Sansui 310 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Sansui

Model: 310

SN: 822081042

Today, I'm showcasing a Sansui 310 receiver that has come in for restoration. It was designed as an entry-level receiver in Sansui's early solid-state lineup. This receiver is less common than other models, but its warm, smooth sound and classic design make it appealing to those who appreciate vintage aesthetics.

Sansui 310 was introduced to the market in 1972. It produces 15 watts per channel into 8 ohms with no more than 1% total harmonic distortion. The damping factor at 8 ohms is 30.

Tone Control, Power Amplifier & Power Supply Board (F-1380)

Power supply circuit

The power supply circuit has 8 aluminum electrolytic capacitors: C004, C005, C006, C007, C008, C009, C010, and C011.

All aluminum e-caps except C005 were replaced with Nichicon UPW/UHE low-impedance capacitors. I didn't replace the main filter capacitor in this unit. The original capacitor has a very large lead spacing of 22mm. Modern capacitors have a maximum lead spacing of 10mm. So, it is very difficult to find a suitable modern e-cap that will fit. In any case, the original filter cap is still in very good condition with a small tolerance and low ESR.

The original e-caps were tested with an Atlas ESR70 capacitance meter; the results are below.

Test results on original capacitors removed from the power supply circuit:

C004: rated capacitance – 330uF, measured – 444uF, ESR – 1.16Ω, deviation: +35%

C005: rated capacitance – 1500uF, measured – 1820uF, ESR – 0.79Ω, deviation: +21%

C006: rated capacitance – 220uF, measured – 286uF, ESR – 1.21Ω, deviation: +30%

C007: rated capacitance – 470uF, measured – 761uF, ESR – 1.47Ω, deviation: +62%

C008: rated capacitance – 100uF, measured – 129uF, ESR – 1.76Ω, deviation: +29%

C009: rated capacitance – 330uF, measured – 438uF, ESR – 1.18Ω, deviation: +33%

C010: rated capacitance – 470uF, measured – 574uF, ESR – 1.16Ω, deviation: +22%

C011: rated capacitance – 100uF, measured – 146uF, ESR – 2.42Ω, deviation: +46%

Tone control circuit

The tone control circuit has 4 aluminum electrolytic capacitors (C707, C708, C711, C712) installed in the signal path. 

To reduce the noise generated in the circuit, the ordinary 1uF capacitors were replaced with film polyester WIMA caps, and the ordinary 3.3uF capacitors were replaced with modern low-leakage Nichicon UKL caps. 

Test results on original capacitors removed from the tone control circuit:

C707: rated capacitance – 1uF, measured – 1.3uF, ESR – 6.4Ω, deviation: +30%

C708: rated capacitance – 1uF, measured – 1.3uF, ESR – 5.8Ω, deviation: +30%

C711: rated capacitance – 3.3uF, measured – 4.7uF, ESR – 7.5Ω, deviation: +42%

C712: rated capacitance – 3.3uF, measured – 5.2uF, ESR – 8.7Ω, deviation: +58%

The tone control circuit has two 2SC871 transistors installed in positions TR701 and TR702. This transistor is on my list of transistors prone to failure. I replaced them with a modern low-noise Fairchild KSC1845. Watch the pinout when replacing transistors. The original 2SC871 transistor is BCE, and the new one (KSC1845) is ECB.

Power amplifier circuit

The power amplifier circuit has 12 aluminum electrolytic capacitors: C801, C802, C805, C806, C807, C808, C809, C810, C813, C814, C817, and C818.

The ordinary 1uF capacitors installed in the signal path were replaced with film polyester WIMA caps to reduce the noise. The ordinary 10uF capacitors were replaced with modern low-leakage Nichicon UKL caps for the same purpose. The coupling capacitors C817 and C818 were replaced with Nichicon LKG caps. The remaining aluminum e-caps were replaced with Nichicon UPW/UPM low-impedance capacitors.

