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1991-06-22
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87 lines
AR2500 report, parts 2 and 3
by KB1MZ
The second unit arrived in about 10 days. It also had the non-continuous
coverage, though it was about 50 MHz "better". THe 45 MHz bleed through
and the lack of image rejection on the 800-1300 MHz band were about the
same. Sensitivity on the 800-1300 MHz band was much better. The tuning
knob also exhibited the same erratic performance in the
counter-clockwise direction. (apparently this is a "feature" common to
the AR2500) At this point the radio did everything that I wanted. Then
about three weeks, the < 550 MHz band went "deaf" by about 40 dB, but
the 800-1300 was still fine. My SWAG was that the MC5800 front end chip
died.
This unit went back to ACE, and at this time is languishing away,
waiting to be repaired (ACE person said 4 to 6 weeks, and that there
were currently no more 2500's available for replacement). I tried to pin
them down on particulars, but the lady on the other end of the phone was
polite, but vague, promising that their repair tech would call me the
next day (he hasn't yet). I get the impression that there is a serious
design problem with the 2500 and they are trying to fix it without
calling too much attention to the fact. I'll gently nudge them about
once a week until I get either the radio or my money back. Maybe I
should have saved up some more dough and gone for the AR3000.
-< round three... >-
I finally got the AR2500 back from ACE. THe deaf <550 MHz problem was
fixed (new 5800 IC, and they did some real tacky ECO's in a feeble
attempt to solve the squelch and 45.03 MHz IF bleedthrough problems.
They hung a couple transistors, resistors and some wires around the
inside, and as a result, the squelch action is a little better. Then
they stuck a wad of aluminum sticky tape over part of the VCO/Loop
filter area, ostensibly as a shield. But there was NO contact with the
ground foil, and, in fact this "tent" was loose, rolling around the
inside of the scanner like the proverbial loose cannon. Did I mention
that the radio was dead on arrival? When I plugged it into the power
pack, and turned it on, *NOTHING* happened. As dead as a rock. Nil.
Nada. After calming down, and rejecting the idea of asking the USAF to
"nuke" ACE, I opened up the radio, and found that one of the plugs going
to the front panel/control boards was not plugged in. I don't know if
the ACE "tech" was in a hurry that day and overlooked this minor detail,
or perhaps the UPS delivery crew had a little game of hackey-sack with
my unit. Anyway, I put the plug back in, removed that deadly little
aluminum tape "time bomb" and tried again. This time, it worked. Better
than before, but still far from perfect. In another feeble attempt to
fix the hole between 550 and 800 MHz, they replaced the VCO assembly.
This was apparent by the soldering iron burns and globs of flux around
the area, that wasn't there before (I admit, I peeked). Now this gap is
600 to 790 MHz. Better, but still not to spec.
The tuning knob is still erratic at best in the CCW direction, giving
random results. It works fine as long as I tune upwards in frequency.
The 45 MHz bleedthru was still there, and the local NHSP Troop B on
45.020 comes thru loud and clear. No matter what frequency the 2500 just
happens to be tuned to. I figured this out: It's not a bug, its a
feature! Just like having a free priority channel! How clever of those
AOR folks! Wow!
Another feature that I discovered was that the antenna didn't have to be
actually connected securely to the BNC connector on the back. Just
touching it to the outer shell of the BNC, or to the attenuator switch
handle did the trick just fine. How clever! Further examination of this
phenomenon disclosed that the outer shell of the BNC was not even
connected to ground. Anywhere. Cold solder joint.
Anyway, I decided to bite the bullet, and see if I could solve some of
these problems. First, I grounded the BNC shell. Better. I took a real
plunge, and removed the cheap BNC and put in a mil-spec Amphenol BNC,
and removed that pesky little attenuator switch and board, and wired the
antenna jack directly to where it should go. Better yet! Next, I took
some sticky-back copper foil and stuck it in the inside of the
upper-half of the plastic cabinet, and made a good solid contact to the
BNC outer shell. This provided a bit of shielding, and kept the
internally generated signals inside, and kept out most of the 45 MHz
NHSP out of my IF strip. I know, the warranty's blown, but now it almost
works as it should. Besides, I accomplished in an evening what ACE
couldn't do in 10 weeks.
It does what I want it to do, in the main bands of interest (118-137 and
225-400 MHz). I can live with the hole in the UHF TV spectrum, and the
very marginal HF/shortwave performance. Would I do this again? No way,
man.