[TenTec] Omni VI testing XTAL OSC module outside of radio

Ken Brown kenradiobrown at gmail.com
Sat Jul 8 22:17:02 EDT 2017

Actually what you said about "each band having its own crystal oscillator"
is not entirely correct. There is only one oscillator for band selection,
with one of several crystal switched in to select what frequency that one
oscillator works on. I know that may be sort of picky to say that, but it
makes a big difference when troubleshooting. Since mine is not working on
any band, I think I can conclude it is a part common to operation on all
bands that has failed. That is why I am looking at the oscillator
semiconductors and not suspecting a bad crystal or tuning inductor.


On Sat, Jul 8, 2017 at 4:06 PM, Ken Brown <kenradiobrown at gmail.com> wrote:

> Hi Gary,
> The crystal oscillator is getting +13.5 vdc and VREG +10 VDC AND band
> select voltage. Still it does not generate anything, regardless of which
> band is selected. I am suspecting a semiconductor failure. I tried 2N3906
> for both Q1 and Q3 and still nothing. Maybe 2N3906 is not good enough HF
> wise to work, but it is NPN and available. MPS5179 and MPS6514 I do not
> have readily available. Maybe the varactor D26 is bad.
> I am using a portable "all band" receiver with a BFO to listen for the
> signals. This works for confirming the 5.0 -5.5 osc is working and the 20
> MHz reference and the 9 MHz BFOs. I can hear them on the little receiver
> and wobble them with my finger on the PCB where each oscillator is, Or in
> the case of the 5.0 to 5.5 MHz oscillator I can tune it with the main VFO
> knob on the rig and follow it with the receiver.
> So if the xtal osc is generating I should be able to detect it too, and I
> do not. That is with the top off of the box it is in, mounted inside the
> Omni VI, or with the whole board removed and connecting bench power
> supplies for +13, +10 and about +8 for band select. Also have tried
> sniffing with a spectrum analyzer and don't see anything coming from the
> xtal osc.
> This should not be too hard to fix,and I really want that Omni VI working
> again. It is just too good a rig to give up on just because one oscillator
> isn't working.
> Thanks for your help.
> Ken N6KB
> On Sat, Jul 8, 2017 at 2:11 PM, Gary F <dukeshifi at comcast.net> wrote:
>> The 5 to 5.5 synthesizer will operate with no reference (out of lock, of
>> course).
>> The reference oscillator in an Omni VI is either on the CPU board or
>> pendant board on the chassis wall facing into the CPU area.
>>  Radio will receive even if the reference is missing, it will only
>> receive one frequency, and will be very unstable.  It will not transmit.
>> The way the radio works IS that the 5.0 to 5.5 synthesizer picks up the
>> on the 20 MHz reference and locks.
>> The crystal oscillators for each band still operate with no reference,
>> they will just be off frequency. Thus you do not have a single crystal
>> oscillator problem. Each band has its own crystal oscillator that gets
>> selected by the data from the CPU.
>> If you are not getting band select data to the crystal oscillator board,
>> look at the serial to bus driver chip on the CPU board because that is
>> where band select originates.
>> I don't recall the chip number but it is right by the connector that
>> connects between the CPU and the  rust all local oscillator board.
>> If your CPU were dead, the display would not change when you turn the
>> main tuning knob so it sounds like the CPU is OK.
>>  my iPad
>> > On Jul 8, 2017, at 4:50 PM, Ken Brown <kenradiobrown at gmail.com> wrote:
>> >
>> > My Omni VI is dead, and it seems like it is because the band selecting
>> > crystal oscillator is not generating a signal.
>> >
>> > Voltmeter readings indicate that the board is getting the correct DC
>> > supplies , the +REG and +13.5v as well as the appropriate band select
>> > control voltages on connectors 82 and 83. Still I cannot detect a strong
>> > signal at the XTAL oscillator frequency for any band selected. This
>> board
>> > is sort of hard to get to, and I would like to troubleshoot it on the
>> bench
>> > and hopefully confirm it is working before I put it back in the radio.
>> >
>> > Since the fault is in the crystal oscillator, I think if I get that
>> working
>> > I'll have it fixed. I don't think I need to supply the 5.0 to 5.5 MHz
>> input
>> > to troubleshoot the crystal oscillator. I think if I give it DC supply
>> > voltages and band select voltage I ought to be able to get it to
>> generate a
>> > signal when on the test bench. With nothing going  to connector 84 the
>> > frequency will not be phase locked or precisely controlled but there
>> should
>> > still be a strong oscillation very near the correct frequency for
>> whichever
>> > band I connect the band select voltage to connector 82 or 83 for.
>> >
>> > I would like to hear from anyone who has tried this. Do I need to put a
>> DC
>> > voltage on connector 84 to get it to work at all? Or should it oscillate
>> > with nothing connected to 84, and just be sort of free running at the
>> > crystal frequency with no correction voltage?
>> >
>> > Thanks, Ken N6KB
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