This is a couple of hacks for the AOR AR3000 scanner. They tamper the IF section,
(a) increasing the AM selectivity and (b) apparently solving a signal leakage
problem.
The point is that, by design, the IF bandwidth for the AM,CW,SSB and NFM modes is
intentionally 30 KHz throughout the IF chain, down to the 2.4/12 Khz ceramic
filters which just precede the decoders. While 12 Khz is acceptable for NFM and
2.4 for SSB, a bandwidth as large as 12 Khz for AM degrades much the unit
performance in busy bands like SW. Moreover, probabily due to an impedance
mismatch, (design error) there is a significant signal leaking across the
filters. Strong offtuned signals interfere with the centered ones until some
25-30 Khz apart, even in the 2.4 Khz SSB mode. After these mods the dynamic
selectivity would still not be termed superb (a little blocking persists), but at
least comparisons with <200$ SW receivers will become definitely ridicolous. In
effect, the AR3000 in SW cries out for an AM 6 Khz mode.
The mod (a) involves adding an additional 455 Khz/6 Khz width ceramic filter and
patching a little around. The mod (b) just implies soldering a couple of
condensers on the printed circuit board [pcb].
Most of the directions are common for the two mods, since the same section is
involved. The mods are not difficult, but require a high confidence on working on
SMD. I used no special tool but a cheap pencil soldering iron, pliers and
screwdrivers, but I TAKE NO RESPONSIBILITY for how you may screw your scanner up,
ok?
It would be strongly advisable to have the etch layouts from the service manual
and the schematics of the unit available, in order to understand what you are
doing. I would even reccommend to xerox the various layers of the layouts of the
main board on colored transparencies, for handy reference. I try to sketch the
most relevant points in ASCII, anyway.
For the mod (a), we will be adding the following external circuit
___________________________________o {A}
| | |
\ \ |
/ 22 Kohm / 2.2 Kohm |
\ \ |
D / ......... / D ---
{1} o__|/|___|__:_o o_:__|__|\|__o {2} --- 10 nF
|\| : CF : |/| |
:.q...p.: |
| | |
|___|_____________________|
|
|
=== {G}
in parallel to the existing filters. This just duplicates the existing ones, with
a new bandwidth. Then the AM steering signal has to be tweaked, so that the new
filter is switched on in place of the 12 Khz one in AM, while the switching
remains normal for the other modes. This reduces just to cutting one track and
soldering one shunt on the pcb. Other options would be possible, in the sense
that additional filters of any bandwidth between ~0 and 15 Khz could be added for
any mode, but in my opinon the present choice is optimal. Personally, I didn't
like the idea of having to select manually the bandwidth (an external switch is
much less neat), and I even tried out the existing 2.4 Khz filter for AM too, but
I didn't like the result (way too narrow for broadcast). Steering the SSB filter
in AM too is perfectly possible, but due to the tracks layout, and the need of
_not_ switching in the SSB decoder, involves a little more cutting and patching
the pcb. I won't describe it here. Perhaps an additional extra-narrow filter
could be considered for CW, but I'd think that the higher size and cost would not
be worth on an unit of this class.
In the above schematic, CF is a 455/6 khz ceramic filter (e.g., Murata CFS455H),
D are any signal diodes, and values of the R and C are not even critical. BTW,
the bandwidth @ 6dB codes for 455 khz filters are (letter after the figure 455):
A=35, B=30, C=25, D=20, E=15, F=12, G=9, H=6, I=4, J=2.9, K=2.4 .
The unit originally mounts a 455F and a 455K (the bigger, metallic shielded one).
It might be a little difficult to find filters other than the most common consumer
ones, which are the D and the E, and you might have to turn to surplus sources.
It is most practical to build the new circuit directly on the pins of the new
filter, to lodge it into the unit glueing it to a corner of the pcb, and to
connect it to the relevant points with 4 flying wires. It's strongly
reccommendable that the signal wires which connect points {1} and {2} to the pcb
are shielded and grounded only on the filter side.
Operation:
----------
-disconnect the power cord! Turn the scanner uspide down. Unscrew the bottom cover
(2 screws on the bottom and 2 on the backside) and remove it. Disconnect the
loudspeaker connector.
