[TenTec] Omni C + new Ham = Oops.

[Rick - DJ0IP / NJ0IP]

Paul,

Normally it is next to impossible to blow the power transistors when using
the correct Ten-Tec power supply - unless the overload current protection
circuit of the power supply is malfunctioning.

You might first check it.  I used to check mine by paralleling a couple of
Bilux auto headlight lamps.  Wire them in parallel with just the normal
lights (low beam) wired.  The power supply should continue at full power.
Then jumper the high beam in parallel with the other two.  This should
trigger the overload circuit and shut the power supply down.

Also, now might be a good time to invest in a dummy load and a good watt
meter.

The only thing between the 13.8v and the transistors is a small RF Choke.  I
doubt that it would have blown, but you never know.  Check it with an Ohm
meter (after removing power and bleeding off any residual voltage to the
chassis).

To check the output of the driver, you can also use a light bulb if you have
nothing else.
Use a tail light from a car or even one of the interior lamps.
Connect it across the output of the driver (after removing it from the input
side of the finals), and TURN THE POWER ALL THE WAY DOWN.  Key the tx (in
lock) and slowly run the power control up.  The lamp should begin to light.
How bright it gets depends of course on which lamp you are using.  DO NOT DO
THIS FOR VERY LONG.  Just briefly to see if the driver is putting out any
power.

Of course a 50 Ohm load and the scope would be a better way to do it.

My buddy Murphy says it's the transistors.  
He's not a very likeabe guy but he's usually right.

73
Rick, DJ0IP
 

-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Paul Harker
Sent: Monday, October 14, 2013 11:29 PM
To: tentec at contesting.com
Subject: [TenTec] Omni C + new Ham = Oops.

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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