Realistic 12-650 Tech Page

This page is designed to give you quick, accurate technical information about the Realistic 12-650 AM/FM portable radio. Sold in the early 1980's, this radio was probably meant to counter the GE Superadio, which was sold around the same time. In my opinion, it is the best AM/FM portable that Radio Shack ever made, and is well worth picking up at thrift sales or on eBay, where I recently got one for $20. New, they were about $60. Be aware, however, that even fully modded - they are not quite as good as GE Superadios, which can be had for similar prices.

Here are quick links to my disassembly instructions and modifications:

• Disassembly Instructions
• Modifications

The 12-650 employs tuned RF design on both AM and FM. The AM section has an IF amplifier that contains one ceramic filter (integrated in the first IF can), and two LC IF filters. With no modifications, the radio is capable of good sensitivity and selectivity, but the limited number of IF stages makes it a bit prone to overload by nearby local stations - a result of Radio Shack's minimalist approach to IF design, something that I think hurt them in the long run. There are modification to improve the AM selectivity - and I cover these in the 12-650 modification page. FM IF is similar, with most of the selectivity depending on one 280 kHz ceramic filter. Selectivity on FM can be improved as well.

My 12-650 has been heavily used, and even more heavily modified by a very inexperienced version of me over 30 years ago. I recently acquired a second 12-650, which allowed me to go back and see what the original unit's performance and construction was like.

The 12-650 is from the "boom box" era, and therefore it needed to look large and loud - hence a faux speaker underneath a sheer grill. Unfortunately, the real speaker and power amp driving it are NOT powerful, so it is not as loud in noisy environments like the beach as one would hope. The dial is wide and graphically well made - and fairly accurate. It is a large and attractive unit, so its use as a prop is insured. I noted a 12-650 on a shelf in an Allstate "mayhem" commercial in an office setting.

Here is a closeup of most of the board, some of it is obscured by the dial, but the dial bracket has slots and holes so you can get to alignment points. Good thinking - Radio Shack! You might notice a few of my modifications, but I'll go into them later.

Just some general notes:

• The plastic tuning capacitor is normal sized for portable radios, and does have six sections to accomodate tuned RF on AM and FM, but as typical for these six section tuning caps, only four trim caps can be accomodated on the top. The others are on the PC board.
• The yellow IF can is the same type used on the 12-655. It contains a ceramic filter and should NOT be disassembled. Although the design of the AM section is different from the 12-655, performance is almost as good after the narrow ceramic filter modification.

This radio is an excallent candidate for covering the expanded band - the oscillator trim cap has plenty of head room to reach to 1700 kHz and even beyond. RF and Antenna trim adjustments are also very easy. Dial calibration is affected by the time you reach 1200 kHz. I haven't bothered with a different dial legend, I just memorize where the stations are and tune by context (what stations are nearby).

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