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

Yamaha CR-620 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Yamaha

Model: CR-620

SN: 509018

A classic '70s receiver recently arrived on my bench for a complete restoration. The Yamaha CR-620 Natural Sound AM/FM receiver is a mid-range model from the renowned Yamaha CR-XX20 series, which includes the CR-220, CR-420, CR-620, CR-820, CR-1020, CR-2020, and CR-3020. This model was manufactured from 1977 to 1980. It produces 35 watts per channel into 8 ohms with no more than 0.05% total harmonic distortion. The damping factor at 8 ohms is 40. 

Main C. Board - 1, Amplifier (NA-06922-1)

The power supply & protection circuit has 18 aluminum electrolytic capacitors (C802, C803, CC804, C805, C806, C807, C808, C809, C810, C811, C812, C813, C814, C815, C816, C820, C825, C826). The two main filter capacitors (C823 and C824) are also installed on this board.

All original electrolytic capacitors (except the filter capacitors) were replaced with Nichicon UPW/UPM low-impedance capacitors. The original filter capacitors (C823 and C824) were replaced with Nichicon LKG caps. The LKG series was developed for high-quality audio equipment and is, in my opinion, the best replacement for original filter capacitors on the market.

The original e-caps were tested using an Atlas ESR70 capacitance meter, and the results are shown below. Note that many capacitors exhibit high ESR and/or capacitance values that fall outside factory specifications.

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

C802: rated capacitance – 1000uF, measured – 824uF, ESR – 0.72Ω, deviation: -18%

C803: rated capacitance – 100uF, measured – 110uF, ESR – 1.04Ω, deviation: +10%

C804: rated capacitance – 47uF, measured – 54uF, ESR – 2.3Ω, deviation: +15%

C805: rated capacitance – 470uF, measured – 538uF, ESR – 0.86Ω, deviation: +14%

C806: rated capacitance – 47uF, measured – 55uF, ESR – 1.12Ω, deviation: +17%

C807: rated capacitance – 47uF, measured – 57uF, ESR – 1.16Ω, deviation: +21%

C808: rated capacitance – 220uF, measured – 262uF, ESR – 1.06Ω, deviation: +19%

C809: rated capacitance – 100uF, measured – 98uF, ESR – 1.28Ω, deviation: -2%

C810: rated capacitance – 100uF, measured – 109uF, ESR – 0.96Ω, deviation: +9%

C811: rated capacitance – 100uF, measured – 96uF, ESR – 0.74Ω, deviation: -4%

C812: rated capacitance – 100uF, measured – 98uF, ESR – 0.68Ω, deviation: -2%

C813: rated capacitance – 22uF, measured – 23uF, ESR – 2.1Ω, deviation: +5%

C814: rated capacitance – 22uF, measured – 24uF, ESR – 2.1Ω, deviation: +9%

C815: rated capacitance – 4.7uF, measured – 4.4uF, ESR – 14.3Ω, deviation: -6%

C816: rated capacitance – 33uF, measured – 8uF, ESR – 6.4Ω, deviation: -76%

C820: rated capacitance – 47uF, measured – 62uF, ESR – 3.4Ω, deviation: +32%

C825: rated capacitance – 10uF, measured – 12uF, ESR – 8.6Ω, deviation: +20%

C826: rated capacitance – 10uF, measured – 11uF, ESR – 14.3Ω, deviation: +10%

Test results on the original filter capacitors:

C823: rated capacitance – 6800uF, measured – 3365uF, ESR – 1.89Ω, deviation: -51%

C824: rated capacitance – 6800uF, measured – 3374uF, ESR – 1.56Ω, deviation: -50%

All original transistors installed in the power supply & protection circuit were replaced to improve the reliability of the power supply. Below is a list of original and replacement parts that I have used.

