Marantz 2252B Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Marantz

Model: 2252B

SN: U090603

Today I'm showcasing a Marantz 2252B vintage receiver that came in for restoration. The owner bought this receiver new in 1980 at Pacific Stereo in Houston, TX. The unit arrived in pretty rough condition with multiple issues.

1. All pots and switches are very dirty. The sound is scratchy and distorted.
2. The left channel is faint.
3. The tuner selectivity is bad.
4. The protection relay is in bad condition (a lot of arcing when engaged).
5. The power amplifier & power supply board has some discoloration due to excess heat.
6. Three dial and both meter lamps are burned out.
7. The dial pointer lamp is burned out.
8. The Stereo indicator lamp is burned out.
9. The lamp reflector is broken into several pieces.
10. Four plastic feet are completely destroyed.

Marantz 2252B was manufactured from 1977 to 1980. It produces 52 watts per channel into 8 ohms with no more than 0.05% total harmonic distortion. The optional walnut veneer cabinet for this model is WC-122.

According to the factory stamp on the chassis, this unit was manufactured in March 1978.

Okay, it’s time to rock and roll. Let's bring it back to life, step by step to its initial condition. This receiver deserves it.

Power Amplifier & Power Supply Board (P700)

The power supply, protection circuit, and power amplifier in Marantz 2252B are all located on a single PCB. This design is similar to that in Marantz 2238B which I restored about a year ago. To service this board one should remove four power transistors, disassemble the heat sink from the chassis, remove the speaker switch extension shafts, and finally unsolder several very short wires between the PCB and two filter capacitors. Sounds like a lot of fun, doesn't it?

The regulated power supply circuit has six aluminum e-caps C733, C734, C735, C737, C738, and C739. All of them were replaced with low impedance and high-reliability Nichicon UPW/UPM caps. 

The original e-caps removed from this board were tested with an Atlas ESR70 capacitance meter and the results are below. Three out of six original e-caps failed the test (open circuit/low capacitance)!

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

C733: rated capacitance – 470uF, measured – 431uF, ESR – 0.01Ω, deviation: -8%

C734: rated capacitance – 100uF, measured – 114uF, ESR – 0.04Ω, deviation: +14%

C735: rated capacitance – 33uF, measured – open circuit/low capacitance

C737: rated capacitance – 100uF, measured – open circuit/low capacitance

C738: rated capacitance – 10uF, measured – open circuit/low capacitance

C739: rated capacitance – 1000uF, measured – 1114uF, ESR – 0.02Ω, deviation: +11%

I also noticed the PCB discoloration around two transistors Q750 and Q751. One of them (Q750) was replaced in the past with a 2SC1567 transistor. I decided to replace both transistors to improve the reliability of the power supply circuit. The Q750 was replaced with a new Fairchild KSC2690AYSTU and Q751 with a new Fairchild KSC2383YTA.

The protection circuit has two aluminum e-caps C730, C731, and one low leakage e-cap C740. The low leakage e-cap is soldered on the back (foil) side of the PCB. Two aluminum e-caps were replaced with low impedance Nichicon UPW caps. The e-cap C740 was replaced with a modern low leakage Nichicon UKL cap.

Test results on original capacitors removed from the protection circuit:

C730: rated capacitance – 47uF, measured – 49uF, ESR – 0.34Ω, deviation: +4%

C731: rated capacitance – 330uF, measured – 416uF, ESR – 0.08Ω, deviation: +26%

C740: rated capacitance – 10uF, measured – 11uF, ESR – 0.46Ω, deviation: +10%

The e-cap C740 soldered on the back (foil) side of the PCB

The PCB discoloration was also observed around three transistors (Q743, Q744, and Q745) and 1/4W resistors (R771, R774, R775). I decided to replace all of them as well. The original transistors Q743 and Q744 were replaced with new Fairchild KSC945CYTA. While the original Q745 was replaced with a new Fairchild KSC2383YTA. All original transistors were tested with Atlas DCA55 semiconductor analyzer. Two of them (Q743 and Q744) are still in spec but the Q745 has a very low current gain of 51. According to the datasheet, this transistor should have a current gain of at least 120 (2SC1318, rank R or S). The original 1/4W resistors were replaced with new 1/2W KOA Speer metal film resistors.

The main amplifier circuit has eight low leakage e-caps C701, C702, C705, C706, C719 thru C722, and three aluminum e-caps C709, C710, and C729. All original low leakage e-caps were replaced with modern low leakage Nichicon UKL capacitors. Pay attention to the polarity of e-cap C701. The polarity on the silkscreen is backward! Also, there is a discrepancy with four low leakage capacitors C719 thru C722. According to the schematic, these capacitors are rated at 22uF/16V. The service manual says they are 10uF/16V. The original capacitors installed in these positions were 22uF/16V. So, I replaced them with new Nichicon UKL capacitors (22uF/16V). The remaining aluminum e-caps (C709, C710, and C729) were replaced with low impedance Nichicon UPW caps.

