[Elecraft] [K2] VCO Not Oscillating

Mon Apr 28 22:46:30 EDT 2014

Yes, leads 1 & 3 should come out the top of the core. However, you also need to make sure the windings are in the same direction, like in figure 6-16, otherwise you are applying negative instead of positive feedback, and the VCO won't oscillate.

Also, make sure leads 1 & 2 are both red, and go through the core for 16 turns. Leads 3 & 4 should both be green, and consist of 4 turns. If you get the windings backwards, then the feedback loop won't have sufficient gain, and the VCO won't oscillate.

Finally, the other problem that can occur with toroids is that the insulation on the magnet wire wasn't stripped back enough, and there is a poor joint. If this happens, the VCO won't oscillate :-)

There are other reasons why the VCO may not oscillate (eg. loading due to solder bridges, components in backwards etc.), but toroids in particular can be quite tricky and confusing, which is why Don suggests to look there first.

73, Matt VK2RQ

> On 29 Apr 2014, at 11:56 am, Joseph Baxley via Elecraft <elecraft at mailman.qth.net> wrote:
> 
> Don,
> 
> Thanks for the reply.  I should have mentioned that I checked the windings on T5.  Pins 1 and 3 should be coming out of the top of the core, no?  That's what I have.  I also used the yellow core for the transformer.  
> 
> Thanks,
> Joseph
> On Monday, April 28, 2014 8:43 PM, Don Wilhelm-4 [via Elecraft] <ml-node+s365791n7588092h87 at n2.nabble.com> wrote:
> 
> Joseph, 
> 
> The first thing to check is T5.  The direction of the windings is 
> critical, and the leads must be in the proper holes as shown in the 
> diagrams in the manual.  In other words, the sense (direction) of the 
> green wire turns on T5 must be the same as the direction of the red wire 
> winding. 
> 
> If that is correct, the VCO will oscillate, and things usually fall 
> 'into line' after that once L30 is adjusted for the proper voltage at 
> the left end of R30. 
> 
> 73, 
> Don W3FPR 
> 
>> On 4/28/2014 8:22 PM, Joseph Baxley via Elecraft wrote: 
>> 
>> Hi, 
>> 
>> I've been trying for the last four days to figure out what is wrong with my 
>> rig.  I'm at Phase 2 Test and Alignment and got to testing the frequency of 
>> the VCO.  It had a reading of 00000.  Also the voltage on R30 was 7.87V. 
>> This is consistent with the VCO not oscillating.  The reference oscillator 
>> works fine and was within the expected ranges when I looked at it.  What I 
>> was able to notice was that U4 on the RF board has two pins with voltage 
>> values that are out of spec.  Pin 1 has 0.9V and pin 13 has 0V.  If I read 
>> the table they should be 2.1 and 4V respectively.  It also seems that the 0V 
>> on pin 13 is causing a 0V reading on pin 6 of U6.  Otherwise U6 is in spec. 
>> I got out my oscilloscope and got a Vrms of 1.13V on U4 pin 1.  There wasn't 
>> anything on pins 3 or 6 of U3.  The voltages for Q17, Q16, and Q18 are as 
>> follows: 
>> 
>> Q17   E: 0V 
>>           B: 7.13V 
>>           C: 0.08V 
>> 
>> Q16   E: 0V 
>>           B: 0V 
>>           C: 7.13V 
>> 
>> Q18   G: 0.08V 
>>           S: 6.64V 
>>           D: 0V 
>> 
>> I have been measuring and testing voltages and components for the last four 
>> days and have run out of things to look at for the moment.  If any of you 
>> can help me with what my problem is, I would be most grateful. 
>> 
>> Sincerely, 
>> Joseph 
>> KI4ITG 
>> 
>> 
>> 
>> -- 
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