Pioneer SX-626 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-626

SN: TF3744403

Today I'm showcasing a Pioneer SX-626 vintage receiver that came in for restoration. The SX-626 was manufactured from 1972 to 1974. It produces 20 watts per channel into 8 ohms with no more than 1% total harmonic distortion. The damping factor at 8 ohms is 40. The retail price in 1974 was $329.95 (Ref. Stereo Review, Stereo Directory 1974, page 48).

According to the Pioneer database, this particular unit was manufactured in June 1973.

Power Supply Board (AWR-006D)

The power supply board has 4 aluminum electrolytic capacitors: C5, C9, C10, and C12. 

All of them were replaced with Nichicon UPW/UPM low-impedance capacitors.

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 board:

C5: rated capacitance – 100uF, measured – 115uF, ESR – 4.1Ω, deviation: +15%

C9: rated capacitance – 330uF, measured – 298uF, ESR – 1.46Ω, deviation: -10%

C10: rated capacitance – 330uF, measured – 325uF, ESR – 1.21Ω, deviation: -2%

C12: rated capacitance – 100uF, measured – 111uF, ESR – 1.45Ω, deviation: +11%

I also replaced all transistors on this board to improve the reliability of the power supply. Below is a list of original and replacement transistors that I have used.

Q1: NPN, 2SD313 (original), replaced with a new Fairchild KSC2073TU

Q2: NPN, 2SC945 (original), replaced with a new Fairchild KSC2383YTA

Q3: NPN, 2SD313 (original), replaced with a new Fairchild KSC2073TU

Power supply board - before and after

Head Amplifier Board (W21-004-B)

The head amplifier board features two solid tantalum capacitors (C1, C2) installed in the input signal path, two sky-blue Sanyo coupling capacitors (C7, C8) installed in the output signal path, and four aluminum electrolytic capacitors (C9, C10, C15, C16).

The original tantalum capacitors were replaced with low-leakage Nichicon UKL caps. Two coupling capacitors were replaced with high-quality film polyester WIMA caps. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW/UPM caps.

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

C1: rated capacitance – 10uF, measured – 11uF, ESR – 3.4Ω, deviation: +10%

C2: rated capacitance – 10uF, measured – 12uF, ESR – 2.9Ω, deviation: +20%

C7: rated capacitance – 0.47uF, measured – 0.51uF, ESR – N/A, deviation: +9%

C8: rated capacitance – 0.47uF, measured – 0.44uF, ESR – N/A, deviation: -6%

C9: rated capacitance – 3.3uF, measured – 3.9uF, ESR – 2.8Ω, deviation: +18%

C10: rated capacitance – 3.3uF, measured – 3.6uF, ESR – 2.2Ω, deviation: +9%

C15: rated capacitance – 100uF, measured – 110uF, ESR – 1.4Ω, deviation: +10%

C16: rated capacitance – 100uF, measured – 109uF, ESR – 1.1Ω, deviation: +9%

The original NPN transistors installed on this board are the infamous 2SC1312. The aging of these transistors usually leads to static noise. I replaced them with modern, low-noise Fairchild KSA1845s. Note the pinout of the new transistors. The original transistor is BCE, and the new one is ECB.

Head amplifier board - before and after

Control Amplifier Board (AWG-008-A)

The control amplifier board has 12 aluminum electrolytic capacitors (C3, C4, C7, C8, C11, C12, C19, C20, C23, C24, C25, C26), and two sky-blue Sanyo coupling capacitors (C27, C28) installed in the output signal path.

The original coupling capacitors were replaced with low-leakage Nichicon UKL caps. The ordinary electrolytic capacitors C11 and C12 were also replaced with low-leakage Nichicon UKL capacitors to reduce noise in the circuit. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW/UPM caps.

