Pioneer SX-770 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-770

SN: RL3311218

I bought this receiver a few months ago from a kind old gentleman for a fair price. The receiver is in excellent cosmetic condition and I always wanted to have this model in my private collection. 

The SX-770 was introduced in 1970 just before the silver face era and was manufactured in 1970-71. It produces 15 watts per channel into 8 ohms with no more than 0.8% total harmonic distortion. The list price in 1970 was $249.95 (Ref. High Fidelity Magazine, April 1970, page 33).

As Pioneer states in its brochure: "The SX-770 has a uniquely attractive smoked dial glass. AM and FM indicators light up only during the reception, and the tuning needle glides over a tuning scale illuminated in blue. With its oiled walnut cabinet and black front panel, the SX-770 makes a handsome addition to the décor of any room". And I totally agree with that! It has a fantastic design, clear and warm sound, and a so-called "McIntosh look". I think it is a very cool receiver from the early 70's.

According to the Pioneer database, this particular unit was manufactured in December 1971. So, it was exactly 50 years ago!

Power Supply Board (W16-024)

The power supply board (W16-024) has four aluminum electrolytic capacitors C4 thru C7. I replaced them with low impedance and high-reliability Nichicon UPW/UPM caps. All original e-caps removed from this board were tested with Atlas ESR70 capacitance meter and the results are below. All of them are outside the factory capacitance tolerance +/- 20%.

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

C4: rated capacitance – 100uF, measured – 128uF, ESR – 0.08Ω, deviation: +28%

C5: rated capacitance – 220uF, measured – 297uF, ESR – 0.01Ω, deviation: +35%

C6: rated capacitance – 220uF, measured – 288uF, ESR – 0.01Ω, deviation: +31%

C7: rated capacitance – 100uF, measured – 151uF, ESR – 0.26Ω, deviation: +51%

The NPN transistor Q2 installed on this board is 2SC870. This is a well-known transistor prone to failure. I replaced it with a modern low noise Fairchild KSC1845. Watch the pinout on the replacement transistor. The original transistor (2SC870) is BCE and the new one (KSC1845) is ECB.

Power supply board - before and after

Control Amplifier Board (W15-033)

Four original coupling capacitors (C1, C2, C7, C8) installed in the signal path on this board are failure-prone sky blue Sanyo e-caps. As expected, the measured capacitance of all e-caps is outside of the factory capacitance tolerance. The coupling capacitor C8 is still within the factory tolerance but its ESR is abnormally high. For more information about sky blue Sanyo e-caps refer to my previous post on Pioneer SX-828 restoration. Two e-caps installed in the input signal path (C1, C2) were replaced with film polyester WIMA MKS2 caps. And other two e-caps installed in the output signal path (C7, C8) were replaced with modern low leakage Nichicon UKL capacitors. The measured capacitance of the remaining four aluminum e-caps are also outside of the factory capacitance tolerance and I replaced them with low impedance Nichicon UPW/UPM caps.

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

C1: rated capacitance – 0.47uF, measured – 0.61uF, ESR – N/A, deviation: +30%

C2: rated capacitance – 0.47uF, measured – 0.79uF, ESR – N/A, deviation: +68%

C5: rated capacitance – 100uF, measured – 139uF, ESR – 5.9Ω, deviation: +39%

C6: rated capacitance – 100uF, measured – 153uF, ESR – 0.84Ω, deviation: +53%

C7: rated capacitance – 2.2uF, measured – 2.7uF, ESR – 2.8Ω, deviation: +23%

C8: rated capacitance – 2.2uF, measured – 2.1uF, ESR – 13.6Ω, deviation: -5%

C17: rated capacitance – 47uF, measured – 66uF, ESR – 0.24Ω, deviation: +40%

C18: rated capacitance – 47uF, measured – 65uF, ESR – 0.21Ω, deviation: +38%

Two NPN transistors installed on this board are 2SC870 (Q1, Q2) and I replaced them with a modern low noise Fairchild KSC1845.

TREBLE and BASS control potentiometers were cleaned and lubricated with DeoxIT FaderLube 5% spray.

Control amplifier board - before and after

Push Switch Unit B (W15-087)

There is only one electrolytic capacitor C13 on this board. The original e-cap installed in this position is a notorious sky blue Sanyo e-cap. I replaced it with a film polyester WIMA MKS2 cap. 

Test results on original capacitors removed from the push switch unit B:

C13: rated capacitance – 0.47uF, measured – 0.58uF, ESR – N/A, deviation: +23%

A small metal mounting plate covers three push switches and limits access to the openings for cleaning and lubricating. Fortunately, this plate can be easily removed to get better access to these switches.