TR802: NPN, 2SD234 (original), replaced with a new Fairchild MJE15032G

TR803: NPN, 2SD400 (original), replaced with a new Fairchild KSC2690AYSTU

TR804: NPN, 2SD400 (original), replaced with a new Fairchild KSC2690AYSTU

TR805: NPN, 2SC1918 (original), replaced with a new Fairchild KSC1845FTA

TR806: NPN, 2SC1918 (original), replaced with a new Fairchild KSC1845FTA

TR807: PNP, 2SA844 (original), replaced with a new Fairchild KSA992FBTA

TR808: NPN, 2SC1918 (original), replaced with a new Fairchild KSC1845FTA

TR809: PNP, 2SB544 (original), replaced with a new Fairchild KSA1013YBU

The main amplifier circuit has two low-leakage capacitors (C701, C702) installed in the input signal path, two bi-polar capacitors (C743, C744), and 8 aluminum electrolytic capacitors (C705, C706, C715, C716, C721, C722, C735, C736).

The original low-leakage capacitors were replaced with modern low-leakage Nichicon UKL caps. Two bi-polar capacitors and 4 ordinary e-caps (C721, C722, C735, C736) with a nominal capacitance of 1uF were replaced with high-quality WIMA film polyester caps. The remaining aluminum e-caps were replaced with Nichicon UPW low-impedance capacitors.

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

C701: rated capacitance – 47uF, measured – 24uF, ESR – 1.92Ω, deviation: -49%

C702: rated capacitance – 47uF, measured – 26uF, ESR – 1.85Ω, deviation: -45%

C705: rated capacitance – 220uF, measured – 167uF, ESR – 1.49Ω, deviation: -24%

C706: rated capacitance – 220uF, measured – 175uF, ESR – 1.22Ω, deviation: -20%

C715: rated capacitance – 47uF, measured – 58uF, ESR – 3.1Ω, deviation: +23%

C716: rated capacitance – 47uF, measured – 61uF, ESR – 3.2Ω, deviation: +30%

C721: rated capacitance – 1uF, measured – 1.4uF, ESR – 6.3Ω, deviation: +40%

C722: rated capacitance – 1uF, measured – 1.5uF, ESR – 4.6Ω, deviation: +50%

C743: rated capacitance – 1uF, measured – 1.3uF, ESR – 5.1Ω, deviation: +30%

C744: rated capacitance – 1uF, measured – 1.2uF, ESR – 6.5Ω, deviation: +20%

C735: rated capacitance – 1uF, measured – 1.3uF, ESR – 8.6Ω, deviation: +30%

C736: rated capacitance – 1uF, measured – 1.3uF, ESR – 6.7Ω, deviation: +30%

The first stage of the power amplifier circuit is a differential amplifier composed of two amplifying transistors with a shared emitter: TR701/TR703 and TR702/TR704. The original transistors installed in these positions are PNP 2SA872s, which are not on my list of infamous transistors prone to failure. However, it is always prudent to test the transistors in the differential amplifier to confirm they remain well-matched.

Testing revealed that the mismatch between the original transistors in the left channel is relatively small (~6%). In contrast, the mismatch in the right channel is significantly higher (~19%). To enhance the reliability of the differential amplifier, I opted to replace the original 2SA872 transistors in each pair with closely matched (within 1%) modern Fairchild KSA992 low-noise transistors. The new transistors have the same pinout as the original ones.

Test results on original 2SA872 transistors

TR701 (left channel): hfe - 364, Vbe - 0.770V 

TR703 (left channel): hfe - 342, Vbe - 0.768V

TR702 (right channel): hfe - 361, Vbe - 0.780V 

TR704 (right channel): hfe - 304, Vbe - 0.777V

Test results on new KSA992 transistors

TR701 (left channel): hfe - 480, Vbe - 0.758V 

TR703 (left channel): hfe - 480, Vbe - 0.760V

TR702 (right channel): hfe - 486, Vbe - 0.758V 

TR704 (right channel): hfe - 487, Vbe - 0.759V

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

Main C. Board - 1, Amplifier - before and after

Main C. Board - 2, Tone Control (NA-06922-2)

The tone control board has 6 low-leakage capacitors (C601, C602, C611, C612, C613, C614) installed in the signal path, two low-impedance capacitors (C619, C620), and 3 ordinary aluminum electrolytic capacitors (C609, C610, C631).

