hi,
>>
>> I'm Sarah, AF6FH! I just got my license a couple weeks ago, and I'm
>> now assembling K2# 0622, which will be my first HF radio. I started
>> assembly on Wednesday night, and so far, everything's been going just
>> fine.
>>
>> But...
>>
>> I'm having a little issue with setting the AGC Threshold.
>>
>> The instructions (pg. 48) say to set the RF GAIN control fully
>> clockwise, measure the voltage on pin 5 of U2 on the control board,
>> and adjust trimmer R1 for a reading of 3.80 volts.
>>
>> I did my voltage measurements with my own (cheap) DMM, not the
>> internal voltmeter.
>>
>> My concern is that, by adjusting R1, the highest I could get the
>> voltage to go was 3.59v, so for the moment, that's where it's set.
>>
>> I measured the voltage on the LM29307-8, and it has 12.23V at the
>> input, but only 7.53V at the output. The 78M05 has 4.93V at its
>> output.
>>
>> When I rotate the AF GAIN control, I get a range of 4.68V (fully
>> counter-clockwise) to 3.59V (fully clockwise) on pin 5 of U2.
>>
>> My power supply is a small benchtop power supply; it's putting out
>> 12.4V, and reports the K2 is drawing about 100mA. When I measure the
>> input voltage across the K2 input jack, I get the same 12.4V.
>>
>> All of the previous checks were fine, and the following step, S-Meter
>> Alignment, seemed to work just fine.
>>
>> So, my question is, do I have a problem? Should I be concerned?
>>
>> Thanks,
>>
>> Sarah AF6FH
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