Push switch unit B - before and after

Head Amplifier Board (W15-032)

The head amplifier board has eight aluminum electrolytic capacitors: C1, C2, C7, C8, C9, C10, C15, and C16. Four of them are installed in the signal path. The original e-caps C1 and C2  installed in the input signal path are just ordinary aluminum e-caps. I replaced them with modern low leakage Nichicon UKL capacitors to improve a signal-to-noise ratio. The original e-caps C7 and C8 installed in the output signal path are notorious sky blue Sanyo e-caps. These were replaced with high-quality film polyester WIMA MKS2 caps. The remaining four aluminum e-caps were replaced with low impedance Nichicon UPW caps.

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

C1: rated capacitance – 10uF, measured – 13uF, ESR – 3.2Ω, deviation: +30%

C2: rated capacitance – 10uF, measured – 13uF, ESR – 4.1Ω, deviation: +30%

C7: rated capacitance – 0.47uF, measured – 0.59uF, ESR – N/A, deviation: +26%

C8: rated capacitance – 0.47uF, measured – 0.61uF, ESR – N/A, deviation: +30%

C9: rated capacitance – 33uF, measured – 48uF, ESR – 1.1Ω, deviation: +46%

C10: rated capacitance – 33uF, measured – 47uF, ESR – 1.2Ω, deviation: +42%

C15: rated capacitance – 100uF, measured – 119uF, ESR – 0.01Ω, deviation: +19%

C16: rated capacitance – 100uF, measured – 163uF, ESR – 0.01Ω, deviation: +63%

Four NPN transistors installed on this board are 2SC871 (Q1, Q2) and 2SC870 (Q3, Q4). Both of them are well-known transistors prone to failure. I replaced them with a modern low-noise Fairchild KSC1845. Before installation, the new transistors were carefully matched by current gain and base-emitter voltage. Pay attention to the current gain in KSC1845 transistors while selecting them for replacement. The minimum current gain of the 2SC870 transistor is 250 whereas the 2SC871 has a minimum current gain of 350. So, make sure that both KSC1845 transistors installed in Q1 and Q2 positions have a current gain of at least 350 or higher. Also, watch the pinout on replacement transistors. The original 2SC870/871 transistor is BCE and the new one (KSC1845) is ECB.

Head amplifier board - before and after

Main Amplifier Board (W15-085)

The main amplifier board has twelve radial electrolytic capacitors C1 thru C8, C11 thru C14, and two axial coupling capacitors C17, C18. The original e-caps C3, C4, C5, C6 installed in the signal path are just ordinary aluminum e-caps. I replaced them with modern low leakage Nichicon UKL capacitors to improve a signal-to-noise ratio. The original axial coupling capacitors C17 and C18 installed in the output signal path were replaced with new Vishay axial e-caps (021 ASM series). The remaining eight aluminum e-caps were replaced with low impedance Nichicon UPW caps.

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

C1: rated capacitance – 100uF, measured – 139uF, ESR – 0.31Ω, deviation: +39%

C2: rated capacitance – 100uF, measured – 156uF, ESR – 0.24Ω, deviation: +56%

C3: rated capacitance – 10uF, measured – 12uF, ESR – 3.1Ω, deviation: +20%

C4: rated capacitance – 10uF, measured – 12uF, ESR – 2.2Ω, deviation: +20%

C5: rated capacitance – 22uF, measured – 27uF, ESR – 0.65Ω, deviation: +23%

C6: rated capacitance – 22uF, measured – 27uF, ESR – 0.75Ω, deviation: +23%

C7: rated capacitance – 47uF, measured – 65uF, ESR – 0.97Ω, deviation: +38%

C8: rated capacitance – 47uF, measured – 63uF, ESR – 0.92Ω, deviation: +34%

C11: rated capacitance – 47uF, measured – 64uF, ESR – 0.91Ω, deviation: +36%

C12: rated capacitance – 47uF, measured – 63uF, ESR – 1.24Ω, deviation: +34%

C13: rated capacitance – 100uF, measured – 144uF, ESR – 0.18Ω, deviation: +44%

C14: rated capacitance – 100uF, measured – 149uF, ESR – 0.18Ω, deviation: +49%

C17: rated capacitance – 1000uF, measured – 1422uF, ESR – 0.01Ω, deviation: +42%

C18: rated capacitance – 1000uF, measured – 1270uF, ESR – 0.01Ω, deviation: +27%

Two NPN transistors (Q1, Q2) installed on this board are 2SC870 and I replaced them with low noise Fairchild KSC1845's.

It's hard to take a good picture of this board because it couldn't be completely released from the chassis. Some wires soldered to the board are very short. This board is still pretty convenient to service but just with limited access.

