Why does a BFO oscillator need 0.1Hz accuracy?
Barry N1EU
On Wed, Jul 30, 2014 at 1:30 PM, Bob McGraw - K4TAX <RMcGraw@blomand.net>
wrote:
> Sorry I can't agree that using a receiver would be adequate for alignment.
> My experience in repairing Paragon and Omni VI radios, a frequency counter
> with an accuracy of 0.1 Hz or better is required for the BFO adjustments.
> For proper alignment, one must follow the steps in exact sequence as listed
> in the manual under the section TX Audio.
>
> There is interaction between the alignment steps and I always go through
> the steps in sequence at least 2 or 3 times before completion. This
> includes the CW, LSB, USB and FSK Space and FSK Mark modes. In performing
> the alignment procedure one will likely find the BFO switching is not
> taking place thus providing the transmitter off-set thus the true reason
> for the fault.
>
> Although I no longer do repair work, except for my self, I have referred
> this to a person that does repair Paragon and Omni VI radios. He is not
> associated with Tentec.
>
> 73
> Bob, K4TAX
>
>
>
>
> ----- Original Message ----- From: "Barry N1EU" <barry.n1eu@gmail.com>
>
> To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
> Sent: Wednesday, July 30, 2014 6:18 AM
> Subject: Re: [TenTec] Omni VI Mystery
>
>
>
> Instead of a freq counter, you can use a general coverage receiver with
>> fairly precise digital readout. I've used a K3 and Orion subrx for this
>> purpose. The Omni 6 BFO test point level is low enough to feed directly
>> into the rx antenna input through a capacitor.
>>
>> Back on day 1, I encouraged you to check your BFO oscillator alignment.
>>
>> Barry N1EU
>>
>>
>> On Wed, Jul 30, 2014 at 1:43 AM, Jim Allen <jim.allen@longhornband.net>
>> wrote:
>>
>> Some will remember the thread a few months back in which I described the
>>> problem I had been having making contacts with the Omni VI opt 3 I have.
>>> I
>>> realized there was a problem with the radio when I used my 706MKIIG at 5
>>> watts one day in a QSO party with the same antenna and made contacts all
>>> over the place no problem.
>>>
>>> It turns out the radio is off somehow by about the amount of the
>>> sidetone,
>>> so when I zero beat a CW station (match tone set in sidetone) and
>>> transmit,
>>> the other station hears no tone. My signal is below his audible range
>>> and/or outside his passband.
>>>
>>> We did some testing here between the 706, the Omni and other stations and
>>> determined that the problem could be gotten around by setting the XIT on
>>> to
>>> -whatever the sidetone was, in my case, usually 700Hz. That made the
>>> radio
>>> work, except for the transmit frequency being off, and hopping around
>>> with
>>> QSK, but it put my signal at the frequency that other stations would hear
>>> it.
>>>
>>> I was not content to leave it like that so sent it to TenTec to check the
>>> alignment and/or correct whatever was occurring to cause this. To my
>>> chagrin, Paul reported that the radio was just fine, working as designed,
>>> no problems, aligned just fine working per factory spec, nothing needed,
>>> etc., and sent it back. At least he didn't charge me for this.
>>>
>>> About that time, I found a Omni VI+, with full filters and speaker/power
>>> supply at a price that I found attractive, so I bought it. I've been
>>> using
>>> it for more than a month, works great, just the way you expect, and have
>>> no
>>> trouble making contracts, in the context of a 100 watt radio with a so-so
>>> antenna anyway. I used it for Field Day at 5 watts output to a homebrew
>>> S9v31 vertical and was very pleased with the performance, given the
>>> circumstances.
>>>
>>> This Omni VI still bugs me though, so I set up all three radios and did
>>> some testing. I set all three radios on 14.030, using the no output on
>>> the
>>> two Omnis and a dummy load on the 706. When I transmit using the 706, I
>>> hear that signal on 14.030 in the Omni VI+, and at 14.030.4 zerobeat on
>>> the
>>> Omni VI, in both cases matching the tone using that function on the CW
>>> button. Both sidetones are set at 700Hz.
>>>
>>> When I set the Omni VI to 14.030 and transmit, I zero that on the Omni
>>> VI+
>>> at 14.030.04, a 40 Hz difference. On the 706, I zero the signal at
>>> 14.030.07. There is a slight difference in the calibration of these 3
>>> radios, and they sometimes vary 10 Hz or so anyway.
>>>
>>> When I hear the Omni VI+ and zerobeat that signal in the Omni VI, it is
>>> at
>>> 14.030.42. In a QSO, I would hear the Omni VI+ calling and zero that
>>> signal. When I called using the Omni VI, on the zero frequency, I hear
>>> it
>>> in the VI+ as a very low pitch, almost a buzz. When I turn on XIT, at
>>> -700Hz and call, the Omni VI+ hears it with a higher audio tone, and
>>> zeros
>>> the Omni VI signal at 14.029.79.
>>>
>>> It looks to me like perhaps the Omni VI is off calibration slightly, but
>>> that doesn't explain why it doesn't transmit with the proper offset as
>>> expected, without XIT being on anyway.
>>>
>>> I tried a QSO with these three radios. I call CQ using the 706, I "hear"
>>> and zero that signal on the Omni + at 14.030, the same as the 706. I
>>> reply
>>> using the Omni VI+ and hear the reply in the 706, with a ~700 hz. tone. I
>>> hear and zero the same signal from the 706 using the Omni VI, at
>>> 14.030.39. When I reply to the 706, the 706 hears a strong signal at a
>>> very low audio tone, inaudible. When I click XIT -7 on the Omni VI, I
>>> hear
>>> it in the 706 just fine.
>>>
>>> I have consulted off list with a very clever and exceedingly
>>> knowledgeable
>>> Tentec user, who probably wishes to remain anonymous, and he has
>>> suggested
>>> I check the alignment myself. This fellow also suggested a way to
>>> determine if one or more of the parts in the affected circuit have gone
>>> bad. I do not have a frequency counter, and do not know where to obtain
>>> access to one. What kind of frequency counter ought to be used for this?
>>>
>>> Is there anyone who works on these radios, other than Tentec, who has the
>>> test gear etc to resolve this?
>>>
>>> There has to be an explanation why the VI doesn't work like the VI+ in
>>> this
>>> respect.
>>>
>>> TIA
>>>
>>> 73 de W6OGC Jim Allen
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>>>
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>
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