[TenTec] Omni C + new Ham = Oops.
paul.r.harker at gmail.com
Mon Oct 14 21:24:03 EDT 2013
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.
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
> 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
> > see. I much appreciate the tuning tips. Being a newbie, I've yet to
> > all the goodies like noise bridge / antenna analyzer, but I'll see if I
> > 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
> > 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
> > 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>
> >>> I am a new member of the amateur radio community, and my first HF rig
> >> an
> >>> Omni C. I was attempting to tune an antenna with a MFJ949E, with the
> >>> 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,
> >>> 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
> >>> 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
> >>> can be deceiving when doing an in-circuit test.
> >>> So... I looked for any sign of a fusible link or other protective
> >>> 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
> >>> also be appreciated.
> >>> Much thanks,
> >>> Paul
> >>> AC8OI
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