Sansui 661 Receiver Restoration

Unit: AM/FM Stereo Receiver
Manufacturer: Sansui

Model: 661

SN: 023123523

I bought this receiver a few months ago from the original owner along with a Sansui QS-500 quadraphonic rear channel amplifier and Sansui RA-500 reverberation amplifier. All three units have never been serviced before but are still in great cosmetic condition. It is certainly worth restoring.

The Sansui 661 is a great mid-range receiver. It is third from the top-of-the-line receivers in the Sansui lineup at the time: 881, 771, 661, 551, 441, 331, 221. It has an attractive champaigne-gold front panel and an all-wood cabinet. It was manufactured from 1970 to 1976 and produces 20 watts per channel into 8 ohms with no more than 0.5% total harmonic distortion. The frequency response is pretty impressive from 15Hz to 30kHz and the damping factor is 60. The output power transistors in Sansui 661 are protected from damage with several elaborate power protection devices. These include four quick-acting fuses and a positive-action relay circuit. The latter also protects speakers from damage and eliminates the annoying "popping" noise. As Sansui states this is a budget-priced stereo receiver that delivers expensive tonal quality and performance. Its power amplifier is of a very advanced all-stage direct-coupled OCL design providing RMS continuous power of 27 fully usable watts per channel (total music power 110 watts) with a surprisingly wide power bandwidth of 15 to 40,000Hz with very low overall distortion. Don't be confused with output power in Sansui 661. There was a power output war going on in the early 70's and no standard for power output was settled in the Hi-Fi industry. Each manufacturer was making its own calculations of power output to advertise the product. Only in 1974, the Federal Trade Commission instated its Amplifier Rule to combat the unrealistic power claims made by many Hi-Fi amplifier manufacturers.

According to the Sansui database, this particular unit was manufactured in December 1973.

Power Circuit Board F-1500A

The Sansui 661 is a service-friendly receiver. Each board has very good access and plenty of space to work on. The power circuit board F-1500A has a power supply circuit, protection circuit, the slot for the driver circuit board, and four power transistors. The driver circuit board is secured in the slot with a massive metal bracket.

The power supply circuit provides +13.2V DC for the Tuner board and +23.0V DC for the Pre-amplifier board. It also provides balanced power to the Power amplifier from the bridge rectifier and two filter capacitors C16/C17. This circuit has five aluminum electrolytic capacitors C02, C03, C04, C05, and C06. I replaced all of them with low impedance Nichicon UPM/UPW caps. Two large filter capacitors C16/C17 were tested in-circuit with Atlas ESR70 capacitance meter and have almost zero ESR. So, I didn't change them. In general, the filter capacitors rarely fail in vintage gears, and I usually don't replace them unless their ESR is high.

The protection circuit has five aluminum electrolytic capacitors C07, C08, C09, C10, and C11. Two of them, C10 and C11 are bipolar e-caps. I replaced the bipolar e-cap C09 with a high-quality film polyester WIMA MKS2 cap and another bipolar e-cap C11 with a new bipolar Nichicon UES cap. The other three aluminum e-caps C07, C08, and C09 were replaced with low impedance Nichicon UPW caps.

All electrolytic capacitors removed from this board were tested with Atlas ESR70 capacitance meter and results are below. Most of them are still within the factory capacitance tolerance except for the e-caps C09 and C11.