All the original low-leakage capacitors, along with the two low-impedance capacitors, were replaced with modern Nichicon UKL low-leakage capacitors. The remaining aluminum electrolytic capacitors were upgraded to Nichicon UPW low-impedance capacitors.

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

C601: rated capacitance – 10uF, measured – 9uF, ESR – 3.4Ω, deviation: -10%

C602: rated capacitance – 10uF, measured – 9uF, ESR – 2.2Ω, deviation: -10%

C609: rated capacitance – 47uF, measured – 59uF, ESR – 4.5Ω, deviation: +26%

C610: rated capacitance – 47uF, measured – 61uF, ESR – 3.2Ω, deviation: +30%

C611: rated capacitance – 47uF, measured – 24uF, ESR – 3.4Ω, deviation: -49%

C612: rated capacitance – 47uF, measured – 23uF, ESR – 4.2Ω, deviation: -51%

C613: rated capacitance – 100uF, measured – 33uF, ESR – 5.9Ω, deviation: -67%

C614: rated capacitance – 100uF, measured – 23uF, ESR – 6.2Ω, deviation: -77%

C619: rated capacitance – 100uF, measured – 28uF, ESR – 4.2Ω, deviation: -72%

C620: rated capacitance – 100uF, measured – 26uF, ESR – 2.8Ω, deviation: -74%

C631: rated capacitance – 10uF, measured – 11uF, ESR – 5.4Ω, deviation: +10%

Main C. Board - 2, Tone Control - before and after

Tuner C. Board - 1 (NA-06923-1)

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 circuit has 22 aluminum electrolytic capacitors: C112, C113, C114, C115, C202, C203, C204, C207, C208, C210, C211, C212, C223, C224, C302, C303, C306, C312, C313, C401, C402, C406.

All capacitors with a nominal capacitance of 0.22uF were replaced with Kemet film polyester caps, and the capacitors with nominal capacitances of 0.47uF and 1uF were replaced with WIMA film polyester caps. The remaining aluminum electrolytic capacitors were upgraded to Nichicon UPW low-impedance capacitors.

Test results on the original capacitors removed from the tuner circuit:

