[TenTec] Omni C + new Ham = Oops.

[Paul Harker]

Scope on disconnected and terminated RF input line shows nice pretty sine
wave which varies in amplitude with the drive control and by frequency with
the appropriate controls.

-- Paul

On Oct 14, 2013 8:26 PM, "Mike Bryce" <prosolar at sssnet.com> wrote:

> You can use a shortwave rec if you have access to one
>
>
>
> Scope!!!!
>
> Unplug the driver (small coax that goes into pa) and load down with a 50
> ohm resistor (47 is fine)
>
> And with the drive/alc all the way up see what's there
>
> Mike wb8vge
>
>
> Sent from my iPad
>
> On Oct 14, 2013, at 7:26 PM, Paul Harker <paul.r.harker at gmail.com> wrote:
>
> > Mike and John,
> >
> > Thanks for the replies.
> >
> > John -- I'll take a look at the drivers -- I'd rather it be them, but
> we'll
> > see. I much appreciate the tuning tips. Being a newbie, I've yet to
> aquire
> > all the goodies like noise bridge / antenna analyzer, but I'll see if I
> can
> > borrow them from the local club.
> >
> > Mike --  There is zero additional current on TX reflected by the ammeter
> > remaining at idle current. The AGC light does not light with DRIVE set
> max
> > in both LOCK mode and in SSB with microphone input.
> >
> > Again, being a new ham I don't have a second radio to check for
> > transmission. I could rig an attenuating probe for my scope to check the
> > output directly, but all other signs say it is dead.
> >
> > On to checking on the drivers... I was a bit surprised the power supply
> > didn't go overcurrent, since typical failure mode for output transistors
> is
> > to short. A driver failure would be a good possibility.
> >
> >
> >
> >
> > On Mon, Oct 14, 2013 at 6:45 PM, Mike Bryce <prosolar at sssnet.com> wrote:
> >
> >> Don't jump to conclusions just because one is nearby.
> >>
> >> Are you pulling current on tx?
> >>
> >> If so how much?
> >>
> >> It's possible the low level driver went out.
> >>
> >> Can you hear yourself in another receiver?
> >>
> >> Mike wb8vge
> >>
> >>
> >> Sent from my iPad
> >>
> >> On Oct 14, 2013, at 5:28 PM, Paul Harker <paul.r.harker at gmail.com>
> wrote:
> >>
> >>> I am a new member of the amateur radio community, and my first HF rig
> is
> >> an
> >>> Omni C. I was attempting to tune an antenna with a MFJ949E, with the
> mode
> >>> select switch (SSB-R / SSB-N / CW / LOCK) switch in the LOCK position,
> >> and
> >>> DRIVE set just high enough to allow tuning.
> >>>
> >>> Suddenly the antenna tuner needles dropped to zero, and it appears I no
> >>> longer have RF output. The Model 255 power supply did not shut down,
> and
> >>> the receiver is still functioning. I am assuming that I managed to blow
> >> the
> >>> MRF458 output transistors due to doing _something_ wrong. Maybe just a
> >>> matter of fiddling too long and letting things get too hot. I don't
> know.
> >>>
> >>> Measuring the MRF458 with a meter suggests they might be bad, but it
> >> isn't
> >>> a case of a dead short, and in the past I've found that power
> transistors
> >>> can be deceiving when doing an in-circuit test.
> >>>
> >>> So... I looked for any sign of a fusible link or other protective
> circuit
> >>> in the Omni schematics and didn't see any, but I thought I'd ask the
> >>> experts before pressing "sumbit" on a transistor order.
> >>>
> >>> And any hints tips corrections to keep me from repeating my bungle
> would
> >>> also be appreciated.
> >>>
> >>> Much thanks,
> >>>
> >>> Paul
> >>> AC8OI
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> >>>
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