All original e-caps removed from this board were tested with an Atlas ESR70 capacitance meter and the results are below. As can be seen, the original e-cap C701 failed the test (open circuit/low capacitance)! This capacitor is installed in the input signal path and was the main reason for the faint sound in the left channel.

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

C701: rated capacitance – 3.3uF, measured – open circuit/low capacitance

C702: rated capacitance – 3.3uF, measured – 2.2uF, ESR – 7.1Ω, deviation: -33%

C705: rated capacitance – 47uF, measured – 29uF, ESR – 12.4Ω, deviation: -38%

C706: rated capacitance – 47uF, measured – 42uF, ESR – 0.59Ω, deviation: -11%

C709: rated capacitance – 47uF, measured – 54uF, ESR – 0.31Ω, deviation: +15%

C710: rated capacitance – 47uF, measured – 50uF, ESR – 0.11Ω, deviation: +6%

C719: rated capacitance – 22uF, measured – 24uF, ESR – 0.86Ω, deviation: +9%

C720: rated capacitance – 22uF, measured – 22uF, ESR – 0.94Ω, deviation: 0%

C721: rated capacitance – 22uF, measured – 22uF, ESR – 1.36Ω, deviation: 0%

C722: rated capacitance – 22uF, measured – 17uF, ESR – 3.1Ω, deviation: -23%

C729: rated capacitance – 220uF, measured – 222uF, ESR – 0.02Ω, deviation: +1%

The polarity of e-cap C701 is backward on the silkscreen

As I mentioned earlier, the original protection relay was in bad condition. I unsoldered it from the PCB and removed the cover. As expected all contacts were toasted. I replaced it with a new Omron relay.

Original protection relay - all contacts are in bad condition

Power amplifier & power supply board - before servicing

Power Transistors

Four power transistors were removed, degreased, and tested with Atlas DCA55 semiconductor analyzer. The measured DC current gain on all transistors was in spec according to the datasheet. However, 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.

I applied a fresh thermal compound and replaced the old thermal pads with new Mica ones which is always a good idea when working on any vintage receiver. The old grease can frequently dry, causing the transistors to suffer from poor heat dissipation. This can result in overheating and the destruction of the device.

Four power transistors - before and after

Phono Amplifier Board (P400)

The phono amplifier board has six low leakage e-caps (C401, C402, C407, C408, C415, C416) and four aluminum e-caps (C411, C412, C421, C422). All low leakage e-caps were replaced with modern low leakage Nichicon UKL caps. The remaining aluminum e-caps were replaced with low impedance Nichicon UPW/UPM caps. This board has no transistors prone to failure or noise. So, no transistors were replaced.

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

C401: rated capacitance – 2.2uF, measured – 3.1uF, ESR – 5.8Ω, deviation: +41%

C402: rated capacitance – 2.2uF, measured – 2.9uF, ESR – 2.8Ω, deviation: +32%

C407: rated capacitance – 22uF, measured – 22uF, ESR – 0.91Ω, deviation: 0%

C408: rated capacitance – 22uF, measured – 23uF, ESR – 0.86Ω, deviation: +5%

C411: rated capacitance – 47uF, measured – 52uF, ESR – 0.22Ω, deviation: +11%

C412: rated capacitance – 47uF, measured – 54uF, ESR – 0.13Ω, deviation: +15%

C415: rated capacitance – 2.2uF, measured – 2.0uF, ESR – 2.3Ω, deviation: -9%

C416: rated capacitance – 2.2uF, measured – 2.0uF, ESR – 2.3Ω, deviation: -9%

C421: rated capacitance – 100uF, measured – 100uF, ESR – 0.02Ω, deviation: 0%

C422: rated capacitance – 47uF, measured – 54uF, ESR – 0.43Ω, deviation: +15%

Phono amplifier board - before and after

Tone Amplifier Board (PE01)

It is pretty easy to service the tone amplifier board in Marantz 2252B. This board is mounted on the chassis with four nuts. Just unscrew them and the board is released for service. This design is way better than that implemented in Marantz 2238B where the servicing of the tone board is a real PITA.

The tone amplifier board has six low leakage e-caps (CE05, CE06, CE19, CE20, CE21, CE22), two non-polar e-caps (CE23, CE24), and three aluminum e-caps (CE03, CE04, CE25). The original low leakage e-caps (CE19, CE20) and two non-polar e-caps (CE23, CE24) were replaced with high-quality film polyester WIMA MKS2 caps. The remaining low leakage e-caps were replaced with modern low leakage Nichicon UKL capacitors. Three original aluminum e-caps were replaced with low impedance Nichicon UPW caps.