Main amplifier board - before and after

Muting Unit (W18-026)

The muting unit has two electrolytic capacitors C1 and C2. The e-cap C2 installed in the signal path was replaced with a low leakage Nichicon UKL capacitor. The e-cap C1 was replaced with a low impedance Nichicon UPW cap.

Test results on original capacitors removed from the muting unit:

C1: rated capacitance – 4.7uF, measured – 6.5uF, ESR – 2.6Ω, deviation: +38%

C2: rated capacitance – 2.2uF, measured – 3.7uF, ESR – 2.5Ω, deviation: +68%

Three NPN transistors (Q1, Q2, Q3) installed on this board are 2SC870. These were replaced with low noise Fairchild KSC1845's.

Muting unit - before and after

MPX Unit (W13-026A)

There are at least two different versions of the MPX circuit in the Pioneer SX-770: W13-024 and W13-026. This particular unit has version W13-026A. Five electrolytic capacitors are installed on this board: C1, C2, C4, C8, and C9. The e-cap C1 installed in the signal path was replaced with a low leakage Nichicon UKL capacitor. Two e-caps C8 and C9 installed in the signal path before the low pass filter are notorious sky blue Sanyo e-caps. These were replaced with film polyester WIMA MKS2 caps. The remaining e-caps were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from the MPX unit:

C1: rated capacitance – 2.2uF, measured – 3.3uF, ESR – 2.8Ω, deviation: +50%

C2: rated capacitance – 10uF, measured – 12uF, ESR – 3.1Ω, deviation: +20%

C4: rated capacitance – 3.3uF, measured – 4.6uF, ESR – 2.6Ω, deviation: +39%

C8: rated capacitance – 0.47uF, measured – 0.55uF, ESR – N/A, deviation: +17%

C9: rated capacitance – 0.47uF, measured – 0.57uF, ESR – N/A, deviation: +21%

The NPN transistor installed in position Q1 is the notorious 2SC870. I replaced it with a modern low noise Fairchild KSC1845 transistor. The other four NPN transistors (Q2, Q3, Q4, and Q5) installed on this board are prone to fail 2SC711. These were also replaced with a modern low noise Fairchild KSC1845 transistor.

MPX unit - before and after

Tuner Unit (W31-002)

The tuner unit has six aluminum electrolytic capacitors: C8, C19, C30, C32, C33, and C35. One of them (C33) is a notorious sky blue Sanyo e-cap. I replaced it with a film polyester WIMA cap. The e-cap C30 installed in the signal path was also replaced with a WIMA cap. The remaining aluminum capacitors were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from the tuner unit:

C8: rated capacitance – 10uF, measured – 13uF, ESR – 3.3Ω, deviation: +30%

C19: rated capacitance – 4.7uF, measured – 5.5uF, ESR – 3.4Ω, deviation: +17%

C30: rated capacitance – 0.47uF, measured – 0.53uF, ESR – N/A, deviation: +13%

C32: rated capacitance – 4.7uF, measured – 5.4uF, ESR – 3.4Ω, deviation: +15%

C33: rated capacitance – 1uF, measured – 1.2uF, ESR – 2.2Ω, deviation: +20%

C35: rated capacitance – 100uF, measured – 142uF, ESR – 0.32Ω, deviation: +42%

Tuner unit - before and after

Off-Board Parts

The original axial electrolytic capacitor C12 (220uF/6.3V) installed across the signal meter leads was replaced with a new Vishay axial e-cap (021 ASM series).

The original axial electrolytic capacitor C21 (4.7uF/16V) installed between the muting & MPX units was replaced with a low leakage Nichicon UKL capacitor. Note that even though the original e-cap is still within the factory capacitance tolerance the measured ESR is very high (see test data below).

The filter capacitor C15 (3300uF/50V) was tested in-circuit with Atlas ESR70 capacitance meter. It is still within the factory capacitance tolerance and has almost zero ESR. But I decided to replace it anyway with a new Nichicon LKG e-cap (3300uF/63V) as a part of preventive maintenance. The new Nichicon LKG e-cap is the same diameter as the original one but shorter. Thus, the same clamp can be used to attach it to the chassis.

Test results on original capacitors C12, C15, and C21:

C12: rated capacitance – 220uF, measured – 304uF, ESR – 0.24Ω, deviation: +38%

C15: rated capacitance – 3300uF, measured – 3890uF, ESR – 0.01Ω, deviation: +18%

C21: rated capacitance – 4.7uF, measured – 5.5uF, ESR – 12.8Ω, deviation: +17%

Original and new e-caps C21 and C12

Original and new filter capacitor C15

Dial and Program Indicator Lamps

Four program indicator lamps in the Pioneer SX-770 are Bayonet base lamps (8V, 150mA). I replaced them with new incandescent lamps. The original tuning meter lamp is also a Bayonet base lamp. This lamp is mounted very close to the plastic tuning meter and over time discolors it due to extensive heat. I replaced it with a warm white LED lamp. The LED lamp has a similar color temperature as the original incandescent lamp but produces much less heat. This will protect the plastic from discoloration.