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

C02: rated capacitance – 1000uF, measured – 992uF, ESR – 0.12Ω, deviation: -1%

C03: rated capacitance – 47uF, measured – 52uF, ESR – 0.08Ω, deviation: +11%

C04: rated capacitance – 47uF, measured – 52uF, ESR – 0.11Ω, deviation: +11%

C05: rated capacitance – 100uF, measured – 106uF, ESR – 0.11Ω, deviation: +6%

C06: rated capacitance – 100uF, measured – 107uF, ESR – 0.31Ω, deviation: +7%

C07: rated capacitance – 33uF, measured – 38uF, ESR – 0.12Ω, deviation: +15%

C08: rated capacitance – 220uF, measured – 280uF, ESR – 0.11Ω, deviation: +27%

C09: rated capacitance – 1.0uF, measured – 1.4uF, ESR – 1.35Ω, deviation: +40%

C10: rated capacitance – 1.0uF, measured – 0.9uF, ESR – 2.3Ω, deviation: -10%

C11: rated capacitance – 47uF, measured – 58uF, ESR – 0.49Ω, deviation: +23%

Power Transistors

The original Hitachi power transistors 2SC1030 were removed, cleaned, and tested with Atlas DCA55 semiconductor analyzer. The measured DC current gain was in spec according to the datasheet. However, one transistor TR01 from the left channel was found leaky. The measured collector leakage current was 0.016mA which is substantial for Si transistor. A leakage current is expected to be less than 0.001mA for the Si transistor. I didn't have another 2SC1030 in stock and decided to replace all four power transistors with new ON Semiconductor MJ21194G transistors. The new transistors are specifically designed for high-power audio output and have excellent gain linearity. In addition, the total harmonic distortion measured on two matched pairs of MJ21194G transistors can be as low as 0.08% according to the datasheet. So, it is a perfect choice as a substitution transistor for the original Hitachi 2SC1030.

Below are the test results on the original and new power transistors. It should be noted that Atlas DCA55 semiconductor analyzer provides the accurate reading for DC current gain only on low power transistors. A high-power transistor requires a much higher collector current and collector-emitter voltage to accurately measure its current gain. However, it is still a very useful device for comparing transistors of a similar type for the purposes of gain matching or fault-finding.

Test results on original power transistors 2SC1030:

Left channel: TR01 (gain - 43, Vbe - 0.581V, leakage - 0.016mA), TR03 (gain - 35, Vbe - 0.598V)

Right channel: TR02 (gain - 43, Vbe - 0.574V), TR04 (gain - 46, Vbe - 0.591V)

Test results on new closely matched power transistors MJ21194G:

Left channel: TR01 (gain - 54, Vbe - 0.599V), TR03 (gain - 59, Vbe - 0.594V)

Right channel: TR02 (gain - 52, Vbe - 0.597V), TR04 (gain - 54, Vbe - 0.601V)

Leaky transistor TR01 from the left channel - substantial leakage current

Power Circuit Board F-1500A - before and after

Driver Circuit Board F-1499A

As I mentioned earlier the driver circuit board in Sansui 661 can be easily removed from the slot for service. This board has two low leakage electrolytic capacitors C01/C02 installed in the signal path and six aluminum e-caps C05/C06, C09/C10, C13/C14. I replaced the low leakage e-caps with high-quality film polyester WIMA MKS2 caps. And six aluminum e-caps were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from the driver circuit board:

C01: rated capacitance – 0.47uF, measured – 0.57uF, ESR – N/A, deviation: +21%

C02: rated capacitance – 0.47uF, measured – 0.56uF, ESR – N/A, deviation: +19%

C05: rated capacitance – 220uF, measured – 288uF, ESR – 0.11Ω, deviation: +31%

C06: rated capacitance – 220uF, measured – 275uF, ESR – 0.09Ω, deviation: +25%

C09: rated capacitance – 33uF, measured – 36uF, ESR – 0.16Ω, deviation: +9%

C10: rated capacitance – 33uF, measured – 37uF, ESR – 0.22Ω, deviation: +12%

C13: rated capacitance – 100uF, measured – 104uF, ESR – 0.07Ω, deviation: +4%

C14: rated capacitance – 100uF, measured – 110uF, ESR – 0.04Ω, deviation: +10%

Four NPN transistors TR05, TR06, TR09, and TR10 installed on the driver circuit board are Sony 2SC1124. The current gain in the 2SC1124 transistor degrades with age. The modern substitution is a Fairchild KSC2690A transistor. I replaced two original Sony transistors TR05 and TR06 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. 

