Pioneer SX-650 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-650

SN: WL3633228S

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

Initial Troubleshooting

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

Test results on Sanken SP-40W power modules:

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

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

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

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

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

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

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

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

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

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

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

Test results on integrated circuit Q6:

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

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

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

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

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

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

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

pin 8: schematic: ground, measured: 0V

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

pin 8: schematic: ground, measured: 0V

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

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

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

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

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

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

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

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

Old and new integrated circuit Q6 (HA1196)

16 pin DIP socket for integrated circuit Q6

New HA1196 chip installed on the socket

Power Amp - 1 Board

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

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

Power Amp - 2 Board

The original coupling capacitors C21 and C22 installed on the power amp - 2 board (#GWR 101) are sky blue Sanyo electrolytic caps. These e-caps become very leaky over time and should be always replaced in any vintage gear. For more information about sky blue Sanyo e-caps refer to my previous post on Pioneer SX-828 restoration. I replaced them with high-quality film polyester Kemet caps. The other 15 e-caps installed on this board were replaced with new Nichicon UPW/UPM caps. I also removed and tested two transistors Q7 and Q8 mounted on the heat sinks with Atlas DCA55 semiconductor analyzer. Both transistors passed the test and the measured DC current gain was in spec according to the datasheet. I applied a new silicone thermal compound between each transistor and the heat sink before installation on the board.

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

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

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

The old thermal grease is almost completely dried out

Tone Control Board

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

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

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

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

Tuner and AF Board

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

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

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

Tuner and AF board - before and after

Phono equalizer circuit - before and after

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

Output Power Test

At the end of my restoration, I loaded this receiver with a low inductance 8Ω/100W dummy resistor for each channel, connected my oscilloscope across the speaker terminals, and applied a sine-wave signal of 1 kHz to the AUX jacks. The output sine-wave signal was perfectly symmetrical on both channels with no clipping up to 15.73 VRMS (left channel) and 16.63 VRMS (right channel). So, one can calculate that the output power is (15.73x15.73)/8=31W (left channel) and (16.63x16.63)/8=35W (right channel). It is very close to factory specifications. A very small difference between the left and right channels is attributed to the imperfection/aging of the balance potentiometer. Keep in mind that this baby is almost 43 years old but still in perfect working condition!

Output power test

As usual, all the knobs and the front panel were gently cleaned in warm water with dish soap. All knobs were also slightly polished by Mothers Mag & Aluminum polish to remove some small spots of aluminum oxidation. All pots and switches have been cleaned with DeoxIT 5% contact cleaner and lubricated with DeoxIT FaderLube 5% spray.

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

Pioneer SX-650 - before restoration

Pioneer SX-650 - after restoration

Demo video after repair & restoration

Pioneer SX-780 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-780

SN: AF3653494Y

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

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

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

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

AF Amplifier Board (GWK-118)

Power Supply Circuit

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

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

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

The original e-caps were tested with an Atlas ESR70 capacitance meter; the results are below.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Protection Circuit

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

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

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

Test results on original capacitors removed from the protection circuit:

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

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

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

Phono Amplifier Circuit

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

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

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

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

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

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

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

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

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

Control Section

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

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

Test results on original capacitors removed from the control section:

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

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

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

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

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

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

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

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

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

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

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

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

FM Muting Circuit

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

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

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

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

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

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

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

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

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

VU Meters Circuit

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

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

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

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

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

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

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

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

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

AF amplifier board - before and after

Tuner Board (AWE-099)

Be aware, that replacing the original capacitors in the tuner section may require subsequent tuner alignment. I do not recommend servicing the tuner section of any vintage receiver unless you have all the necessary equipment and sufficient experience in tuner alignment.

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

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

Test results on original capacitors removed from the tuner board:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Dial Lamps

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

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

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

DC Offset & Power Meters Adjustments

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

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

DC offset on the left and right channels after restoration

Output Power Test

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

Output power test

As usual, all the knobs and the front panel were gently cleaned in warm water with dish soap. All controls have been cleaned with DeoxIT 5% contact cleaner and lubricated with DeoxIT FaderLube 5% spray.

