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

Mon Jul 13 23:00:59 EDT 2020

OK Dave -

I will check the relays as you suggest. In troubleshooting capacitors I
messed up
C74. I pulled it off the board and sent a request to Elecraft for a
replacement.
That may solve the problem. In the meantime I will check the coils on the
K13 to K15
relays and let you know what I find.

I did not see Don's post. I will check it out. So I will be waiting a bit
to fix this problem,

73 and thanks!
Keith N8CEP

On Mon, Jul 13, 2020 at 10:36 PM Dave Van Wallaghen <dave at w8fgu.com> wrote:

> 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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> >>>
> >>>
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