Pioneer SX-434 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-434

SN: WB3943648Y

Today I'm showcasing a Pioneer SX-434 receiver that came in for restoration. The SX-434 is Pioneer's budget-priced stereo receiver. It produces only 15 watts per channel into 8 ohms but has an excellent build quality and a very sensitive FM tuner with low noise FET and 3-gang frequency linear variable capacitor. The total harmonic distortion is less than 0.8%. The damping factor at 8 ohms is 25. The SX-434 was manufactured from 1974 to 1976. It is a very nice-looking receiver with a warm and rich sound. The retail price in 1975 was $239.95 (Ref. Stereo Review, Stereo Directory 1975, page 45). 

According to the Pioneer database, this particular unit was manufactured in February 1976.

Power Supply Board (AWR-062)

The power supply board has 6 electrolytic capacitors C7, C8, C9, C10, C11, and C12. 

All of them were replaced with low-impedance and high-reliability Nichicon UPW/UPM caps. 

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:

C7: rated capacitance – 330uF, measured – 394uF, ESR – 0.52Ω, deviation: +19%

C8: rated capacitance – 100uF, measured – 116uF, ESR – 0.31Ω, deviation: +16%

C9: rated capacitance – 100uF, measured – 105uF, ESR – 0.42Ω, deviation: +5%

C10: rated capacitance – 100uF, measured – 127uF, ESR – 1.12Ω, deviation: +27%

C11: rated capacitance – 100uF, measured – 118uF, ESR – 0.55Ω, deviation: +18%

C12: rated capacitance – 220uF, measured – 218uF, ESR – 0.32Ω, deviation: -1%

This board is easy to service but pay attention to the old nylon snap lock supports. Over time they become brittle and can be easily broken if too much force is used to separate the board from the chassis.

Power supply board - before and after

AF Amplifier Board (AWK-034)

The cleaning and lubricating of all pots and switches are routine work on any vintage gear. Some models have very easy access to pots/switches by design, and it usually takes just 10-15 minutes to thoroughly clean all of them. However, in other models, this routine work could be a real challenge. The Pioneer SX-434 is one of them. Cleaning the pots and switches in this model is impossible without disassembling the AF amplifier board from the chassis. In addition, it is a little bit tricky to take it out due to the large heat sink with an unusual shape. The function switch was loosened and the headphone jack was removed from the chassis to facilitate this process. Moreover, the power supply board was also removed from the chassis since one screw holding the heat sink is located just under the power supply board! Anyway, after the board was successfully disassembled from the chassis I thoroughly cleaned all pots and switches with DeoxIT 5% contact cleaner and lubricated them with DeoxIT FaderLube 5% spray.

The AF amplifier board has 10 notorious sky-blue Sanyo e-caps (C1, C2, C17, C18, C23, C24, C35, C36, C37, C38) installed in the signal path, two low-leakage e-caps (C11, C12), and 10 ordinary aluminum electrolytic capacitors (C41, C42, C43, C44, C51, C52, C55, C56, C57, C58).

All Sanyo coupling capacitors were replaced with high-quality film polyester WIMA caps. The original low-leakage e-caps were replaced with modern low-leakage Nichicon UKL caps. Two original aluminum e-caps C56 and C58 were replaced with Nichicon UFG caps. The remaining aluminum capacitors were replaced with low-impedance Nichicon UPW caps.

