S. King Marantz 170DC Stereo Amplifier Restoration

Marantz 170DC Stereo Power Amplifier

The Marantz 170DC is a 85Watt output at .03% THD+N non conventional amplifier in a sense of its design. The 170DC is a true DC designation versus the typical DC coupled nomenclature units as it amplifies input DC thru out the gain stages. A trait some would say, such as Marantz, is perfect for the naturally chaotic or asymmetric nature of  recorded music versus a typical reference sine wave. Produced during the late 70’s from 1978 to 1980 it boast a unique meter design compared to the popular 250 series with extended bezel and silver patina. What I admire about the 170DC is its truly simplistic design and quality build. 

AC/DC and Dual Supply Rails

The power supply is a very simplistic approach versus a lot of other Marantz designs, featuring a single 5A Full Wave Bridge rectifier for the  -/+50V rails and a AC Full Wave single rectifier in the protection.

The original build uses a single dual capacitor which have fairly high failure rates and in my experience not the same build quality as single capacitors in a bank. In this case the single dual 12,000MFD capacitor was replaced with a custom made bank for a total of 33,600MFD for an increase of about 6KMFD per rail for better AC/DC filtering and deep subsonic required power reserves. 

-/+ Rail Confirmation

P700 Drivers

The input driver design of the 170DC also differs from many traditional Marantz designs. With a AC/DC selectable input and gain potentiometer signal is presented to a differential JFET device 2SK109 which is essentially a dual JFET and proceeded by a 2nd tier dual differential NPN stage. With matched commentary class output stage in a class A configuration.This allows for an almost auto adjusting bias design with no adjustments.

The secondary PNP differential  stage were gain matched and thermally coupled. the and the JFET devices plotted for any obvious failure modes. 

Secondary Differential Matching

Remaining small signal devices were replaced with low noise, modern TO92L fairchild devices for better dissipation. New pre drivers and drivers were cross referenced with modern, robust TO220 type OnSemi/Fairchild devices and thermally coupled with synthetic Wakefield compound. New precision Bourns potentiometer installed for DC adjustments. 

The capacitor is this stage were replaced with a audio grade KT/KA and low impedance PW Nichicon type with an increase in operating voltage. Films were also replaced with high grade WIMA polypropylene and MKP PP films. 

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AFTER

In addition, a entire new bank of NPN/PNP output Transistors were installed using robust, modern TO-3 OnSemi devices coupled with new synthetic Wakefield compound.

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SM Verified DC Offset of P700

Square Wave Performance Report

PN00 Meter & Protection Assembly

The PN00 assembly contains both protection and meter control circuits. The 170DC protection is fairly advanced in the 170DC with a overload protection design that limited gain current in the P700 stage when excessive output voltage/current is detected. Another feature is a automatic cutoff when DC voltage is detected at the output or peak to peaks below 5Hz. Finally a inrush (on/off) muting circuit. The operating rail is developed with a single 1A full wave rectifier diode array on board.

Unlike many meter displays of the era, the 170DC is very accurate display due to accepting variations in AC line input and driver impedance loads. The 170DC also features a dynamic shunt design protection the meter magnetic coils form excessive current. 

The electrolytic capacitors here in both stage were all replaced with a low impedance, high temp Nichicon PW with an increase in operating voltage. All small signal devices were replaced with low noise modern TO-92L Fairchild devices with better dissipation. The 1A bridge diodes were replaced with Ultra-Fast, Soft Recovery Vishay axial diodes. All meter calibration pots were replaced with a precision Bourns potentiometers. A new 24V Omron Relay was installed.

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AFTER

Lamps & Coupling Switch

The coupling switch is design to allow a DC or AC coupled input line such as when older pre-amplifiers with a DC potential output are used, out of phase connections. The switch is bypassed with two 10MFD bipolar capacitors. These were replaced with a audio grade bipolar MUSE Nichicon with a increase in operating voltage. The switch was heavily cleaned and protectant applied. 

The 8V AC lamps were replaced with a  custom SMD 8V LED fuse type and new high quality, heavy vellum filters applied to both meter assemblies. 

AUDIO NOTIZEN