Pioneer SX-727 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-727

SN: TE3729669

One of my customers brought in this beautiful vintage receiver for restoration. It has been taken with care and is still in a perfect cosmetic condition but has some issues typical for 40-50 years old electronic equipment. Most original incandescent bulbs burned out, the stereo indicator bulb was replaced before but burned out again, all pots and switches are dirty and/or oxidized due to age. The SELECTOR switch was mostly affected in this receiver and didn't work properly when a PHONO source was selected. As a result, it was impossible to get a clear sound from a turntable. I restored this beautiful receiver and serviced all boards except a tuner section. Well, the tuner board was also serviced but I just replaced the electrolytic capacitors installed in a signal path on this board. It is usually safe to do and will not require any tuner re-alignment.

The Pioneer SX-727 was introduced in 1972 and delivers 37 watts per channel into 8 ohms with total harmonic distortion under 0.5%. It is a great mid-range receiver with a very sensitive and selective FM section offering distortion-free reception. A low noise field effect transistor in the first stage and a two-step RF amplifier provide an excellent sensitivity of 1.8 micro-volts (!) and a high signal-to-noise ratio of 70dB. I was able to get a stable stereo reception even without any external FM antenna attached! The SX-727 has terminals to connect two turntables, two tape decks, three pairs of speaker systems, and a microphone. The walnut-finish wooden cabinet looks beautiful and the build quality is excellent. A test report on the Pioneer SX-727 completed by the Hirsch-Houck Lab was published in Stereo Review magazine (July 1972, p. 34). The list price in 1972 was $349.95 (Ref. Stereo Review, March 1972, page 1). According to the Pioneer database, this unit was manufactured in May 1973.

Power Supply Board

The power supply circuit (#AWR-011) in Pioneer SX-727 has a similar design as a power supply circuit (#AWR-010) in Pioneer SX-828. And as might be expected both boards have similar problems. Two transistors Q1 and Q3, and the resistor R9 are running very hot on this board under normal operating conditions. The heat sink attached to transistors Q1 and Q3 is secured by a nylon screw. Due to high temperature and aging, this screw becomes brittle over time and easily breaks down. The metal oxide resistor R9 has a rated power of 2W which is usually not enough to efficiently dissipate the electrical power as heat. As a result, the part of PCB around this resistor is often found damaged. I replaced resistor R9 with a flameproof 3W metal oxide resistor to improve its heat dissipation. Also, it always makes sense to replace transistors Q1 and Q3 with Fairchild MJE transistors due to better thermal characteristics in comparison to original transistors. Five electrolytic capacitors C6, C7, C9, C10, and C11 were replaced with low impedance and high-reliability Nichicon UPW/UPM caps. The maximum operating temperature of new electrolytic capacitors is +105C which is also beneficial in power supply circuits. The original e-caps installed in vintage gears from the 70's have a maximum operating temperature of +85C.

Power supply board (#AWR-011) - PCB discoloration due to excessive heat from resistor R9

Original Sanyo power transistor and modern Fairchild MJE substitution

Power supply board (#AWR-011) - before and after

Protection Board

There are two versions of the protection board in Pioneer SX-727: #AWM-011 (early version) and #AWM-027 (later version). This receiver has the later version of the protection board. Also, the later version (#AWM-027) has several modifications and this particular one has two capacitors C1 and C2 as 0.22uF/10V electrolytic caps instead of Mylar caps. These two caps are the notorious sky blue Sanyo e-caps. These e-caps have a bad reputation to become very leaky over time. For more information about sky blue Sanyo e-caps refer to my previous post on Pioneer SX-828 restoration. I replaced C1 and C2 with high-quality film polyester Kemet caps. The remaining e-caps C3 thru C6 were replaced with low impedance and high-reliability Nichicon UPW caps.

Protection board (#AWM-027) - before and after

Head Amplifier Board

The head amplifier board in Pioneer SX-727 has six NPN transistors Q1 thru Q6. According to the service manual, these transistors could be either 2SC871 or 2SC458. However, in some units, the 2SC1312 transistor can be also found in these positions. All of these transistors are known to become very noisy over time. I replaced six original 2SC1312 transistors installed in this receiver with modern low noise Fairchild KSC1845. All new transistors were carefully matched by current gain and base-emitter voltage within 1%. Watch the pinout on replacement transistors while servicing this board. The original transistor is BCE and the new one is ECB.

Electrolytic capacitors C1/C2 and C5/C6 installed in the signal path were replaced with low leakage Nichicon UKL caps. The remaining e-caps C3/C4 and C15 were replaced with low impedance and high-reliability Nichicon UPW caps.

Head amplifier board (#W21-002) - before and after

AF Amplifier Board

The most difficult circuit board to work on in Pioneer SX-727 is an AF amplifier board (#AWK-010). This is a double solder PCB and requires a lot of patience while unsoldering and soldering the electronic components. There is always a risk of overheating and damaging this board while unsoldering electronic components. So, take your time and be very careful while servicing the AF amplifier board. 