I didn't replace the other two Sony transistors TR09 and TR10 due to the difference in packaging between 2SC1124 and KSC2690A. The packaging type of the original Sony 2SC1124 is TO-202 while the Fairchild KSC2690A transistor has a packaging type of TO-126. It would be impossible to provide a good thermal contact between two pairs of transistors TR09/TR07 and TR10/TR08 which is critical to compensate for any temperature drift under normal operating conditions. So, I left them alone.

Two PNP transistors TR11 and TR12 installed on the driver circuit board are 2SA706. This transistor is also known for current gain deterioration with age. I replaced both of them with Fairchild KSA1220A transistors. Again, watch the pinout on replacement transistors. The original transistor is EBC and the new one is ECB.

I also replaced four green dot diodes D01 thru D04 on this board with Fairchild 1N4148 diodes. The original diodes DS-430 are still okay but their reliability is in question. So, I usually prefer to replace them to avoid any issues in the future.

Driver Circuit Board F-1499A - before and after

Pre-Amplifier Circuit Board F-1501

The pre-amplifier circuit board F-1501 has two solid tantalum C01/C02, four low leakage C15/C16, C25/C26, and four aluminum C09/C10, C601/C602 electrolytic capacitors. The tantalum and low leakage e-caps are installed in the signal path. I replaced them with high-quality film polyester WIMA MKS2 caps. The remaining four aluminum capacitors were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from the pre-amplifier circuit board:

C01: rated capacitance – 1.5uF, measured – 1.5uF, ESR – 6.2Ω, deviation: 0%

C02: rated capacitance – 1.5uF, measured – 1.4uF, ESR – 6.6Ω, deviation: -7%

C09: rated capacitance – 100uF, measured – 112uF, ESR – 0.28Ω, deviation: +12%

C10: rated capacitance – 100uF, measured – 110uF, ESR – 0.29Ω, deviation: +10%

C15: rated capacitance – 0.47uF, measured – 0.43uF, ESR – N/A, deviation: -9%

C16: rated capacitance – 0.47uF, measured – 0.43uF, ESR – N/A, deviation: -9%

C25: rated capacitance – 0.47uF, measured – 0.47uF, ESR – N/A, deviation: 0%

C26: rated capacitance – 0.47uF, measured – 0.46uF, ESR – N/A, deviation: -2%

C601: rated capacitance – 100uF, measured – 117uF, ESR – 0.08Ω, deviation: +17%

C602: rated capacitance – 100uF, measured – 114uF, ESR – 0.08Ω, deviation: +14%

Six NPN transistors TR01 thru TR06 installed on this board are notorious 2SC1222 transistors. They usually get very noisy over time. I replaced them with modern low noise Fairchild KSC1845 transistors. The old and new transistors have the same pinout, i.e. ECB.

This point during the restoration is the best time to clean and lubricate all pots and switches located on this board. It is almost impossible to reach these pots and switches without removing this board once again. I used the DeoxIT 5% contact cleaner and DeoxIT FaderLube 5% spray for cleaning and lubricating, respectively.

Pre-Amplifier Circuit Board F-1501 - before and after

Dial, Meter, and Stereo Indicator Lamps

The Sansui 661 has three dial bulbs mounted inside the plastic illuminator box. This box can be easily released from the chassis if two small screws on the top of it are loosened. However, to reach these screws the front panel should be removed as well. I replaced the original incandescent bulbs with warm white LED lamps to maintain the original look and decrease heat.

Old incandescent dial bulbs

New warm white LED lamps installed

The signal meter lamp can be replaced even easier than the dial lamps. Just remove the plastic holder behind the meter and you would get plenty of space to reach the lamp. I replaced the original incandescent bulb for the signal meter with warm white LED lamp.

Plenty of space to replace a signal meter lamp - new warm white LED lamp installed

The stereo indicator bulb burned out in this unit. I replaced it with a new incandescent lamp.