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

C1: rated capacitance – 1uF, measured – 1.0uF, ESR – 4.4Ω, deviation: 0%

C2: rated capacitance – 1uF, measured – 1.2uF, ESR – 3.6Ω, deviation: +20%

C11: rated capacitance – 3.3uF, measured – 4.2uF, ESR – 8.8Ω, deviation: +27%

C12: rated capacitance – 3.3uF, measured – 3.9uF, ESR – 10.6Ω, deviation: +18%

C17: rated capacitance – 1uF, measured – 1.3uF, ESR – 5.8Ω, deviation: +30%

C18: rated capacitance – 1uF, measured – 1.2uF, ESR – 4.8Ω, deviation: +20%

C23: rated capacitance – 0.33uF, measured – 0.38uF, ESR – N/A, deviation: +15%

C24: rated capacitance – 0.33uF, measured – 0.37uF, ESR – N/A, deviation: +12%

C35: rated capacitance – 0.47uF, measured – 0.52uF, ESR – N/A, deviation: +11%

C36: rated capacitance – 0.47uF, measured – 0.52uF, ESR – N/A, deviation: +11%

C37: rated capacitance – 0.47uF, measured – 0.55uF, ESR – N/A, deviation: +17%

C38: rated capacitance – 0.47uF, measured – 0.54uF, ESR – N/A, deviation: +15%

C41: rated capacitance – 220uF, measured – 242uF, ESR – 0.36Ω, deviation: +10%

C42: rated capacitance – 220uF, measured – 240uF, ESR – 0.42Ω, deviation: +9%

C43: rated capacitance – 47uF, measured – 57uF, ESR – 0.67Ω, deviation: +21%

C44: rated capacitance – 47uF, measured – 57uF, ESR – 0.77Ω, deviation: +21%

C51: rated capacitance – 220uF, measured – 235uF, ESR – 0.32Ω, deviation: +7%

C52: rated capacitance – 220uF, measured – 233uF, ESR – 0.26Ω, deviation: +6%

C55: rated capacitance – 47uF, measured – 55uF, ESR – 0.83Ω, deviation: +17%

C56: rated capacitance – 330uF, measured – 355uF, ESR – 1.31Ω, deviation: +8%

C57: rated capacitance – 220uF, measured – 239uF, ESR – 0.26Ω, deviation: +9%

C58: rated capacitance – 330uF, measured – 357uF, ESR – 0.22Ω, deviation: +8%

This board has a bunch of small signal transistors that are prone to failure.

Original Hitachi 2SC1344 NPN transistors installed in positions Q1, Q2, Q3, Q4, Q5, and Q6 often create static noise in the circuit. They also have a relatively high failure rate due to early degradation. I replaced them with new Fairchild KSC1845 transistors. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

The first stage of the AF amplifier circuit is a differential amplifier consisting of two transistors with a common emitter: Q10 & Q12 (left channel) and Q11 & Q13 (right channel). The original PNP transistor installed in these positions is 2SA725. This transistor is infamous for creating a shot noise in the circuit. I replaced each pair of original 2SA725 transistors with a new close-matched pair of modern low-noise Fairchild KSA992 transistors. Each pair of new KSA992 transistors was carefully matched (within ~1%) by current gain and base-emitter voltage. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

Test results on original 2SA725 transistors

Q10 (left channel): hfe - 497, Vbe - 0.770V 

Q12 (left channel): hfe - 457, Vbe - 0.772V

Q11 (right channel): hfe - 510, Vbe - 0.761V 

Q13 (right channel): hfe - 547, Vbe - 0.763V

Test results on new KSA992 transistors

Q10 (left channel): hfe - 424, Vbe - 0.756V 

Q12 (left channel): hfe - 421, Vbe - 0.758V

Q11 (right channel): hfe - 419, Vbe - 0.755V 

Q13 (right channel): hfe - 418, Vbe - 0.757V

Original Panasonic 2SC1318 NPN transistors installed in positions Q14, Q15, Q16, and Q17 often suffer from thermal stress. I replaced them with new Fairchild KSC2383 transistors.

A complementary pair to the 2SC1318 transistor is a Panasonic 2SA720. This transistor is installed in positions Q18 and Q19. I replaced the original 2SA720 transistors with new Fairchild KSA1013s.

AF amplifier board - disassembled from the chassis

AF amplifier board - before and after

Tuner Board (AWE-052)

The tuner board has 6 notorious sky-blue Sanyo capacitors (C36, C48, C49, C50, C59, C60) and 10 aluminum electrolytic capacitors (C18, C21, C25, C26, C28, C32, C33, C39, C44, C61).

The original Sanyo e-cap C36 was replaced with a film polyester Kemet capacitor. The other Sanyo capacitors and two ordinary aluminum e-caps C21 and C25 were replaced with film polyester WIMA caps. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW caps.

Test results on original capacitors removed from the tuner board:

C18: rated capacitance – 4.7uF, measured – 5.2uF, ESR – 2.4Ω, deviation: +11%

C21: rated capacitance – 0.47uF, measured – 0.47uF, ESR – N/A, deviation: 0%

C25: rated capacitance – 1uF, measured – 1.3uF, ESR – 2.6Ω, deviation: +30%

C26: rated capacitance – 10uF, measured – 12uF, ESR – 1.64Ω, deviation: +20%

C28: rated capacitance – 100uF, measured – 117uF, ESR – 0.52Ω, deviation: +17%

C32: rated capacitance – 4.7uF, measured – 5.2uF, ESR – 2.9Ω, deviation: +11%

C33: rated capacitance – 47uF, measured – 56uF, ESR – 1.31Ω, deviation: +19%

C36: rated capacitance – 0.1uF, measured – 0.1uF, ESR – N/A, deviation: 0%

C39: rated capacitance – 10uF, measured – 12uF, ESR – 1.59Ω, deviation: +20%

C44: rated capacitance – 10uF, measured – 12uF, ESR – 2.2Ω, deviation: +20%

C48: rated capacitance – 0.33uF, measured – 0.35uF, ESR – N/A, deviation: +6%

C49: rated capacitance – 1uF, measured – 1.3uF, ESR – 4.6Ω, deviation: +30%

C50: rated capacitance – 0.47uF, measured – 0.42uF, ESR – N/A, deviation: -11%

C59: rated capacitance – 0.33uF, measured – 0.34uF, ESR – N/A, deviation: +3%

C60: rated capacitance – 0.33uF, measured – 0.32uF, ESR – N/A, deviation: -3%

C61: rated capacitance – 220uF, measured – 239uF, ESR – 0.36Ω, deviation: +9%

The original PNP transistors installed in positions Q8 and Q9 are notorious 2SA725 transistors. I replaced them with modern low-noise Fairchild KSA992. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

