DYNACO Dynaco-400-Rebuild free download

File name Dynaco-400-Rebuild.pdf

Dynaco 400 Assimilation The Dynaco 400 was the Tyranasaurus Rex of power amps in 1973. I wanted one badly but I was bringing up children and settled for a more affordable Dynaco Stereo 80, which pumped out music for the next twenty-odd years. It also survived three teenaged boys. In 1998, I spotted a Dynaco 400 in a used equipment shop, paid way too much and dragged it home--all sixty pounds! Unfortunately, my `Dynasaur 400' was seriously flawed. It had a nasty habit of going DC and destroying speakers. I spent more money unsuccessfully attempting to repair a unit with too many previous ham-handed repairs and modifications. Eventually I decided it was too old and complicated to fix and too big to use as a doorstop. Resistance was futile; it needed to be assimilated with newer technology. I decided on a rebuild that would retain the outward appearance of this classic amplifier while replacing the innards with new components. After some looking around, I chose to upgrade using a couple of compact 200 W all-MOSFET amplifier modules from Marchand Electronics in Rochester N.Y. You can buy the PM224 modules as fully assembled and tested units, as kits with the PC boards and all parts or as bare PC boards. See the Marchand site for details, specifications and schematics. The rebuild is straightforward. Strip everything but the massive Hammond power transformer from the old chassis. Remove the faceplate and hinge the front part of the chassis forward. Clean the wiring from all front panel controls. Only the power switch and pilot light will be functional in the new amplifier. The old Dynaco yields about two hundred dollars in parts--a 1,000 VA transformer, heat sink, rectifier bridge, circuit breaker, switch, speaker fuses, connectors and a sturdy chassis with lovely big rubber feet. On a good day, you can pick up an old, one-channel-good Dyna 400 for fifty dollars on E-Bay; you'll have talk to the Teamsters about getting it to your place. Despite the utility of the Dynaco's original fifteen amp power switch, I chose to use a triac to turn the amp on, believing that MOSFETS are more susceptible to switching spikes and noise than the transistors in the old amp. Audiophiles are, as Nelson Pass notes, a superstitious group. The triac power-on circuit is the same as that used in the Pass/Thagard A75. The voltage regulator circuit is taken from Pass' `Complementary Zen' design. Th