Around 1995 I purchased the Optimus STA-300, shortly after my Pioneer SX-626 died. I was on a tight budget and the STA-300 had what I needed which was an FM receiver and a CD input.

I remember first hooking it up to my speakers and hearing how similar it sounded to the Pioneer. I was impressed that such a small receiver at a relatively low price point could deliver similar sound. Perhaps it was the cheap GE/RCA all in one unit I had which set low expectations of modern inexpensive equipment.

Radio Shack started selling the STA-300 in late September 1991 and listed it in the catalog from 1992 through 1997. Perhaps that is why (at the time of this post) there are so many available on auction sites at a bargain price.

Minor Repair Needed

I was able to get an STA-300 for a bargain off eBay. The one I acquired was in great condition, worked, and was $35 plus shipping. I connected an AirPort Express (2nd Gen) to the CD input to be able to stream to it for testing. Between songs I would hear a slight 60Hz hum from the speakers.

I suspected the filter capacitors in the power supply section were going bad, so I pulled them out to test them. This was a bit of a challenge since the PCB for this receiver is not easily removable from the chassis.

I ended up removing the back and front panels then lifting the board up a bit to get to the leads for the caps underneath. To make matters more difficult, these caps were glued in place.

I used a small jewelers screwdriver and IPA to peel up some of the glue and the caps came free.

These filter capacitors are 4,700 µF with a 35 volt rating. Both tested okay, with low ESR.

Rather than put these back into the receiver, I bought replacements and put those in.

I powered up the receiver via my dim bulb tester just in case I had a short or other issue:

The bulb barely lit, which means no high current draw was happening and the unit’s display lit up.

I hooked up the speakers and heard no background hum! Either the original caps were starting to go, or the 3.5mm to RCA patch cable I used had bad shielding.

Either way, I was glad the hum was gone.

Memory Backup Super Cap

The AM/FM memories are backed up using a super cap which provides a small amount of voltage when the receiver is powered off or unplugged.

There are several options for replacement of the super cap for the memory backup. The original is 0.047F at 5.5 volts.

Many of the replacements are a different package type, but will fit. I found one with the original package style:

Mine seems to be working still, memories for the FM stations I programmed in are holding (for the time being). However, I mainly use this for streaming, so I am not sure if I would get around to replacing the super cap if I find it does not hold the memories any longer.

There is a tap off the power transformer for a 5 volt backup voltage, which is independent of the power switch. Memories will hold as long as the AC line is connected, regardless of the condition of the super cap.

The service manual mentions this cap will provide up to 72 hours of backup for the memories on loss of AC power.

Amplifier Design

This receiver does not use basic components for the power amplifier like most vintage 70s and earlier HiFi amplifiers. The STA-300 like almost every other amplifier made after the late 70s uses integrated circuits or “packs” which serve as a single IC for the power amplifier section.

The STA-300 uses two UPC1188H power amplifiers (one for each channel), which appears to be a common IC for the period. Further, these are still available today. This is good, because if these ever failed, they could easily be replaced.

For many vintage and modern receivers, sometimes these packs are not sold anymore, or they were not commonly used by many brands or models. In this case, they can be replaced only with donor parts from another receiver.

Interestingly, Radio Shack rates this receiver at 15 watts per channel into 8 ohms. The data sheet for the UPC1188H indicates 20 watts. It also notes that the impedance can go as low as 6 ohms. This is good because I do have some speakers which are 6 ohms.

Further, the data sheet indicates there is overcurrent and over temperature protection, making these quite durable, at least by design.

The heat sink for the power amplifier is rather large, which helps move heat away.

The power supply transformer is hefty and a majority of the weight of this receiver is from the transformer. There is a fuse on the AC side for overcurrent protection.

In addition, there is thermal protection on the power transformer, via a transistor thermally coupled to the side of it. If it rises to 105 Celsius it will turn off the input signal to the power amplifiers.

One annoyance with this receiver is the pop on/off sound from the speakers when turning the power switch on or off.

I included a copy of the service manual for the STA-300 below. The service manual is very useful and includes voltage test points and the alignment procedure.

Final Thoughts

The STA-300 is a great receiver for small areas, and even though the power output is only 15 watts, it can drive small and efficient large speakers and is comparable to my Pioneer SX-626 in terms of sound.

Radio Shack most often rebadged other brand models or had them custom manufactured based either partially or entirely on major brand designs. I am curious what brand(s) this receiver might have borrowed its design from.

With a comprehensive service manual and readily available parts (as of the writing of this post), this receiver should be able to be kept running for awhile to come.