New stereo indicator lamp installed

DC Offset and Bias Adjustments

The DC offset and bias adjustments are pretty straightforward and clearly described in the service manual. I adjusted the DC offset on each channel as close to zero volts as possible with the trimming resistors VR01 and VR02, respectively. The bias current was adjusted to ~19mA with the trimming resistors VR03 and VR04, respectively. Before adjustment, all trimming resistors were cleaned and lubricated with DeoxIT FaderLube 5% spray.

DC offset on the left and right channel after restoration

Bias current 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 15.37 VRMS (left channel) and 15.10 VRMS (right channel). So, one can calculate that the output power is (15.37 x 15.37)/8=29.5W (left channel) and (15.10 x 15.10)/8=28.5W (right channel). The output power exceeds a factory specification for this model which is not too bad for 48 years old gear!

Output power test - meets and even exceeds a factory specification!

As usual, all the knobs and the front panel were gently cleaned in warm water with dish soap. The final result can be seen in the photos below. The receiver looks brand new again and sounds fantastic! Please watch a short demo video at the end of this post. Thank you for reading.

Sansui 661 - after restoration

Demo video after repair & restoration

Pioneer SX-828 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-828

SN: TE3718523

Today I'm showcasing a Pioneer SX-828 stereo receiver that came in for restoration. I restored another SX-828 a few months ago and one can read the restoration process in detail here: Pioneer SX-828 receiver restoration. However, the restoration I completed earlier on another SX-828 describes only servicing of two boards along with dial and meter lamps replacement. That was a customer request since he had a limited budget for restoring his receiver. In this post, I am going to describe the servicing of all boards in Pioneer SX-828 except a tuner section. Some information will probably duplicate my previous post but I will try to avoid it as much as possible.

Below are some photos I took during the restoration process, complete with comments and recommendations. This particular receiver came in with various electronic and mechanical issues. I will describe each of them in the corresponding section. I hope this post will be a good reference for everyone who has this receiver and wants to restore it to a factory spec. I usually don't make any modifications to the circuit design trying to stick with the original circuit topography. However, some particular electronic components I replaced in this unit will definitely improve the original sound quality. In general, the Pioneer SX-828 is a very well-built receiver but not the easiest to work on. However, it is always worth the effort. 

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

Initial Evaluation and Troubleshooting

As I mentioned earlier this particular receiver came in with various electronic and mechanical issues. 

(a) The original STEREO and FM indicator lamps have been replaced in the past but it was done carelessly. I re-soldered the wires and insulate them with shrink tubing instead of electric tape.

(b) Also, all wires from the SPEAKER/POWER switch have been cut in the past, re-wired, and insulated by electric tape. I decided to re-work it to keep the receiver in good shape. To my surprise, almost all of them were not soldered at all and each connection was kind of loosened. I re-soldered all those wires and insulate them with shrink tubing to get a good and secure connection.

Wires from the SPEAKER/POWER switch - before and after

(c) The original dial pointer bulb burned out in this unit and I replaced it with a new incandescent lamp. This work requires a lot of patience and punctuality. It is very easy to break the thin plastic pointer during the lamp replacement. I used an X-ACTO #2 Knife and small pliers to release it from the metal case.

Dial pointer lamp - before and after

(d) Previously someone tried to re-string the dial cord in this receiver but did it wrong. As a result, the dial pointer was moving to the left when the tuning knob was rotating clockwise, and to the right when the tuning knob was rotating counterclockwise. I reworked it according to the instructions in the service manual. This procedure is quite simple and can be easily done on this model. Just carefully follow the instructions in the service manual and preferably use a new dial cord.

