[TenTec] OMNI-C frequency unstable

[Ron]

Thanks Mike for the good leads and the encouragement. I've been up late
tonight working on her and put it back together and worked PSK31 with it for
about 30 minutes and it behaved well.  Hmmm... now if I just knew what I did
that actually solved it I'd tell you.  Any way, I sprayed all the POTS and
unplugged and plugged in circuit boards, especially the VFO amp and the
Oscillator boards.  Those have band switches and are harder to remove, but
all for a good cause eh?  Also unplugged the crystals and stuff.  Checked
out the diodes in the circuits and generally poked and prodded everything.
Put the lids on and hooked it up and so far it is behaving nicely.  No noise
on the receiver and the heat sinks are almost to hot to hold on to.  I just
don't know what it is that caused the problem, but for now she is up and
running like a new rig, so if it is working don't fix it, right?
73 and thanks.
Ron
KA7U


-----Original Message-----
From: tentec-bounces at contesting.com [mailto:tentec-bounces at contesting.com]On
Behalf Of Mike Hyder -N4NT-
Sent: Saturday, August 13, 2005 9:40 PM
To: Discussion of Ten-Tec Equipment; ron at morell.us
Subject: Re: [TenTec] OMNI-C frequency unstable

Hi, Ron--

The first thing to do with an Omni-C that has almost any trouble is to
remove and reseat each printed circuit card (one at a time).  While each
card is out of the rig, inspect it closely for bad solder joints or other
obvious trouble.  Then if there is a molex connector with a jumper on the
rear panel, remove the jumper plug and reinsert it.

If the instability is only on one band, troubleshoot that band's oscillator.

If the instability is on all bands then here are the things I would check in
the order I'd check them:

1.  There ought to be a potentiometer internal to the radio which sets the
offset tuning.  This is to balance the control on the front panel.  A dirty
potentiometer will create trouble for you.

2.  The next thing is to put a frequency counter on the output of the BFO
crystal oscillator.  This oscillator has detailed instructions in the manual
about how to align the padder capacitors.  Another fellow suggested using
DeOxit and I concur with him that the wipers of those padder capacitors
ought to be treated with DeOxit.  If that BFO oscillator is unstable, you'll
find it in this step.

3.  About the last thing would be the PTO.  Remember that the PTO has a
switching voltage applied to it which is used on 10 meters, I think, to move
its frequency a bit to avoid birdies (at least this is the way the Tritons
worked).  With your counter, you ought be able to see if the PTO is unstable
in its frequency output.  If so, then you will want to monitor the different
voltages applied to the PTO to see if one of them is causing your trouble.

I'd appreciate your telling all of us what you find wrong with that rig.

73, Mike N4NT

----- Original Message -----
From: "Ron" <ron at morell.us>
To: "TenTecReflector" <tentec at contesting.com>
Sent: Saturday, August 13, 2005 12:08 PM
Subject: [TenTec] OMNI-C frequency unstable


OK, I'm stumped.  My OMNI-C lost 4 diodes on the SWR/TR board a while back
and I replaced them and the AGC circuit works again, but after some use on
CW the receiver will start to drift up and down in frequency by 100 to 200
cycles and then it will start to make noise in addition.  During transmit
this produces a warbly chirp.  Turn the radio off for 10 minutes or so and
it will start up and run fine for a while.  The power supply seems fine but
I've tried another and the same effect is occasionally observed.  I have an
OMNI D that does not seem to have a problem with either power supply.  If I
connect and use the external VFO, the same problem can occur, so I do not
believe the PTO needs rebuilt or any of that type of thing.  I'm fairly
certain the problem is either voltage regulation or an oscillator that warms
up and becomes unstable, hence the noise.

So any suggestions of what to look for and where to look, in this old but
beautiful OMNI C ?  I bought her new long ago and now I see it in the Ten
Tec museum, and some call her vintage, but I hate to let this rig go down.
Any suggestions of what could cause this anomaly will be appreciated.
Ron
KA7U
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