Tuner board - before and after

Dial, Meter, and Dial Pointer Lamps

Replacing the dial and tuning meter lamps in SX-434 is a very simple task. To release the lamp housing that holds the dial and meter lights, simply unscrew the two small screws at the top of the lamp housing. And then carefully remove it from the chassis. I replaced the original dial lamps with modern warm white LED lamps to maintain the original look and reduce heat. The original incandescent light bulb installed behind the tuning meter usually overheats the plastic of the meter and discolors it. As a result, the plastic often turns yellowish.  I replaced the original tuning meter bulb with a cool blue LED lamp to solve this problem and better match the tuning meter with the rest of the dial.

New LED-type dial and meter lamps installed

The stereo indicator bulb burned out in this unit and I replaced it with a new incandescent bulb. This is a very delicate work since the original rubber holder dries out over time and eventually becomes very brittle. I used an X-ACTO #2 Knife to remove the original bulb from the holder carefully trimming around the bulb. The diameter of the new incandescent bulb is smaller than the original one and to compensate for the difference I put on a heat shrink on a new bulb. After that, the new incandescent bulb was inserted into the holder and secured with super glue.

A new stereo indicator lamp installed

The dial pointer bulb burned out in this unit as well and it was probably the most challenging work for me in this model. It is not so easy to replace a burned-out dial pointer bulb in Pioneer SX-434. Ideally, the dial pointer holder should be removed from the dial panel before bulb replacement. But in that case, the holder should be disassembled from the dial string. I didn't want to break the factory sealing and decided to replace the burned bulb without disassembling the dial pointer holder from the dial string. Well, I wouldn't say it is an easy task but it is doable with extra care and with the right tools. I used an X-ACTO #2 Knife again and small pliers to release a plastic dial holder from the metal part. Be extra cautious during this step if you need to replace a dial pointer bulb in this model. It is very easy to break a plastic needle. I was patient and lucky. The plastic needle was not broken and I successfully replaced the old bulb with a new one! 

Original dial pointer lamp - removed from the plastic holder

A new dial pointer lamp installed

DC offset and Bias Adjustments

The service manual describes step-by-step the power amplifier alignment but these instructions could be confusing for someone who is not familiar with this model. The problem is that this receiver has no trimmers to adjust the DC offset or Bias. Instead, it has three jumpers per channel which should be cut to make a correct alignment. 

The first step is to check the DC offset. To do this I connected my DC voltmeter between terminal #27 and the ground. The measured voltage was ~17mV which is perfectly normal. Then, I connected the DC voltmeter between terminal #24 and the ground. The measured voltage was ~13mV which is again in an acceptable range. Ideally, the measured voltage should be as close as possible to zero volts. But in real life, any DC offset less than 30mV would be just fine. So, I didn't cut any jumpers and left them alone. However, if the measured voltage is above 40-50mV then the correct jumper lead should be cut to get the DC offset close to zero volts. Jumper C should be cut if the DC voltmeter reads a positive voltage between terminal #27 and the ground (i.e. > +50mV). Jumper E should be cut if the DC voltmeter reads a negative voltage between terminal #27 and the ground (i.e. < -50mV). Jumpers D or F should be cut if a positive or negative voltage is measured between terminal #24 and the ground.

DC offset on the left and right channels after restoration

The second step is to check Bias.  According to the service manual, the bias should be above 5mV otherwise the jumper lead should be cut. I connected my DC voltmeter between terminals TP1 and 27. The measured voltage was ~23mV which is above the minimal voltage stated in the service manual. The measured voltage between terminal number TP2 and 24 was ~28mV which is again in an acceptable range. So, no jumpers were cut here as well. If the measured voltage between terminal number TP1 (TP2) and 27 (24) is less than 5mV then the jumper A (B) should be cut.

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 receiver 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 11.45 VRMS (left channel) and 11.64 VRMS (right channel). It corresponds to the output power of 16.4W on the left channel and 16.9W on the right channel.

Output power test

As usual, all the knobs and the faceplate 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. As I mentioned earlier this receiver has a very warm and rich sound. And it looks very cool too! Please watch a short demo video at the end of this post. Thank you for reading.

Pioneer SX-434 - after restoration

Demo video after repair & restoration

Pioneer SX-650 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-650

SN: WL3633228S

Today I'm showcasing a Pioneer SX-650 receiver that came in for restoration. The SX-650 was manufactured from 1976 to 1978. It produces 35 watts per channel into 8 ohms with no more than 0.3% total harmonic distortion. The list price in 1976 was around $300. This particular unit came in with several faulty components. However, the receiver is still in excellent cosmetic condition. The wood case, faceplate, and all knobs are almost flawless. It is certainly worth restoring. According to the Pioneer database, this unit was manufactured in December 1976.

