[TenTec] Omni C + new Ham = Oops.

[Mike Bryce]

rick and Paul

My magic sticks seems to point in that direction, too.

without seeing if they have standby current, it seems they are open, but still there are drivers in the the pa, too.

while it is a bitch to get into, a scope on the output of the internal drivers should show RF.

however that being said, if the pa is not pulling current from the supply, the odds of taking out both pa transistor and the two low level drivers is kinda of hard.

It too would wiggle the band switch back and forth,

if he were drawing current, but no rf, I'd suspect a band filter, but that old radio doesn't use relays to select output filters


Mike Bryce
prosolar at sssnet.com



On Oct 15, 2013, at 2:51 AM, Rick - DJ0IP / NJ0IP wrote:

> 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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