[Elecraft] Fwd: Re[2]: K2 7747 VCO alignment problem

Mon Jul 13 22:35:36 EDT 2020

Keith,

I saw Don respond to this and he is right that K15 would be the common 
denominator if not for 15m. You can check to see if the coils are intact 
if you measure the resistance between pins 1 and 10 for each. It should 
be around 230 ohms or so. If you find one open or well away from that 
resistance, then it would indicate a bad coil on that relay.

You can also measure the intended path through the relays for each band 
and make sure there are no unintended paths. If you can read the 
schematic, it shows the path with each relay in it's reset state. You 
would need to consult the table in Appendix B to see which relay(s) are 
active for a respective band and you can then measure the continuity of 
the intended path. You should also check any unintended paths which 
could indicate a problem.

As Don stated, this is not a fun problem to find and will take some good 
debugging skills. So far, everything you've indicated seems to suggest 
the PLL oscillator and VCO circuits are working properly, but the range 
selection area is not producing the proper capacitance to allow the VCO 
to produce the proper frequency on those bands you indicated.

If reading the schematic is a problem, let us know and I am sure between 
Don and myself, we can give you some step by step instructions for 
tracing through this problem.

73,
Dave W8FGU

------ Original Message ------
From: "Keith Hamilton" <n8cep.keith at gmail.com>
To: elecraft at mailman.qth.net
Sent: 7/13/2020 9:34:03 PM
Subject: [Elecraft] Fwd: Re[2]: K2 7747 VCO alignment problem

>---------- Forwarded message ---------
>From: Keith Hamilton <n8cep.keith at gmail.com>
>Date: Mon, Jul 13, 2020 at 9:19 PM
>Subject: Re: Re[2]: [Elecraft] K2 7747 VCO alignment problem
>To: Dave Van Wallaghen <dave at w8fgu.com>
>
>
>Dave
>
>Yes the frequency changes when turning the VFO knob when measuring with CAL
>FCTR on TP1.
>
>I have checked the markings on C71, C72, C73, and C74 (C75 is not installed
>yet). They are
>all correct and I resoldered them to be sure.
>
>I am beginning to think I might have a bad relay K13, K14 or K15. My good
>bands are 80,40 and
>15 meters. My bad bands are 30,20,17,12 and 10 meters. Looking at the relay
>table in appendix B
>I am not sure which relay could be bad. How can I test them? Should I try
>to replace them?
>73,
>Keith N8CEP
>
>
>On Mon, Jul 13, 2020 at 4:58 PM Dave Van Wallaghen <dave at w8fgu.com> wrote:
>
>>  Ok Keith, just to make sure things are where we want on the bands that
>>  seem to work, does the frequency change when turning the VFO knob when
>>  measuring with CAL FCTR on TP1? If they do, then things are working ok.
>>
>>  For the bands that are locked at 8.28v, that is the max output from the
>>  integrator circuit (U6B). This means that there is no lock between the PLL
>>  Oscillator and the VCO circuit and the PLL chip (U4) is trying to drive the
>>  varactors to provide more capacitance to the VCO circuit. The most likely
>>  cause will be a misplaced capacitor in the VCO range selector (C71 - C75)
>>  or a problem in the VCO circuit itself. But because this may be band
>>  dependent, I would look first in that range selection circuit. Verify that
>>  those caps are the proper values. They are easily misread and will cause
>>  the type of thing you are seeing.
>>
>>  Also verify those varactor diodes are the proper type and in the right
>>  places. Soldering issues are typically the leading cause of problems, so
>>  check that also. If all looks right to you, you may want to signal trace
>>  through that selection circuit to verify the proper relays and active for
>>  the bands not working. There is a relay table in Appendix B that shows you
>>  which relays are active for each band.
>>
>>  This is a brand new build, correct?
>>
>>  73,
>>  Dave, W8FGU
>>
>>
>>  On 7/13/2020 4:41:07 PM, "Keith Hamilton" <n8cep.keith at gmail.com> wrote:
>>
>>  Thanks for your help Dave!
>>
>>  Yes. I was able to get 6 volts on R30 by turning L30. At first the voltage
>>  was too high
>>  so I adjusted the turns on T5 by spreading the red windings further apart.
>>  Then I
>>  was able to get to 6 volts.
>>
>>  I did a CAL FCTR with the probe on TP1 and the frequency set to 4000,
>>  The reading is 8912.22
>>
>>  73 Keith N8CEP
>>
>>
>>  On Mon, Jul 13, 2020 at 3:59 PM Dave Van Wallaghen <dave at w8fgu.com> wrote:
>>
>>>  Hi Keith,
>>>
>>>  Were you able to get 6v on R30 by tuning L30? Did you have to bring the
>>>  voltage down or up by moving the windings on T5?
>>>
>>>  If you would, set the VFO to 4 MHz and do a CAL FCTR on TP1 and let me
>>>  know what the reading is. I'm just looking for a couple of clues before
>>>  we proceed.
>>>
>>>  73,
>>>  Dave, W8FGU
>>>
>>>
>>>  On 7/13/2020 2:52:11 PM, "Keith Hamilton" <n8cep.keith at gmail.com> wrote:
>>>
>>>  >I am having trouble with the K2 VCO alignment. I have moved the T5
>>>  windings
>>>  >and obtained 6 volts at the left end of R30. I then went on to the chart
>>>  >(table 6-1) to measure the voltages at the listed frequencies. My
>>>  readings
>>>  >are ok on 80 40 and 15 meters (between 1.5 to 7.5 volts) but on 30,20,17
>>>  >,12 and 10 meters the voltage is 8.29 volts and does not change.
>>>  >
>>>  >I have checked the solder connections of T5 and the value of C72 (271).
>>>  The
>>>  >diodes D21,D22,D23, D24, D25 and D26 are correctly installed. I also went
>>>  >over the solder connections of K13, K14 and K15.
>>>  >
>>>  >Now I'm stuck. Any help will be appreciated.
>>>  >
>>>  >Keioth Hamilton N8CEP
>>>  >______________________________________________________________
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