Warm white LED lamp behind the tuning meter

To replace four dial lamps in the SX-770 the front panel and dial scale should be removed. It is actually very easy to do in comparison to most Marantz receivers from the 70's. The dial scale is mounted on the chassis with eight screws. I replaced the original incandescent lamps with warm white LED lamps.

Original dial lamps - easy to replace

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 46.10V. Then I set the collector voltage of transistor Q11 to half of the voltage measured at pin +B1 with trimming resistor VR1. Similarly, the collector voltage of transistor Q12 was set to half of the voltage measured at pin +B1 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 was increased until the audio output on the oscilloscope began to start clipping. The trimming resistor VR1 (VR2) was slightly adjusted for equal clipping on each channel.

The idle current was set to ~20mA in each channel with trimming resistor VR3 (VR4). This current can be set by measuring the voltage drop across emitter resistor R33 (R34). Since the nominal resistance of R33 (R34) is 0.5Ω, the voltage drop should be ~10mV.

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 10.77 VRMS (left channel) and 10.61 VRMS (right channel). It corresponds to the output power of 14.5W on the left channel and 14.1W 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 final result can be seen in the photos below. This receiver has a magnificent look. The sound is beautiful, clear and lovely warm. This is a very underrated receiver from the early 70's. I am very happy to have it in my private collection in excellent cosmetic condition. Please watch a short demo video at the end of this post. Thank you for reading.

Pioneer SX-770 - after restoration

Demo video after repair & restoration

Pioneer SA-7500 II Stereo Integrated Amplifier Restoration

Unit: Stereo Integrated Amplifier

Manufacturer: Pioneer

Model: SA-7500 II

SN: XH36111130 

Today I'm showcasing a Pioneer SA-7500 II stereo integrated amplifier that came in for restoration. The SA-7500 II was manufactured from 1975 to 1978. It produces 40 watts per channel into 8 ohms with no more than 0.3% total harmonic distortion. The rear panel of SA-7500 II has input jacks for two turntables, two tape decks, a tuner, and an auxiliary source.

This particular unit is in perfect cosmetic condition and has no issues with electronics. I completed just routine servicing, i.e. cleaning and lubricating all the controls, replacing all electrolytic capacitors, bias adjustment checking, and final output power test. According to the Pioneer database, this particular unit was manufactured in August 1977.

Control Amplifier Board (AWG-047)

The control amplifier board AWG-047 is mounted to the sub-panel. In general, this board can be serviced without removing it from the chassis. However, two variable resistors (Bass and Treble) are assembled on the board and it is much easier to clean them when the board is being removed. So, I cleaned and lubricated both variable resistors with DeoxIT FaderLube 5% spray. The Volume variable resistor is assembled on its own small board and also mounted to the sub-panel. I cleaned it with DeoxIT FaderLube 5% spray as well. Finally, all the switches were cleaned with DeoxIT 5% contact cleaner and then lubricated with DeoxIT FaderLube 5% spray.

The original low leakage e-caps C5, C6, C9, and C10 installed in the signal path were replaced with modern low leakage Nichicon UKL capacitors. The remaining aluminum e-caps C19 and C20 were replaced with low impedance and high-reliability Nichicon UPW caps. All original e-caps removed from this board were tested with Atlas ESR70 capacitance meter and results are below. All of them except e-cap C5 are outside the factory capacitance tolerance +/- 20%.

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

C5: rated capacitance – 47uF, measured – 54uF, ESR – 1.49Ω, deviation: +15%

C6: rated capacitance – 47uF, measured – 57uF, ESR – 1.23Ω, deviation: +21%

C9: rated capacitance – 3.3uF, measured – 4.2uF, ESR – 1.14Ω, deviation: +27%

C10: rated capacitance – 3.3uF, measured – 4.3uF, ESR – 1.21Ω, deviation: +30%

C19: rated capacitance – 47uF, measured – 59uF, ESR – 0.05Ω, deviation: +26%

C20: rated capacitance – 47uF, measured – 57uF, ESR – 0.16Ω, deviation: +21%

Control amplifier board (AWG-047) - before and after

AF Amplifier Board (AWK-079)

The power supply, protection, phono equalizer, and power amplifier circuits are assembled on one board (AWK-079).

The power supply and protection circuit has 10 aluminum e-caps: C45 thru C48, C54 thru C56, and C58 thru C60. All of them were replaced with low impedance Nichicon UPW/UPM caps.

