Excellent troubleshooting!
Besides the experience you gained, you saved a lot of money by NOT sending the
radio out for repair.
Mike Bryce, WB8VGE
the heathkit shop
SunLight energy systems
J e e p
o|||||||o
On Oct 22, 2013, at 11:37 AM, Paul Harker wrote:
> Much thanks to all who contributed to the thread regarding my OMNI C. I'll
> save you the long and whining road of my internal torment, but I ended up
> going with a matched pair of Motorola MRF458s (same as original). I took
> the middle road between "it's not worth fixing" and "better than original
> parts".
>
> Including a shipping and a pair of 3.3 ohm resistors to replace two which
> had developed a tan, the repair was just under $70 -- not including a much
> needed upgrade to my soldering station.
>
> Final assembly and checkout revealed another problem, possibly what caused
> the meltdown in the first place. The 13.x volts to the PA goes to a
> pass-thru connector consisting of a nut, screw, insulators, and two
> star-washer type solder lugs, one on the inside an one on the outside.
>
> The solder lug on the inside had developed a high resistance (hundreds of
> ohms) between itself and the pass-thru nut/bolt. This resulted in a low
> voltage to the finals, but proper voltage to the driver stage which has a
> separate power pass-thru. It wouldn't be hard to see how that could blow
> the finals.
>
> I replaced the MRF458s without incident, cleaned up the 13.x VDC power feed
> through, and had previously found and replace a leaky sense diode in the
> SWR meter circuit. All seems back to normal, except for my frayed nerves
> whenever I turn up the DRIVE control.
>
> Thanks again to all who contributed answers to my original thread!
>
> Paul
> AC8OI
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