Initial Troubleshooting

The previous owner told me that one channel is highly distorted and the other channel sounds very weak. First of all, I decided to check the operating voltages on Sanken SP-40W Power Modules to make sure that nothing was wrong with them. There is no modern substitution for those modules and the new modules available on eBay are usually counterfeit from China. A quick test showed that all measured voltages on both modules were very close to the reference voltages on the schematic. Well, at least these power modules are not burned out.

Test results on Sanken SP-40W power modules:

pin 1: schematic: 36V, left channel: 36.2V, right channel: 36.1V

pin 2: schematic: 1.2V, left channel: 1.3V, right channel: 1.3V

pin 3: schematic: 65mV, left channel: 63mV, right channel: 58mV

pin 4: schematic: 33mV, left channel: 37mV, right channel: 31mV

pin 5: schematic: -36V, left channel: -36.1V, right channel: -36.1V

pin 6: schematic: -1.2V, left channel: -1.2V, right channel: -1.2V

pin 7: schematic: -6mV, left channel: 9.8mV, right channel: 6.2mV

pin 8: schematic: N/A, left channel: 0V, right channel: 0V

The next test I completed was to trace the input signal through the balance/volume control, tone control, power amplifier - 1, power amplifier - 2, and down to the speaker's terminals. This test will show me where the signal is distorted or attenuated. So, I connected my function generator to the AUX terminals, applied a sine-wave signal of 1 kHz, and traced the input signal with my oscilloscope throughout all boards down to the speaker's terminals. Surprisingly but the sine-wave signal was clear and without any distortion throughout all boards. I connected my cell phone to the AUX terminals and the receiver played music with a crystal clear sound. It looks like the amplifier in this unit has no issues.

So, the last step was to check the tuner performance. I connected the FM antenna to 300 Ohm terminals of SX-650 and switched to FM broadcasts. Tuned the knob to the local FM station and immediately realized that the left channel was very weak and the right channel was highly distorted. At the same time, the signal strength was very strong and the stereo indicator lamp came on. Well, there is definitely some issue in the tuner circuit.

I turned off the receiver and tested all transistors and diodes in the tuner circuit. All semiconductor devices were normal. Then, I turned on the receiver again and checked the operating voltages on all transistors and integrated circuits (ICs). The measured voltages on all six transistors Q1 thru Q4, Q7, and Q8 were very close to the reference voltages on the schematic. Two ICs Q5 and Q9 also had the correct operating voltages on all pins. However, the integrated circuit Q6 was in question. The measured voltage on pins 4, 6, and 7 didn't match the reference voltage on the schematic.

Test results on integrated circuit Q6:

pin 1: schematic: 14.0V, measured: 14.0V

pin 2: schematic: 3.0V, measured: 3.1V

pin 3: schematic: 7.5V, measured: 7.7V

pin 4: schematic: 11.0V, measured: 4.8V

pin 5: schematic: 11.0V, measured: 11.2V

pin 6: schematic: 7.3V, measured: 9.5V

pin 7: schematic: 7.3V, measured: 4.9V

pin 8: schematic: ground, measured: 0V

pin 9: schematic: 0.1V, measured: 1.0V

pin 10: schematic: 2.3V, measured: 2.5V

pin 11: schematic: N/A, measured: 2.4V

pin 12: schematic: N/A, measured: 0V

pin 13: schematic: 2.2V, measured: 2.5V

pin 14: schematic: 2.4V, measured: 2.5V

pin 15: schematic: 2.4V, measured: 2.5V

pin 16: schematic: 3.0V, measured: 3.2V

The integrated circuit Q6 (HA1196) installed on Pioneer SX-650 is an MPX decoder. Pins 4 and 5 are the left and right channel outputs, respectively. And pins 6 and 7 provide feedback control. The sound from the left channel is very weak when the receiver is switched to FM broadcasts. It makes sense since the measured voltages on both pins (4 and 7) don't match with the reference voltages on the schematic. The right channel has a sound but it is highly distorted. It also makes sense since the measured voltage on pin 5 is close to the reference voltage but there is a problem with feedback control on pin 6. So, it looks like a faulty IC. I unsoldered the IC Q6 from the board and installed a new HA1196 chip on the 16-pin DIP socket. Then, I turned on the receiver, tuned the knob to the local FM station, and... The left channel was still very weak and the right channel was still highly distorted. Huh! It looks like there is another faulty component on the tuner board. I carefully inspected all passive electronic components around the HA1196 chip and found that the polystyrene film capacitor C41 (510pF) is faulty. I replaced it with a new polystyrene cap and there you go! The clear sound from FM broadcasting was restored. No more distortions. The measured voltages on all pins match with the reference voltages on the schematic. Yep, the troubleshooting of the tuner circuit is always a tricky task. Now it's time to service other boards.