-unscrew the lowermost board, which is the IF/audio/power supply board (6 screws).
Locate the two ceramic filters (approximately on the center of the board) and L29,
on the visible side. Identifiers are printed on the board. Disconnect gently the
thin coax wire socketed to J1. Turn gently the board upside down to see the SMD
component side. All interventions are on this side of the pcb.
For the mod (a):
-locate the following section on the pcb:
[[ overview: reference to the integrated circuits - o indicates pin 1 ]]
----------
| 3357 | IC1
o---------
____
|4 |
|0 |
|9 | IC2
|4 |
---o
....
:::: filters (other side)
::::::::::::::
:::::::::::::: ____
|4 |
|0 |
|9 | IC4
|4 |
---o
----------
| 4066 | IC5
o---------
vvvvvvvvvvv
fuse on this side
[[ Enlargment of the filters section: (I sketch only the relevant components) ]]
.........
: O O_:_ <-{4} (primary)
: \[C]/ :
: : L29
________________:_O O : (secondary)
H .........
[1K]
H
o\ <-{1} \
H {3}-> [10K]H[10n]
[D15] | |
__H ..H....................... o <-{2}
| : O O : H
: : [D17]
: CF1 455F H[R90]H H
: | : |
: O===O===0===O x :H |
..............................|.[D29]...|..
: O H H O :
: :
: CF2 Murata CFJ455K :
: :
: O O :
...........................................
(in this drawing O represents a pin, H the soldered terminal of an SMD component,
the outlines are dotted and lines are tracks. The double-diodes are recognizable
for their rectangular 3-pinned case, and resistors should have their value printed
on)
-Cut the track between R90 (1.5Kohm) and D29 in the point 'x'. This isolates the
steering diodes of the 455F (half D15 and half D17).
-solder a shunt wire between the free side of R90 and pin 4 of IC4. (in alternative,
the shunt can be soldered to the pin of D19 connected to the said pin 4 of IC4).
This pin carries the command signal for NFM; this way the steering diodes of the
455F remain powered up in NFM.
-prepare the new filter, with connection wires of the exact lenght to reach the pcb.
As said above, I think the best location for it is to glue it to the corner in which
the fuse is.
-join the points {1} and {2} of the new filter with the corresponding points on the
pcb. Be careful to trim and to tape the ends of the shielded wire, so that the shield
cannot touch any track.
-solder the connection {A} to pin 6 of IC4, which carries the AM steering signal.
-solder the ground connection to any ground point. I'd reccommend the ground track
on the edge of pcb in the vicinity of IC4.
And now for the mod (b).
I don't have a precise explanation of why the mod works, but it does, and well
enough to reccommend it. Apparently, with the new components there is a much better
impedance match between the output of IC1 and the ceramic filters, which prevents
or accidentally cancels the leakage.
-solder a 10 nF capacitor (or greater) between points {1} and {4}, and a 68 pf one
between points {2} and {3}. You might try slightly higher values for the latter,
but these would result in an attenuation of the good signal together with the
spurious. The new capacitors have just to be small in size, and to be soldered
parallel to the board, since the available height is limited.
On the AR3000A, at least on Marc Gauw's unit, a similar patch was already applied
by the factory. In that case, a shunt wire directly connects the primary of L29
with the common pin of D15, while two resistances and one capacitor attached to
the secondary of L29 are missing, in contrast with the AR3000 and its schematic
on the service manual. This tells us that AR itself became at some stage aware of
the problem, and developed that hack solution. In any event, the bypass of L29
makes almost irrelevant its adjustment.
I have discovered the problem and the mod while communicating with Marc about
replacement ceramic IF filters. In particular, I could check the leakage and
its cancellation with both IF filters excluded.
-remount the board, rescrew, reconnect connectors, close the unit and enjoy.
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 forwarded me
a copy of the service manual, two years ago, and to Marc Gauw <marc@simplex.nl>
with whom I developed this mods, for a lot of relevant email discussion and help.
The filter my AR now mounts, btw, was kindly provided by Marc.
Enrico Segre, <segre@polito.it>
ps: check out also my hack for improving the audio bass fidelity...