C112 rated capacitance – 1uF, measured – 1.4uF, ESR – 14.9Ω, deviation: +40%

C113: rated capacitance – 10uF, measured – 12uF, ESR – 8.2Ω, deviation: +20%

C114: rated capacitance – 47uF, measured – 61uF, ESR – 2.1Ω, deviation: +30%

C115: rated capacitance – 0.47uF, measured – 0.54uF, ESR – N/A, deviation: +15%

C202: rated capacitance – 100uF, measured – 128uF, ESR – 1.28Ω, deviation: +28%

C203: rated capacitance – 0.47uF, measured – 0.61uF, ESR – N/A, deviation: +30%

C204: rated capacitance – 10uF, measured – 10uF, ESR – 7.3Ω, deviation: 0%

C207: rated capacitance – 10uF, measured – 11uF, ESR – 11.6Ω, deviation: +10%

C208: rated capacitance – 0.22uF, measured – 0.21uF, ESR – N/A, deviation: -5%

C210: rated capacitance – 1uF, measured – 1.4uF, ESR – 8.4Ω, deviation: +40%

C211: rated capacitance – 220uF, measured – 256uF, ESR – 5.4Ω, deviation: +16%

C212: rated capacitance – 220uF, measured – 251uF, ESR – 4.5Ω, deviation: +14%

C223: rated capacitance – 0.47uF, measured – 0.61uF, ESR – N/A, deviation: +30%

C224: rated capacitance – 0.47uF, measured – 0.79uF, ESR – N/A, deviation: +68%

C302: rated capacitance – 1uF, measured – 1.6uF, ESR – 6.4Ω, deviation: +60%

C303: rated capacitance – 10uF, measured – 10uF, ESR – 15.3Ω, deviation: 0%

C306: rated capacitance – 1uF, measured – 1.5uF, ESR – 6.7Ω, deviation: +50%

C312: rated capacitance – 0.22uF, measured – 0.23uF, ESR – N/A, deviation: +5%

C313: rated capacitance – 3.3uF, measured – 3.4uF, ESR – 9.5Ω, deviation: +3%

C401: rated capacitance – 100uF, measured – 126uF, ESR – 2.8Ω, deviation: +26%

C402: rated capacitance – 10uF, measured – 9uF, ESR – 8.8Ω, deviation: -10%

C406: rated capacitance – 33uF, measured – 44uF, ESR – 1.87Ω, deviation: +33%

The phono amplifier is also located on this board. It has 4 low-leakage capacitors (C501, C502, C517, C518) installed in the signal path and 3 aluminum electrolytic capacitors (C515, C516, C520).

The original low-leakage capacitors were replaced with modern Nichicon UKL low-leakage capacitors, and the remaining aluminum electrolytic capacitors were upgraded to Nichicon UPW low-impedance capacitors.

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

C501 rated capacitance – 10uF, measured – 11uF, ESR – 3.8Ω, deviation: +10%

C502 rated capacitance – 10uF, measured – 10uF, ESR – 3.1Ω, deviation: 0%

C515 rated capacitance – 470uF, measured – 564uF, ESR – 1.33Ω, deviation: +20%

C516 rated capacitance – 470uF, measured – 546uF, ESR – 1.12Ω, deviation: +16%

C517 rated capacitance – 2.2uF, measured – 2.1uF, ESR – 4.2Ω, deviation: -5%

C518 rated capacitance – 2.2uF, measured – 2.1uF, ESR – 4.6Ω, deviation: -5%

C520 rated capacitance – 47uF, measured – 61uF, ESR – 1.31Ω, deviation: +30%

This board also houses four low-leakage capacitors (C671, C672, C677, C678) from the tone control circuit. All of them were replaced with modern Nichicon UKL low-leakage capacitors.

Test results on the original capacitors removed from the rest of the tone control circuit:

C671 rated capacitance – 10uF, measured – 10uF, ESR – 2.3Ω, deviation: 0%

C672 rated capacitance – 10uF, measured – 11uF, ESR – 1.9Ω, deviation: +10%

C677 rated capacitance – 2.2uF, measured – 2.1uF, ESR – 3.8Ω, deviation: -5%

C678 rated capacitance – 2.2uF, measured – 2.2uF, ESR – 3.7Ω, deviation: 0%

Tuner C. Board - 1 (NA-06923-1) - before and after

Power Transistors

The original PNP power transistor in the left channel was open, while the NPN power transistor in the right channel exhibited a relatively low current gain of 29. To address these issues, I replaced all the original transistors with new Onsemi MJ21193G (PNP) and MJ21194G (NPN) transistors. These modern transistors are optimized for high-power audio applications and offer excellent gain linearity. A fresh thermal compound and new Mica pads were applied between each power transistor and the heat sink.

left channel:

TR723 (2SD371, NPN), hfe - 84, Vbe - 0.626V

TR725 (2SB531, PNP), open collector-base

right channel:

TR724 (2SD371, NPN), hfe - 29, Vbe - 0.629V

TR726 (2SB531, PNP), hfe - 110 Vbe - 0.635V

Original and new power transistors

Dial and Meter Lamps

Two meters in the CR-620 are illuminated with three incandescent lamps. One of them was burned out. I replaced all of them with new incandescent lamps.

There is no backlit to the dial scale in CR-620 but the dial pointer is illuminated. This design makes it relatively easy to replace the burned pointer bulb. One just needs to unscrew two small screws on the top of the dial pointer unit and the lamp will be released.

Meter lamps - old and new

Dial pointer lamp - original and new

Bias Adjustment

The bias for the left channel is measured between TP1 (-) and TP2 (+), while the bias for the right channel is measured between TP3 (-) and TP4 (+). It should be adjusted to 8mV using the trimming resistors VR701 and VR702.