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

CE03: rated capacitance – 22uF, measured – 24uF, ESR – 0.21Ω, deviation: +9%

CE04: rated capacitance – 22uF, measured – 25uF, ESR – 0.27Ω, deviation: +14%

CE05: rated capacitance – 4.7uF, measured – 5.7uF, ESR – 3.5Ω, deviation: +21%

CE06: rated capacitance – 4.7uF, measured – 5.8uF, ESR – 3.4Ω, deviation: +23%

CE19: rated capacitance – 1uF, measured – 0.9uF, ESR – 2.6Ω, deviation: -10%

CE20: rated capacitance – 1uF, measured – 0.9uF, ESR – 2.9Ω, deviation: -10%

CE21: rated capacitance – 3.3uF, measured – 3.3uF, ESR – 2.1Ω, deviation: 0%

CE22: rated capacitance – 3.3uF, measured – 3.3uF, ESR – 1.6Ω, deviation: 0%

CE23: rated capacitance – 1uF, measured – 1uF, ESR – 0.72Ω, deviation: 0%

CE24: rated capacitance – 1uF, measured – 1uF, ESR – 0.65Ω, deviation: 0%

CE25: rated capacitance – 220uF, measured – 195uF, ESR – 0.01Ω, deviation: -11%

Tone amplifier board - before servicing, after cleaning, and after servicing

Filter Capacitors

Two large filter capacitors were tested with Atlas ESR70 capacitance meter.  Both caps are still within the factory capacitance tolerance and have very low 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.

In-circuit test on two filter capacitors - both are still in spec

C005-1: rated capacitance – 10000uF, measured – 9931uF, ESR – 0.02Ω, deviation: -1%

C005-2: rated capacitance – 10000uF, measured – 9952uF, ESR – 0.02Ω, deviation: -1%

Miscellaneous Issues

• All the dial and both meter lamps were replaced with new warm white LED lamps. For more details, please refer to my previous post regarding the restoration of Marantz 2238B.
• The burned dial pointer lamp was replaced with a new incandescent lamp.
• The burned Stereo indicator lamp was also replaced with a new incandescent lamp.
• The main variable capacitor was cleaned with a non-residue contact cleaner. Then, the CAIG FaderLube was applied to each contact. It restored the selectivity of the tuner.
• I did not find another lamp reflector and decided to seal the broken one. It took some time and patience, but the final result was good. It is strong enough again and does its job.
• Four new Marantz feet made from strong BASF A3W Nylon 6/6 material were installed.
• The old power cord was replaced with a new one. The strain relief bushing was replaced as well.

Power Supply Adjustment

The DC voltage between pins J722 (+) and J728 (ground) was adjusted to +35V with the trimming resistor R787.

DC voltage between pins J722 and J728 under no signal condition

Bias and DC Offset Adjustments

The bias is measured across two emitter resistors (wire wound type, 0.39Ω/5W). The voltage drop across resistors R763 and R765 should be adjusted to ~24mV DC with the trimming resistor R731. This is a bias on the left channel. And on the right channel, the voltage drop across resistors R764 and R766 should be adjusted to ~24mV DC with the trimming resistor R732.

The DC offset is measured between pins J717 and J714 (ground) on the left channel and between pins J718 and J714 (ground) on the right channel. It should be adjusted as close to zero volts as possible with the trimming resistors R711 and R712, respectively.

Bias on the left and right channels after restoration

DC offset on the left and right channels after restoration

Output Power Test

The final output power test was performed at the end of my restoration. The amplifier was loaded with a low inductance 8Ω/100W dummy resistor for each channel. The oscilloscope was connected across the speaker terminals and a sine-wave signal of 1kHz was applied to the AUX jacks. The output sine-wave signal was perfectly symmetrical on both channels with no clipping up to 20.66 VRMS (left channel) and 20.48 VRMS (right channel). It corresponds to the output power of 53.4W on the left channel and 52.4W on the right channel.

Output power test

All the knobs and the faceplate 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 receiver looks great again and sounds fantastic! Please watch a short demo video at the end of this post. Thank you for reading.

Marantz 2252B - after restoration

Demo video after repair & restoration

Upgrading the STK-0050 Modules in Pioneer SX-780

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-780

SN: YD3600495S

Another Pioneer SX-780 came in for a full restoration on my bench. The original owner purchased it new in 1978 and used it for many years enjoying the music. The unit is still in excellent cosmetic condition but had been sitting in storage for several decades. The owner recently pulled it from the storage, but the receiver quit working after running for a few hours.