All original coupling capacitors C13/C14, C21/C22, and C27/C28 installed on this board are the notorious sky blue Sanyo electrolytic caps. I replaced e-caps C13/C14 and C21/C22 with low leakage Nichicon UKL caps. The e-caps C27/C28 are rated at 1uF/25V and I replaced them with high-quality film polyester WIMA MKS2 caps. In general, I replace all electrolytic capacitors with capacitance from 0.1uF to 1uF with film polyester caps. The remaining eight e-caps on this board were replaced with low impedance and high-reliability Nichicon UPW caps.

Two NPN transistors Q5 and Q6 installed on this board are 2SC1312. I replaced them with modern low noise Fairchild KSC1845. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB. I also replaced two PNP transistors Q3 and Q4 with modern low noise Fairchild KSA992. The original PNP transistors are 2SA725 and become very noisy over time.

The access to the pots and switches installed on the AF amplifier board is very limited when it is assembled to the chassis. But everything can be easily cleaned when it is disassembled from the chassis. I used the DeoxIT 5% as a contact cleaner and DeoxIT FaderLube 5% spray for subsequent lubrication. 

AF amplifier board (#AWK-010) - before servicing

AF amplifier board (#AWK-010) - after servicing

Power Amplifier Board

The power amplifier board (#AWH-011) has six electrolytic capacitors C7 thru C10, C15, and C16. I replaced two e-caps C7 and C8 with low leakage Nichicon UKL caps. The remaining four e-caps were replaced with low impedance and high-reliability Nichicon UPW/UPM caps. As can be seen from the test results four out of six e-caps on this board are out of factory spec. 

Test results on original capacitors removed from Power Amplifier board:

C7: rated capacitance – 22uF, measured capacitance – 31uF, deviation: +41% (out of spec)

C8: rated capacitance – 22uF, measured capacitance – 30uF, deviation: +36% (out of spec)

C9: rated capacitance – 220uF, measured capacitance – 307uF, deviation: +40% (out of spec)

C10: rated capacitance – 220uF, measured capacitance – 323uF, deviation: +47% (out of spec)

C15: rated capacitance – 100uF, measured capacitance – 106uF, deviation: +6% (Okay)

C16: rated capacitance – 100uF, measured capacitance – 106uF, deviation: +6% (Okay)

Two NPN transistors Q5 and Q6 installed on the power amplifier board are Sony 2SC1124. The current gain in the 2SC1124 transistor degrades with age. The modern substitution is a Fairchild KSC2690A transistor. I replaced the original Sony transistors with KSC2690A to prevent any issues in the future. Watch the pinout on replacement transistors. The original transistor is EBC and the new one is ECB.

Power amplifier board (#AWH-011) - before and after

Dial and Meter Lamps

Two fuse-type lamps installed behind the tuning and signal strength meters are relatively easy to change in Pioneer SX-727. Just unscrew two small screws on the top and the lamp holder will be easily released from the chassis. The original incandescent bulbs installed in the lamp holder usually overheat the plastic on the meters and discolors it. As a result, the plastic often becomes yellowish. One possible solution to overcome this issue is to install cool blue LED lamps. These LED lamps are brighter than original incandescent bulbs but generate almost no heat and virtually last forever.

Original incandescent bulbs installed behind the tuning and signal strength meters

New cool blue LED lamps installed

Yellowish plastic due to extensive heat from original incandescent bulbs

To replace 5 dial lamps I pulled off all knobs from shafts, removed nuts and washers from their shafts, removed a faceplate, and then carefully removed four clips holding a dial scale. This process is similar to that I described in detail during the Pioneer SX-828 restoration. 

New cool blue LED lamps installed

The original stereo indicator bulb in this unit was replaced by somebody in the past but burned out again. It looks like the technician or whoever he was doesn't possess good soldering skills as can be seen in the photo below. I installed a warm white LED lamp here and secured it with super glue. The new LED lamp will virtually last forever.

Stereo indicator bulb replaced by somebody in the past - sloppy work!

Stereo indicator bulb replaced with warm white LED lamp

Bias and DC Offset Adjustments

At the end of my restoration, I checked and adjusted the Bias and DC offset on the power amplifier unit. The service manual has no description for these adjustments but this procedure is pretty much straightforward.

The bias is measured between pins 11 and 13 on the left channel, and between pins 12 and 14 on the right channel. It should be adjusted to ~20mV with trimmer VR3 and VR4, respectively. 

The DC offset is measured between pin 5 and ground on the left channel, and between pin 6 and ground on the right channel. It should be adjusted as close to zero volts as possible with trimmer VR1 and VR2, respectively.

Bias on the left and right channel after restoration

DC offset on the left and right channel after restoration

The final result can be seen in the photos below. The receiver looks beautiful and has enough power to fill a large room with great sound. Please watch a short demo video at the end of this post. Thank you for reading.

Pioneer SX-727 - before restoration

Pioneer SX-727 - after restoration

Demo video after repair & restoration

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