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

Wed May 28 00:54:32 EDT 2008

Some advice is being sought for the RF Power amp in my Omni 546 series C. 

First the history of what happened and why.

As mentioned earlier on this list I obtained my Omni-C unused, new in a box over a month ago. I replaced all the colored striped 0.1 uF film capacitors because they, without exception, had cracked at the sides ( all the others were fine ). I also replaced all electrolytics. 

Last Thursday I was happily working CW when the transmitter died on me, one moment all was fine and next moment the power supply tripped. Restarted and on receive read 4 Amp current while normally this was about 1 - 1.5 Amp. While staring at this and scratching my head the power supply tripped again and this time there was no resetting: the finals were drawing in excess of 18 Amp. (put a resistor in series to limit the current and measured again).

The finals have died (MRF 458). While I am waiting for the replacements (MRF 484) to arrive (a friend obtained them for me and got the MRF 476 drivers too , saving $$'s on shipping) I investigated what might have happened.

There were four candidates and one scenario came out on top. I build the TenTec 1553 keyer kitset (found one in Australia) and this one sends a dit when powered up. I am powering this from the back of the transceiver and often I did not switch from CW back to SSB before switching the rig off. Result was a nicely transmitted dit when I powered the rig on.

This, imho, resulted in a timing sequence error for RF - DC - Antenna that was creating stress on the base of the finals because the keyer uses 5 - 12 Volt and hence gives a signal far before the rest is really powered up / is able to act. E.g. the ALC chain is likely not yet properly up and / or antenna swicthing may not yet be in place and / or DC is not yet up to full voltage reducing the amount of power the base of the finals can absorb. ( found this clue at the website of a RF power amp manufacturer ).

Another item may well have aggrevated the situation - The low level driver board's schematic has in the manual for C10 ( there are a couple of C10's in this schematic, some typo's there but only one fits the bill ) was labelled as 4.7 uF where I removed a 33 uF. I assumed the schematic was right, ( especially after I found on the board I did just prior an electrolytic with the + at ground ) and plonked in a 4.7 uF.

Unfortunately this capacitor is part of the shaping and turning on of TX RF to the drivers / finals :-( \. IMHO we can attribute this failure of the finals to an unfortunate combination of events, none of which is to blame TenTec or their design.

(other candiates I investigated were: Having a roller inductor in ATU  perhaps not making contact when tuning, using too high a power (50 watt) in final tune up, heavy rain and me using twin TV wire (240 Ohm) and perhaps a changed impedance, changing a day earlier the coax connection: inserted a 90 degree"elbow" between chassis and coax cable. Neither roller inductor or connector showed any sign of corrosion / non contact / discoloring or having any oil / grease on it.)

While I am having the transceiver out of action (actually I can connect the low level board directly to the output filtering and get approx 1 watt output) I am looking at the RF amplifier and am wondering about a few items.

If I look at the Motorola AN762 application notes then I see that a similar design can produce between 100 and 180 watt output with only minor changes to transistors, feedback loop and changing number of secondary windings on the output transformer. It also struck me that the Omni is capable of more than sufficient drive for the finals, perhaps a bit on the "too much so" side. (not to forget that the low level driver board can produce about 1 watt on its own)

If I am working CW I find that the drive knob is sitting at the most at about 9 o'clock. On SSB with the 215P microphone I am more sitting at about 10 o'clock.

Personally I am more a low power type of person and would like to reduce the output a bit. Prefer an approx 50 watt level minimum on all bands but staying as close as possible to that and not wanting to go above 65 - 70 watts at the most.

Hence two questions that I have (not being an RF designer and some of you being far more versed in this subject matter than I am).

1) Is there an easy way to control the drive level better by changing / adding some resistors to the ALC circuitery?

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 ruggid then no problem ;-)

Many thanks for reading this far and many thanks in advance to those who can contribute to this, 

73, Marinus, ZL2ML
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