home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
QRZ! Ham Radio 18
/
QRZ Ham Radio Callsign Database - Volume 18.iso
/
modfiles
/
ar3000.txt
< prev
next >
Wrap
Text File
|
2001-11-01
|
5KB
|
108 lines
This is a hack I have done on the AR3000 scanner. My one is a first series one, but
I guess the 3000A has the very same motherboard, so it should apply to the new one
too.
One defect I've found of this scanner is that its audio completely lacks basses
in the AM/NFM/SSB/CW modes. I've gone to the service manual, and I've found that this
is intentional. The audio line passes through a notch filter. This is stated
to be 300-3000Hz. I do not believe much to the upper limit (the 5Khz whistle
on shortwaves is quite strong), but I find the lower one annoying and unjustified.
Therefore I removed that cutoff. After the intervention, I find a much improved
audio quality, both in speech and in broadcast, and I haven't yet found a signal
which makes me regret it. And if I even wanted, now I can add an external equalizer.
The mod itself is simple, but involves working on SMD, so it's quite DELICATE.
Do it at your risk. I'd reccommend having a copy of the PC layouts and of the
schematics from the service manual, in order to understand what you're doing.
I found useful making a coloured xerox copy of the etch layouts on transparencies,
so to overlay them.
Theory: the notch filter for AM/NFM/CW/SSB is built around the double op-amps
in IC7-IC8 on the if/audio board. The lower cutoff is determined primarily by
R130(10k), R139(4.7k), C130(22n), C131(22n), C137(22n) (I don't swear on the
correctness of the identifiers since my schematic is borderline readable)
C128
,---||---.
|-/\/\/\-| ,________.
| R143 | | |
| |\ | | |\ |
---|-\ | C130 C131 R139 | 2| \ | R142
2| \___|____||____||____/\/\/\_|__|- \__|___/\/\/\_
| /1 || | || | | /1
---|+/ / |___||_____|+/
3|/ R \ || | 3|/
1 / C137 |
IC8 3 \ _______| IC7
0 /
|
_
=
What I did was: to remove R130; to solder a 0.1uF condenser in parallel to C130+C131;
to remove C137 and to shunt it with a jumper.
Operation: unscrew the cover and nake the receiver. Unplug the loudspeaker connector.
The audio/if board is the lowest one, and has to be unscrewed and turned upside-down,
so that the SMD side is accessible. To this extent, at least one of the coax
connectors (J1,J10) has to be unplugged. Locate the components to be attacked (again
I recommend having the etch layout; I try to sketch, but it's not as clear). The
region is on the SMD side opposite to IC9 on the component side. IC9 is recognizable
since is a voltage regulator, it has 3 pins and gets hot.
^^^^^^^^
(ceramic filters)
(components side)
__
/ \
|| ||=|| || (hole)
\__/ || ||
C131 22k
IC7 || ||
C130 100k
|| || || || || ||
||=|| || || 220p ||
|| R130
||C137|| IC8 ||
||
|| || || || 220p
||
( IC 11 )
(components side)
vvvvvvvv
(Sorry but I can't draw any better)
I suggest to turn the radio on, and to try to shunt C130+C131 and C137 leaning
a O.1uF condenser on the proper tin points, in order to locate them. The sound
should get richer of basses in both cases.
Then unsolder R130 and C137. It requires a quick and firm hand since the components
are ~4mm long, and glued to the board. Solder a short shunt in place of C137.
Solder a 0.1uF condenser (or greater) on the extremes of C130+C131. A little
ceramic one does it; even if it is not as neat as replacing with another
SMD, there is plenty of room.
I did all the job with no other equipment than a cheap and thin tip soldering iron
and a pair of pincers, but, again, entirely YOUR RESPONSIBILITY.
Rescrew, close, enjoy... Of course the effect is much better with an external
speaker.
I'd like to hear comments or to know if anyone has tried other hacks on the same
box. Please email me. Once more, I take NO RESPONSIBILITY whatsoever... :)
... In my case it worked well, though.
A final note: I'm indebted to Henry Laviers <hl1@acpub.duke.edu>, who sent me
a copy of the service manual two years ago.
Enrico Segre, segre@polito.it