Ten Tec Omni-A(D) erratic frequency on TX?

Discussion in 'Radio Circuits, Repair & Performance' started by W8SJB, Feb 14, 2018 at 3:56 AM.

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  1. W8SJB

    W8SJB Ham Member QRZ Page

    Hi all,

    I recently purchased an old Ten-Tec Omni-A(with an AADE digital Freq. counter installed and original dial removed) from a silent key sale that is having some issues on transmit. Did a little searching and can't really find anything that stands out in the forums and my lousy Google-foo isn't finding anything helpful either.. Hopefully someone has experience or can give me a starting point to troubleshoot?

    Receive seems to be okay, but the issue comes into play on transmit.. It appears(on the freq. counter display, and reports from me trying to check into various nets on 40M) that while transmitting on SSB, my frequency is oscillating quite a bit.. several Hz. and am told it makes my audio sound fine one second, then distorted the next. If I key up on CW into the dummy load, I believe it stays steady as long as I have the key down and the drive not too high(20 watts-ish on the wattmeter). Also does it if I turn the drive up on a key down CW, and if I hold down the dits on the keyer with the WPM turned up.
    Wanted to move up into a little more solid state(my other rig with the exact antenna/tuner/dummyload, etc.) from my trusty old SB-100 and clear up the desk a little with just the TT and the TT power supply.
    Is it possible that I am bleeding RF back into the radio for some reason? I dont' currently have an earth ground installed(I know I should and it's on the adgenda as soon as the ground thaws a little.. the old Heathkit didn't seem to care much); Could this be causing something wonky?

    My ALC led is also not illuminating, and am going to take a look at the low-level driver board whenever I get a chance(I tested the LED and it's good) and see if there is something amiss with a resistor or diode.. SSB waveform looks good on my Heath station monitor(the only piece of "test" equipment I have other than a DMM).

    Thanks for any insight you guys might have,

    Sam, W8SJB.
     
  2. KV4PD

    KV4PD Ham Member QRZ Page

    Issues like that with the Omni series are many times related to the plug-in board interconnects on the chassis. Before I did anything else, I would remove each one separately and clean the pins on the board. Might be just that simple.
     
  3. W8SJB

    W8SJB Ham Member QRZ Page

    Also on the agenda.. Had it open once so I could check the ALC led and make sure there was nothing obvious burnt or bulging when I got it and you could tell it was owned by a smoker.. nothing like 40 year old tobacco smell!! :eek:
    Can't open it up until later, but I retract my previous statement.. it does NOT fluctuate if I have it on Lock and crank it up to 100w into the dummy load(but does if I really over-drive it).
     
    Last edited: Feb 14, 2018 at 1:25 PM
  4. W8SJB

    W8SJB Ham Member QRZ Page

    Okay.. so a solution so simple that I missed it I believe has presented itself... My former rig is my SB-100, witch requires a Hi-Z mic.. My Ten Tec Manual also calls for a High Impedance microphone.. unless someone has changed around the circuitry in the Ten Tec, Their definition of Hi-Z does not jive well with either the Shure Microphone company, or Heathkit circa 1965 LOL.. Swapped the button under the mic to lo impedance and my issue seems to have gone away(for now, more testing to be done).. Checked in to ECARS on 7255 and got good audio reports. Thanks all for reading and hopefully my solution might help someone else someday :)

    Sam,
    W8SJB.
     
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Could be a simple mic drive issue that you solved by switching the mic output impedance setting which makes sense as you couldn't get ALC response with the mic set to high impedance output. It could also be RFI on the mic line that also improved by switching to the low impedance output thus shunting more of the mic line RFI to ground. If you have some ferrite clamp on chokes around you could see if the symptoms change when you twist the mic cord a few times through the choke. Of course you could also just run with what works ;)
     

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