Quick testing revealed that both original STK-0050 modules in this unit are toasted. The original Sanyo power modules are obsolete and no longer available on the market. And the "new" STK-0050 modules available on eBay are usually counterfeit from China. So, after discussion with the owner, we agreed to upgrade the toasted original STK modules with custom-made modules available from eBay seller "delta9electronics" located in Canada (no affiliation, but in my opinion his design and build quality are the best on the market).

A few years ago I serviced another SX-780 and published the restoration process in detail on my blog here: Pioneer SX-780 receiver restoration. In this post, I am going to focus only on the replacement of the original Sanyo power modules. Everything else in this unit was restored in the same manner as described in my previous post.

According to the Pioneer database, this particular unit was manufactured in April 1978.

STK-0050 Modules Troubleshooting

The original STK-0050 modules were unsoldered and tested with the digital multimeter in diode mode. Both modules were found to be toasted.

Test results on original STK-0050 modules (DMM in diode mode):

Left channel

pin 1 (-) and 3 (+): open circuit

pin 8 (-) and 0 (+): open circuit

Right channel

pin 1 (-) and 3 (+): open circuit

pin 8 (-) and 0 (+): 1.4V (Okay)

With the original STK-0050 modules removed from the PCB, I soldered two 1kΩ resistors to verify the front-end circuit. Each 1kΩ resistor was soldered between pins 0 and 8, and pins 1 and 3, respectively. The receiver was powered up and the DC voltage was measured at each pin. As can be seen from the test results below, the voltage measured at pins 1 and 0 was approximately equal but opposite in polarity. It indicates the correct behavior of the STK driver circuits.

Test results with the original STK-0050 modules removed and 1kΩ resistors soldered in place:

pin 1: left channel: -3.9V, right channel: -4.2V

pin 2: left channel: -39.8V, right channel: -39.9V

pin 3: left channel: 0V, right channel: 0V

pin 8: left channel: 0V, right channel: 0V

pin 9: left channel: +39.9V, right channel: +39.7V

pin 0: left channel: +4.0V, right channel: +4.2V

For reference, below are the equivalent circuit of the original STK-0050 module and custom-made replacement module from eBay seller "delta9electronics".

Original STK-0050 module - equivalent circuit

Custom-made replacement module - equivalent circuit

Installation of Replacement STK Modules

It is very easy to install the replacement modules in Pioneer SX-780. The new modules are larger than the original STK-0050 modules due to the wide aluminum heat sink. However, there is plenty of space there, and the mounting holes have the same spacing as in the original STK modules. 

So, the original STK-0050 modules were unsoldered from the PCB, the heat sink was cleaned and degreased, and the replacement modules were installed. A thin film of silicone thermal compound (Wakefield-Vette, 120 series) was applied to the back side of the aluminum heat sink. The jumper wires were bent and soldered to the PCB.

Replacement STK module

Original STK-0050 modules - unsoldered from the PCB

STK replacement modules are installed and soldered

DC offset and Bias Adjustments

After verifying that there are no shorts, I powered up the SX-780 using the Dim Bulb Tester (DBT). The light bulb went bright for a second and then dimmed down almost completely. The protection relay clicked after a few seconds as well. 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 replacement STK modules also allow adjusting the Bias in the SX-780 across the two 0.22Ω emitter resistors (pins 3 and 8). I adjusted the bias to ~20mV on each channel with a blue Bourns 500Ω trimming resistor.

DC offset on the left and right channel after restoration

Bias on the left and right channel after restoration

For reference, I also measured the voltage at each pin of replacement STK modules. These results confirm a normal operation of the output stage of AF power amplifier.

Test results on replacement STK modules:

pin 1: left channel: -1.2V, right channel: -1.2V

pin 2: left channel: -39.5V, right channel: -39.5V

pin 3: left channel: 0V, right channel: 0V

pin 8: left channel: 0V, right channel: 0V

pin 9: left channel: +39.5V, right channel: +39.5V

pin 0: left channel: +1.3V, right channel: +1.4V

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 19.36 VRMS (left channel) and 19.18 VRMS (right channel). It corresponds to the output power of 46.9W on the left channel and 46.0W on the right channel.

Output power test

The final result can be seen in the photos below. The receiver looks brand new again and sounds awesome. I would highly recommend replacing the original toasted STK modules with custom-made ones instead of counterfeit modules from China. 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

Pioneer SX-850 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-850

SN: WJ2901167M

A nice Pioneer SX-850 stereo receiver came on the bench for full restoration. The SX-850 is a high-end offering from Pioneer's most famous product line, the SX-X50 series. It was initially introduced in 1976 and made through 1978. It delivers a continuous power output of 65 watts per channel into an 8 ohms load from 20 to 20 kHz with no more than 0.1% total harmonic distortion. The SX-850 has a super-quiet phono equalizer in a 3-stage direct-coupled design to achieve a wide dynamic range and +/- 0.2dB RIAA equalization accuracy. The unit is heavy and weighs about 42 pounds without a package. As Pioneer states in its 1976's brochure "It's the Pioneer SX-850 - the one you can trust to keep faith with your musical needs". 