Dial cord is stringed incorrectly

Dial cord is stringed correctly

(e) Both channels in this receiver were cut out occasionally. And it was predominately at low volume. The BALANCE and VOLUME pots were very dirty and also contributed to the scratchy sound. The other switches (LOUDNESS, AUDIO MUTING, TAPE MONITOR, LOW and HIGH FILTER) had contaminated/oxidized contacts as well. I thoroughly cleaned all pots and switches with DeoxIT 5% contact cleaner and lubricated with DeoxIT FaderLube 5% spray. The scratchy sound has gone after that but it didn't solve a problem with channels occasionally cutting out. I decided to trace the input signal from AUX terminals through all boards down to the speaker's terminals to see if I can catch at which point the signal cuts out. 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: INPUT SELECTOR, AF amplifier IN/OUT, Main amplifier IN/OUT, etc. At each testing point, I played with all switches and pots to see if they would distort or attenuate somehow the input signal. The signal was clear and without any distortion or attenuation until I reached the protection relay. The picture below shows the sine-wave signal detected at two testing points. The pink curve represents the signal measured at pin 9 of the protection relay (IN) and the yellow curve shows the signal detected at pin 5 (OUT). As one can see the sine-wave signal is highly distorted at pin 5 which means that most probably the relay's contacts are worn and don't provide a good contact anymore.   

Original protection relay - the signal measured at pin 9 (pink curve, IN) and pin 5 (yellow curve, OUT)

I replaced the original relay with a new Omron (part # LY2F-DC24) and the signal was restored across the protection relay (see the picture below). The replacement relay has mounts and pins on opposite sides. I added the standoff supports to mount a new relay on the chassis.

Original protection relay

New Omron protection relay

New and old protection relays

New Omron protection relay - the signal measured at pin 9 (pink curve, IN) and pin 5 (yellow curve, OUT). Both curves perfectly overlap each other.

Power Supply Board AWR-010-B

The power supply board AWR-010-B in Pioneer SX-828 is notorious for two transistors Q1 and Q3 running extremely hot (by design) under normal operating conditions. Each transistor is mounted on a heat sink but its size is too small to efficiently dissipate the excessive heat. Also, the metal oxide resistor R12 connecting the collector of Q1 to the collector of Q3 is running very hot as well. The rated power of R12 is 2W and its resistance is 330Ω. The voltage drop across R12 is usually measured from 20 to ~25V. The electrical power dissipated by this resistor can be calculated based on Ohm’s law: P = (V*V)/R = (25*25)/330 = 1.89W. And it is very close to the rated power of R12. The excessive heat coming from transistor Q3 and resistor R12 usually results in discoloration of PCB around these electronic components. In extreme cases, the PCB can be severely damaged.

I replaced transistors Q1 and Q3 with Fairchild MJE transistors due to better thermal characteristics in comparison to the original transistors. The resistor R12 was replaced with a flameproof 3W metal oxide resistor to improve its heat dissipation as well. Eight electrolytic capacitors C5 thru C10, C12 and C13 were replaced with low impedance and high-reliability Nichicon UPW caps. The maximum operating temperature of new electrolytic capacitors is +105C which is beneficial in power supply circuits. The original e-caps installed in vintage gears from the 70's have a maximum operating temperature of +85C. 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 still within a factory spec. However, the measured capacitance of C5 is almost 50% lower than its rated capacitance.

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

C5: rated capacitance – 1000uF, measured – 506uF, ESR – 0.06Ω, deviation: -49%

C6: rated capacitance – 100uF, measured – 102uF, ESR – 0.12Ω, deviation: +2%

C7: rated capacitance – 100uF, measured – 100uF, ESR – 0.12Ω, deviation: 0%

C8: rated capacitance – 100uF, measured – 107uF, ESR – 0.12Ω, deviation: +7%

C9: rated capacitance – 100uF, measured – 105uF, ESR – 0.13Ω, deviation: +5%

C10: rated capacitance – 220uF, measured – 175uF, ESR – 0.26Ω, deviation: -20%

C12: rated capacitance – 220uF, measured – 262uF, ESR – 0.11Ω, deviation: +19%

C13: rated capacitance – 470uF, measured – 502uF, ESR – 0.13Ω, deviation: +7%

Power Supply board AWR-010-B - PCB discoloration due to excessive heat coming from Q3 and R12