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

C45: rated capacitance – 330uF, measured – 372uF, ESR – 0.08Ω, deviation: +13%

C46: rated capacitance – 330uF, measured – 373uF, ESR – 0.08Ω, deviation: +13%

C47: rated capacitance – 100uF, measured – 117uF, ESR – 0.12Ω, deviation: +17%

C48: rated capacitance – 2.2uF, measured – 2.1uF, ESR – 4.9Ω, deviation: -5%

C54: rated capacitance – 330uF, measured – 356uF, ESR – 0.01Ω, deviation: +8%

C55: rated capacitance – 330uF, measured – 344uF, ESR – 0.01Ω, deviation: +4%

C56: rated capacitance – 220uF, measured – 227uF, ESR – 0.02Ω, deviation: +3%

C58: rated capacitance – 47uF, measured – 52uF, ESR – 0.24Ω, deviation: +11%

C59: rated capacitance – 100uF, measured – 107uF, ESR – 0.08Ω, deviation: +7%

C60: rated capacitance – 220uF, measured – 263uF, ESR – 0.01Ω, deviation: +20%

The phono equalizer circuit has four low leakage e-caps installed in the input (C1, C2) and output (C23, C24) signal path, two filtering aluminum e-caps (C61, C62), and two DC blocking aluminum e-caps (C9, C10) in the RIAA feedback loop. The e-caps installed in the input signal path were replaced with high-quality film polyester WIMA MKS2 caps. The e-caps installed in the output signal path were replaced with modern low leakage Nichicon UKL capacitors. Pay attention to the polarity of e-caps C23 and C24 installed in the output signal path. The polarity on the silkscreen is backward! The remaining four aluminum e-caps were replaced with low impedance Nichicon UPW caps.

The polarity of e-caps C23 & C24 is backward on the silkscreen

Test results on original capacitors removed from the phono equalizer circuit:

C1: rated capacitance – 1uF, measured – 1.1uF, ESR – 4.4Ω, deviation: +10%

C2: rated capacitance – 1uF, measured – 1.1uF, ESR – 5.1Ω, deviation: +10%

C9: rated capacitance – 220uF, measured – 264uF, ESR – 0.04Ω, deviation: +20%

C10: rated capacitance – 220uF, measured – 270uF, ESR – 0.05Ω, deviation: +23%

C23: rated capacitance – 2.2uF, measured – 2.8uF, ESR – 2.1Ω, deviation: +27%

C24: rated capacitance – 2.2uF, measured – 2.7uF, ESR – 2.4Ω, deviation: +23%

C61: rated capacitance – 220uF, measured – 265uF, ESR – 0.04Ω, deviation: +21%

C62: rated capacitance – 220uF, measured – 254uF, ESR – 0.06Ω, deviation: +16%

Phono equalizer circuit - before and after

The power amplifier circuit has two low leakage e-caps C31, C32 installed in the signal path, and two aluminum e-caps C35, C36 installed in the negative feedback loop. The e-caps installed in the input path were replaced with modern low leakage Nichicon UKL capacitors. And the remaining two aluminum e-caps were replaced with low impedance Nichicon UPW caps.

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

C31: rated capacitance – 2.2uF, measured – 2.8uF, ESR – 2.1Ω, deviation: +27%

C32: rated capacitance – 2.2uF, measured – 2.7uF, ESR – 2.1Ω, deviation: +23%

C35: rated capacitance – 47uF, measured – 56uF, ESR – 0.74Ω, deviation: +19%

C36: rated capacitance – 47uF, measured – 55uF, ESR – 0.79Ω, deviation: +17%

AF amplifier board (AWK-079) - before and after

Bias Adjustments

The bias in Pioneer SA-7500 II is measured across two emitter resistors R57/R59 on the left channel (terminals TP1 and TP2) and across R58/R60 on the right channel (terminals TP3 and TP4). According to the service manual, the DC voltmeter reading tolerance is from 10mV to 70mV. This model has no trimmers for bias adjustments. Instead, it has one jumper per channel which should be cut if the voltage is less than 10mV. I measured the bias in each channel and the DC voltmeter reading was close to the middle of factory tolerance. So, no jumper leads were cut.

Bias on the left and right channel 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 18.96 VRMS (left channel) and 19.23 VRMS (right channel). It corresponds to the output power of 44.9W on the left channel and 46.2W 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 knobs were also slightly polished by Mothers Mag & Aluminum polish to remove some small spots of aluminum oxidation.

The final result can be seen in the photos below. This amplifier looks really very cool: clean and shiny. The sound is rich and very detailed. Please watch a short demo video at the end of this post. Thank you for reading.