Test results on integrated circuit Q6 after HA1196 chip and capacitor C41 replacement:

pin 1: schematic: 14.0V, measured: 14.0V

pin 2: schematic: 3.0V, measured: 3.1V

pin 3: schematic: 7.5V, measured: 7.7V

pin 4: schematic: 11.0V, measured: 11.1V

pin 5: schematic: 11.0V, measured: 11.1V

pin 6: schematic: 7.3V, measured: 6.7V

pin 7: schematic: 7.3V, measured: 6.7V

pin 8: schematic: ground, measured: 0V

pin 9: schematic: 0.1V, measured: 1.0V

pin 10: schematic: 2.3V, measured: 2.5V

pin 11: schematic: N/A, measured: 2.4V

pin 12: schematic: N/A, measured: 0V

pin 13: schematic: 2.2V, measured: 2.3V

pin 14: schematic: 2.4V, measured: 2.5V

pin 15: schematic: 2.4V, measured: 2.5V

pin 16: schematic: 3.0V, measured: 3.3V

Old and new integrated circuit Q6 (HA1196)

16 pin DIP socket for integrated circuit Q6

New HA1196 chip installed on the socket

Power Amp - 1 Board

The power amp - 1 board (#GWH 101) has two tantalum capacitors C1 and C2 installed in the signal path. I replaced them with high-quality film polyester WIMA MKS2 caps. Other 6 e-caps C7 thru C10, C13, and C14 were replaced with low impedance and high-reliability Nichicon UPW caps. The protection relay S1 installed on this board is very clean inside and I didn't notice any contact degradation. So, I didn't replace it. The modern substitution for this relay is Omron MY4-02-DC24.

Power Amp - 1 (#GWH 101) - before and after

Power Amp - 2 Board

The original coupling capacitors C21 and C22 installed on the power amp - 2 board (#GWR 101) are sky blue Sanyo electrolytic caps. These e-caps become very leaky over time and should be always replaced in any vintage gear. 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 caps. The other 15 e-caps installed on this board were replaced with new Nichicon UPW/UPM caps. I also removed and tested two transistors Q7 and Q8 mounted on the heat sinks with Atlas DCA55 semiconductor analyzer. Both transistors passed the test and the measured DC current gain was in spec according to the datasheet. I applied a new silicone thermal compound between each transistor and the heat sink before installation on the board.

Sky blue Sanyo coupling capacitors C21 and C22 - replaced with film polyester Kemet caps

Power Amp - 2 (#GWR 101) - before and after

Transistors Q7 and Q8 were removed from the board and tested with Atlas DCA55 semiconductor analyzer

The old thermal grease is almost completely dried out

Tone Control Board

The tone control board (#AWG049) has only 8 electrolytic capacitors but this board is not easily accessible to work on. I would strongly recommend removing the faceplate and the dial frame before servicing this board. I don't think it would be possible to install any new electronic components without removing the faceplate and the dial frame. Even when these two parts are removed it is still a bit tricky to install some particular capacitors.

This board has two tantalum capacitors C1 and C2 which were replaced with high-quality film polyester WIMA MKS2 caps. Two low leakage e-caps C13 and C14 were replaced with new low leakage Nichicon UKL caps. The remaining four electrolytic capacitors were replaced with low impedance and high-reliability Nichicon UPW caps.

Faceplate and dial frame removed - Tone control board is behind the pots

Tone control board - before and after - very limited space to work on

Tuner and AF Board

On the board (#AWE075) I replaced all tantalum capacitors C43 thru C50 installed in the signal path after the multiplex decoder circuit. The capacitors C43 and C44 were replaced with low leakage Nichicon UKL caps. Four capacitors C45 thru C48 were replaced with high-quality film polyester Kemet R82 series, and the last two C49 and C50 with film polyester WIMA MKS2 caps.

The phono equalizer and microphone circuits are both a part of the tuner and AF board. The phono equalizer has 8 e-caps: C76, C77, C84, C85, C90, C91, C94, and C95. Four of them C76, C77, C90, and C91 are tantalum caps. I replaced them with low leakage Nichicon UKL caps. The remaining 4 caps were replaced with low impedance and high-reliability Nichicon UPW caps. 

The microphone amp has four e-caps: C100, C101, C104, and C105. The e-caps C100 and C104 are tantalum caps and were replaced with high-quality film polyester Kemet R82 and WIMA MKS2 caps, respectively. The remaining 2 caps were replaced with Nichicon UPW caps.

Tuner and AF board - before and after

Phono equalizer circuit - before and after

Microphone amp circuit - before and after - very limited space to work on

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 1 kHz to the AUX jacks. The output sine-wave signal was perfectly symmetrical on both channels with no clipping up to 15.73 VRMS (left channel) and 16.63 VRMS (right channel). So, one can calculate that the output power is (15.73x15.73)/8=31W (left channel) and (16.63x16.63)/8=35W (right channel). It is very close to factory specifications. A very small difference between the left and right channels is attributed to the imperfection/aging of the balance potentiometer. Keep in mind that this baby is almost 43 years old but still in perfect working condition!

