The crystals are phase locked by the control board. I'd expect it to
remain the same from 28.0 to 28.5 since the crystal and reference remain
unchanged over that range. Probably a slight tweak of the coil L17 one
way or the other will solve the problem. It sets the voltage controlled
range of the crystal for the 28.0 band. I'm sure there's an optimum
adjustment to get the voltage on the varicap (good place to measure
would be at the junction of C20, R23, and R20, R26 and R25 also connect
to that junction in mid range, probably 3 or 4 volts.
Likely some of the oddities you see are due to slight temperature changes.
73, Jerry, K0CQ
On 3/11/2011 1:24 PM, Don Rasmussen wrote:
> Hi Guys,
> My OMNI 564 started acting a little odd today, I thought I would see if
> anyone has any suggestions as far as things to consider before packing the
> unit up for shipment to the factory for service.
> What happens is, when the radio is started from cold at 28.400 everything is
> normal listening on SSB. After some time, the VCO will seem to lose lock and
> we lose receive audio and no transmit. This will come and go for 30 minutes.
> If I arrow up 100KHZ the radio will return to a normal state. If I dial back
> down towards 28.400 the radio will tune normally until it gets somewhere near
> 28.440 where it will go silent.
> That is to say after the radio went silent at 28.400 and I tune up to 28.480
> it would remain silent. However, if it went silent at 28.400 and I arrow up
> or tune up 150 khz and then tune back down, when I arrive at 28.480 the radio
> will be working.
> I was looking at the L.O Mixer board (page 4-14 in the manual) and I can see
> where there are 10 crystals and one would be selected at 28.0, 28.5, 29.0 and
> 29.5 so maybe we have a marginal crystal in the 28.0 position. However I
> wonder why the crystal is okay at 28.475 all of the time and failing at
> 28.400. You would assume the crystal would fail up until the 28.500 point
> where the next crystal in sequence would be selected.
> There is a variable inductor at L17, wondering about that one.
> All of the other bands and frequencies work 100%, just this one small bugaboo
> in this one spot and so far only when the radio is cold.
> Any ideas on things to check would be welcome.
> Don wb8yqj/6
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