[TenTec] Omni VI+ Intermittent Power Amp?

[Dr. Gerald N. Johnson]

On Sun, 2007-03-11 at 17:55 -0700, Peter Hoon wrote:
> Fellows,
> 
> Have you got suggestions for the case of intermittent power out from an Omni
> VI Plus?
> 
> First year (May 2006 - March 2007) since I am the second owner, no
> intermittent noticed; always got full 100 watts on all bands (10 M a little
> low, around 90 W out).
> 
> Now in second year of ownership.  Very clean unit; complete and successful
> receiver alignment matches ARRL MDS.
> 
> Suddenly, while operating on 40M CW, into perfectly 1:1 tuned loop via
> Dentron tuner, can only get 7 to 9 watts out max on all bands.
> 
> Next day, try again, and get full power again on all bands for next 5 days.
> Completely normal operation.
> 
> Hook to 300 W dummy load on bench.
> 
> Adjust bias pots to confirm linear class AB  output on SSB; Speech sounds
> fine on adjacent receiver after adjustment; normal full power out after bias
> pot adjustments.
> 
> All of a sudden, with no changes anywhere, can only get 7 to 9 watts out
> again for next 24 hours.
> 
> Thinking it might be thermal, wait overnight for cold transceiver.  In
> morning, cold, only 7 to 9 watts out. Two tries, same low power result.
> 
> Remove power amp module from rear.  Remove blue T wire and coax input from
> jack 79. Careful inspection of top circuit board, shows no loose screws,
> finals are clean, no obvious cold solder joints; replace blue wire and coax
> input.
> 
> I have MRF 454 finals.  Throughout testing, heat sink is hardly even warm to
> touch.  Power meter on Dentron tuner and on Omni VI Plus match each other;
> not a measurement problem.
> 
> Turn power out-put pot, front panel,  from min to max, and only 7 to 9 watts
> out.
> 
> Turn power output pot back to min again, move up to max, and bingo,
> completely normal 100 watts out on all bands!
> 
> There were no changes.  What the heck is going on!
> 
> Would like to know if anyone elase has experienced this kind of transmit
> intermittent with an Omni VI plus.  What did you find was the cause?
> 
> Am about to go back up and wiggle some more components and wires.
> 
> May also remove power amp circuit board and check the underside for cold
> solder joints.
> 
> Thanks for suggestions and ideas; will report findings for posterity.
> 
> Peter
> VE1CHS
> Nederland, Colorado
> 
Check the voltage on the wiper of the power adjust pot. That last move
makes me think its the pot.

73, Jerry, K0CQ