Marantz 250 Power Amplifier
Today we are show casing a rare Marantz 250 Power Amplifier. Originally manufactured in several Mark variants and the 250M and produced from 1971 to 1976 with optional case and rack mounts. The major differences in the MK variants is the driver topography and rectification assembly.
Power Supply/Protection Assembly
The regulated power supply share a assembly with the protection circuit. Comprised of a full wave rectification path the AC 86.5V is rectified to a stable 58.5 driver rail voltage and filter through two 20,000MFD filter capacitors. Each of the capacitors was replaced with a high grade Nippon 22,000/100V filter capacitor with new bleeder resistors. The remaining inlet is and secondaries feed a AC rail for the camping.
All of the original Axial electrolytic were replaced with Nichicon TVX (85C) capacitors and both CR rectification diodes to Ultra-Fast UF400X type diodes. The original TO-69 type MOTO transistor was replaced with a more reliable MJE TO-220 type transistor.
The integrated protection is a two fold, AC in rush protector upon start up and DC deviation detection circuit peak at 4.5Vdc (-/+ present at the outputs where as the output section is tied directly to the 24V relay wipers. The original relays due to their open frame nature present many problems in the 240/250 designs. A new low noise Omron MY2 24V was installed, significant modifications need to be done prior to relay as the coil and wiper contacts are reversed. As photo’d below you can see each coil lead was de-solder and buckled to the wiper, a very very delicate procedure!
Output from the driver sections is driven to the meter assembly to Q403 small signal transistor, protected by two small value diodes. buckled to Q404 driver, pushed a bridge rectification network that supplies the DC rails for the meter operation. All of the capacitors were updated to a low impedance high reliability Nichon PW and zeners updated. Several of the 1/4W resistors had drifted and were replaced with carbon film KOA type. Often times these meters will become sticky with age due to the original lubricant, these meters were no exception and still are "reliable" at bests, the 250 is synonyms with issues, even rebuilt.
The driver pcbs is a classic push-pull topography using parked NPN/PNP TO-3 outputs Moto SJ2520/2519 pairs. The initial stage is comprised on a NPN/PNP compliment in a Sziklai design. The output is then coupled to the thermally coupled differential transistor pairs Q501/502. The gain current is driven tot he pre-drivers Q507/508 TO-69 transistors and applied to driver compliment Q510/11.
The differential pair transistors were replaced with thermally coupled performance oriented ZTK795 transistors to stabilize the DC offset and thermally coupled together. Pre-Drivers were updated to a 1220/2690 compliment TO-220 to replace the MOTO to-69 transistors and drivers to a modified TO-220 mount MJE components. The bias Q521 was updated to a ztx transistors, due to the solid construction the potentiometers were pulled and deeply cleaned and lubricated on this model. All of the original axial were replaced with a audio grade Nichicon KA capacitors an MPK and input 1MFD film capacitors along with a high end WIMA polypropylene film capacitor.