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[AMPS] Alpha 87A

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Subject: [AMPS] Alpha 87A
From: John_Nelson@compuserve.com (John Nelson)
Date: Tue, 18 Nov 1997 07:18:31 -0500
I've had an Alpha 87A since December 1995. In that time it's been used into
a
Cushcraft A4S for the HF bands and assorted wires for LF. To power it, I
installed a dedicated mains spur from the consumer unit, with integral
filtering and VDR protection (we live in a very rural area and the mains
supply
is erratic and noisy, with kilovolt transients an everyday occurrence). In
view
of what I suspected might be a degree of vulnerability on the part of the
PIN
diode switching to high VSWR, I also built and installed an external SWR
trip
in the master antenna feedline as backup to the one inside the 87A. The
standard driver is an FT-990. According to the hours counter I mounted in
the
filter box, the amp has done 305hr so far.   
 
By and large I've been pleased with the 87A. It's been reliable and given
few
operational problems, the only major irritant being an occasional tendency
when
on 28MHz to trip out and show the failure code for "locked" 10m operation.
Initially I was concerned about cooling; a temperature probe in the exhaust
air
consistently showed about 95 degrees C key-down. However, I had expected
that
because of the reduced blower efficiency on 50Hz mains -- US companies tend
to
forget that most of the rest of the world doesn't use 60Hz! So I
immediately
installed a muffin fan with air filter where there's provision to do so on
the
rear drop. I also blocked the air holes at upper right of the cabinet with
clingfilm, to force all the inlet air past the transformer and into the
grid
compartment. Since then the unit has run pretty well stone-cold at all
times.
It's coped perfectly well with the odd silliness on my part (like trying to
transmit into an open circuit, or the wrong antenna) and also hasn't
objected
to occasional overdrive, tripping out correctly on all occasions. I've had
no
'hard' faults at all.    
 
Minor gripes? I notice that the anode (plate) voltage is higher than the
data-sheet maximum value for a 3CX800 when in standby, but that probably
doesn't matter. The initial tuning and loading procedure is tedious and
easy to
get wrong, and tends to deter you from running at other than the power
level
for which you last tuned up. The rapid-bandchange facility is very useful
and
flexible, although I wish they'd gone the extra mile and made the amplifier
completely self-tuning and loading. The manual is awful; it's badly written
and
edited, and doesn't even mention some controls (such as the high/low anode
voltage selector switch). It reminds me of the manual for a Krell audio
amplifier, which must be the worst in the world. Doesn't the profession of
technical author exist in the USA? <g>. For UK use the mains supply is
wrongly
wired and fused (we have 0-240, not 110-0-110) but that's easy to fix. It
also
doesn't meet the requirements for CE marking, but to a US user that's
irrelevant. 
 
Other than that, I like it and would certainly buy another one. I'm
currently
considering stabilising the heater supply to the 3CX800s, using a circuit
similar to one I published some years ago for 4CX250s using a power-FET
pass
element, but apart from that I haven't yet felt the need to make major
modifications. 
 
John Nelson
GW4FRX
 
 
      
 
  

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