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 pots and switches have been cleaned with DeoxIT 5% contact cleaner and lubricated with DeoxIT FaderLube 5% spray.

The final result is amazing. The receiver looks brand new. The sound is wonderful and it has plenty of power for home stereo needs. Please watch a short demo video at the end of this post. Thank you for reading.

Pioneer SX-650 - before restoration

Pioneer SX-650 - after restoration

Demo video after repair & restoration

Pioneer SX-780 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-780

SN: AF3653494Y

I recently repaired another Pioneer SX-780 and decided to update this old post with better photos and more details about the restoration of this model. This may be useful for those who want to restore their own SX-780 and bring it back to life. This model is relatively easy to troubleshoot and service compared to the top models of this product line. But once properly restored, it sounds amazing. So it's always worth a try if you have good soldering skills and can read schematics.

The Pioneer SX-780 is a great mid-range receiver from one of Pioneer’s last series of receivers before they switched to digital meters and black faceplates. It features a "high output, low distortion power amplifier, a high fidelity equalizer amplifier with low noise and high gain, a high stability tuner section, tone controls with tone defeat function, built-in protection circuits, and independent power meters". 

The Pioneer SX-780 was manufactured from 1978 to around 1980. It was initially built in Japan, but later production switched to South Korea, as evidenced by the varying tags on the back of the unit. It produces 45 watts per channel into 8 ohms with no more than 0.05% total harmonic distortion. The list price in 1978 was $325.00 (Ref. Stereo Review, August 1978, page 143). 

According to the Pioneer database, this unit was manufactured in June 1980.

AF Amplifier Board (GWK-118)

Power Supply Circuit

The power supply circuit has 14 aluminum electrolytic capacitors: C301, C302, C303, C304, C305, C306, C307, C308, C309, C310, C311, C314, C315, and C316. 

All of them were replaced with Nichicon UPW/UPM low-impedance capacitors. Those e-caps are designed for switching power supplies and can operate in the temperature range from -55 to +105 C.

Pay attention to the capacitor C311. The schematic shows this capacitor as 100uF/16V despite the original capacitor installed on the board being 220uF/16V. I replaced this e-cap with a Nichicon UPW capacitor (220uF/25V).

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:

C301: rated capacitance – 100uF, measured – 107uF, ESR – 0.58Ω, deviation: +7%

C302: rated capacitance – 100uF, measured – 107uF, ESR – 0.75Ω, deviation: +7%

C303: rated capacitance – 100uF, measured – 113uF, ESR – 0.74Ω, deviation: +13%

C304: rated capacitance – 100uF, measured – 110uF, ESR – 0.66Ω, deviation: +10%

C305: rated capacitance – 47uF, measured – 55uF, ESR – 0.34Ω, deviation: +17%

C306: rated capacitance – 47uF, measured – open circuit/low capacitance

C307: rated capacitance – 100uF, measured – 112uF, ESR – 0.89Ω, deviation: +12%

C308: rated capacitance – 100uF, measured – 115uF, ESR – 0.78Ω, deviation: +15%

C309: rated capacitance – 2.2uF, measured – 2.9uF, ESR – 2.3Ω, deviation: +32%

C310: rated capacitance – 2.2uF, measured – 2.7uF, ESR – 1.88Ω, deviation: +23%

C311: rated capacitance – 220uF, measured – 234uF, ESR – 0.54Ω, deviation: +6%

C314: rated capacitance – 470uF, measured – 394uF, ESR – 0.22Ω, deviation: -16%

C315: rated capacitance – 220uF, measured – 260uF, ESR – 0.48Ω, deviation: +18%

C316: rated capacitance – 47uF, measured – 56uF, ESR – 0.58Ω, deviation: +19%

Protection Circuit

The protection circuit in SX-780 is responsible for muting when the POWER switch is turned on and off. The delay is determined by the time constants of the timing capacitor C317 and the constant-current circuit. The transistor Q26 turns the relay S7 on and off, thereby protecting the speakers. 

The protection circuit has 3 aluminum electrolytic capacitors: C317, C318, and C319. 

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

Test results on original capacitors removed from the protection circuit:

C317: rated capacitance – 33uF, measured – 39uF, ESR – 0.97Ω, deviation: +18%

C318: rated capacitance – 10uF, measured – 11uF, ESR – 2.2Ω, deviation: +10%

C319: rated capacitance – 47uF, measured – 48uF, ESR – 0.92Ω, deviation: +2%

Phono Amplifier Circuit

The phono amplifier circuit has 4 low-leakage capacitors (C103, C104, C117, C118) installed in the signal path, and two aluminum electrolytic capacitors (C105, C106). 

