Tuesday, October 11, 2016

Sansui 1000A Receiver Restoration

Sansui 1000A Receiver

Today were showcasing a Sansui 1000A Tube Receiver.  The 1000A came in several variations and each variation also contain about 2-3 versions of the schematic. The 1000A is not for the faint of heart to tackle and can be quite time consuming.

The 1000A features a Tube/Transistor topology, paired with a push-pull compliment of two 7591 power tubes a channel and a nice 6A/AX7 signal tubes. One of the real prized sections of the early and tube models of Sansui was the use of Hashimoto Electric output transformers and iron who still produce extremely high end amplifiers, etc.  The 1000A is regulated to a 117VAC line level so we recommend all our customer to use a voltage regulator/conditioner when operating their restored units.

One of the first issue is the 1000A utilized oil/paper and large value axial capacitors, which of the time were quite poor in construction. Also noted with 120V mains we can see upwards of 480Vdc at the plates which actually starts to exceed even the original voltage ratings. 

 All of the 7591 couple films were replaced with higher voltage, audio grade MKP polypropylene film capacitors along with the associated grid resistors which are often found to be quite out of tolerance due to the heat radiated int his stage. This is also the time we installed a 10OHM at the cathode of the 7591 to ground for a more reliable bias point which using our bias formula and the 7591 dissipation factor were looking at about 32mA each cathode or 325mV across the resistor (325/10) 32.5mA;  this is where a matched pair of tubes comes in handy.

The associated bias network capacitors were all replaced with a modern higher voltage Nichicon TVX and .33-.22 MKP audio grade Polypropylene film capacitors. A high temp (105C) low impedance radial PW Nichicon 33MFD was replaced with a higher voltage rating at the terminals versus an axial  package.

The original SW-05 diodes (X3) were replaced with an ultra-fast modern UF400# type diode and to each 220/300V capacitors which were recapped, each diode was bypassed at the caps with a .0051 PPT film capacitor.  

At this location the headphone ballast resistor is housed, on this model someone had improperly driven the 1000A causing the resistor to radiant massive amounts of heat stripping the shielding off the associated leads.  The multi-pin resistor was replaced with high grade Eagle non-inductive film resistors at 10W, we found under the ballast a AC Primary lead which had actually burnt through! What a disaster waiting to happen!



Solid State Pre-Amplifier RIAA EQ
The 1000A features a Solid State Pre-Amplifier stage with incredibly low noise BJT’s, very nice devices for quite operation, these are not point to point but PCB sections at the side of the unit.  The SM indicates preferred performance with MM cartridges and recommends the use a a input transformer for inputs of under 2mV. Each PCB was repopulated with low impedance high temp (105C) PW Nichicons and the BJT’s removed and tested for leakage, input resistors were replaced with 1% films and out of spec resistors. 


The original paper/films were replaced with audio grade WIMA polypropylene film capacitors and Panasonic ECW polypropylene film capacitors at a higher 630 voltage including coupling capacitors for the controls and bypass films. 

Audio Report

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