Actually according to the 564 manual and the schematics in it, the low
pass filters are switched with relays. The band pass filters (used for
both transmit and receive) are diode switched and unique to each band.
Further the 10 and 12 meter LO pass filter is the same. Another is
shared between 15 and 17, and another between 30 and 40. That works
because the filter can cut off close to the high edge of the upper band
and still have enough attenuation at the second harmonic of the lower
band which the push pull PA will have minimized making the third
harmonic the critical one for filtering.
So loss of a relay or a dirty relay contact in the low pass filter can
kill transmit on two bands like 10 and 12, or 15 and 17. Its only in the
circuit on transmit so it won't kill receive.
73, Jerry, K0CQ
On 11/27/2010 2:20 PM, Ken Brown wrote:
>
>> Low or no output on a band or bands can be dirty switch contacts. 10,
>> 12, and 15 haven't had much use in the past few year. It might be a
>> quarter drop of deoxit on each contact with some vigorous switch
>> operation might clean them up, or the problem could be a poor solder
>> joint that went open or nearly open.
>>
> The Omni VI uses diode switching for all of that. I don't think there is
> a single mechanical switch contact (besides the band select push button)
> involved with the band selection. If it indicates that it has switched
> to the band you want, then that pushbutton is working fine. The parallel
> band select lines that go to various boards may need cleaning and the
> bad solder joint is a possibility.
>>
>> Its not possible to be more specific because there can be so many ways
>> to loose a band or bands (and the crystal is not above suspicion nor the
>> crystal socket when crystals are used to heterodyne the VFO) unless one
>> has repaired many Omni VI with the same problem. I doubt there's any RF
>> difference between VI and VI+.
>>
> Omni VI does not use crystal sockets. A bad crystal, or misaligned
> inductor in the crystal oscillator circuit could be the cause. Don't
> mess with those inductors unless you have the right equipment to get
> yourself out of the trouble you may cause. The locations of the coils
> used for each band is not immediately obvious, and there is a good
> chance you'll end up adjusting one that did not need adjusting. Make
> sure you use the perfectly shaped soft plastic twiddle stick for the
> inductor slug or you will likely break the slug. You will need an
> oscilloscope with a high impedance probe (like a 10 X probe that only
> loads the circuit with 10 Meg ohms + small capacitance) to monitor the
> PLL loop in order to properly set those slugs. And when you have that
> equipment, the description of the waveform you are looking for is more
> than I'm prepared to put in this message.
>
> DE N6KB
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