[TenTec] Omni 546 Series C RF power amp advice sought (long post)

[Marinus Loewensteijn]

> (Dr. Gerald N. Johnson)

> Some 90 degree UHF elbows (not made my Amphenol) cause a very high SWR
> at 2m, seems they use a coil spring to go around the corner that acts
> like an RF choke or trap.


Jerry,


Thanks for this tidbit, the elbow has been removed and it will not go back in.


> 20 dB gain is a lot of gain for a single stage 100 watt PA. Many do only
> 10 or 12 dB. Are there two stages on the PA board?


The Omni has two stages, one with the MRF 476 and one with the MRF 458 / MRF 454 (direct replacement). At present TenTec advises to use the SD1405 which I believe just has turned to being "no longer manufactured".



The Motorolo AN762 notes refer to a single output stage ( one version is also mentioned in the ARRL handbook of 1987 and it is in a number of other versions too ).



> Lowering the ALC threshold is one way to control the power. Reducing the
> drive control while monitoring the power output is another. The trouble
> with lowering the ALC threshold (a front panel setting on my Corsair II)
> is that it often allows a full power output spike at the leading edge of
> the first dit or dot before the ALC takes over. And depending on ALC to
> control power causes some distortion of the output signal because ALC
> reduces the drive when the output envelope starts to exceed the ALC
> threshold. Some would call using ALC a crutch for the operator unwilling
> to monitor the peak power output directly. And in some radios its the
> only handy way to control drive and still have a relatively leveled
> power output despite varying positions of the mouth and microphone or
> differences in transmitter stage gains for different frequencies.
>
I have heard about this spike at the start of transmission too. Using full QSK will then certainly make matters worse but when I looked at the shape with the original capacitor values I did not see a spike. Ofcourse there was no check done after I did replace the electrolytics so it may well have been there due to the 4.7Uf cap instead of the 33 uF.



>
>> 2) The Motorola AN762 documentation has for 100 Watt and 140
>> Watt output a ratio of the output windings of 1:4 and for the
>> 180 watt output a ratio of 1:5. Can I reduce the present, in
>> the TenTec in Omni implemented, 1:4 ratio to 1:3 thereby reducing
>> available output without upsetting the rest of the feedback
>> circuitry or affecting the stability / life expectancy of the
>> componentry? If it becomes more rugged then no problem ;-)
>
> Lowering the turns ratio of that output transformer raises the load
> impedance seen at the transistor collectors and so raises the voltage
> swing on those collectors making the collector voltage breakdown more
> likely. And that higher impedance won't do much for PA linearity. I'd
> leave the output transformer alone and lower the drive.


That is very worthwhile to know and I'll take this advise on-board and leave it well alone.



>
> You could always insert a 3 dB resistive attenuator in the RF drive
> line, say between the 1 watt driver and the PA board. Or to expand the
> RF drive control range, wire a resistor of value equal to the
> potentiometer value in series with the ungrounded end of the drive
> control. That would make setting the drive level easier.


I think I have a 50 Ohm, 10 Watts,  6 dB stripline attenuator somewhere (going up to 1 Ghz) so can try that out. Will 6dB be "too much reduction"?



(  Motorola has the AN779L design which has the MRF 476 as driver and the MRF 475 as finals. It is 20 Watts out and is rated as 25 dB gain. In the documentation is mentioned a 100 mW drive is sufficient. )



Jerry many thanks for your adivice which certainly helps me bringing this Omni back up to factory reliability and standards again.



73, Marinus, ZL2ML

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