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

The restoration of the smaller brothers of SX-850 can be found on my blog here:

Pioneer SX-450 restoration

Pioneer SX-650 restoration

Pioneer SX-750 restoration

Power Supply (AWR-101)

The power supply board in this unit has been serviced before by somebody. And I wouldn't say that this work was done with high quality.

(a) The electrolytic capacitor C18 was replaced with a Jamicon e-cap and C21 with an ITT e-cap. Both brands are definitely not top-notch manufacturers. Moreover, the negative lead of e-cap C18 was not soldered to the PCB at all! 

(b) Two no-name transistors were installed in positions Q8 and Q9. 

(c) The original transistor installed in position Q5 was replaced with another 2SC1318 but the technician didn't cut off the leads after the soldering! 

(d) Finally, a 2A fuse was installed in position FU6 instead of a 1.5A fuse.

The negative lead of e-cap C18 is not soldered at all!

A correct transistor was installed in position Q5 but the leads were not cut off - sloppy work!

The power supply board (AWR-101) has twelve aluminum electrolytic capacitors C10 thru C16, C18 thru C21, and C23. All of them were replaced with low impedance and high-reliability Nichicon UPW/UPM caps. The max operating voltage was increased one step up on several e-caps. 

Note that the electrolytic capacitor C14 installed on this board (AWR-101-C) has a rated capacitance of 47uF/25V instead of 220uF/10V as on the schematic. I checked it against another SX-850 sitting on my bench and waiting for the restoration, and it also has the same rated capacitance in that position, i.e. 47uF/25V. So, it looks like this modification was not well documented by Pioneer which is actually pretty typical.

The original e-caps removed from this board were tested with an Atlas ESR70 capacitance meter and the results are below. Three out of twelve original e-caps are outside the factory capacitance tolerance +/- 20%. And the other three e-caps failed the test (open circuit/low capacitance)! These results are very similar to what I observed on the Pioneer SX-1010 power supply board (AWR-054-A). Both boards work under extreme operating conditions and become pretty well toasted after 45+ years of operation.

Test results on original capacitors removed from power supply board:

C10: rated capacitance – 470uF, measured – 368uF, ESR – 0.01Ω, deviation: -22%

C11: rated capacitance – 470uF, measured – 355uF, ESR – 0.01Ω, deviation: -24%

C12: rated capacitance – 2200uF, measured – 1986uF, ESR – 0.01Ω, deviation: -10%

C13: rated capacitance – 1000uF, measured – 1102uF, ESR – 0.02Ω, deviation: +10%

C14: rated capacitance – 47uF, measured – 55uF, ESR – 1.93Ω, deviation: +17%

C15: rated capacitance – 470uF, measured – 322uF, ESR – 0.02Ω, deviation: -32%

C16: rated capacitance – 47uF, measured – open circuit/low capacitance

C18: rated capacitance – 47uF, measured – 47uF, ESR – 0.42Ω, deviation: 0%

C19: rated capacitance – 47uF, measured – open circuit/low capacitance

C20: rated capacitance – 100uF, measured – 115uF, ESR – 0.02Ω, deviation: +15%

C21: rated capacitance – 47uF, measured – 52uF, ESR – 0.78Ω, deviation: +11%

C23: rated capacitance – 100uF, measured – open circuit/low capacitance

To improve the reliability of the power supply, I also decided to replace all transistors on this board. They should be retired after working hard for the last 45+ years. Below is a list of original and replacement transistors I used. Four transistors Q1, Q2, Q8, and Q9 are mounted on the heat sink. The old thermal pads were replaced with new Mica ones. The old thermal compound was refreshed with a new silicone thermal compound (Wakefield-Vette, 120 series).

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

Q2: PNP, 2SB507 (original), replaced with a new Fairchild KSA940TU

Q3: NPN, 2SC1318 (original), replaced with a new Fairchild KSC2383YTA

Q4: PNP, 2SA720 (original), replaced with a new Fairchild KSA1013YBU

Q5: NPN, 2SC1318 (original), replaced with a new Fairchild KSC2690AYSTU

Q6: PNP, 2SA720 (original), replaced with a new Fairchild KSA1013YBU

Q7: NPN, 2SC1318 (original), replaced with a new Fairchild KSC2383YTA

Q8: NPN, 2SD313 (original), replaced with a new Fairchild KSC2073TU

Q9: NPN, 2SD313 (original), replaced with a new Fairchild KSD526Y

Q10: NPN, 2SC869 (original), replaced with a new Fairchild KSC2383YTA

Power supply board - before servicing

Power supply board - after servicing

Protection Unit (AWM-062-0)

