[TenTec] Omni C + new Ham = Oops.

[Mike Bryce]

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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>>> TenTec at contesting.com
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