Pioneer SA-7500 II - after restoration

Demo video after repair & restoration

Pioneer SX-750 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-750

SN: YC3683249S

Today I'm showcasing a Pioneer SX-750 stereo receiver that came in for restoration. The SX-750 was and still is one of the most popular and well-respected receivers from the 70's. It was initially introduced in 1976 and made through 1978. The receiver produces 50 watts per channel into 8 ohms with no more than 0.1% total harmonic distortion. The unit is quite heavy and weighs about 30 pounds without a package. As Pioneer states in its 1976's brochure "for a practically-priced integrated receiver, Pioneer's SX-750 delivers an astonishing amount of state-of-the-art perfection. It is innovative, attractive, and value-packed". And it's hard to argue! According to the Pioneer database, this particular unit was manufactured in March 1978.

The restoration of the smaller brothers of SX-750 can be found on my blog here: Pioneer SX-450 receiver restoration and Pioneer SX-650 receiver restoration.

This particular receiver came in for a restoration with a broken original Speaker Selector/Power switch. The owner took it up to a local repair shop to fix this issue but the technician just simply bypassed the original switch with a new one. The new switch was assembled on the left side panel. This is definitely not the best solution to the problem but this switch is a common issue on those Pioneer receivers. The switch can't be fixed by cleaning and there are no more new replacements available on the market. The best solution would be to disassemble the switch, clean up carbonized contacts with very fine sandpaper and then assemble it. I have tried to repair such a switch several times in the past but the reliability of the repaired switch is always in question. I usually prefer to replace it with a new switch if it is available. So, the owner decided to leave it alone and replace it probably later.

Bypassed Speaker Selector/Power switch - installed in the repair shop

Power Supply & Protection Assembly (AWR-099)

The power supply & protection circuits are assembled on one board (AWR-099). This board has 17 aluminum electrolytic capacitors: C1 thru C13, and C15 thru C18.

 

The original coupling capacitors C1 & C2 installed on this board are failure-prone sky blue Sanyo e-caps. As expected, the measured capacitance of these two e-caps is outside of factory capacitance tolerance. For more information about sky blue Sanyo e-caps refer to my previous post on Pioneer SX-828 restoration. I replaced them with high-quality film polyester Kemet capacitors.

The original aluminum e-caps C17 and C18 are rated at 330uF/50V. However, these two capacitors are running very close to their maximum rated voltage in the circuit. As a result, they are aging very fast and can completely fail at any time. I replaced them with 330uF/63V low impedance and high-reliability Nichicon UPM caps. It will substantially improve the reliability and extend a capacitor lifespan.

As can be seen from the test results below, at least five more original e-caps from this board are also outside of factory capacitance tolerance. I replaced all remaining aluminum e-caps with low impedance Nichicon UPW/UPM caps.

Test results on original capacitors removed from the power supply & protection board:

C1: rated capacitance – 0.22uF, measured – 0.29uF, ESR – N/A, deviation: +32%

C2: rated capacitance – 0.22uF, measured – 0.32uF, ESR – N/A, deviation: +46%

C3: rated capacitance – 330uF, measured – 362uF, ESR – 0.12Ω, deviation: +10%

C4: rated capacitance – 330uF, measured – 374uF, ESR – 0.11Ω, deviation: +13%

C5: rated capacitance – 3.3uF, measured – 4.2uF, ESR – 0.8Ω, deviation: +27%

C6: rated capacitance – 220uF, measured – 218uF, ESR – 0.08Ω, deviation: -1%

C7: rated capacitance – 220uF, measured – 151uF, ESR – 0.12Ω, deviation: -31%

C8: rated capacitance – 100uF, measured – 73uF, ESR – 0.12Ω, deviation: -27%

C9: rated capacitance – 100uF, measured – 95uF, ESR – 0.13Ω, deviation: -5%

C10: rated capacitance – 100uF, measured – 95uF, ESR – 0.14Ω, deviation: -5%

C11: rated capacitance – 4.7uF, measured – 6.1uF, ESR – 1.48Ω, deviation: +30%

C12: rated capacitance – 220uF, measured – 138uF, ESR – 0.14Ω, deviation: -37%

C13: rated capacitance – 100uF, measured – 95uF, ESR – 0.04Ω, deviation: -5%

C15: rated capacitance – 100uF, measured – 84uF, ESR – 0.16Ω, deviation: -16%

C16: rated capacitance – 100uF, measured – 90uF, ESR – 0.06Ω, deviation: -10%

C17: rated capacitance – 330uF, measured – 261uF, ESR – 0.01Ω, deviation: -21%

C18: rated capacitance – 330uF, measured – 185uF, ESR – 0.01Ω, deviation: -44%

The original PNP transistor 2SB507 installed on this board is running pretty hot under normal operating conditions (by circuit design) but has no heat sink. I mounted a low-profile heat sink on this transistor to improve heat dissipation. The heat sink is small enough and doesn't touch any nearby components. The manufacturer's part number is 577404B00000G (Aavit).