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

C3: rated capacitance – 47uF, measured – 63uF, ESR – 1.78Ω, deviation: +34%

C4: rated capacitance – 47uF, measured – 61uF, ESR – 1.52Ω, deviation: +30%

C7: rated capacitance – 47uF, measured – 49uF, ESR – 1.68Ω, deviation: +4%

C8: rated capacitance – 47uF, measured – 50uF, ESR – 1.84Ω, deviation: +6%

C11: rated capacitance – 4.7uF, measured – 5.7uF, ESR – 2.8Ω, deviation: +21%

C12: rated capacitance – 4.7uF, measured – 5.6uF, ESR – 2.8Ω, deviation: +19%

C19: rated capacitance – 10uF, measured – 14uF, ESR – 1.9Ω, deviation: +40%

C20: rated capacitance – 10uF, measured – 14uF, ESR – 1.4Ω, deviation: +40%

C23: rated capacitance – 100uF, measured – 133uF, ESR – 1.73Ω, deviation: +33%

C24: rated capacitance – 100uF, measured – 134uF, ESR – 1.67Ω, deviation: +34%

C25: rated capacitance – 47uF, measured – 62uF, ESR – 1.3Ω, deviation: +32%

C26: rated capacitance – 47uF, measured – 60uF, ESR – 0.86Ω, deviation: +28%

C27: rated capacitance – 2.2uF, measured – 2.4uF, ESR – 3.7Ω, deviation: +9%

C28: rated capacitance – 2.2uF, measured – 2.7uF, ESR – 1.6Ω, deviation: +23%

The original PNP transistors (Q3 and Q4) installed on this board are notorious 2SA725. The aging of these transistors usually leads to shot noise. I replaced both of them with a modern low-noise Fairchild KSA992. Note the pinout of the new transistors. The original transistor is BCE, and the new one is ECB.

The original NPN transistors (Q5 and Q6) installed on this board are the infamous 2SC1312. The aging of these transistors usually leads to static noise. I replaced them with modern, low-noise Fairchild KSA1845s. Note the pinout of the new transistors. The original transistor is BCE, and the new one is ECB.

Control amplifier board - before and after

Power Amplifier Board (AWH-002-D)

The power amplifier board has 11 aluminum electrolytic capacitors (C1, C2, C3, C4, C5, C6, C7, C8, C11, C12, C21) and two coupling capacitors (C13, C14).

The ordinary e-caps installed in the signal path (C1, C2, C5, C6) were replaced with low-leakage Nichicon UKL caps. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW/UPM caps. I left the two original coupling capacitors on this board since their measured capacitance is within ±5% with low ESR.

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

C1: rated capacitance – 3.3uF, measured – 4.8uF, ESR – 3.4Ω, deviation: +46%

C2: rated capacitance – 3.3uF, measured – 4.7uF, ESR – 2.7Ω, deviation: +42%

C3: rated capacitance – 100uF, measured – 116uF, ESR – 0.78Ω, deviation: +16%

C4: rated capacitance – 100uF, measured – 115uF, ESR – 0.41Ω, deviation: +15%

C5: rated capacitance – 3.3uF, measured – 5.0uF, ESR – 3.4Ω, deviation: +52%

C6: rated capacitance – 3.3uF, measured – 4.4uF, ESR – 3.4Ω, deviation: +33%

C7: rated capacitance – 100uF, measured – 113uF, ESR – 0.58Ω, deviation: +13%

C8: rated capacitance – 100uF, measured – 112uF, ESR – 1.08Ω, deviation: +12%

C11: rated capacitance – 100uF, measured – 103uF, ESR – 1.47Ω, deviation: +3%

C12: rated capacitance – 100uF, measured – 102uF, ESR – 1.65Ω, deviation: +2%

C13: rated capacitance – 2200uF, measured – 2108uF, ESR – 0.43Ω, deviation: -4%

C14: rated capacitance – 2200uF, measured – 2127uF, ESR – 0.48Ω, deviation: -3%

C21: rated capacitance – 100uF, measured – 115uF, ESR – 1.75Ω, deviation: +15%

Power amplifier board - before and after

FM/AM Tuner Board (AWE-011)

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 board has 5 sky-blue Sanyo capacitors (C53, C55, C57, C66, C67) and 13 aluminum electrolytic capacitors (C23, C27, C29, C30, C33, C36, C38, C45, C54, C58, C61, C65, C73).

