Market Electronics held off on using transistors while other companies made the transition. Nevertheless, in the late '60s they set Battle and Dixon to the task of creating the first transistor version of their product. Once the two were satisfied, beginning in the 1970s, the solid-state Echoplex was offered by Maestro[ and designated the EP-3. This unit offered echo, sound-on-sound, a bunch of little convenience tweaks, and a sound that has come to be preferred by many professionals and tape echo aficionados. Having been produced from 1970 to 1991, this unit enjoyed the longest production run of all the Echoplex models. Right about the time of the public introduction of the EP-3, Maestro was taken over by Norlin industries, then the parent company to Gibson Guitars. In the mid-1970s Market created an upgrade to the EP-3 (designated the EP-4) adding features such as an LED input meter, tone controls, and a compander circuit to lower the noise of the tape playback. Unfortunately, in a production mistake, the expander circuit was place in across both the direct signal and the tape playback causing the guitar's sustain to be cut off prematurely. As a result the EP-4 hasn't enjoyed the reputation of the EP-3 and was even disliked by Echoplex creator Mike Battle. Modification procedures are available on the Internet that allow the owner to remove the compander board entirely and thus make the EP-4 sound much like the EP-3. Mike's final consulting with Market yielded the EM-1 Groupmaster which offered a four-channel input mixer section and a mono output section. In the mid-'70s Maestro also created the ES-1 Sireko (pronounced "Sir-Echo"), a simplified, echo-only unit with a bin-loop cartridge system.
At the end of the '70s, Norlin folded their Maestro brand and Market Electronics was forced to find another distributor for their products. They found that distributor in Harris Teller, a Chicago musical wholesaler. Units built for Harris Teller carried an Echoplex badge that omitted the Maestro name. In 1984, Harris Teller bought out the Echoplex name and all the back stock of Echoplex parts from Market Electronics. Harris Teller used the back stock to assemble reissues of the EP-3, EP-4, and tube EP-2 (which they designated the EP-6t). In 1991, the thirty-year run of electro-mechanical Echoplex production finally came to an end. Towards the middle of that decade the Echoplex brand was applied by Gibson to its line of digital looping units.
Today the Echoplex still enjoys its reputation for a distinct echo sound and feel that is modeled by many effects manufacturers in their digital units. In fact, "sounds just like an Echoplex" has become a major sales point for guitar echo units. Vintage Echoplex units are available at vintage guitar dealers and through EBay and a support micro-industry has sprung up offering parts, supplies, and repairs. The sound of the solid state EP-3 preamp has become so desired as a tone shaper for guitar that several effects manufacturers have created preamp-only emulation circuits.