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, unfortunatley, 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 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, since it will be used by an active child.

Actually, the Optimus radio is built better than the GE, using thicker, more rugged plastic - reminiscant of the GE SR-2, which I also own. I am getting pretty good at quickly opening Radio Shack radios, after owning several. The front and side knobs should be removed, including the switch latches (unlike the SR-3, where they are underneath the cover). There are six long chassis screws, three on each side, and one short one in the battery compartment. It is not necessary to remove the screws holding the handle.

This is where it gets a little complicated, Radio Shack radios are notorious for needing a little dexterity and having "tricks" to opening. The case separates at the bottom, and it is necessary to flex the earphone jack side outward a little to clear the earphone jack. Immediately afterwards, the two halves should be manipulated to minimize stress on the on/off switch, located on the top of the unit. After that, the two chassis halves can be separated - and you have access to the inner chassis.

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

Click on either image to get a much larger version, that may be good enough for component identification.

I try very hard not to jump to the "more stuff is better" conclusion - which led me to buy a junky "12 transistor" Jade brand radio in 7th grade, when I should have bought Radio Shack's 8 transistor tuned RF radio instead. But what I see here is more IF/RF cans on the GE radio, along with many other components. The 12-603 board is open by comparison, and 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, which I will discuss later. An example of how components have been cost reduced in the 12-603 is the lower tuning voltage set point. In the GE, it is a pot. In the 12-603, it is a fixed value. Reduces cost, but also eliminates the ability to scale the tuning voltage low end to exactly match the dial.

Since the majority of DX'ers will be tempted to purchase the unit for AM reception, I thought that I would concentrate on that aspect. The 12-603 has a 120 mm (4 3/4 inch) instead of 200 mm (7 7/8 inch) in the GE. As almost all of you will know, longer is better - as the ferrite is the only thing that can concentrate flux into the tiny loop antenna. The longer and thicker the rod, the more magnetic flux from the station is transferred to the loop - and the more distant the station that can be received.

Substituting a longer ferrite rod in the 12-603 is a bit tricky - but the same techniques needed to replace the wide FM ceramic filter also give access to the ferrite rod. The plastic "chassis" can be removed from the case by unscrewing some deeply set phillips screws through holes in the PC board, and near the tuning dial assembly. It is posssible to separate the circuit board from the plastic "chassis" by removing the screws holding the board to the plastic on the front, two screws by the ferrite bar in back, and unscrewing the two screws holding the on-off switch and AC/DC switch. Be warned - the plastic posts are a tight fit with the small daughter PC boards in a couple of spots, particularly the power switch board.

Once a longer ferrite rod was installed, sensitivity of the 12-603 improved quite a bit, but still not to the level of the GE Superradio 3. I think that this is due to the winding of the antenna in the SR3 - it winds turns along more of the ferrite bar. The 12-603, in contrast, uses a paper form like most AM radios.

FM performance is also improved by substituting a 150 kHz ceramic filter (near the center of the board) for the filter supplied. Again, it is still not up to SR3 standards, but stations up to 100 miles away are easily receivable.

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 (3) 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.