[TenTec] Omni VI instability, TCXO worth it?

Tue Jul 5 22:19:28 EDT 2016

I agree, that the “Master” should affect all bands, in most cases (I’ll get to the exception later).

Remember what it is doing. It is “soft steering” the crystal oscillators to keep them operating at the frequency the the dial says it is operating.

It does this just lake any other PLL does it, by comparing a division of the master against a division of the crystal to produce an error signal. That error signal tunes the crystal oscillator to lock it up in phase with the reference master.

If one (or more) of the crystals is operating so far off frequency that the error signal reaches the maximum (or minimum) that it can reach, then the crystal oscillator cannot lock up. 

If things are operating this way, it is quite easy to understand why the suspect bands can wander in and out of lock, in what seems like a whimsical way.

This state of operation could even induce something that sounds like chirp on CW because the voltage on the line supplying the crystal oscillators may pull just a bit with a CW note and thus cause the out of-lock oscillator to chirp.

Since the recurring problem seems to be only 20, 30 and 40 meters, and presumably all have been previously ‘tweaked” to make them work, I believe this adds credibility to this conjecture. If they are tweaked to the limit of the ability of the loop, this is exactly what will happen.

I really do not think a new reference master oscillator will cure this problem, I think your repair should be a lot less costly than that. 

I may still have a crystal board from a defunct Omni 6 and could perhaps offer replacement crystals for those bands.

There is ONE way in which the master could cause this problem one some bands and not on others. If the crystal oscillators for the three offending bands had been adjusted to limits, and then the master were to drift off in the right direction, the same problem would occur. You would run out of correction range in the PLL. 

Bands whose crystals were closer to their centerlines in operating frequency would likely not suffer this fate since the PLL cold still lock, albeit at an error voltage not at the normal centerline voltage.

Put a counter on the output of the master and see what it is. See if it changes as the set warms up (these early ones always did, to some extent). If it does NOT drift, likely its oven is burned out (as often occurred).

If you know how, unlock the loop and read the frequencies of the respective band crystals to see how close they are to their marked frequencies.

In reality, with proper recalibration of the trimmers, you COULD operate this radio without the master and you would essentially have the same frequency scheme as was used in the Omni 5. You’d always have some readout error. That was what the reference master oscillator and PLL were added to correct. If you don’t care about being a few KHz off from the dial reading, and stay away from band edges, you’ll be happy like all Omni 5 owners are.

Gary

W0DVN

> On Jul 5, 2016, at 6:39 PM, Greg S via TenTec <tentec at contesting.com> wrote:
> 
> Soooooo...... maybe I am totally off BASE, but if the "master"  oscillator is the issue, wouldn't he be noticing the problem on all bands, with going higher in frequency getting worse?? 
> 
> 73-
> Greg, KC8HXO
> --------------------------------------------
> On Mon, 7/4/16, Peter Klein <pklein at threshinc.com> wrote:
> 
> Subject: [TenTec] Omni VI instability, TCXO worth it?
> To: tentec at contesting.com
> Date: Monday, July 4, 2016, 8:14 PM
> 
> I queried service at tentec.com
> by email about the following issue last 
> Tuesday. I have heard nothing so far, so I thought I'd get
> some advice here.
> 
> Background:  I live in the Seattle area. I have modest
> wire and vertical 
> antennas. I'm not a contester or hard-core competitive DXer.
> I do try to 
> work DX when I can, and I might jump into a contest to snag
> a new 
> country. But mostly I do HF digital (PSK, Olivia, MFSK),
> casual CW, or 
> occasionally SSB.
> 
> I've owned my Omni VI for 20 years. It has the Option 1 DSP
> upgrade. 
> Over the years, the radio has several times developed a
> frequency 
> instability issue. Over a time ranging between less than a
> second to a 
> few seconds, the transceiver moves a few 10s of Hz off
> frequency, then 
> "snaps" back. It's mostly done this on 30m, occasionally on
> 20m, and now 
> on 40m. Lately, it happens on 40m, with a short "period," so
> if I am 
> listening to a good, pure CW signal, it sounds like it has
> old-fashioned 
> "chirp."
> 
> The frequency shift occurs on both transmit and
> receive.  When it 
> happens, if I switch to another band, it does not happen
> there. It is 
> intermittent. Sometimes it appears random, and sometimes
> appears 
> temperature-related (for example after transmitting for a
> while, or if 
> the radio has been just turned on in the last half-hour or
> so).
> 
> When it first happened (1997), Ten Tec Service advised me to
> tweak the 
> trimmer coil to the 30m mixer crystal 1/8 to 1/4 turn in
> either 
> direction until the "drift and snap" stopped, and just live
> with the 
> slight display error. I did, and later did the same thing to
> the 40m 
> coil when that band acted up. It worked in both cases.
> 
> When it started to do the frequency dance on 20m, I sent
> back to the 
> factory for a tune-up. This was in October 2013. Now, it's
> doing it 
> again on 40m.
> 
> I spoke to TenTec's service manager a few months ago. He
> said that the 
> problem might be solved by installing a temperature
> compensated crystal 
> oscillator (the TCXO was never suggested before).  He
> said that I could 
> install this myself if I could solder a couple of wires. He
> quoted me a 
> price of $200 to send me the necessary circuit board and
> instructions. 
> He said it was up to me whether I wanted to spend $200 on
> "an older 
> radio."  He was also dead-set against my tweaking the
> trimmer coils to 
> the individual band crystals.
> 
> I decided to wait and see if leaving the power supply on
> 24/7 would 
> solve the issue. Nope. And while I was waiting, Ten-Tec was
> sold and has 
> new service policies.
> 
> I would like to install the TCXO, *IF* it is very likely
> that it will 
> solve the problem, and I can get at least another couple of
> years of 
> good service out of the radio.  On the other hand, the
> radio is 20 years 
> old. So if it's just as likely that I will continue to have
> the problem, 
> then maybe the TCXO isn't worth it, and I should think about
> a new radio.
> 
> So, what would you do if you were me?
> 
> Thanks and 73,
> --Peter, KD7MW
> 
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