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 18.51 VRMS (left channel) and 18.75 VRMS (right channel). It corresponds to the output power of 42.8W on the left channel and 43.9W 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 receiver looks great and sounds fantastic! Please watch a short demo video at the end of this post. Thank you for reading.

Yamaha CR-620 - after restoration

Demo video after repair & restoration

Technics SU-7700 Stereo Integrated Amplifier Restoration

Unit: Stereo Integrated Amplifier

Manufacturer: Technics

Model: SU-7700

SN: AB7524A036

Today I'm showcasing a Technics SU-7700 stereo integrated amplifier that came in for restoration. The SU-7700 was manufactured from 1976 to 1979. It produces 50 watts per channel into 8 ohms with no more than 0.08% total harmonic distortion. The damping factor at 8 ohms is 50.

The Technics SU-7700 is a big brother of Technics SU-7300. Both amplifiers share a similar design with two attractive large-scale VU meters and a precise 41-step volume control. In addition, the SU-7700 is equipped with a muting relay which operates in conjunction with a protective circuit.

Main Amplifier Board (SUP 10392A)

The main amplifier board contains the power supply circuit, muting/protection switching circuit, phono amplifier, main amplifier, and VU meters circuit.

The power supply circuit provides +23V DC (pin #7) and -15V DC (pin #4) for the phono amplifier integrated circuits IC101 and IC102. The voltage at pins 7 and 4 is controlled by Zener diodes D101 and D102, respectively. The bridge rectifier and two filter capacitors (C401, C402) provide the power to the main amplifier circuit.

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

Test results on the original filter capacitors:

C401: rated capacitance – 10000uF, measured – 10550uF, ESR – 0.53Ω, deviation: +6%

C402: rated capacitance – 10000uF, measured – 10420uF, ESR – 0.55Ω, deviation: +4%

The local filtering capacitors C129 and C130 were replaced with low impedance Nichicon UPW caps. 

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

C129: rated capacitance – 100uF, measured – 67uF, ESR – 1.14Ω, deviation: -33%

C130: rated capacitance – 330uF, measured – 149uF, ESR – 1.06Ω, deviation: -55%

The muting/protection switching circuit has one bi-polar capacitor C301 and two aluminum electrolytic capacitors (C302, C303). 

The original bi-polar capacitor was replaced with a new Nichicon UEP bi-polar cap. The other two aluminum e-caps were replaced with low-impedance Nichicon UPW capacitors.

Test results on original capacitors removed from the muting/protection switching circuit:

C301: rated capacitance – 220uF, measured – 332uF, ESR – 0.97Ω, deviation: +51%

C302: rated capacitance – 3.3uF, measured – 2.3uF, ESR – 5.5Ω, deviation: -30%

C303: rated capacitance – 220uF, measured – 235uF, ESR – 2.1Ω, deviation: +7%

The phono amplifier circuit has 4 low-leakage capacitors installed in the signal path (C101, C102, C125, C126) and 6 aluminum electrolytic capacitors (C113, C114, C115, C116, C117, C118). 

The original low-leakage capacitors installed in the input signal path (C101, C102) were replaced with new low-leakage Nichicon UKL caps. The other pair of low-leakage e-caps installed in the output signal path (C125, C126) were replaced with WIMA film polyester capacitors. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW capacitors.