The protection unit in Pioneer SX-850 has the same design as in Pioneer SX-1010 introduced to the market in 1974. There are two coupling capacitors (C1, C2) and four aluminum electrolytic capacitors (C3 thru C6) installed on this board. Two coupling capacitors are the notorious sky blue Sanyo e-caps. I replaced them with high-quality film polyester Kemet caps. The remaining aluminum e-caps were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from protection unit board:

C1: rated capacitance – 0.22uF, measured – 0.37uF, ESR – N/A, deviation: +68%

C2: rated capacitance – 0.22uF, measured – 0.34uF, ESR – N/A, deviation: +55%

C3: rated capacitance – 330uF, measured – 369uF, ESR – 0.08Ω, deviation: +12%

C4: rated capacitance – 330uF, measured – 391uF, ESR – 0.06Ω, deviation: +18%

C5: rated capacitance – 4.7uF, measured – 5.3uF, ESR – 2.1Ω, deviation: +13%

C6: rated capacitance – 100uF, measured – 110uF, ESR – 0.3Ω, deviation: +10%

The relay driver transistor Q7 suffers from the current spike every time the relay turns off. Eventually, it fails due to severe degradation. I replaced the original 2SC1384 transistor installed in this position with a new Fairchild KSC2690. Also, a fly-back safety diode (1N4004G) was added to the circuit to prevent potential output damage. The safety diode was soldered between pins 9 (anode, "+") and 10 (cathode, "-").

Protection unit - before and after

Tone Amplifier Board (AWG-039A)

The tone amplifier board has ten solid tantalum capacitors (C13, C14, C15, C16, C23, C24, C25, C26, C29, C30), two low leakage electrolytic capacitors (C19, C20), and two aluminum e-caps (C21, C22). 

All tantalum capacitors with the rated capacitance of 2.2uF were replaced with film polyester WIMA MKS2 caps while those with the rated capacitance of 0.22uF were replaced with film polyester Kemet caps. Two remaining tantalum capacitors with the rated capacitance of 10uF as well as original low leakage e-caps were replaced with modern low leakage Nichicon UKL caps. Finally, two aluminum filtering capacitors (C21, C22) were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from tone amplifier board:

C13: rated capacitance – 2.2uF, measured – 2.2uF, ESR – 1.8Ω, deviation: 0%

C14: rated capacitance – 2.2uF, measured – 2.2uF, ESR – 3.2Ω, deviation: 0%

C15: rated capacitance – 2.2uF, measured – 2.2uF, ESR – 2.3Ω, deviation: 0%

C16: rated capacitance – 2.2uF, measured – 2.3uF, ESR – 1.6Ω, deviation: +5%

C19: rated capacitance – 100uF, measured – 122uF, ESR – 0.18Ω, deviation: +22%

C20: rated capacitance – 100uF, measured – 116uF, ESR – 0.2Ω, deviation: +16%

C21: rated capacitance – 47uF, measured – 41uF, ESR – 0.19Ω, deviation: -13%

C22: rated capacitance – 47uF, measured – 42uF, ESR – 0.18Ω, deviation: -11%

C23: rated capacitance – 2.2uF, measured – 2.2uF, ESR – 1.96Ω, deviation: 0%

C24: rated capacitance – 2.2uF, measured – 2.2uF, ESR – 1.38Ω, deviation: 0%

C25: rated capacitance – 0.22uF, measured – 0.22uF, ESR – N/A, deviation: 0%

C26: rated capacitance – 0.22uF, measured – 0.22uF, ESR – N/A, deviation: 0%

C29: rated capacitance – 10uF, measured – 11uF, ESR – 1.1Ω, deviation: +10%

C30: rated capacitance – 10uF, measured – 11uF, ESR – 0.6Ω, deviation: +10%

Two original NPN transistors (Q1 and Q2) installed on this board are 2SC1312. This transistor is known to become very noisy over time. I replaced both original transistors with modern low noise Fairchild KSA1845. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

Tone amplifier board - before and after

Flat Amplifier Board (AWG-038)

The flat amplifier board has six low leakage e-caps installed in the signal path (C1, C2, C5, C6, C23, C24), two solid tantalum capacitors (C17, C18), two low leakage e-caps (C25, C26), and three aluminum filtering e-caps (C13, C14, C27).

Two low leakage e-caps (C1, C2) installed in the input signal path were replaced with film polyester WIMA MKS2 caps. The other six low leakage e-caps and two tantalum capacitors were replaced with modern low leakage Nichicon UKL caps. The remaining three aluminum filtering capacitors (C13, C14, C27) were replaced with low impedance Nichicon UPW/UPM caps.

