Posted by
Don Wilhelm on
Sep 30, 2017; 10:12pm
URL: http://elecraft.85.s1.nabble.com/K2-BFO-test-failed-tp7634802p7634814.html
Mike,
How did you check the value of the resistor. If you used your DMM while
it was in-circuit, then you have an open L33. The BFO would oscillate,
but the range would be restricted as yours is or more narrow.
First thing, check capacitors C173 and C174 to be certain you have the
correct values installed. Those blue capacitors with the yellow
printing on them are difficult to read, so use a magnifier and lots of
light.
Then check D37 and D38 to be certain they are oriented correctly - the
slightly angled side is the "rounded" side.
Make certain RP6 is well soldered - reflow with a hot iron (750 degF)
and apply a bit of extra solder - you really one need the flux. The
iron dwell time should be about 3 seconds, but watch for the solder to
flow out onto both the solder pad and the component lead to assure good
soldering - up to 5 seconds dwell will not hurt anything.
Then when doing the BFO Hi and BFO LO frequency checks make sure RP6 pin
7 goes to quite near 5 volts and also quite near zero volts.
The above are the easiest items to check.
Keep in mind that the resistor which holds L33 is really an insulator,
it serves no electrical function.
You are correct that a bad BFO crystal could cause the problem, but to
change them (change them as a matched pair), you have to lift L33. Not
too bad is you are careful, leave the leads connected to the resistor
leads, heat the solder pad and pull the resistor lead out - repeat for
the other side and lift L33 and the resistor off as an assembly.
So try the easy stuff first, and if that does not cure it, contact
Elecraft Support and request a pair of BFO crystals and a new L33 and
5.1 meg resistor to replace them.
73,
Don W3FPR
On 9/30/2017 5:40 PM, Mike Maiorana wrote:
> Thanks Don.
> I triple-checked the inductor wires and they are well soldered to the
> pads. I also verified the value of the resistor across it, 5.1 Mohms.
> I reflowed the solder anyway, but no change in the BFO frequency range.
>
> If L33 wasn't connected would the oscillator even run?
>
> If one of the two crystals was bad (or not soldered properly) would
> you also see a reduction in BFO range?
> What about if one of the varactors had failed?
>
> Any further advice will be greatly appreciated.
> Thanks and 73
> Mike M.
> KU4QO
>
> On Sat, Sep 30, 2017 at 9:57 AM, Don Wilhelm <
[hidden email]
> <mailto:
[hidden email]>> wrote:
>
> Mike,
>
> The inadequate BFO range is usually a result of the L33 toroid
> leads not being well soldered. The toroid wires are very fine and
> sometimes are not visible to the naked eye, so use magnification
> to examine them. When re-soldering, use a pointed tool to bring
> the fine toroid wires down onto the solder pad instead of further
> up on the securing resistor lead.
>
> For adjusting the 4 MHz Reference Oscillator (that has nothing to
> do with the BFO range), refer to the K2 Dial Calibration article
> on my website www.w3fpr.com <
http://www.w3fpr.com> for the most
> accurate method of adjusting the Reference Oscillator.
>
> Be aware that the 4 MHz Reference oscillator can drift in normal
> operation, so soon after adjusting it, run CAL PLL and CAL FIL
> while it is still "on frequency". That oscillator is not used in
> operation for the K2 frequency display. In other words, simply
> adjusting the 4 MHz Reference does nothing for normal operation.
>
> 73,
> Don W3FPR
>
>
> On 9/30/2017 9:35 AM, Mike Maiorana wrote:
>
> Hello all. I have a K2, 6000+ serial number, that I'm trying
> to get working
> correctly. Unknown history. A quick test showed the filters
> were not
> properly adjusted (could hear both sidebands of carrier on
> some bands). I
> thought it would be best to do a top-down alignment.
>
> I adjusted the 4 MHz oscillator, verified the PLL reference
> oscillator
> range (12.43 kHz), VCO test and VCO alignment. No problems up
> to that
> point.
>
> I'm having an issue at the BFO test. The BFO high frequency is
> 4916.04 kHz
> and the low frequency is 4913.82 kHz. That's a difference of
> 2.19 kHz, well
> below the 3.6 kHz stated minimum.
>
>
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