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

[Dr. Gerald N. Johnson]

On Wed, 2008-05-28 at 12:54 +0800, Marinus Loewensteijn wrote:
> 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 switching 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 ).

Rather if the low pass filters were relay switched, the RF drive caused
the transistor collector voltage to swing higher than the transistors
could stand, a collector failure, rather than a base failure. However I
think the Omni C low pass filters are selected by the band switch. So
only the antenna relay wasn't actuated when that early dit from the
keyer produced RF drive. TenTec PAs have been designed to handle all
abuse handed to them with the only protection being a power supply
breaker or current limit shut down. This needs transistors with extra
voltage handling capability and extra heat sinking to handle the heat
from a mismatch that reflects through the output matching as a low
impedance.
>  
> Another item may well have aggravated 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 labeled 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 candidates 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.)

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.
>  
> 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).

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?
>  
> 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.

Turn down the drive. Its as simple as that.
>  
> 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 circuitry?

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've had little trouble when wanting to save battery power on Field Day
with a couple different radios running a 100 watt radio at 25 watts
output, merely by backing off the drive while monitoring the output with
a wattmeter.
>  
> 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.

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.
>  
> Many thanks for reading this far and many thanks in advance 
> to those who can contribute to this, 
>  
> 73, Marinus, ZL2ML

73, Jerry, K0CQ