Paul's Sansui G-7700 Receiver

Sansui G-7700 Receiver

Today were showcasing a customers Sansui G-7700 Receiver. The G-Series had several variants released during the period with the G-X700 being the mid-way point. Touting a pure DC amplifier section, Peak LED and digital tuning display with their quartz-locking feature.

F-2980 Power Supply/Protection

The Sansui G-7700 and other G-series feature this similar layout, with a MAIN-PCB in the centre of the unit populated as the power supply and driver section F-2980.  Many of these G-series present the same faults here from an over heating power supply and caustic glue. The glue used to mount the large vertical capacitors in the power supply becomes oxidized over time and causes the exposed leads of components such as resistors to oxidize and break as well as excessive radiant heat from the nearby regulator devices (x3.)

One of the first issues to address as mentioned above was removing all the glue and replacing about half dozen or so resistors with new 1/2 carbon film KOA resistors and several diodes with axial modern Vishay/Fairchild diodes.   All three regulators were replaced with a robust ST, TO-220 modern regulators manufactured using their new construction standards along with new performance Wakefield thermal compound.

All of the electrolytic capacitors were replaced with a high temp (105C), low impedance Nichicon PW and VP Bipolar capacitors with an increase in operating voltage. New axial Vishay Ultra-Fast, Soft recovery diodes were installed as well.

BEFORE

AFTER

The protection network integrated into the F-2980 is similar to the recent SX-780 I did with a HA12002 monolithic IC used to monitor dc deviation, current consumption in conjunction with varistors for thermal run-away.  This also controls the front “Safe” operation mode indicator. A NOS relay was installed from a reputable retailed in the UK and new filter capacitors. 

F-2980 Driver Stage

Shared on the same MAIN-PCB as the PS the G-7700 has an interesting design. Using an initial Dual FET 2SK150 devices the differential stage, current mirror/differential, and a two-stage OCL Darlington design.

All of the electrolytic here were replaced with high temp (105C), low impedance Nichicon PW capacitors with an increase in operating voltages. 1MFD< capacitors and mylar were replaced with high grade WIMA polypropylene ilm capacitors. New precision Bourns potentiometers installed, new modern, low noise TO-92 Fairchild (see underside adaption for smoother action while biasing protocol) for a matched constant current stage (see BETA mapping below), new modern low noise, low distortion audio applicable On-Semi TO-126 type NPN/PNP pre-drivers/current devices were installed.  Original drivers were removed and tested with new performance Wakefield thermal compound applied. (high volt. broad spectrum drivers should be cross references VERY carefully if replaced to prevent oscillation issues..note the often reference MJE15032/33 are NOT good references @ 30Mz)

BEFORE

AFTER

BIAS Confirmation

(SM error, Correction at .66x5mA @ 3.3mV)

NPN BETA Matching

F-2855 Tone 

The tone control features a defeat option with dual Bass and Treble attenuations, a 41-stepped precision volume attenuator is used for gain control which is always nice to see, though not potted..

The tone is designed around an initial NPN differential gain stage.

All of the electrolytic capacitors here were replaced with audio KT and MUSE (Bipolar’s) along with VP Nichicons with an increase in operating voltages. All the small signal devices were replaced with modern TO-92 low noise Fairchild devices. 

BEFORE

AFTER

F-2857 MIC AMP

The G-7700 features a separate driver for microphone line level and dual dubbing options.

The small signal devices here were replaced with modern TO-92 low noise Fairchild devices. Electrolytic’s were replaced with audio grade KT and low impedance PW Nichicons with an increase in operating voltages, a new high grade WIMA polypropylene .47MFD film was installed as well.

BEFORE

AFTER

F-2865 EQ RIAA

The RIAA is placed under the main chassis carriage using a metal standoff as shielding. Built using an array of precision resistors for low noise (SN @ 78dB) comprised again of a PNP gain stage…..also the G-7700 has two phono inputs!

All of the small signal devices were replaced with low noise, modern TO-92 Fairchild devices. Electrolytic’s were replaced with low impedance and audio grade Nichicon KT capacitors and 470MFD VP Nichicon Bipolar with an increase in operating voltages.

BEFORE

AFTER

G-7700 TESTED RIAA CURVE RESPONSE

(EXTREMELY GOOD RESPONSE AFTER THE REBUILD!)

F-3060 Digital Display Power Supply

The digital display requires its own power supply source, with a 5/13V output for the display.  Their are several variations of the supply, all suffering the same issues from the large wire-wound resistors radiating enough heat to scour the PCB. Each resistor should be increase in watt rating as well as foil foot prints exposed and expanded for a good anchor over time.

New robust ST, TO-220 modern regulators manufactured using their new construction standards along with new performance Wakefield thermal compound were installed. All electrolytic capacitors were replaced as well with high temp (105C), low impedance PW Nichicon capacitors with an increase in operating voltages as well as emitter values and several carbon film 3.9K 1/2W resistors.

BEFORE

AFTER

F-3000 Digital Display

Using a FL type Tube display, it broadcast the FM/AM RF tuning indication and locking  features, it utilizes the MSM5540 monolithic IC with a advanced circuit NJM4558 OPs and BA658 monolithic devices.

All of the electrolytic capacitors here were replaced with a high temp (105C), low impedance PW and VP Bipolar Nichicon capacitors with an increase in operating voltages. The FL assembly was carefully dissembled and cleaned. 

BEFORE

AFTER

F-2988 RF Stages

The G-7700 features a very fine tuned RF stage, with a emphasis around their “Digital Quartz Locked Tuning System”.  The FM uses a 4-gang tuning capacitor with a dual MOS-FET front end. The AM features a “Frequency linear, double-ganged tuning capacitor, along with a high performance monolithic IC in the IF circuit”.

In both stage all of the electrolytic capacitors here were replaced with a high temp (105C), low impedance PW Nichicon capacitors with an increase in operating voltages.

New Custom 8V SMD Warm White LED’s installed for the highlight

AUDIO NOTIZEN