Posted by
Matthew Ellis on
Jul 17, 2012; 7:35pm
URL: http://elecraft.85.s1.nabble.com/K2-Alignment-and-Test-Part-II-VCO-Test-fails-tp7559122p7559318.html
Don,
Thanks to your pointing me in the right direction I appear to have
tracked down my issue: Capacitor R68 (10pf, next to K15) was duff
(probably damaged when I inserted it as the holes on the pcb are wider
than the lead spacing on the capacitor). The final clue was when I
broke an unrelated capacitor lead off which showed how easily they can
be damaged (I've put hundreds of capacitors into circuits without issue
before), The other clue was that when I touched my frequency meter
probe to TP1 I got a very brief reading before it dropped to zero, and
while it would continue to read zero if I touched tp1 repeatedly, if I
left it for a while (a minute or so) then touched the probe again, I
once again got a brief reading. This suggested to me that T5 was
probably OK (your telling me it was a Hartley oscillator helped there as
I then looked it up to get a better idea of the operation).
Lets hope it doesn't take me two weeks to get each of the various tests
to pass!
Thanks again,
Matthew
On 15/07/2012 14:14, Don Wilhelm wrote:
> Matthew.
>
> Zero Hz. says that Q18 is not oscillating.
> To help you visualize a little better, forget about the PLL part and
> concentrate on Q18 only. That is just a Hartley oscillator.
>
> You have already shorted out the VFO ALC with the short across C60 -
> and that is a good debugging step. The tuned circuit inductance
> consists of L30 in parallel with the red winding of T5 and the
> capacitance is made up of the varactors and capacitors switched in and
> out of the circuit by K13, K14 and K15. That is really all there is
> to it as far as basic oscillation is concerned.
> Once it oscillates, then you can consider the other "appendages" like
> the R30 voltage and the VFO ALC.
>
> So, go over the circuit once again with that in mind. There is a
> reason Q18 is not oscillating - Several possibilities: the green
> winding of T5 is wound backwards, the diodes are oriented backwards,
> The varactors are mounted backwards, Q18 is oriented incorrectly, Q18
> is damaged,
>
> Note also that the path to TP1 includes U3 (which also could be
> failing). Since you have an external frequency counter, check for
> oscillation at the source or Q18 rather than just at TP1.
>
> You should first be trying to just get Q18 to oscillate. Once that is
> working, the other parts come into play to make the frequency of
> oscillation correct.
>
> 73,
> Don W3FPR
>
>
>
> On 7/15/2012 3:44 AM, Matthew Ellis wrote:
>> Hi,
>>
>> I'm struggling to track down an issue with the VCO test in part 2 of the
>> alignment for my K2 (page 64 in my manual).
>>
>> The 4MHz Oscillator calibration looks OK as the frequency on TP3 shows
>> as 12096hz stable (both on the internal test probe and an external freq
>> counter). The PLL test is in range as well. But I get nothing (0hz) on
>> TP1 for the VCO test with both the internal probe and a standalone
>> frequency meter.
>>
>> I've checked and re-wound/re-fitted Transformer 5, gone over my assembly
>> with a magnifying glass a few times (both top and bottom side of the
>> pcb), confirmed all the components in the VCO area are present and
>> correctly oriented. I've also used a signal scanner on the i2c lines to
>> confirm the pll ic is getting the 'change freq' instructions. Most
>> voltages appear good compared to the manual, although the voltage on Q16
>> C and Q17 G is off (initially 8v then dropped to near 0v after i
>> re-soldered the joints of resistor R9). I have shorted C60 to
>> completely bypass the VFO ALC (suggestion from Elecraft).
>>
>> All prior tests were fine, everything in expected range. Also the
>> correct frequency shows on pins 1 and 2 of U4 (MC145170), and voltages
>> look good on U3,4 & 6.
>>
>> I'm in the UK, so sending it back to Elecraft is not my preferred option
>> (time/cost).
>>
>> Has anyone got experience of issues in this area?
>>
>> Thanks,
>>
>>
>
______________________________________________________________
Elecraft mailing list
Home:
http://mailman.qth.net/mailman/listinfo/elecraftHelp:
http://mailman.qth.net/mmfaq.htmPost: mailto:
[hidden email]
This list hosted by:
http://www.qsl.netPlease help support this email list:
http://www.qsl.net/donate.html