Sansui G-8700DB
Today were showcasing a customers Sansui G-8700DB receiver. The G-8700DB (Dobly) was one of the larger series released which include the digital display circuit. The G-8700DB has some unique features such as two RIAA inputs and a Microphone flat amplifier stage and the two classic massive knobs in the front! It should be noted, like many Sansui’s…the schematic is rife with errors.
F-3070 AC/DC Rectification & Unreg. Power Supply
The unregulated power supply utilizes an array of transformers secondaries to achieve the -/+ 70/50 and 24V rails.
A unique design is the separate Pre-Amp/RF and DUT power supplies employed. One of the most crucial and critical stages in any design is clean and pure power. The potential for intrinsic AC potential and noise is abound in many older rectification designs due to the limits of production back then. One of the biggest benefits a unit can be supplied is clean, low noise rectification of the AC to DC voltage. Modern Ultra-Fast and Schottky diodes employ extremely fast switching transients and uber low noise potential along with being robust and discrete form factors.
In this restoration both of the RB type rectification devices were replaced with with modern, Ultra-Fast (UF), low noise high efficiency axial Vishay UF4### diodes in a custom PCB bridge form factor for a cleaner rectification prices. The associated smoothing 1000/470MFD capacitors were replaced with low impedance, high temp (105C) Nichicon PW capacitors with an increase in operating voltages. Along with the 40Vac secondaries to one the bridge devices, the 40Vac source is piggybacked to the HA12002 protection IC detailed below for the soft start delay.
The original 12,000MFD filter capacitors for the rail supplies were both replaced with an increase in capacity Cornel 15,000MFD capacitors with the lowest possible profile due to the proximal vertically mounted filter capacitors which radiate significant heat.
F-3071 RF Regulated Power Supply
The RF stage regulated power supply is housed on the top of the chassis near the driver and AC/DC stage off centre. The three main regulating devices were replaced with modern On-Semi MJE and KSC/KSA TO-220 devices with new thermal compound applied. The associated TO-92 devices were all replaced with modern, better dissipating Fairchild TO-92L/TO-92 low noise devices. A precision Bourns trimmer replaced the original adjustment point. The electrolytic capacitors were all replaced with a low impedance, high temp (105C) Nichicon PW capacitors with an increase in operating voltages along with several high grade WIMA polypropylene film capacitors.
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F-3073 AF/Pre-Amplifier Regulated Power Supply
This regulated power supply circuit for the AF stages also contains a channel of the protection output circuit. As above the four main regulating devices were replaced with modern On-Semi MJE and KSC/KSA TO-220 devices with new thermal compound applied. The associated TO-92 devices were all replaced with modern, better dissipating Fairchild TO-92L/TO-92 low noise devices this includes the PNP TO-126 device for the protection circuit. The electrolytic capacitors were all replaced with a low impedance, high temp (105C) Nichicon PW and VP Bipolar capacitors with an increase in operating voltages along with several high grade WIMA polypropylene film capacitors. The varistor MV diodes were replaced with series modern general 4148 type axial diodes.
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F-3072 Protection
The protection circuit is design around the common protection IC HA12002 SIP8 device. The HA12002 was a common IC protection device due to its versatility in design and multiple AC dampening/DC shunting and over current protections incorporated into a single and circuit array. This circuit is unique dependent on the 40Vac to Pin#5 of the HA12002 to establish all other concerning voltage references.
The two, protection and soft start relays were both replaced with modern, high quality MY series Omron relays. The associated diodes were replaced with modern axial 4148 general diodes and new Vishay UF4### type rectifier diodes. As above the TO-92 devices were replaced with better dissipating Fairchild TO-92L devices and a new TO-126 MJE (PS rated) device. The electrolytic capacitors were all replaced with a low impedance, high temp (105C) Nichicon PW and VP Bipolar capacitors with an increase in operating voltages along with several high grade WIMA polypropylene film capacitors.
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F-3074 Symmetric Drivers
The G-8700DB driver design is extremely conservative in output regards, capable of MUCH more than rated potential at 8OHM’s. The F-3074 uses the Sansui common dual JFET 2SK97 differential stage with a current stage, thermally coupled transistor and a pre-driver output stage. (PCB indicated wrongs device form factor**)
Both of the adjustment pots were replaced with precision Bourns trimmers. All associated TO-92 devices were replaced with low noise Fairchild TO-92 devices, populated TO-126 pre-drivers/bias devices were all replaced with low noise/low distortion Fairchild TO-126 KSC/KSA devices. The drivers were replaced with heavy duty, high grade MJE On-Semi TO-220 devices with new thermal compound for a significant reduction in final stage distortion! Fusible were replaced with 150OHM 1/2W KOA carbon film resistors.
Old & New Components
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Idle Current (leerlaufstrom 5mV)
F-3080 Tone + F-3076 Mic Amplifier
The G-8700DB flat amp is comprised of a NPN initial gain differential stage with integrated Bass/Mid/Treble attenuators and buffer output.
All of the Bipolar electrolytic were replaced with high grade Nichicon MUSE BP and VP Nichicons with a increase in operating voltages. The remaining electrolytic were replaced with a low impedance Nichicon PW and Fine-Gold/ KT Audio grades with an increase in operating voltages. Low capacity type were replaced with a high grade WIMA polypropylene film capacitors. The populated TO-92 devices were all replaced with a modern, low noise Fairchild TO-92 device. Te varistor diodes were all replaced with series axial modern 4148 diodes.
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F-3076
Uniquely the G-8700DB features a flat amp Mic gain stage for inline recording from the tone/driver stages. This board can be a bit of a PITA to access properly but is highly recommended due to the populated failure prone 2SA devices.
All the small signal devices were replaced with modern, low noise Fairchild TO-92 devices. Electrolytic’s were replaced with low impedance Nichicon PWs with an increase in operating voltages. 1MFD< were all replaced with a high grade WIMA polypropylene film capacitor.
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F-2865 RIAA EQ
Constructed with the chassis to act as shielding the RIAA EQ is made of a very simple PNP/NPN gain stage.
All of the capacitors were replaced with a Fine-Gold (FG) Nichicon FG and low impedance PW Nichicon with an increase in operating voltages. The associated films were replaced with high grade WIMA polypropylene film capacitors. Small signal devices were all replaced with low noise modern Fairchild TO-92 devices.
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F-3088 Digital Display & F-2688 Dolby NR
The G-8700DB uses a digital display FL for the RF indicators. There are at least two version of the F-3XXX assembly in active rotation, with many Sansui’ displays failing from a defective or damaged MSM4550“ type IC.
The electrolytic on this PCB were all replaced with a low impedance Nichicon PW and VP Bipolar with an increase in operating voltages. The MSM IC pins were reflowed and hardware pins.
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Though limited in its functions it still is efficient as a noise reduction circuit (at best), the Dolby NR for original Dolby FM, still though active in the signal chain. The electrolytic here were replaced with Nichicon PW low impedance type and modern high grade WIMA polypropylene for the 1MFD< film capacitors.
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New SMD LED’s Installed
New Upgraded Speaker Output Hardware Installed
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AUDIO NOTIZEN
