[Elecraft] Fwd: Re[2]: K2 7747 VCO alignment problem
Don Wilhelm
donwilh at embarqmail.com
Wed Jul 15 15:28:29 EDT 2020
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 <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
>>>>> >______________________________________________________________
>>>>> >Elecraft mailing list
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>>>>> >Help: http://mailman.qth.net/mmfaq.htm
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>>>>> >
>>>>> >This list hosted by: http://www.qsl.net
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>>>>> >Message delivered to dave at w8fgu.com
>>>>>
>>>>>
>>> ______________________________________________________________
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>>
>>
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