Test results on original capacitors removed from the power amplifier circuit:

C801: rated capacitance – 1uF, measured – 1.3uF, ESR – 6.4Ω, deviation: +30%

C802: rated capacitance – 1uF, measured – 1.3uF, ESR – 5.2Ω, deviation: +30%

C805: rated capacitance – 100uF, measured – 148uF, ESR – 2.4Ω, deviation: +48%

C806: rated capacitance – 100uF, measured – 155uF, ESR – 1.8Ω, deviation: +55%

C807: rated capacitance – 10uF, measured – 13uF, ESR – 19.5Ω, deviation: +30%

C808: rated capacitance – 10uF, measured – 16uF, ESR – 11.1Ω, deviation: +60%

C809: rated capacitance – 33uF, measured – 45uF, ESR – 1.73Ω, deviation: +36%

C810: rated capacitance – 33uF, measured – 40uF, ESR – 1.56Ω, deviation: +21%

C813: rated capacitance – 100uF, measured – 140uF, ESR – 1.58Ω, deviation: +40%

C814: rated capacitance – 100uF, measured – 146uF, ESR – 2.2Ω, deviation: +46%

C817: rated capacitance – 1000uF, measured – 1178uF, ESR – 1.18Ω, deviation: +18%

C818: rated capacitance – 1000uF, measured – 1247uF, ESR – 1.44Ω, deviation: +25%

This circuit also has two 2SC632 prone-to-failure transistors installed in positions TR801 and TR802. I replaced them with a modern, low-noise Fairchild KSC1845. Make sure the new transistors have a current gain >350. Also, when replacing transistors, pay attention to the pinout. The original 2SC632 is a BCE, and the new transistor is an ECB.

Tone control, power amplifier & power supply board - before and after

Head Amplifier Board (F-1379)

The head amplifier board has two low-leakage capacitors (C601, C602) installed in the input signal path, two solid electrolytic capacitors (C613, C614) installed in the output signal path, and 6 ordinary aluminum electrolytic capacitors (C605, C606, C609, C610, C611, C612).

To reduce noise in this circuit, the original low-leakage capacitors were replaced with modern low-leakage Nichicon UKL caps. The original solid e-caps were replaced with film polyester WIMA caps. The ordinary aluminum e-caps C611 and C612 were upgraded to modern low-leakage Nichicon UKL caps to further minimize noise. The remaining aluminum e-caps were replaced with Nichicon UPW low-impedance capacitors.

Test results on original capacitors removed from the head amplifier board:

C601: rated capacitance – 3.3uF, measured – 3.1uF, ESR – 1.46Ω, deviation: -6%

C602: rated capacitance – 3.3uF, measured – 3.3uF, ESR – 1.39Ω, deviation: 0%

C605: rated capacitance – 100uF, measured – 150uF, ESR – 0.91Ω, deviation: +50%

C606: rated capacitance – 100uF, measured – 151uF, ESR – 1.08Ω, deviation: +51%

C609: rated capacitance – 47uF, measured – 70uF, ESR – 1.52Ω, deviation: +49%

C610: rated capacitance – 47uF, measured – 72uF, ESR – 1.39Ω, deviation: +53%

C611: rated capacitance – 10uF, measured – 15uF, ESR – 1.85Ω, deviation: +50%

C612: rated capacitance – 10uF, measured – 15uF, ESR – 2.1Ω, deviation: +50%

C613: rated capacitance – 0.68uF, measured – 0.65uF, ESR – N/A, deviation: -4%

C614: rated capacitance – 0.68uF, measured – 0.65uF, ESR – N/A, deviation: -4%

This circuit also has four 2SC632 prone-to-failure transistors installed in positions TR601, TR602, TR603, and TR604. I replaced them with a modern, low-noise Fairchild KSC1845. Again, ensure the new transistors have a current gain >350 and pay close attention to the pinout.

Head amplifier board - before and after

FM, MPX, and AM Unit (F-1353)

Be aware, that replacing the original capacitors in the tuner section may require subsequent tuner alignment. I do not recommend servicing the tuner section of any vintage receiver unless you have all the necessary equipment and sufficient experience in tuner alignment.

