Pioneer SX-790 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Pioneer

Model: SX-790

SN: YF3600217S

Take a look at what’s on my bench today! This is the rare Pioneer SX-790 receiver from the late 1970s. This model you don’t come across very often. The SX-790 is the blackout version of the SX-780, featuring a slightly different appearance while maintaining similar internal specifications. This receiver is truly a gem for any audio enthusiast.

The SX-790 was manufactured from 1978 to 1980. It produces 45 watts per channel into 8 ohms with no more than 0.05% total harmonic distortion. The damping factor at 8 ohms is 30.

Although the Pioneer SX-790 is essentially identical to the SX-780 model I serviced a few years ago, I felt it was worthwhile to document this restoration as well. This post can serve as a helpful reference for anyone looking to restore his own SX-790. Let’s dive in!

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

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.

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 – 60uF, ESR – 1.09Ω, deviation: -40%

C302: rated capacitance – 100uF, measured – 68uF, ESR – 3.4Ω, deviation: -32%

C303: rated capacitance – 100uF, measured – 69uF, ESR – 1.53Ω, deviation: -31%

C304: rated capacitance – 100uF, measured – 74uF, ESR – 1.56Ω, deviation: -26%

C305: rated capacitance – 47uF, measured – 45uF, ESR – 1.98Ω, deviation: -4%

C306: rated capacitance – 47uF, measured – 45uF, ESR – 1.81Ω, deviation: -4%

C307: rated capacitance – 100uF, measured – 90uF, ESR – 1.09Ω, deviation: -10%

C308: rated capacitance – 100uF, measured – 91uF, ESR – 1.81Ω, deviation: -9%

C309: rated capacitance – 2.2uF, measured – 2.6uF, ESR – 3.6Ω, deviation: +18%

C310: rated capacitance – 2.2uF, measured – 2.6uF, ESR – 2.9Ω, deviation: +18%

C311: rated capacitance – 100uF, measured – 104uF, ESR – 1.13Ω, deviation: +4%

C314: rated capacitance – 470uF, measured – 408uF, ESR – 0.62Ω, deviation: -13%

C315: rated capacitance – 220uF, measured – 200uF, ESR – 0.72Ω, deviation: -9%

C316: rated capacitance – 47uF, measured – 45uF, ESR – 0.94Ω, deviation: -4%

Protection Circuit

The protection circuit in SX-790 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 – 34uF, ESR – 1.48Ω, deviation: +3%

C318: rated capacitance – 10uF, measured – 13uF, ESR – 2.1Ω, deviation: +30%

C319: rated capacitance – 47uF, measured – 58uF, ESR – 1.45Ω, deviation: +23%

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 – 2.6uF, ESR – 6.4Ω, deviation: +18%

C104: rated capacitance – 2.2uF, measured – 2.6uF, ESR – 5.1Ω, deviation: +18%

C105: rated capacitance – 470uF, measured – 498uF, ESR – 1.94Ω, deviation: +6%

C106: rated capacitance – 470uF, measured – 502uF, ESR – 1.61Ω, deviation: +7%

C117: rated capacitance – 2.2uF, measured – 2.4uF, ESR – 5.6Ω, deviation: +9%

C118: rated capacitance – 2.2uF, measured – 2.3uF, ESR – 6.1Ω, deviation: +5%

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 – 5.9uF, ESR – 2.8Ω, deviation: +26%

C202: rated capacitance – 4.7uF, measured – 6.1uF, ESR – 2.9Ω, deviation: +30%

C207: rated capacitance – 1uF, measured – 1.2uF, ESR – 6.6Ω, deviation: +20%

C208: rated capacitance – 1uF, measured – 1.1uF, ESR – 7.2Ω, deviation: +10%

C213: rated capacitance – 47uF, measured – 51uF, ESR – 2.1Ω, deviation: +9%

C214: rated capacitance – 47uF, measured – 51uF, ESR – 2.2Ω, deviation: +9%

C217: rated capacitance – 4.7uF, measured – 5.8uF, ESR – 8.2Ω, deviation: +23%

C218: rated capacitance – 4.7uF, measured – 5.7uF, ESR – 7.8Ω, deviation: +21%

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

C228: rated capacitance – 0.22uF, measured – 0.21uF, ESR – N/A, deviation: -5%

C229: rated capacitance – 2.2uF, measured – 2.9uF, ESR – 6.3Ω, deviation: +32%

C230: rated capacitance – 2.2uF, measured – 2.8uF, ESR – 6.2Ω, deviation: +27%

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 – 0.55uF, ESR – N/A, deviation: +17%