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

C1: rated capacitance – 2.2uF, measured – 2.4uF, ESR – 5.7Ω, deviation: +9%

C2: rated capacitance – 2.2uF, measured – 2.7uF, ESR – 4.8Ω, deviation: +23%

C5: rated capacitance – 4.7uF, measured – 5.3uF, ESR – 1.7Ω, deviation: +13%

C6: rated capacitance – 4.7uF, measured – 5.4uF, ESR – 1.6Ω, deviation: +15%

C13: rated capacitance – 220uF, measured – 182uF, ESR – 0.07Ω, deviation: -17%

C14: rated capacitance – 220uF, measured – 179uF, ESR – 0.02Ω, deviation: -19%

C17: rated capacitance – 10uF, measured – 10uF, ESR – 0.67Ω, deviation: 0%

C18: rated capacitance – 10uF, measured – 11uF, ESR – 1.1Ω, deviation: +10%

C23: rated capacitance – 4.7uF, measured – 5.7uF, ESR – 1.4Ω, deviation: +21%

C24: rated capacitance – 4.7uF, measured – 5.3uF, ESR – 1.6Ω, deviation: +13%

C25: rated capacitance – 100uF, measured – 112uF, ESR – 0.26Ω, deviation: +12%

C26: rated capacitance – 100uF, measured – 105uF, ESR – 0.38Ω, deviation: +5%

C27: rated capacitance – 100uF, measured – 88uF, ESR – 0.12Ω, deviation: -12%

Two original PNP transistors (Q5 and Q6) installed on this board are notorious 2SA725. The failure of this transistor usually results in a shot noise. I replaced both of them with a modern low noise Fairchild KSA992. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

Flat amplifier board - before and after

Switch Assembly (AWS-094)

Only one aluminum electrolytic capacitor (C1) is installed on this board. I replaced it with a low impedance Nichicon UPW cap. Don't forget to clean and lubricate all switches on this board before mounting it back to the chassis!

Test results on the original capacitor removed from the switch assembly

C1: rated capacitance – 220uF, measured – 281uF, ESR – 0.13Ω, deviation: +28%

Switch assembly - before and after

Equalizer Amplifier Board (AWF-011)

The equalizer amplifier circuit in the Pioneer SX-850 is a three-stage direct-coupled design using a one-stage PNP transistor to achieve high gain amplification. Distortion is kept low by applying sufficient negative feedback. 

The original PNP transistors (Q1 and Q2) installed in the first stage are the notorious 2SA725. The failure of this transistor usually results in a shot noise. I replaced both of them with a modern low noise Fairchild KSA992. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB. Also, keep in mind that these transistors are supposed to be gain-matched (orange dot on the top). Below are test results on original transistors removed from the board along with test results on new closely matched transistors.

Test results on original transistors 2SA725:

Left channel: Q1 (gain - 376, Vbe - 0.778V)

Right channel: Q2 (gain - 409, Vbe - 0.771V)

Test results on new closely matched transistors KSA992:

Left channel: Q1 (gain - 412, Vbe - 0.758V)

Right channel: Q2 (gain - 415, Vbe - 0.758V)

The original NPN transistors (Q3 thru Q6) installed on this board are 2SC1313. These are also known as noisy/leaky transistors. I replaced them with modern low noise Fairchild KSA1845. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

Two coupling capacitors (C1, C2) installed in the input signal path are notorious sky blue Sanyo e-caps. I replaced them with film polyester WIMA MKS2 caps to improve the signal-to-noise ratio. Two low leakage e-caps (C11, C12) were replaced with modern low leakage Nichicon UKL caps. And the remaining aluminum electrolytic capacitors (C7, C8, C17, C18) were replaced with low impedance Nichicon UPW/UPM caps.

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

C1: rated capacitance – 1uF, measured – 1.1uF, ESR – 2.2Ω, deviation: +10%

C2: rated capacitance – 1uF, measured – 1.1uF, ESR – 4.3Ω, deviation: +10%

C7: rated capacitance – 330uF, measured – 417uF, ESR – 0.06Ω, deviation: +26%

C8: rated capacitance – 330uF, measured – 422uF, ESR – 0.05Ω, deviation: +28%

C11: rated capacitance – 3.3uF, measured – 4.0uF, ESR – 4.2Ω, deviation: +21%

C12: rated capacitance – 3.3uF, measured – 3.8uF, ESR – 4.2Ω, deviation: +15%

C17: rated capacitance – 100uF, measured – 124uF, ESR – 0.09Ω, deviation: +24%

C18: rated capacitance – 220uF, measured – 274uF, ESR – 0.05Ω, deviation: +25%

Equalizer amplifier board - before and after

Power Amplifier Board (AWH-059)

The SX-850 power amplifier is an all-stage direct-coupled pure complementary circuit. It is an OCL circuit with a balanced positive and negative power supply and center point DC potential kept at 0V.

