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

Yamaha CA-1000 Stereo Integrated Amplifier Restoration

Unit: Stereo Integrated Amplifier

Manufacturer: Yamaha

Model: CA-1000

SN: 23974

Today I'm showing a beautiful Yamaha CA-1000 stereo integrated amplifier that came in for restoration. This amplifier was manufactured from 1974 to 1976. It produces 70 watts (Class B) or 15 watts (Class A) per channel into 8 ohms. The total harmonic distortion is 0.1%. The damping factor at 8 ohms is 70. This amplifier is relatively small but heavy due to two large heat sinks, a massive transformer, and two large filter capacitors (18000uF/63V).

Servicing the Yamaha CA-1000 can be tricky for those without experience and/or the right tools, so think twice before attempting this project (many boards are inaccessible without disassembling the front panel chassis).

Power Supply Board (NA-06332)

The power supply board has 2 bi-polar capacitors (C711, C713) and 8 aluminum electrolytic capacitors (C705, C706, C707, C708, C709, C710, C714, C715).

The original bi-polar capacitor C711 with a nominal capacitance of 1uF was replaced with a WIMA film polyester cap. Another bi-polar cap C713 was replaced with a modern Nichicon UEP bi-polar cap. The e-cap C715 was replaced with a modern low-leakage Nichicon UKL cap. The remaining aluminum e-caps were replaced with Nichicon UPW/UPM low-impedance capacitors.

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 board:

C705: rated capacitance – 220uF, measured – 247uF, ESR – 0.34Ω, deviation: +12%

C706: rated capacitance – 220uF, measured – 238uF, ESR – 0.26Ω, deviation: +8%

C707: rated capacitance – 220uF, measured – 242uF, ESR – 0.36Ω, deviation: +10%

C708: rated capacitance – 220uF, measured – 240uF, ESR – 0.46Ω, deviation: +9%

C709: rated capacitance – 100uF, measured – 114uF, ESR – 0.66Ω, deviation: +14%

C710: rated capacitance – 100uF, measured – 132uF, ESR – 0.52Ω, deviation: +32%

C711: rated capacitance – 1uF, measured – 0.9uF, ESR – 4.2Ω, deviation: -10%

C713: rated capacitance – 100uF, measured – 107uF, ESR – 1.39Ω, deviation: +7%

C714: rated capacitance – 220uF, measured – 259uF, ESR – 0.46Ω, deviation: +18%

C715: rated capacitance – 47uF, measured – 45uF, ESR – 0.49Ω, deviation: -4%

The original Hitachi 2SC458 transistor installed in positions TR705, TR707, and TR709 is on my list of prone-to-failure transistors. I replaced all original 2SC458s with new Fairchild KSC1845 transistors. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

The three fusible resistors (FR701, FR702, FR705) were replaced with new Stackpole Electronics carbon film resistors. The other fusible resistor (FR703) was replaced with new Vishay fusible metal film resistors.

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

Power supply board - before and after

Microphone Amplifier Board (NA-06584)

The microphone amplifier board has 6 solid tantalum capacitors (C461, C462, C471, C472, C475, C476) installed in the signal path, and 6 aluminum electrolytic capacitors (C463, C464, C465, C466, C473, C474).

All original tantalum 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 microphone amplifier board:

C461: rated capacitance – 220uF, measured – 233uF, ESR – 0.51Ω, deviation: +6%

C462: rated capacitance – 220uF, measured – 228uF, ESR – 0.43Ω, deviation: +4%

C463: rated capacitance – 47uF, measured – 57uF, ESR – 0.88Ω, deviation: +21%

C464: rated capacitance – 47uF, measured – 54uF, ESR – 0.82Ω, deviation: +15%

C465: rated capacitance – 220uF, measured – 268uF, ESR – 0.91Ω, deviation: +22%

C466: rated capacitance – 220uF, measured – 262uF, ESR – 0.64Ω, deviation: +19%

C471: rated capacitance – 4.7uF, measured – 5.6uF, ESR – 0.72Ω, deviation: +19%

C472: rated capacitance – 4.7uF, measured – 5.4uF, ESR – 0.71Ω, deviation: +15%

C473: rated capacitance – 47uF, measured – 50uF, ESR – 1.43Ω, deviation: +6%

C474: rated capacitance – 47uF, measured – 52uF, ESR – 1.16Ω, deviation: +11%

C475: rated capacitance – 4.7uF, measured – 4.7uF, ESR – 1.68Ω, deviation: 0%

C476: rated capacitance – 4.7uF, measured – 5.0uF, ESR – 2.8Ω, deviation: +6%

Four NPN transistors (TR461, TR462, TR463, TR464) installed on this board are Hitachi 2SC1345. This transistor is on my list of transistors that are prone to failure. I replaced all original 2SC1345s with new Fairchild KSC1845 transistors. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

Microphone amplifier board - before and after

Function Board (NA-06583)

The serial number of the function board (NA-06583) installed in this amplifier differs from the serial number in the service manual (NA-06335).