All original low-leakage capacitors were replaced with modern low-leakage Nichicon UKL caps. The remaining aluminum e-caps were replaced with Nichicon UPW low-impedance capacitors.

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

C103: rated capacitance – 2.2uF, measured – 2.1uF, ESR – 1.63Ω, deviation: -5%

C104: rated capacitance – 2.2uF, measured – 2.1uF, ESR – 1.74Ω, deviation: -5%

C105: rated capacitance – 470uF, measured – 528uF, ESR – 0.61Ω, deviation: +12%

C106: rated capacitance – 470uF, measured – 494uF, ESR – 0.47Ω, deviation: +5%

C117: rated capacitance – 2.2uF, measured – 2.3uF, ESR – 1.65Ω, deviation: +5%

C118: rated capacitance – 2.2uF, measured – 2.2uF, ESR – 1.56Ω, deviation: 0%

Control Section

The control section has 8 low-leakage capacitors (C207, C208, C217, C218, C227, C228, C229, C230) installed in the signal path, and 4 aluminum electrolytic capacitors (C201, C202, C213, C214).

The original low-leakage capacitors C207, C208, C227, and C228 were replaced with film polyester WIMA and Kemet caps. The other low-leakage capacitors C217, C218, C229, and C230 were replaced with modern low-leakage Nichicon UKL caps. The remaining aluminum e-caps were replaced with Nichicon UPW low-impedance capacitors.

Test results on original capacitors removed from the control section:

C201: rated capacitance – 4.7uF, measured – 5.7uF, ESR – 1.96Ω, deviation: +21%

C202: rated capacitance – 4.7uF, measured – 5.7uF, ESR – 1.98Ω, deviation: +21%

C207: rated capacitance – 1uF, measured – 0.9uF, ESR – 2.6Ω, deviation: -10%

C208: rated capacitance – 1uF, measured – 0.9uF, ESR – 2.5Ω, deviation: -10%

C213: rated capacitance – 47uF, measured – 58uF, ESR – 1.03Ω, deviation: +23%

C214: rated capacitance – 47uF, measured – 56uF, ESR – 1.01Ω, deviation: +19%

C217: rated capacitance – 4.7uF, measured – 4.8uF, ESR – 3.1Ω, deviation: +2%

C218: rated capacitance – 4.7uF, measured – 5.1uF, ESR – 3.2Ω, deviation: +9%

C227: rated capacitance – 0.22uF, measured – 0.21uF, ESR – N/A, deviation: -5%

C228: rated capacitance – 0.22uF, measured – 0.22uF, ESR – N/A, deviation: 0%

C229: rated capacitance – 2.2uF, measured – 2.2uF, ESR – 1.88Ω, deviation: 0%

C230: rated capacitance – 2.2uF, measured – 2.1uF, ESR – 2.52Ω, deviation: -5%

FM Muting Circuit

The FM muting circuit has two low-leakage capacitors (C401, C403) and 4 aluminum electrolytic capacitors (C402, C404, C405, C406).

The original e-caps C401, C402, C403, and C404 were replaced with film polyester WIMA caps. The remaining capacitors were replaced with Nichicon UPW low-impedance capacitors.

Test results on original capacitors removed from the FM muting circuit:

C401: rated capacitance – 0.47uF, measured – 0.43uF, ESR – N/A, deviation: -9%

C402: rated capacitance – 0.47uF, measured – 0.61uF, ESR – N/A, deviation: +30%

C403: rated capacitance – 0.47uF, measured – 0.43uF, ESR – N/A, deviation: -9%

C404: rated capacitance – 0.47uF, measured – 0.52uF, ESR – N/A, deviation: +11%

C405: rated capacitance – 10uF, measured – 12uF, ESR – 2.1Ω, deviation: +20%

C406: rated capacitance – 220uF, measured – 261uF, ESR – 0.56Ω, deviation: +19%

VU Meters Circuit

The VU meters circuit has two aluminum electrolytic capacitors: C324 and C327. Both of them were replaced with Nichicon UPW low-impedance capacitors.

Test results on original capacitors removed from the VU meters circuit:

C324: rated capacitance – 4.7uF, measured – 5.8uF, ESR – 2.1Ω, deviation: +23%

C327: rated capacitance – 4.7uF, measured – 5.7uF, ESR – 1.9Ω, deviation: +21%

I found one extra e-cap on this board that was not marked on the circuit board schematic. It is just to the right of e-cap C309, and the value is 4.7uF/35V. This is also a result of a production improvement that was not documented by Pioneer. I replaced this cap with a new Nichicon UPW.

Three transistors (Q19, Q20, and especially Q25) in the power supply circuit are running very hot under normal working conditions. Those transistors are mounted on heat sinks, but the size of each heat sink is not enough to efficiently dissipate the heat. I would strongly recommend carefully investigating the PCB around those transistors for any signs of excess heating if you need to service this model. Be aware that those heat sinks are electrically live.