C402: rated capacitance – 0.47uF, measured – 0.58uF, ESR – N/A, deviation: +23%

C403: rated capacitance – 0.47uF, measured – 0.59uF, ESR – N/A, deviation: +26%

C404: rated capacitance – 0.47uF, measured – 0.56uF, ESR – N/A, deviation: +19%

C405: rated capacitance – 10uF, measured – 12uF, ESR – 2.6Ω, deviation: +20%

C406: rated capacitance – 220uF, measured – 233uF, ESR – 1.79Ω, deviation: +6%

VU Meters Circuit

The VU meters circuit has 4 aluminum electrolytic capacitors: C324, C325, C326, and C327. All 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 – 6.5uF, ESR – 2.2Ω, deviation: +38%

C325: rated capacitance – 4.7uF, measured – 6.2uF, ESR – 2.4Ω, deviation: +32%

C326: rated capacitance – 4.7uF, measured – 6.5uF, ESR – 2.2Ω, deviation: +38%

C327: rated capacitance – 4.7uF, measured – 6.3uF, ESR – 2.2Ω, deviation: +34%

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. If you need to service this model, I strongly recommend carefully investigating a PCB around those transistors for any signs of excess heating. 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 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 2 low-leakage capacitors (C38, C40), one solid tantalum capacitor (C37), and 12 aluminum electrolytic capacitors (C27, C28, C34, C35, C41, C43, C44, C53, C60, C61, C71, C76). An additional axial electrolytic capacitor is soldered on the foil side between pin 22 and the negative lead of e-cap C76.

The original low-leakage capacitors C38 and C40 were replaced with film polyester WIMA caps. The original solid tantalum 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 original axial e-cap was replaced with a new Vishay (138 AML series) capacitor. 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 – 2.9Ω, deviation: +27%

C28: rated capacitance – 4.7uF, measured – 6.5uF, ESR – 3.3Ω, deviation: +38%

C34: rated capacitance – 1uF, measured – 1.2uF, ESR – 3.2Ω, deviation: +20%

C35: rated capacitance – 22uF, measured – 25uF, ESR – 1.79Ω, deviation: +14%

C37: rated capacitance – 6.8uF, measured – 7.5uF, ESR – 4.2Ω, deviation: +10%

C38: rated capacitance – 1uF, measured – 1.2uF, ESR – 6.3Ω, deviation: +20%

C40: rated capacitance – 1uF, measured – 1.2uF, ESR – 6.7Ω, deviation: +20%

C41: rated capacitance – 1uF, measured – 1.2uF, ESR – 3.1Ω, deviation: +20%

C43: rated capacitance – 330uF, measured – 362uF, ESR – 0.89Ω, deviation: +10%

C44: rated capacitance – 220uF, measured – 170uF, ESR – 1.73Ω, deviation: -23%

C53: rated capacitance – 220uF, measured – 207uF, ESR – 1.25Ω, deviation: -6%

C60: rated capacitance – 4.7uF, measured – 6.2uF, ESR – 3.6Ω, deviation: +32%

C61: rated capacitance – 3.3uF, measured – 4.2uF, ESR – 3.2Ω, deviation: +27%

C71: rated capacitance – 10uF, measured – 12uF, ESR – 3.5Ω, deviation: +20%

C76: rated capacitance – 10uF, measured – 13uF, ESR – 2.9Ω, deviation: +30%

C# (axial cap): rated capacitance – 10uF, measured – 11, ESR – 1.75Ω, deviation: +10%

The original 2SA726 transistor in positions Q7 and Q8 is notorious for its shot noise. I replaced it with a modern low-noise Fairchild KSA992 transistor. See the pinout for the replacement transistors. The original transistor is BCE, and the new one is ECB. Also, make sure that the new transistors are gain-matched (note the blue dots on the original transistors).

Additional axial e-cap soldered on the foil side - before and after

Tuner board - before and after

Dial Lamps

As I mentioned in my previous post, I prefer to install new incandescent dial bulbs in any SX-x50 or SX-x80 model. In my opinion, the modern warm white LED lamps tend to change the original look of these receivers. To address the issue with the heat coming from these bulbs I installed a strip of foil tape inside the top cover. It reflects the light and 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 20.63 VRMS (left channel) and 20.95 VRMS (right channel). It corresponds to the output power of 53.2W on the left channel and 54.9W on the right channel.

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-790 - after restoration

Demo video after repair & restoration