[TenTec] Omni VI instability, TCXO worth it?

Thu Jul 7 09:35:51 EDT 2016

There is no “can”, per se, in the Omni 6, into which one might insert cotton.

I still doubt that the issue is in the master reference. 

A frequency counter will verify this. Alternately, you could put a voltmeter at the junction of R20, R23 and C20 and just monitor the voltage as you go to different bands. The voltage should change a little but not a lot from band to band. If it does change a lot on the suspect bands compared with bands that work properly, then the crystals are off frequency too much and the PLL cannot compensate reliably.

When the radio is acting up, does it ever show any fault on the display.

When the radio is acting up, will it still transmit?

If you unplug cable number 84 on the crystal board, you will open the PLL loop. 

The CPU may or may to go into PLL fault but the receiver should still work. I am not sure of this because I have never tried it and the manual does not address any PLL fault codes.

This line carries two signals, the DC control voltage for the varactor diode that tunes the signal, and also the RF signal from that same oscillator back to U13, the PLL chip, for comparison with the reference oscillator.

The RF signal goes into the PLL chip to close the loop but there is a data out line from the PLL chip to the CPU. I do not know if that is the out-of-lock line that the CPU might use to determine lock condition but you will know right away if you unplug cable 84 from either end (preferably both ends).

Once you determine whether the radio will work or not in this condition, you will likely find that you are 5 to 10 KHz off frequency. There are a few ways to compensate for this.

1) Ignore it and stay away from band edges.
2) Apply a DC voltage to connector 84 on the crystal board to bring the crystals close to correct frequency. The output of thee 5 volt regulator is pretty clean and you can apply this voltage through a variable trim pot to set frequency. You will need to have an RF choke between the potentiometer wiper and connector 84 on the crystal board because there is RF present there.

Option 2) above will eliminate the need for the master reference signal, so long as the CPU does not fault. The frequency adjustment will be a compromise for the various bands, and there will always be some error, just as there was in the Omni 5.

If you make this a temporary modification, it will serve as a diagnostic since, if all bands except your trouble bands work, then you know that something is amiss with the crystals or their selector diodes. If all bands pull in to roughly the same amount of error in frequency, then likely your master reference was indeed at fault. 

If the master is at fault, permanentize the bypass circuit and use the radio.

73

Gary

W0DVN

> On Jul 7, 2016, at 7:38 AM, Ira Franklin via TenTec <tentec at contesting.com> wrote:
> 
> GM Rick, I will admit I had same doubt about stuffing it with cotton but thought what the heck not try it. Took about 20 minutes start to finish. It did and still does work on the 746pro stable as a rock.   Go figure. 73  Ira K4YMQAlabama
> 
>      From: "rick at dj0ip.de" <Rick at DJ0IP.de>
> To: 'Ira Franklin' <k4ymq at yahoo.com>; 'Discussion of Ten-Tec Equipment' <tentec at contesting.com> 
> Sent: Tuesday, July 5, 2016 2:17 AM
> Subject: Re: [TenTec] Omni VI instability, TCXO worth it?
> 
> That sounds like an April Fool's joke, Ira.
> How on earth might that help?
> Seems it would only result in things getting even hotter.
> 
> 73 - Rick, DJ0IP
> (Nr. Frankfurt, Germany)
> 
> 
> 
> -----Original Message-----
> From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Ira Franklin via TenTec
> Sent: Tuesday, July 05, 2016 9:11 AM
> To: Discussion of Ten-Tec Equipment
> Cc: Ira Franklin
> Subject: Re: [TenTec] Omni VI instability, TCXO worth it?
> 
> I own a TT Omni 6 opt never had that problem nor have I opened the case, What am getting at is that I also own a IC746 pro that developed a problem similar to yours on the TT I was advised to stuff the metal can that has osc  fightly full of cotton and carefully I did that about 4 years ago and never had another problem. Don't know if this will help on TT or not.  Ira K4YMQ
> 
>       From: "rick at dj0ip.de" <Rick at DJ0IP.de>
> To: 'Discussion of Ten-Tec Equipment' <tentec at contesting.com>
> Sent: Tuesday, July 5, 2016 1:28 AM
> Subject: Re: [TenTec] Omni VI instability, TCXO worth it?
>   
> Peter,
> 
> I personally would not do it.
> If you can find a used one for $50 or less, then perhaps.
> 
> Hopefully someone here will have a few tips to enable you to keep using it as is.
> 
> The TCXO was a nice but at this late stage, investing another $200 in a radio that old is probably not a wise idea.
> 
> If you can't fix it, perhaps it is time to buy another radio.
> The $200 is a good start towards that.
> If you can sell it "as is", fine but otherwise you can part it out and sell it on ebay.
> 
> I doubt a good option 1 will bring more than $500 these days and since it is not working properly, it won't bring that.  
> 
> 
> 73 - Rick, DJ0IP
> (Nr. Frankfurt, Germany)
> 
> 
> -----Original Message-----
> From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Peter Klein
> Sent: Tuesday, July 05, 2016 2:14 AM
> To: tentec at contesting.com
> Subject: [TenTec] Omni VI instability, TCXO worth it?
> 
> 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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