I must agree with Doug. There is very likely an unfavorable load
presented to the PA, and it manifests its displeasure by burning up!
Most of us (me included) do the initial tune-up of a commercial
transmitter with a Bird 45 wattmeter feeding a good 50 ohm dummy load.
Transistorized PAs adore dummy loads, because they are resistive. By
definition, such loads are broadband- often from DC to 1 GHz- and the PA
is "comfortable" with such a laid-back, non-confrontational,
non-reactive sink for its RF power.
Ah, but the Real World (duplexers, isolators, and antennas) are not so
forgiving; they are REACTIVE! By definition, such RF devices present
different impedances at different frequencies. We should not be
surprised that a PA that works (or SEEMS to work) perfectly into a dummy
load for hours on end, suddenly goes belly-up when feeding a highly
reactive load such as a duplexer. HELLO? A duplexer is the
quintessential unstable load for a PA, and it seldom presents a 50 ohm
load at any frequency.
Although the use of impedance matchers has been hotly debated in these
pages, along with recipes for cutting cables to "magic" lengths, we must
acknowledge the fact that any PA that works perfectly into a precise 50
ohm load, "may" not work perfectly- or even close- when presented a
reactive load.
So, let's improve our odds! The first step, especially when dealing
with a solid-state PA, is to adjust its output impedance to properly
match a 50 ohm load. I don't care if you do this with a Z-matcher or
with laborious cut-and-try cable lengths, it's a step in the right
direction.
The second step is to ensure that the next component seen by the PA,
whether an isolator or a duplexer, is tuned for a 50 ohm source.
Despite passionate denials by some "experts," you cannot tune a duplexer
to match a wayward impedance. All adjustments on a duplexer affect its
frequency response, and since the frequency is not going to change,
neither should the tuning of a duplexer be tinkered with. A lot of my
work is undoing the "tweaking" performed by self-styled experts who
sincerely believe that a duplexer can be tuned to a PA. NOT! The
results prove my point, and are supported by those firms that make
duplexers.
The bottom line is that a well-made, commercial-grade PA should not die
unless something is very, very wrong. I can live with the rare death
due to natural causes, but repeated toastings... NOT!
73, Eric Lemmon WB6FLY
Doug Bade wrote:
>
> I have limited experience with the MSR2000 however if the PA keeps
> toasting, and if RX sensitivity is low ( MSR2000's were not super hot to
> start with) I would suspect the load being presented to the transmitter may
> have high swr. (Like maybe duplexer, maybe the Isolator ) That would seem
> to be the place I would start...It is possible these factory parts may not
> be re-tuned properly and/or like operation at 444 mhz. In general I know of
> one here ( MSR2000) in Cleveland on 444.xxx which has never toasted a PA to
> my knowledge... While it has limitations due to the general design, as far
> as replacing the Duplexer and Isolator with lower insertion loss versions,
> it has been serving well for a long time. Even if the duplexer was
> mis-tuned, it would seem like the Isolator should protect the
> transmitter... Is it possible it is taking lightning strikes or severe
> discharges???? That would seem more likely, although damage should occur to
> more things...
>
> Doug Bade
> Cleveland Comms.
>
> At 10:00 AM 9/7/2004, you wrote:
> >I have an MSR2000 on 444.550 for the local ham club / county EMA. The PA (a
> >110W model) is out again. We just had this repaired by the local Motorola
> >shop, with whom we've had very good results from in the past. We're running
> >the PA at about 80W, so I'm thinking we didn't "burn it up", but I can't
> >help but wonder if there is anything inherently wrong with the MSR2000 PA
> >for UHF. This is the third time the PA has failed in this particular
> >machine in about a 2-year time frame. (This was a repeater originally tuned
> >for 460 MHZ, and brought down to the ham band.) This particular machine has
> >been a pain in the @$% - RX sensitivity isn't what it should be, PA keeps
> >going out, etc. Seems like we're up at the site every couple of months do
> >make some sort of repair on it.
> >I'm wondering if we should just cut our losses and get another machine, or
> >should I keep plugging away at this repeater? If we do decide to chuck it,
> >how much can I expect to spend to get a comparable replacement - we use this
> >machine during severe weather season for forwarding reports to the NWS, and
> >it's open other times. We want to add remote RX sites (you've probably seen
> >my inquiries about those in the past) but I don't want to start adding
> >remote RX sites until I get the main repeater working reliably.
> >I'm open to suggestions...
> >Thanks,
> >Mark - N9WYS
>
>
> Yahoo! Groups Links
>
>
>
>
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