Low profile heat sink mounted on original PNP transistor Q12

Power supply & protection assembly (AWR-099) - before and after

Filter & Muting Assembly (AWM-094)

The filter and muting circuits are assembled on one board (AWM-094). The access to the foil side of this board is blocked by a bunch of wires. I recommend removing the metal angle supporting this board to get better access. Otherwise, there is a high risk of touching some of these wires with a hot soldering iron. Don't forget to screw back a small ground plate with a wire coming from the power supply & protection board (pin #12) after the servicing is done. Also, re-flow the pad under the screw to improve ground contact.

This board has three solid tantalum capacitors C1, C2, C7, and two aluminum electrolytic capacitors C3 and C5. The capacitors C1 and C2 installed in the signal path after the low pass filter were replaced with modern low leakage Nichicon UKL capacitors. The capacitor C7 was replaced with a high-quality film polyester WIMA MKS2 cap. And the remaining two capacitors C3 and C5 were replaced with low impedance Nichicon UPW/UPM caps.

Test results on original capacitors removed from the filter and muting board:

C1: rated capacitance – 2.2uF, measured – 2.1uF, ESR – 4.1Ω, deviation: -5%

C2: rated capacitance – 2.2uF, measured – 2.2uF, ESR – 3.2Ω, deviation: 0%

C3: rated capacitance – 10uF, measured – 12uF, ESR – 0.56Ω, deviation: +20%

C5: rated capacitance – 100uF, measured – 87uF, ESR – 0.06Ω, deviation: -13%

C7: rated capacitance – 1uF, measured – 1uF, ESR – 1.12Ω, deviation: 0%

Filter & muting assembly (AWM-094) - before and after

Tuner, AF, & Control Assembly (AWE-073)

The tuner, phono equalizer, and microphone circuits are all assembled on the same board (AWE-073).

The phono equalizer circuit has four solid tantalum e-caps installed in the input (C82, C83) and output (C98, C99) signal path, two filtering aluminum e-caps (C84, C85), and two DC blocking aluminum e-caps (C88, C89) in the RIAA feedback loop. All original tantalum capacitors were replaced with low leakage Nichicon UKL caps. The remaining four aluminum e-caps were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from the phono equalizer circuit:

C82: rated capacitance – 2.2uF, measured – 2.3uF, ESR – 1.48Ω, deviation: +5%

C83: rated capacitance – 2.2uF, measured – 2.2uF, ESR – 1.85Ω, deviation: 0%

C84: rated capacitance – 220uF, measured – 278uF, ESR – 0.02Ω, deviation: +26%

C85: rated capacitance – 220uF, measured – 280uF, ESR – 0.02Ω, deviation: +27%

C88: rated capacitance – 100uF, measured – 112uF, ESR – 0.31Ω, deviation: +12%

C89: rated capacitance – 100uF, measured – 112uF, ESR – 0.32Ω, deviation: +12%

C98: rated capacitance – 2.2uF, measured – 2.2uF, ESR – 2.71Ω, deviation: 0%

C99: rated capacitance – 2.2uF, measured – 2.1uF, ESR – 2.22Ω, deviation: -5%

Phono equalizer circuit - before and after

The microphone circuit in the Pioneer SX-750 is a two-stage transistor amplifier. The original PNP transistor installed in the 1st stage is a notorious 2SA725 transistor. This transistor becomes very noisy over time and I replaced it with a modern low noise Fairchild KSA992 transistor. The original NPN transistor installed in the 2nd stage is a 2SC1344 transistor. This transistor is also prone to failure. I replaced it with a modern low noise Fairchild KSC1845 transistor.

According to the service manual, the input (C102) and output (C105) coupling capacitors in the microphone circuit are supposed to be solid tantalum e-caps. However, in this particular unit, the tantalum e-cap was installed only in the output position. In the input position, a solid low leakage aluminum e-cap was installed. I replaced both coupling capacitors with modern low leakage Nichicon UKL caps.

The aluminum e-cap C106 was originally rated at 220uF/35V. I replaced it with a 220uF/50V low impedance Nichicon UPW cap. The maximum voltage rating was increased since this capacitor is running very close to its maximum rated voltage in the circuit.