The first stage of the power amplifier is a differential amplifier consisting of two PNP 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. This transistor gets very noisy over time. I replaced them with modern low noise Fairchild KSA992. Before installation, each pair of KSA992 transistors was carefully matched by current gain and base-emitter voltage. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB. Below are test results on original transistors removed from the board along with test results on new closely matched transistors. As one can see that the differential pair from the left channel is poorly matched.

Test results on original transistors 2SA726:

Left channel: Q1 (gain - 253, Vbe - 0.774V), Q3 (gain - 475, Vbe - 0.775V)

Right channel: Q2 (gain - 361, Vbe - 0.775V), Q4 (gain - 374, Vbe - 0.773V)

Test results on new closely matched transistors KSA992:

Left channel: Q1 (gain - 436, Vbe - 0.759V), Q3 (gain - 439, Vbe - 0.761V)

Right channel: Q2 (gain - 435, Vbe - 0.759V), Q4 (gain - 436, Vbe - 0.761V)

Four coupling capacitors C1 thru C4 installed in the signal path are notorious sky blue Sanyo e-caps. I replaced them with high-quality film polyester WIMA caps. The remaining four aluminum e-caps (C5, C6, C11, C12) were replaced with low impedance Nichicon UPW caps.

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

C1: rated capacitance – 2.2uF, measured – 2.4uF, ESR – 3.1Ω, deviation: +9%

C2: rated capacitance – 2.2uF, measured – 2.7uF, ESR – 1.2Ω, deviation: +23%

C3: rated capacitance – 1uF, measured – 1.3uF, ESR – 3.4Ω, deviation: +30%

C4: rated capacitance – 1uF, measured – 1.2uF, ESR – 2.8Ω, deviation: +20%

C5: rated capacitance – 33uF, measured – 36uF, ESR – 0.78Ω, deviation: +9%

C6: rated capacitance – 33uF, measured – 36uF, ESR – 0.63Ω, deviation: +9%

C11: rated capacitance – 330uF, measured – 368uF, ESR – 0.04Ω, deviation: +12%

C12: rated capacitance – 330uF, measured – 374uF, ESR – 0.05Ω, deviation: +13%

The original trimming resistors VR1/VR2 (10 kΩ) and VR3/VR4 (100 Ω) were replaced with new Bourns potentiometers.

Power amplifier board - before and after

Filter Capacitors

I performed an in-circuit test on two large filter capacitors with Atlas ESR70 capacitance meter.  Both caps are still within the factory capacitance tolerance and have 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.

In-circuit test on two filter capacitors - both are still in spec

C1: rated capacitance – 15000uF, measured – 14300uF, ESR – 0Ω, deviation: -5%

C2: rated capacitance – 15000uF, measured – 15360uF, ESR – 0Ω, deviation: +2%

Dial Lamps

Two out of four original dial lamps were blown out in this unit. Each lamp is very easy to replace since they are inserted into an individual lamp socket. I replaced the original dial lamps with new incandescent lamps (8V, 0.3A, wedge base).

DC offset and Bias Adjustments

At the end of my restoration, I adjusted the DC offset and Bias on the power amplifier according to the service manual. Before adjustment, two jumper plugs were removed and a 5.1kΩ resistor was connected to the POWER IN jacks.

The DC offset is measured between pins 10 and 9 on the left channel, and between pins 25 and 24 on the right channel. It should be adjusted as close to zero volts as possible with the trimming resistors VR1 and VR2, respectively.

The Bias is measured between pins 12 (+) and 13 (-) on the left channel, and between pins 27 (+) and 28 (-) on the right channel. It should be adjusted to ~20mV with the trimming resistors VR3 and VR4, respectively.

DC offset on the left and right channels after restoration

Bias on the left and right channels after restoration

Output Power Test

The final output power test was performed at the end of my restoration. The amplifier was loaded with a low inductance 8Ω/100W dummy resistor for each channel. The oscilloscope was connected across the speaker terminals and a sine-wave signal of 1kHz was applied to the AUX jacks. The output sine-wave signal was perfectly symmetrical on both channels with no clipping up to 23.51 VRMS (left channel) and 23.17 VRMS (right channel). It corresponds to the output power of 69.1W on the left channel and 67.2W on the right channel. A very small difference between the left and right channels is attributed to the imperfection/aging of the volume potentiometer.

Output power test

As usual, all the knobs and the front panel were gently cleaned in warm water with dish soap. The wood case was stained with Howard's Restor-A-Finish. I also slightly polished all knobs with Mothers Mag & Aluminum polish to remove some small spots of aluminum oxidation.

The final result can be seen in the photos below. The receiver looks brand new and sounds awesome. Please watch a short demo video at the end of this post. Thank you for reading.

Pioneer SX-850 - after restoration

Demo video after repair & restoration