All sky-blue Sanyo capacitors (except C53) were replaced with low-leakage Nichicon UKL caps.  Capacitor C53, as well as capacitors C27, C30, C38, C58, and C61, were replaced with high-quality WIMA polyester film capacitors. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW/UPM caps.

Test results on original capacitors removed from the FM/AM tuner board:

C23: rated capacitance – 4.7uF, measured – 5.2uF, ESR – 3.4Ω, deviation: +11%

C27: rated capacitance – 1uF, measured – 1.2uF, ESR – 1.56Ω, deviation: +20%

C29: rated capacitance – 4.7uF, measured – 5.8uF, ESR – 3.8Ω, deviation: +23%

C30: rated capacitance – 1uF, measured – 1.3uF, ESR – 1.59Ω, deviation: +30%

C33: rated capacitance – 100uF, measured – 134uF, ESR – 1.06Ω, deviation: +34%

C36: rated capacitance – 10uF, measured – 13uF, ESR – 1.42Ω, deviation: +30%

C38: rated capacitance – 0.47uF, measured – 0.51uF, ESR – N/A, deviation: +9%

C45: rated capacitance – 10uF, measured – 13uF, ESR – 1.86Ω, deviation: +30%

C53: rated capacitance – 0.47uF, measured – 0.54uF, ESR – N/A, deviation: +15%

C54: rated capacitance – 4.7uF, measured – 6.1uF, ESR – 2.4Ω, deviation: +30%

C55: rated capacitance – 3.3uF, measured – 4.1uF, ESR – 5.2Ω, deviation: +24%

C57: rated capacitance – 3.3uF, measured – 3.9uF, ESR – 4.4Ω, deviation: +18%

C58: rated capacitance – 1uF, measured – 1.2uF, ESR – 3.1Ω, deviation: +20%

C61: rated capacitance – 0.47uF, measured – 0.52uF, ESR – N/A, deviation: +11%

C65: rated capacitance – 100uF, measured – 125uF, ESR – 2.4Ω, deviation: +25%

C66: rated capacitance – 2.2uF, measured – 2.4uF, ESR – 5.9Ω, deviation: +9%

C67: rated capacitance – 2.2uF, measured – 2.7uF, ESR – 3.1Ω, deviation: +23%

C73: rated capacitance – 47uF, measured – 59uF, ESR – 0.82Ω, deviation: +26%

The stereo lamp driver transistor Q19 was replaced with a new Fairchild KSC2383YTA. The original transistor installed in this position is 2SC968, and it often fails.

FM/AM tuner board - before and after

Filter Capacitor

The original filter capacitor was replaced with a new Nichicon LKG e-cap. The LKG series was designed for high-grade audio equipment and, in my opinion, is the best replacement for the original filter and/or coupling capacitors on the market. The new Nichicon LKG e-cap has the same diameter as the original, so the same clamp can be used to attach it to the chassis.

Test results on original filter capacitor:

C3: rated capacitance – 3300uF, measured – 3915uF, ESR – 3.8Ω, deviation: +19%

New Nichicon LKG filter capacitor

Dial and Meter Lamps

The original dial and meter lights on this receiver were replaced with warm white LED lamps to keep the original look.

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 voltage at pin +B1 on the main amplifier board. The measured voltage was 56.60V. Then I set the collector voltage of transistor Q11 to half the voltage measured at pin +B1 with trimming resistor VR1. Similarly, the collector voltage of transistor Q12 was set to half the voltage measured at pin +B2 with trimming resistor VR2. 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. Trimming resistor VR1 (VR2) was slightly adjusted to ensure uniform clipping on each channel.

The Bias was adjusted to ~17mV across each emitter resistor R33 (R34) with the trimming resistor VR3 (VR4).

Voltage at pin +B1

Collector voltage of transistor Q11

Bias on the left and right channels after restoration

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 13.11 VRMS (left channel) and 12.97 VRMS (right channel). It corresponds to the output power of 21.5W on the left channel and 21.0W 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 receiver looks and sounds great again.  Please watch a short demo video at the end of this post. Thank you for reading.

Pioneer SX-626 - after restoration

Demo video after repair & restoration