The final result can be seen in the photos below. The sound is wonderful, very warm, and extremely clean. Please watch a short demo video at the end of this post. Thank you for reading.

Pioneer SX-780 - after restoration

Demo video after repair & restoration

Fisher 202 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Fisher

Model: 202

SN: 15547

Today I am showing a little Fisher 202 receiver that came in for restoration. The Fisher 202 was introduced in 1970 and was the first of the Futura series. It produces 28 watts per channel into 8 ohms with 0.5% total harmonic distortion. The receiver has one phono input, one tape input, and two Aux inputs. The list price in 1970 was $249.95. The optional walnut veneer cabinet for this model is 21-UW.

The previous owner told me that the right channel in this receiver is dead. I checked three fuses on the back side and found that one fuse protecting the right channel is blown. I replaced the blown fuse and powered up this little receiver using my Dim Bulb Tester (DBT) to check that there are no short circuits. For more information on Dim Bulb Tester refer to my previous post on Craig 5502 restoration. The receiver passed the test with DBT and I removed the cover to look inside.

Power Amplifier Board

The power amplifier in Fisher 202 is very easy to service since each channel has a separate driver board inserted in the slot. I removed both boards for inspection and immediately realized that the resistor R10 on the right channel board is severely overheated. I measured its resistance and it was 267Ω. The same resistor on the left board was 183Ω. The schematic says the resistor R10 is supposed to be 180Ω. The overheated resistor from the right channel is definitely not original. I unsoldered the resistor R10 from both boards and replaced it with a new metal film resistor 180Ω/0.5W. I also tested all other electronic components (transistors, diodes, and remaining resistors) on each driver board. No other problems with electronic components were found on those boards. I replaced the electrolytic capacitor C01 with a low leakage Nichicon UKL cap. The remaining three e-caps C02, C03, and C04 were replaced with low impedance and high-reliability Nichicon UPW capacitors.

Test results on original capacitors removed from Power Amplifier board:

C01-L: rated capacitance – 1uF, measured – 1.7uF, deviation: +70%

C01-R: rated capacitance – 1uF, measured – 1.8uF, deviation: +80%

C02-L: rated capacitance – 100uF, measured – 124uF, deviation: +24%

C02-R: rated capacitance – 100uF, measured – 165uF, deviation: +65%

C03-L: rated capacitance – 220uF, measured – 261uF, deviation: +19%

C03-R: rated capacitance – 220uF, measured – 275uF, deviation: +25%

C04-L: rated capacitance – 47uF, measured – 68uF, deviation: +45%

C04-R: rated capacitance – 47uF, measured – 66uF, deviation: +40%

Left and right driver boards - before servicing

Overheated resistor R10 on the right driver board

Resistor R10 installed on left driver board

Driver boards - after servicing

Driver boards installed

Power Transistors

The original Hitachi power transistors 2SC1030 were removed, cleaned, and tested with Atlas DCA55 semiconductor analyzer. The DC current gain was in spec according to the datasheet. Each pair was also very well matched within ~3%. 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 also replaced the old thermal pads with new Mica ones.

Power transistors tested with Atlas DCA55 semiconductor analyzer

Old thermal pad - the thermal grease is almost completely dried out

Original power transistors with new pads and fresh thermal compound

Power Supply Board

The power supply board provides +15.5V DC for the tuner, +31.5V DC for the pre-amplifier and control amplifier, and +62V DC for a power amplifier. This board has 3 electrolytic capacitors C01, C02, and C03. I replaced all e-caps with low impedance and high-reliability Nichicon UPW/UPM caps. The test results on the original e-caps removed from this board are below. All of them are outside of the factory capacitance tolerance +/- 20%.