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

C101: rated capacitance – 3.3uF, measured – 2.7uF, ESR – 4.6Ω, deviation: -18%

C102: rated capacitance – 3.3uF, measured – 2.9uF, ESR – 4.5Ω, deviation: -12%

C113: rated capacitance – 22uF, measured – 11uF, ESR – 3.8Ω, deviation: -50%

C114: rated capacitance – 22uF, measured – 9uF, ESR – 7.8Ω, deviation: -59%

C115: rated capacitance – 47uF, measured – 42uF, ESR – 1.44Ω, deviation: -11%

C116: rated capacitance – 47uF, measured – 48uF, ESR – 1.55Ω, deviation: +2%

C117: rated capacitance – 3.3uF, measured – 2.2uF, ESR – 5.8Ω, deviation: -33%

C118: rated capacitance – 3.3uF, measured – 2.6uF, ESR – 4.1Ω, deviation: -21%

C125: rated capacitance – 1uF, measured – 1uF, ESR – 4.7Ω, deviation: 0%

C126: rated capacitance – 1uF, measured – 0.8uF, ESR – 7.8Ω, deviation: -20%

The main amplifier circuit has 2 low-leakage capacitors installed in the signal path (C201, C202) and 11 aluminum electrolytic capacitors (C211, C212, C213, C214, C223, C225, C226, C261, C262, C265, C266). 

The original low-leakage capacitors were replaced with WIMA film polyester capacitors. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW capacitors.

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

C201: rated capacitance – 1uF, measured – 0.8uF, ESR – 10.1Ω, deviation: -20%

C202: rated capacitance – 1uF, measured – 0.9uF, ESR – 5.9Ω, deviation: -10%

C211: rated capacitance – 33uF, measured – 34uF, ESR – 1.69Ω, deviation: +3%

C212: rated capacitance – 33uF, measured – 35uF, ESR – 1.64Ω, deviation: +6%

C213: rated capacitance – 100uF, measured – 103uF, ESR – 1.52Ω, deviation: +3%

C214: rated capacitance – 100uF, measured – 106uF, ESR – 1.33Ω, deviation: +6%

C223: rated capacitance – 47uF, measured – 52uF, ESR – 1.12Ω, deviation: +11%

C225: rated capacitance – 220uF, measured – 218uF, ESR – 3.1Ω, deviation: -1%

C226: rated capacitance – 220uF, measured – 191uF, ESR – 1.12Ω, deviation: -13%

C261: rated capacitance – 33uF, measured – 35uF, ESR – 1.94Ω, deviation: +6%

C262: rated capacitance – 33uF, measured – 34uF, ESR – 2.1Ω, deviation: +3%

C265: rated capacitance – 47uF, measured – 24uF, ESR – 2.2Ω, deviation: -49%

C266: rated capacitance – 47uF, measured – 23uF, ESR – 2.4Ω, deviation: -51%

The VU meters circuit has 4 aluminum electrolytic capacitors: C351, C352, C353, and C354. All of them were replaced with low-impedance Nichicon UPW capacitors.

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

C351: rated capacitance – 10uF, measured – 11uF, ESR – 3.2Ω, deviation: +10%

C352: rated capacitance – 10uF, measured – 11uF, ESR – 2.7Ω, deviation: +10%

C353: rated capacitance – 10uF, measured – 12uF, ESR – 1.9Ω, deviation: +20%

C354: rated capacitance – 10uF, measured – 11uF, ESR – 2.1Ω, deviation: +10%

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

Main amplifier board - before and after

Bias Adjustments

The idle current (or bias) should be verified whenever a power transistor is replaced. I did not replace the power transistors in this unit, but I checked the bias after completing the rebuild. The bias in the Technics SU-7700 is measured across the emitter resistor R235 on the left channel and R234 on the right channel. According to the service manual, the voltage drop across each resistor should be less than 25 mV. However, if this voltage exceeds 25 mV, the corresponding wire should be cut off (this model does not have trimming resistors for bias adjustment). I checked the voltage drop across each emitter resistor and it was ~33 mV on the left channel and ~34 mV on the right channel. So, I cut both wires and the voltage across each emitter resistor dropped to 6.4 mV on the left channel and 6.5 mV on the right channel. This voltage drop corresponds to the idling current of ~20mA.

I also checked the DC offset and it is close to zero on each channel: 2.7mV (left channel) and 2.1mV (right channel).

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 22.21 VRMS (left channel) and 22.59 VRMS (right channel). It corresponds to the output power of 61.7W on the left channel and 63.8W 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 wood case was treated with Howard's Restor-A-Finish.

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

Technics SU-7700 - after restoration

Demo video after repair & restoration