Legendary PH-1. Based on the serial number, it appears to be from 1979. ・Some op-amps and capacitors have been replaced. ・Modified so the LED stays lit when ON. ・Modified to PSA specifications, so a standard 9V center-negative power supply can be used. No need for an expensive ACA adapter. While not fully original, please consider this if you're looking for a reliable unit for practical use.