All three transistors were replaced with modern Fairchild MJE transistors. A new silicone thermal compound (Wakefield-Vette, 120 series) was applied between each transistor and the heat sink.

The original relay driver transistor Q26 on this board is Panasonic 2SC1384. This transistor is on my list of prone-to-failure transistors and I replaced it with a new Fairchild KSC2690. The new transistor has the same pinout as the original one. 

Finally, the original protection relay was also replaced with a new Omron relay to improve overall reliability.

AF amplifier board - before and after

Tuner Board (AWE-099)

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 3 low-leakage capacitors (C37, C38, C40), and 12 aluminum electrolytic capacitors (C27, C28, C34, C35, C41, C43, C44, C53, C60,C61, C71, C76).

The original low-leakage capacitors C38 and C40 were replaced with film polyester WIMA caps. The third original low-leakage capacitor C37 was replaced with a new low-leakage Nichicon UKL cap. Two ordinary aluminum e-caps (C34, C41) with a nominal capacitance of 1uF were replaced with film polyester WIMA caps. The remaining capacitors were replaced with Nichicon UPW low-impedance capacitors.

Test results on original capacitors removed from the tuner board:

C27: rated capacitance – 2.2uF, measured – 2.8uF, ESR – 1.96Ω, deviation: +27%

C28: rated capacitance – 4.7uF, measured – 5.8uF, ESR – 2.2Ω, deviation: +23%

C34: rated capacitance – 1uF, measured – 1.2uF, ESR – 2.2Ω, deviation: +20%

C35: rated capacitance – 22uF, measured – 29uF, ESR – 0.74Ω, deviation: +32%

C37: rated capacitance – 6.8uF, measured – 6.6uF, ESR – 1.02Ω, deviation: -3%

C38: rated capacitance – 1uF, measured – 1.2uF, ESR – 3.2Ω, deviation: +20%

C40: rated capacitance – 1uF, measured – 1.2uF, ESR – 2.8Ω, deviation: +20%

C41: rated capacitance – 1uF, measured – 1.4uF, ESR – 2.5Ω, deviation: +40%

C43: rated capacitance – 220uF, measured – 252uF, ESR – 0.72Ω, deviation: +15%

C44: rated capacitance – 220uF, measured – 215uF, ESR – 0.51Ω, deviation: -2%

C53: rated capacitance – 220uF, measured – 246uF, ESR – 0.72Ω, deviation: +12%

C60: rated capacitance – 4.7uF, measured – 5.6uF, ESR – 2.4Ω, deviation: +19%

C61: rated capacitance – 3.3uF, measured – 3.4uF, ESR – 1.91Ω, deviation: +3%

C71: rated capacitance – 10uF, measured – 11uF, ESR – 2.4Ω, deviation: +10%

C76: rated capacitance – 10uF, measured – 12uF, ESR – 2.2Ω, deviation: +20%

The original 2SA726 transistor installed in positions Q7 and Q8 is infamous for its shot noise. I replaced it with a modern low-noise Fairchild KSA992 transistor. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

Tuner board - before and after

Dial Lamps

Initially, I installed modern warm white LED lamps instead of old incandescent bulbs but in my opinion, they look too bright on this model. So, I moved to new incandescent bulbs. To reflect the light and diffuse the heat coming from these bulbs I installed a strip of foil tape inside the top cover. It substantially reduces the heat coming from incandescent bulbs.

Tip: there is a small opening at the center of each lamp socket. I used a small Allen wrench and gently pushed each lamp out of the socket. In that case, the brittle socket tabs won't be damaged or broken.

Push each dial lamp out of the socket with a small Allen wrench

DC Offset & Power Meters Adjustments

At the end of my restoration, I checked and adjusted the DC offset of the power amplifier as described in the service manual. No dummy load or input signal is required for this adjustment. The DC offset was adjusted close to zero volts on each channel with trimming resistors VR5 and VR6.

An AC voltmeter and function generator are required to adjust power meters. A sine-wave signal of 1 kHz should be applied to the AUX terminals and the level of this signal should be adjusted so that the voltage on the SPEAKERS terminals reads 20 VRMS. Then, the trimming resistors VR7 and VR8 should be adjusted so that the power meters read 50W.

DC offset on the left and right channels after restoration

Output Power Test

The final output power test was conducted at the end of the restoration. Two low inductance 8Ω/100W resistors connected across each speaker terminal were used as a dummy load. The output sine-wave signal was perfectly symmetrical on both channels with no clipping up to 20.11 VRMS (left channel) and 20.66 VRMS (right channel). The output power on each channel can be calculated based on the following formula: P = (VRMS x VRMS)/8. The max output power (before clipping) is 50.6W on the left channel and 53.4W on the right channel. This corresponds to the factory specifications of this model and even slightly exceeds them.

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. The sound is wonderful, very warm, and extremely clean. Please watch a short demo video at the end of this post. Thank you for reading.

Pioneer SX-780 - after restoration

Demo video after repair & restoration