Power Supply board AWR-010-B - after servicing

Protection Board AWM025-A

There are at least two versions of the protection board in Pioneer SX-828: W28-008 (the early version I believe) and AWM025 (the later version). This particular unit has a later version. The later version has two notorious sky-blue Sanyo e-caps C1 and C2 (0.22uF/10V) installed instead of Mylar capacitors (0.056uF/50V). The sky blue Sanyo e-caps have a bad reputation to get electrically leaky over time and eventually fail shorted. They were used in most Pioneer gears in the early 70's. I replaced them with high-quality film polyester Kemet caps. Also, in the later version two e-caps C3 and C4 were upgraded from 220uF/10V to 330uF/6.3V. These were replaced with low impedance Nichicon UPW caps. The e-cap C6 was also replaced with the Nichicon UPW cap. Another modification in the AWM025 protection circuit in comparison to W28-008 is the additional electrolytic capacitor C5 (4.7uF/25V). I replaced it with a Nichicon UPW cap as well.

Test results on original capacitors removed from the protection board:

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

C2: rated capacitance – 0.22uF, measured – 0.28uF, ESR – N/A, deviation: +27%

C3: rated capacitance – 330uF, measured – 390uF, ESR – 0.08Ω, deviation: +18%

C4: rated capacitance – 330uF, measured – 392uF, ESR – 0.06Ω, deviation: +19%

C5: rated capacitance – 4.7uF, measured – 6.1uF, ESR – 1.1Ω, deviation: +30%

C6: rated capacitance – 100uF, measured – 108uF, ESR – 0.18Ω, deviation: +8%

Protection board AWM025-A - before and after

Head Amplifier Board W21-002-B

The head amplifier board W21-002-B has six NPN transistors Q1 thru Q6. According to the service manual, either 2SC871 or 2SC458 transistors can be installed. This particular unit has the 2SC458 transistors installed. This transistor gets very noisy over time. I replaced each 2SC458 transistor with a 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.

Seven aluminum electrolytic capacitors (C1 thru C6 and C15) are installed on this board.  Four of them C1/C2, C5/C6 are installed in the signal path. For some reason, Pioneer installed just ordinary aluminum e-caps in these positions. I replaced them with low leakage Nichicon UKL caps to improve the signal-to-noise ratio. The remaining e-caps C3/C4 and C15 were replaced with low impedance Nichicon UPW caps. All original e-caps removed from the head amplifier board were tested and the results are below. Most of them are outside of +/- 20% factory capacitance tolerance.

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

C1: rated capacitance – 22uF, measured – 27uF, ESR – 0.46Ω, deviation: +23%

C2: rated capacitance – 22uF, measured – 29uF, ESR – 0.52Ω, deviation: +32%

C3: rated capacitance – 4.7uF, measured – 5.9uF, ESR – 1.23Ω, deviation: +26%

C4: rated capacitance – 4.7uF, measured – 6.0uF, ESR – 1.16Ω, deviation: +28%

C5: rated capacitance – 10uF, measured – 13uF, ESR – 0.47Ω, deviation: +30%

C6: rated capacitance – 10uF, measured – 12uF, ESR – 0.53Ω, deviation: +20%

C15: rated capacitance – 220uF, measured – 259uF, ESR – 0.02Ω, deviation: +18%

Head Amplifier board W21-002-B - before and after

AF Amp Board AWK-012-B

The SX-828 receiver is not the easiest one for servicing. The major problem is an AF amplifier board. This is a double solder PCB and requires a lot of patience while unsoldering and soldering the electronic components. It is very easy to overheat and damage it. So, pay close attention to this board while servicing it.

