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@morell.us>
To: "TenTecReflector" <tentec@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
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|