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

C01: rated capacitance – 100uF, measured capacitance – 161uF, deviation: +61%

C02: rated capacitance – 100uF, measured capacitance – 142uF, deviation: +42%

C03: rated capacitance – 220uF, measured capacitance – 287uF, deviation: +31%

Power Supply board - before and after

Pre-amplifier Board

The pre-amplifier board has 5 electrolytic capacitors: C1/C2, C9/C10, and C16. Two signal path capacitors C1 and C2 were replaced with low leakage Nichicon UKL caps. The remaining e-caps were replaced with low impedance and high-reliability Nichicon UPW/UPM caps. Four NPN transistors 2SC458LG installed on this board are prone to create noise or work improperly. I replaced those transistors (TR01 thru TR04) with modern low-noise Fairchild KSC1845. The new transistors were gain matched within 1%. Watch the pinout on replacement transistors if you need to service this board. The original transistor is BCE and the new one is ECB. The original electrolytic capacitors and transistors removed from the pre-amplifier board were tested to check their long-term reliability. As expected all e-caps are outside of the factory capacitance tolerance +/- 20%.

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

C1: rated capacitance – 10uF, measured – 19uF, deviation: +90%

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

C9: rated capacitance – 100uF, measured – 158uF, deviation: +58%

C10: rated capacitance – 100uF, measured – 160uF, deviation: +60%

C16: rated capacitance – 100uF, measured – 158uF, deviation: +58%

Pre-amplifier board - before and after

Pre-amplifier board - all new Fairchild KSC1845 transistors are gain matched

Control Amplifier Board

The control amplifier board has nine aluminum electrolytic capacitors: C503, C504, C511, C512, C513, C514, C517, C518, and C525. All capacitors except the filtering cap C525 were replaced with low-leakage Nichicon UKL caps. The e-cap C525 was replaced with a low-impedance and high-reliability Nichicon UPW cap. Four noisy 2SC458LG transistors were replaced with modern low-noise Fairchild KSC1845. The new transistors were gain matched within 1%.

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

C503: rated capacitance – 10uF, measured – 19uF, deviation: +90%

C504: rated capacitance – 10uF, measured – 18uF, deviation: +80%

C511: rated capacitance – 4.7uF, measured – 6.1uF, deviation: +30%

C512: rated capacitance – 4.7uF, measured – 6.0uF, deviation: +28%

C513: rated capacitance – 1uF, measured – 1.6uF, deviation: +60%

C514: rated capacitance – 1uF, measured – 1.6uF, deviation: +60%

C517: rated capacitance – 1uF, measured – 1.6uF, deviation: +60%

C518: rated capacitance – 1uF, measured – 1.6uF, deviation: +60%

C525: rated capacitance – 220uF, measured – 282uF, deviation: +28%

Control amplifier board - before and after

Dial and Stereo Indicator Lamps

It is very easy to replace the old dial bulbs in Fisher 202. Just unscrew two small screws on the top of the dial plate and the plate holding all dial lamps would be released. I replaced the original incandescent bulbs with warm white LED lamps to maintain the original look and decrease heat.

Two stereo indicator bulbs burned out. I replaced them with new white LED lamps.

Old incandescent dial bulbs

New warm white LED lamps installed

Old stereo indicator bulbs

New white LED lamps installed

Center Voltage Adjustment

The center voltage adjustment is a straightforward procedure. The multimeter should be connected between pin 5 of the left/right channel driver board and the ground. With no signal input, the DC voltage at pin 5 should be adjusted to 31V. In general, the DC voltage at pin 5 should always be half of the voltage at pin 3. So, I adjusted the center voltage at pin 5 to ~31.5V on each channel since the voltage measured at pin 3 was 63V.

Center voltage adjustment - left channel

Center voltage adjustment - right channel

As a routine procedure on all vintage gears I restore, all the knobs and the faceplate were gently cleaned in warm water with dish soap. Also, all pots and switches 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. Thank you for reading.

Fisher 202 - before restoration

Fisher 202 - after restoration