The tuner section has 13 aluminum electrolytic capacitors: C211, C221, C316, C323, C334, C341, C343, C402, C406, C407, C408, C409, and C412.

The original 1uF e-caps were replaced with film polyester WIMA capacitors. The remaining aluminum e-caps were replaced with Nichicon UPW low-impedance capacitors.

Test results on original capacitors removed from the FM, MPX, and AM unit:

C211: rated capacitance – 10uF, measured – 17uF, ESR – 5.6Ω, deviation: +70%

C221: rated capacitance – 3.3uF, measured – 4.1uF, ESR – 3.9Ω, deviation: +24%

C316: rated capacitance – 1uF, measured – 1.4uF, ESR – 5.4Ω, deviation: +40%

C323: rated capacitance – 3.3uF, measured – 4.5uF, ESR – 4.1Ω, deviation: +36%

C334: rated capacitance – 47uF, measured – 67uF, ESR – 2.1Ω, deviation: +43%

C341: rated capacitance – 100uF, measured – 129uF, ESR – 1.02Ω, deviation: +29%

C343: rated capacitance – 3.3uF, measured – 5.1uF, ESR – 3.8Ω, deviation: +55%

C402: rated capacitance – 3.3uF, measured – 4.2uF, ESR – 3.8Ω, deviation: +27%

C406: rated capacitance – 10uF, measured – 16uF, ESR – 2.2Ω, deviation: +60%

C407: rated capacitance – 1uF, measured – 1.2uF, ESR – 6.4Ω, deviation: +20%

C408: rated capacitance – 10uF, measured – 14uF, ESR – 2.6Ω, deviation: +40%

C409: rated capacitance – 10uF, measured – 15uF, ESR – 5.7Ω, deviation: +50%

C412: rated capacitance – 1uF, measured – 1.3uF, ESR – 6.6Ω, deviation: +30%

FM, MPX, AM unit - before and after

Dial and Meter Lamps

The original dial and meter incandescent bulbs were replaced with warm white LED lamps to maintain the original look and decrease heat.

Audio Adjustments

The service manual has no instructions for main amplifier adjustment. I used the following procedure to adjust the amplifier. 

First of all, I checked the center voltage at fuses F801/F802 on the main amplifier board. The measured voltage was 46.64V. Then I set the collector voltage of transistor TR811 to half of the center voltage with trimming resistor VR801. Similarly, the collector voltage of transistor TR813 was set to half of the center voltage with trimming resistor VR802. After that, the oscilloscope was connected across the speaker terminals, and a sine-wave signal of 1kHz was applied to the AUX jacks. The audio signal level was increased until the audio output on the oscilloscope began to clip. The trimming resistor VR801 (VR802) was slightly adjusted for equal clipping on each channel.

The idling current was set to 15mA in each channel with trimming resistor VR803 (VR804).

Center voltage measured at fuses F801/F802

Collector voltage of transistors TR811 and TR813 (half of the center voltage)

Idling current of the left and right channels

Output Power Test

The final output power test was performed at the end of my restoration. The amplifier was loaded with a low inductance 8Ω/100W dummy resistor for each channel. The oscilloscope was connected across the speaker terminals, and a sine-wave signal of 1kHz was applied to the AUX jacks. The output sine-wave signal was perfectly symmetrical on both channels with no clipping up to 9.82 VRMS (left channel) and 9.87 VRMS (right channel). It corresponds to the output power of 12.1W on the left channel and 12.2W on the right channel.

Output power test

As usual, all the knobs and the front panel were gently cleaned in warm water with dish soap. All controls have been cleaned with DeoxIT 5% contact cleaner and lubricated with DeoxIT FaderLube 5% spray. The wood case was treated with Howard's Restor-A-Finish.

The final result can be seen in the photos below. The sound is beautiful, clear, and warm. Please watch a short demo video at the end of this post. Thank you for reading.

Sansui 310 - after restoration

Demo video after repair & restoration