Keith,
C74 could explain a problem on 30, 20, and 12 meters, but you apparently
have something else awry.
If you do not find a problem with the relay coils, you should next check
the contacts of relays K13, K14 and K15.
On 20 meters, all the relays should be set - meaning you should have
conductivity from pins 3 to 4 and not between pins 2 and 3.
Check the other side of the relays - conductivity between pins 7 and 8,
and open between pins 8 and 9
Then check 80 meters, only K13 should be set - so check K14 and K15
which are reset on that band - no conductivity between pins 4 and 3 and
open between pins 2 and 3
Also check the other side, conductivity between pins 8 and 9 and open
between 7 and 8.
Then switch to 30 meters and you can check the reset state of K13 -
Continuity between pins 2 and 3 and open between pins 3 and 4
Also conductivity between pins 8 and 9 and open between pins 7 and 8.
If all that checks out, look again at the capacitor values of C71, C72,
and C73. I know some of those capacitors are tight tucked between the
relays, but bend them over a bit and use a good light and possibly a
magnifier to read the markings.
73,
Don W3FPR
On 7/13/2020 11:00 PM, Keith Hamilton wrote:
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 <d...@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.ke...@gmail.com>
To: elecraft@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.ke...@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 <d...@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 <d...@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.ke...@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 <d...@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.ke...@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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