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 , who forwarded me a copy of the service manual, two years ago, and to Marc Gauw 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, ps: check out also my hack for improving the audio bass fidelity...