The 10-day recorder seismic system used by the USGS since 1965 has been modified substantially to improve its dynamic range and frequency response, to decrease its power consumption and physical complexity, and to make its recordings more compatible with other NCER systems to facilitate data processing. The principal changes include: 1. increasing tape speed from 15/160 ips to 15/80 ips (reducing running time from 10 days to 5 days with a 14' reel of 1 mil tape), 2. increasing the FM center frequency by a factor of 4, from 84.4 Hz to 337.6 Hz, 3. replacing the original amplifiers and FM modulators with new low-power units, 4. replacing the chronometer with a higher quality time code generator (with IRIG-C) to permit automation of data retrieval, 5. eliminating the amplifier/WWVB radio field case by incorporating these elements, along with the new TCG, in the weatherproof tape-recorder box, 6. reducing the power consumption of the motor-drive circuit by removal of a redundant component. In the new system, the tape-recorder case houses all components except the seismometers, the WWVB antenna, the 70-amp-hour 12-VDC battery (which powers the system for 5 days), and the cables to connect these external elements to the recorder box. The objectives of this report are: 1. to describe the new 5-day-recorder seismic system in terms of its constituent parts and their functions, 2. to describe modifications to parts of the original system that were retained and to document new or replacement components with appropriate circuit diagrams and constructional details, 3. to provide detailed instructions for the correct adjustment or alignment of the system in the laboratory, and 4. to provide detailed instructions for installing and operating the system in the field.