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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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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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

S.Trice Sansui TU-9900 AM/FM Tuner Restore W/ TA7136 Upgrade

Sansui TU-9900 AM/FM Stereo Tuner 

The Sansui TU-9900 is certainly one of the most prolific Sansui tuner’s ever produced. Release in 1974 to match the performance oriented upper end AU Integrated Amplifiers. On this TU-9900 we applied our first TA7136 Upgrade with a high-end Op-Amp Modules.

The TU-9900 features a high sensitivity 5 gang variable capacitor and three dual gate MOSFETs incorporating 3 IF Filters. Wide0Band detector and selection FM IM bandwidth with the known PLL Circuit.  The TU-9900 how impressive output at only 0.08% stereo configuration.

F-2527 Power Supply

The regulated power supply of the TU-9900 is built around a single bridge rectifier 1A and developed to a dual supply output. All Dc filtering input and output coupled electrolytic capacitors were increase to provide optimal DC filtering thru out. The lamps operate of a AC source. 

Due to the obvious water associated oxidization the power supply was stripped/cleaned and repopulated as well as various points of other PCB's and chassis areas.

All small signal devices were replaced with modern TO92/92L Fairchild type with better dissipation. The regulators were both replaced with modern, robust TIP devices with new synthetic Wakefield thermal compound. A new precision Bourns voltage adjustment was installed. The diode arrays were all replaced with a Ultra-Fast, Soft Recovery Vishay type for better transient switching.

The electrolytic as mentioned above were increased in MFD value, with new low impedance Nichicon PW’s with an increase in operating voltages. New high grade polypropylene bypass films were installed as well. 

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Regulated V-/+ Out Confirmation

F-2526 FM MPX W/ TA7136 Upgrades 

One of the biggest improvements for the TU-9900 is the replacement of the original TA7136 modules. Well documented on other articles in our blog, the original monolithic SIP7 TA devices have quite the limited gain and bandwidth like the BA312 devices, though unlike its a dual rail design. We have helped design a direct replacement module to remove the old TA device with a new high end modern Op-Amp for a SUBSTANTIAL improvement in sound quality and presentation. 

With the TA7137 modules were also able to remove some unnecessary components for a better form factor. With the new Op-Amp modules installed we next replaced all the electrolytic and tantalum capacitors with a low impedance PW Nichicon and high grade WIMA polypropylene films, with an increase in operating voltages. 

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F-2524 FM IF & F-2525 FM Discriminator 

The FM IF assembly is located in a shielded well under the chassis for better rejection. The assembly is composed of two six pole linear phase 10.7MHZ filters and a ceramic filter core.  All the ceramics in this stage were replaced with a high grade WIMA polypropylene film type for better sonic qualities. 

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The F-2525 assembly had all of the electrolytic and small value type 1MFD< replaced with a low impedance Nichicon PW and high grade WIMA polypropylene films all with an increase in operating voltages. 

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F-2539 AM IF & F-2559 MPX Noise Amp

Superior AM is achieved with the double-tuned/element design featuring dual coil ceramic filters. In this stage again, all the electronics and tantalums were replaced with a low impedance Nichicon PW and high grade WIMA polypropylene films all with an increase in operating voltages. 

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For the F-2259 MPX Noise stage all small signal devices were replaced with modern low noise TO92 Fairchild devices. Electrolytic were replaced with a high grade WIMA polypropylene (1MFD) and a low impedance PW, as well as the diode filtering 47MFD for the LED.

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LEDS

Both meters have a 8V wheat lamp which fail often, tied to two 22OHM fusible resistors which can be jumped with new 8V Bipins or replaced for tailored output, either way they fusible must be removed and or replaced as they fail often.  The dial display had new custom SMD 8V LED’s installed with the original Blue filters reapplied.