Test results on original capacitors removed from the microphone circuit:

C102: rated capacitance – 2.2uF, measured – 2.6uF, ESR – 4.01Ω, deviation: +18%

C105: rated capacitance – 2.2uF, measured – 2.5uF, ESR – 3.86Ω, deviation: +14%

C106: rated capacitance – 220uF, measured – 200uF, ESR – 0.02Ω, deviation: -9%

Microphone circuit -before and after (sorry, it's very hard to take a good picture of this circuit, two transistors (Q11, Q12) are located behind the rotary switch)

Tone Control Assembly (AWG-046)

The tone control board has two coupling Sanyo e-caps (C1, C2) installed in the input signal path, two low leakage aluminum e-caps (C9, C10) installed in the output signal path, two aluminum e-caps (C3, C4) in the feedback loop, and two filtering aluminum e-caps (C7, C8). The original coupling capacitors C1 and C2 are failure-prone sky blue Sanyo capacitors and I replaced them with high-quality film polyester WIMA MKS2 caps. Two original low leakage capacitors (C9, C10) were replaced with modern low leakage Nichicon UKL caps. And the remaining four aluminum e-caps were replaced with low impedance Nichicon UPW caps.

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

C1: rated capacitance – 0.47uF, measured – 0.73uF, ESR – N/A, deviation: +55%

C2: rated capacitance – 0.47uF, measured – 0.56uF, ESR – N/A, deviation: +19%

C3: rated capacitance – 100uF, measured – 107uF, ESR – 0.31Ω, deviation: +7%

C4: rated capacitance – 100uF, measured – 112uF, ESR – 0.24Ω, deviation: +12%

C7: rated capacitance – 220uF, measured – 244uF, ESR – 0.01Ω, deviation: +11%

C8: rated capacitance – 220uF, measured – 242uF, ESR – 0.02Ω, deviation: +10%

C9: rated capacitance – 4.7uF, measured – 6.5uF, ESR – 4.2Ω, deviation: +38%

C10: rated capacitance – 4.7uF, measured – 6.5uF, ESR – 3.6Ω, deviation: +38%

Tone control assembly (AWG-046) - before and after

Power Amplifier Assembly (AWH-046)

The power amplifier board has two solid tantalum capacitors (C1, C2), and six aluminum e-caps (C7 thru C10, C13, C14). The tantalum capacitors C1 and C2 installed in the signal path were replaced with film polyester WIMA MKS2 caps. The e-caps C7 and C8 installed in the negative feedback loop were replaced with low impedance Nichicon UPW caps. The original decoupling e-caps C9 and C10 are rated at 100uF/35V and I replaced them with 100uf/50V low impedance Nichicon UPW caps. The remaining emitter bypass e-caps C13 and C14 were also replaced with low impedance Nichicon UPW caps.

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

C1: rated capacitance – 1uF, measured – 1uF, ESR – 2.8Ω, deviation: 0%

C2: rated capacitance – 1uF, measured – 1uF, ESR – 4.2Ω, deviation: 0%

C7: rated capacitance – 220uF, measured – 263uF, ESR – 0.14Ω, deviation: +20%

C8: rated capacitance – 220uF, measured – 264uF, ESR – 0.12Ω, deviation: +20%

C9: rated capacitance – 100uF, measured – 90uF, ESR – 0.11Ω, deviation: -10%

C10: rated capacitance – 100uF, measured – 95uF, ESR – 0.12Ω, deviation: -5%

C13: rated capacitance – 220uF, measured – 271uF, ESR – 0.14Ω, deviation: +23%

C14: rated capacitance – 220uF, measured – 252uF, ESR – 0.12Ω, deviation: +15%

Power amplifier board - before and after

Dial and Stereo Indicator Lamps

Three original dial illumination lamps were replaced with new 8V/300A incandescent lamps. The original stereo indicator lamp in this unit was burned out and I replaced it with a new 8V/60mA lamp.

Idle Current Adjustment

The idle current adjustment is clearly described in the service manual. DC voltmeter should be connected between pins 1 and 6 (left channel), or between pins 11 and 16 (right channel). The bias measured across two emitter resistors R31 & R33 (left channel) or R32 & R34 (right channel) should be adjusted to ~30mV with trimmers VR1 or VR2, respectively. Since each emitter resistor is rated at 0.5Ω, the idle current can be calculated to be 30mV/(0.5Ω + 0.5Ω) = 30mA.

Bias on the left and right channel after restoration

Output Power Test

At the end of my restoration, I loaded this receiver with a low inductance 8Ω/100W dummy resistor for each channel, connected my oscilloscope across the speaker terminals, and applied a sine-wave signal of 1kHz to the AUX jacks. The output sine-wave signal was perfectly symmetrical on both channels with no clipping up to 21.23 VRMS (left channel) and 21.22 VRMS (right channel). It corresponds to the output power of 56.3W on the left channel and 56.3W 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 knobs were also slightly polished by Mothers Mag & Aluminum polish to remove some small spots of aluminum oxidation. All the pots and switches were also thoroughly cleaned with DeoxIT 5% contact cleaner and lubricated with DeoxIT FaderLube 5% spray.

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

Pioneer SX-750 - after restoration

Demo video after repair & restoration