Seems like this board has been serviced in the past. Two coupling capacitors C37 and C38 are supposed to be at 1uF/25V but have been replaced with 0.68uF/50V e-caps. I replaced them with high-quality film polyester WIMA MKS2 caps, 1uF/50V. Another pair of coupling capacitors in question are C15 and C16. They are supposed to be at 1.5uF/25V but one of them C15 is at 3.3uF/25V while the C16 is 1.5uF/25V. In addition, these are two different types of e-caps. I tested the e-caps C15 and C16 with an Atlas ESR70 capacitance meter and the ESR of e-cap C16 is 12.8Ω! The ESR of e-cap C15 is 2.4Ω (see the test results for all e-caps removed from this board below). I replaced the e-caps C15 and C16 with low leakage Nichicon UKL caps, 1.5uF/50V. The remaining six coupling capacitors C25/C26, C31/C32, C45/C46 were also replaced with low leakage Nichicon UKL caps. The last ten aluminum e-caps on this board are C11 thru C14, C27 thru C30, C43, and C44. I replaced them with low impedance Nichicon UPW caps.

AF Amplifier board AWK-012-B - before servicing

AF Amplifier board AWK-012-B - blue e-caps C37 & C38 are supposed to be 1uF/25V

AF Amplifier board AWK-012-B - C37 & C38 replaced with film polyester WIMA MKS2 caps

AF Amplifier board AWK-012-B - e-cap C15 has an incorrect capacitance rating

AF Amplifier board AWK-012-B - C15 & C16 replaced with low leakage Nichicon UKL caps

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

C11: rated capacitance – 47uF, measured – 57uF, ESR – 0.45Ω, deviation: +21%

C12: rated capacitance – 47uF, measured – 58uF, ESR – 0.48Ω, deviation: +23%

C13: rated capacitance – 47uF, measured – 50uF, ESR – 0.16Ω, deviation: +6%

C14: rated capacitance – 47uF, measured – 49uF, ESR – 0.18Ω, deviation: +4%

C15: rated capacitance – 1.5uF, measured – 3.3uF, ESR – 2.4Ω, deviation: N/A (wrong cap)

C16: rated capacitance – 1.5uF, measured – 1.5uF, ESR – 12.8Ω, deviation: 0% (very high ESR!)

C25: rated capacitance – 3.3uF, measured – 3.3uF, ESR – 3.0Ω, deviation: 0%

C26: rated capacitance – 3.3uF, measured – 3.5uF, ESR – 1.8Ω, deviation: +6%

C27: rated capacitance – 100uF, measured – 126uF, ESR – 0.36Ω, deviation: +26%

C28: rated capacitance – 100uF, measured – 125uF, ESR – 0.26Ω, deviation: +25%

C29: rated capacitance – 47uF, measured – 50uF, ESR – 0.12Ω, deviation: +6%

C30: rated capacitance – 47uF, measured – 50uF, ESR – 0.11Ω, deviation: +6%

C31: rated capacitance – 2.2uF, measured – 2.4uF, ESR – 2.2Ω, deviation: +9%

C32: rated capacitance – 2.2uF, measured – 2.9uF, ESR – 2.0Ω, deviation: +32%

C37: rated capacitance – 1uF, measured – 0.67uF, ESR – N/A, deviation: N/A (wrong cap)

C38: rated capacitance – 1uF, measured – 0.72uF, ESR – N/A, deviation: N/A (wrong cap)

C43: rated capacitance – 100uF, measured – 94uF, ESR – 0.08Ω, deviation: -6%

C44: rated capacitance – 100uF, measured – 93uF, ESR – 0.08Ω, deviation: -7%

C45: rated capacitance – 3.3uF, measured – 3.3uF, ESR – 2.2Ω, deviation: 0%

C46: rated capacitance – 3.3uF, measured – 3.7uF, ESR – 3.6Ω, deviation: +12%

Four NPN transistors Q5 thru Q8 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 they become very noisy over time.

