The Radio Shack Optimus 12-603 / 12-903 Page

Click on the image above to see an enlarged view ----

I get many inquiries about the Radio Shack Optimus 12-603 and 12-903 radios (now discontinued). People wonder "are they the same as the GE Superradio 3"? I am in a position to answer this question, because I own both. The answer to the question, unfortunatly, is no. The 12-603, and its later re-design, the 12-903 appear to be cost reduced re-designs, based on the GE concept. While there is nothing wrong with cost reduction, and the Radio Shack radios are good radios, they have made some compromises that affect AM DX capability. The 12-603 is actually built better than the GE SR-3, using thicker, more rugged plastic - reminiscant of the GE SR-2, which I also own.

The first clue that I had that the two radios were not identical was obvious even in the store. The arrangement of the secondary controls: "AM/FM", "wide/narrow", and "AFC" is different on the SR-3 and the 12-603:

After doing hardware hacking since I was in seventh grade, I knew that the underlying switches would most likely be soldered onto the PC board, rather than connected with wires. This strongly implied that the circuit board layout is different between the two units. This proved to be the case (see below).

One thing that initially caused me to think I had purchased a defective unit: unlike the GE SR-3, the 12-603 includes an "AC/DC" switch on the back. Initially set to the AC position, the switch prevented me from using the radio with batteries (to minimize AC line noise that would enter the unit if I used the AC cord). When switched to the DC position, the radio worked with batteries. This may cause confusion to the user. One other note related to this switch - it has one of those little dust shields that goes around the shaft inside the radio. I had re-assembled the unit only to find the dust shield sitting on the bench. I then had to take it apart so I could re-install it. Speaking of ---- I have written disassembly instructions for the 12-603.

A visual inspection revealed that the 12-603 circuit board is completely different from the SR3:

Optimus 12-603 circuit board	GE SR-3 circuit board
12-603 Board Rear View

Click on the 12-603 front image to get a much larger version, that may be good enough for component identification.

PC Board observations:

• I try very hard not to jump to the "more stuff is better" conclusion - but what I see here is more IF/RF cans on the GE radio. Therefore, without as many IF stages, the selectivity will not be as good.
• The 12-603 is based on a different "radio on a chip" than the SR-3. In this case, it is a Sanyo LA1260. This absolutely confirms it is not a rebranded Superadio 3.
  
  
  
  

  This picture also shows the location of the FM ceramic filter, to the bottom left of the IC. The LA1260 is an interesting chip - it has no audio amplifier in it, but it does incorporate an on chip FM discriminator, requiring only an external inductor. The block diagram of the IC is below:
  
  

  
  
  
• It uses a SIP package IC for the FM front end, possibly the same one os the SR3. I was able to identify it as an LA1186. The block diagram is below:
  
  
• It uses a separate audio amplifier IC, a KA2201.
• There is a small screened area that probably contains the DC-DC converter for tuning voltage.
• There is one less adjustment in the FM front end.
• There is no lower tuning voltage set pot - it is a fixed value. In the GE, there is a pot. In the 12-603. This limits the ability to scale the tuning voltage low end to exactly match the dial.
• The PC board actually extends up underneath the tuning scale, to support the ferrite rod antenna.

The 12-603 schematic is available here. This is re-drawn from a traced schematic by Superadio III Yahoo group member Peter - many thanks to him for taking the time to trace it out!

The 12-603, like the SR-3, comes poorly aligned, and alignment can improve its performance. I learned many years ago to do "coils low, caps high", and that the tuning dial low and high could be set with the oscillator coil and cap. I don't generally touch FM air coils in wax - too touchy and cranky. I found that the RF cap was slightly off, but the antenna cap was very far off, about 30 degrees brought a dramatic improvement. The AM IF's were slightly off. When touched up, performance improved slightly. Not enough to come anywhere near to the SR-3, but enough to increase the volume on weaker stations. FM alignment did not yield much improvement, but some. It was closer initially to being correct. I found myself wondering why there was no FM antenna cap, and what sort of improvement it would yield if present. After all, the AM antenna cap yielded the most AM improvement.

The picture below shows the alignment points for the 12-603.

Since I don't have the manual, here is the procedure I use to do a quick alignment of the unit.

1. Tune the radio to a known station, preferably one that will line up on a mark on the dial.
2. Being very careful not to de-tune the radio, remove the knobs and open it up to the clamshell figure in the disassembly instructions.
3. Lightly tape a dial legend on top of the black plastic dial. Line up the pointer with the station you identified in step 1. If you don't have a copier for the front of the radio, I've got a crude dial graphic here:
   
   
   
   

   Feel free to grab the graphic and re-scale it on your printer to match the original. It must be the exact same size, or you can't do oscillator adjustments!
   
   

4. Switch the AC/DC power switch to the AC position and power the radio off of AC. Or put the batteries back in and power off of DC. Turn it on.
5. Verify that the radio is still tuned to the station you identified in step 1, and that the paper dial copy is centered properly under the pointer. If it is not, I'm afraid you have to re-assemble, re-tune the station until you get it right! The paper dial should line up exactly under the clear plastic scale. Get it right!
6. Switch band to AM, bandwidth to narrow.
7. Find a really weak station, one barely receivable at all.
8. Adjust AM IF1 and AM IF2 for maximum signal.
9. Tune a known station near the bottom of the AM band. 600 kHz would be ideal. Adjust red AM OSC coil to center the station on the dial marking.
10. Tune a known station near the top of the AM Band. 1700 kHz would be ideal. Adust the AM OSC trim cap to center the station on the dial marking.
11. Repeat previous two steps to get the dial right on both ends.
12. Find a very weak station on the low end of the band.
13. Adjust AM RF coil and AM antenna (behind the dial) for maximum signal.
14. Tune to a very weak station on the high end of the band.
15. Adjust AM RF trim cap and AM antenna trim cap for maximum signal.
16. Repeat the last two steps until no further improvement can be made. AM alignment is complete at this time.

Don't attempt FM alignment unless you have instruments and know what you are doing. If you absolutely must try, a similar procedure can be used - but it is really easy to mess up (especially the FM IF and Detector). Mark the positions in case you want to go back! Spreading and compressing FM coils is a pain in the postierior. There are plastic tools made for this task - it would be a good idea to buy one! The coils low caps high method works fairly well, the problem is that even very weak stations may come in with full quieting - this is FM we are talking about, not amplitude modulation.