AF Amplifier board AWK-012-B - after servicing

Main Amplifier Board AWH-010-D

The main amplifier board AWH-010-D has eight electrolytic capacitors C1, C2, C5, C6, C9 thru C12. Two of them C1 and C2 are installed in the signal path. However, for some reason, Pioneer installed just ordinary aluminum e-caps in these positions. I replaced them with high-quality film polyester WIMA MKS2 caps to improve the signal-to-noise ratio. The remaining six aluminum e-caps were replaced with low impedance Nichicon UPW/UPM caps. As can be seen from the test results six out of eight e-caps on this board are out of factory spec.

Test results on original capacitors removed from main amplifier board:

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

C2: rated capacitance – 1uF, measured – 1uF, ESR – 5.4Ω, deviation: 0%

C5: rated capacitance – 22uF, measured – 28uF, ESR – 1.1Ω, deviation: +27%

C6: rated capacitance – 22uF, measured – 27uF, ESR – 1.1Ω, deviation: +23%

C9: rated capacitance – 220uF, measured – 328uF, ESR – 0.11Ω, deviation: +49%

C10: rated capacitance – 220uF, measured – 284uF, ESR – 0.06Ω, deviation: +29%

C11: rated capacitance – 100uF, measured – 125uF, ESR – 0.04Ω, deviation: +25%

C12: rated capacitance – 100uF, measured – 122uF, ESR – 0.06Ω, deviation: +22%

The first stage of the main amplifier circuit is a differential pair amplifier consisting of two amplifying transistors with a common emitter: Q1/Q3 (left channel) and Q2/Q4 (right channel). The original PNP transistors installed in these positions are notorious 2SA726. They get very noisy over time. I replaced them with modern Fairchild KSA992 transistors. Each pair of KSA992 transistors was carefully matched by current gain and base-emitter voltage before installation. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

Four NPN transistors Q5 thru Q8 installed on the main 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.

Main Amplifier board AWH-010-D - before and after

Dial and Indicator Lamps

Five original dial bulbs in this unit were replaced with warm white LED lamps to maintain the original look and decrease heat. I also replaced two original bulbs behind the signal strength and FM tuning meters with warm white LED lamps. This process was described earlier in my previous post on servicing another SX-828 and can be found here: Pioneer SX-828 receiver restoration.

DC offset and Bias Adjustments

At the end of my restoration, I checked and adjusted the DC offset and Bias on the main amplifier unit. For some reason, this procedure is not described in the original manual but it is pretty much straightforward.

The DC offset is measured between pin 7 and ground on the left channel, and between pin 8 and ground on the right channel. It should be adjusted as close to zero volts as possible with the trimming resistors VR1 and VR2, respectively. To access these trimming resistors the protection shield covering the MC transformer socket should be removed.

The bias is measured across two emitter resistors. These are 5W wire wound resistors with rated resistance of 0.5Ω. The voltage drop across resistors R43 and R45 should be adjusted to ~20mV DC with the trimming resistor VR3 (the DC voltmeter is connected between pins 11 and 17). This is a bias on the left channel. And on the right channel, the voltage drop across resistors R44 and R46 should be adjusted to ~20mV DC with the trimming resistor VR4 (the DC voltmeter is connected between pins 12 and 18). To access these trimming resistors the protection shield covering the heat sink should be removed.

DC offset on the left and right channel after restoration

Bias on the left and right channel after restoration

Output Power Test

The 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 22.13 VRMS (left channel) and 22.67 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 61.2W on the left channel and 64.2W on the right channel. This is very close to the factory specification for this model. A very small difference observed between the left and right channels is due to imperfection/aging of the balance potentiometer. But that is still not too bad for a vintage gear manufactured almost half a century ago!

Output power test - still very close to factory specification

The final result can be seen in the photos below. The receiver looks awesome again, the sound is crystal clear, smooth, and fairly well detailed. The power is more than enough 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-828 - after restoration

Demo video after repair & restoration