I tried my best to identify each electronic component correctly, but some errors may still exist due to the lack of the correct circuit diagram.

The function board has 12 solid tantalum capacitors and 9 aluminum electrolytic caps.

All original tantalum capacitors with a nominal capacitance of 1uF or less were replaced with high-quality WIMA film polyester caps. The other 4 tantalum capacitors with a nominal capacitance of 4.7uF were replaced with low-leakage Nichicon UKL caps. The remaining aluminum e-caps were replaced with Nichicon UPW/UPM low-impedance capacitors.

Test results on original capacitors removed from the function board:

C417: rated capacitance – 1uF, measured – 1.1uF, ESR – 14.7Ω, deviation: +10%

C418: rated capacitance – 1uF, measured – 1.1uF, ESR – 14.5Ω, deviation: +10%

C423: rated capacitance – 10uF, measured – 12uF, ESR – 1.34Ω, deviation: +20%

C424: rated capacitance – 10uF, measured – 11uF, ESR – 1.84Ω, deviation: +10%

C427: rated capacitance – 0.47uF, measured – 0.45uF, ESR – N/A, deviation: -4%

C428: rated capacitance – 0.47uF, measured – 0.44uF, ESR – N/A, deviation: -6%

C429: rated capacitance – 100uF, measured – 123uF, ESR – 0.84Ω, deviation: +23%

C430: rated capacitance – 100uF, measured – 126uF, ESR – 0.79Ω, deviation: +26%

C443: rated capacitance – 220uF, measured – 225uF, ESR – 0.72Ω, deviation: +2%

C444: rated capacitance – 100uF, measured – 114uF, ESR – 0.92Ω, deviation: +14%

C445: rated capacitance – 4.7uF, measured – 5.5uF, ESR – 0.68Ω, deviation: +17%

C446: rated capacitance – 4.7uF, measured – 4.9uF, ESR – 1.05Ω, deviation: +4%

C449: rated capacitance – 4.7uF, measured – leaky/low capacitance

C450: rated capacitance – 4.7uF, measured – 4.9uF, ESR – 1.85Ω, deviation: +4%

C451: rated capacitance – 47uF, measured – 50uF, ESR – 1.35Ω, deviation: +6%

C4xx: rated capacitance – 33uF, measured – 34uF, ESR – 1.91Ω, deviation: +3%

C4xx: rated capacitance – 33uF, measured – 36uF, ESR – 1.77Ω, deviation: +9%

C4xx: rated capacitance – 0.68uF, measured – 0.66uF, ESR – N/A, deviation: -3%

C4xx: rated capacitance – 0.68uF, measured – 0.65uF, ESR – N/A, deviation: -4%

C4xx: rated capacitance – 1uF, measured – 1.0uF, ESR – 2.9Ω, deviation: 0%

C4xx: rated capacitance – 1uF, measured – 1.0uF, ESR – 3.8Ω, deviation: 0%

The six prone-to-failure Hitachi 2SC1345 transistors installed on this board were replaced with Fairchild KSC1845 transistors. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

The two fusible resistors FR401 and FR402 were replaced with new Vishay fusible metal film resistors.

Function board - before and after

Main Amplifier Board (NA-06588)

The serial number of the main amplifier board (NA-06588) installed in this amplifier differs from the serial number in the service manual (NA-06331).

I tried my best to identify each electronic component correctly, but some errors may still exist due to the lack of the correct circuit diagram.

Yamaha CA-1000 has two main amplifier boars. Each board is mounted directly on the heatsink with two power transistors. 

Each main amplifier board has 2 bi-polar capacitors (C601, C603) and one aluminum electrolytic capacitor C609.

The original bi-polar capacitor (C601, 2.2uF/25V) installed in the input signal path was replaced with a WIMA film polyester cap. Another bi-polar capacitor (C603, 47uF/50V) was replaced with a new bi-polar Nichicon UES cap. The remaining aluminum e-cap C609 was replaced with a Nichicon UPW low-impedance capacitor.

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

left channel:

C601: rated capacitance – 2.2uF, measured – 2.1uF, ESR – 3.9Ω, deviation: -5%

C603: rated capacitance – 47uF, measured – 49uF, ESR – 1.1Ω, deviation: +4%

C609: rated capacitance – 220uF, measured – 223uF, ESR – 0.66Ω, deviation: +1%

right channel:

C601: rated capacitance – 2.2uF, measured – 2.0uF, ESR – 4.6Ω, deviation: %

C603: rated capacitance – 47uF, measured – 48uF, ESR – 0.76Ω, deviation: %

C609: rated capacitance – 220uF, measured – 216uF, ESR – 0.82Ω, deviation: %

The first stage of the power amplifier circuit is a differential amplifier consisting of two amplifying transistors with a common emitter: TR602/TR603. The original PNP transistor installed in these positions is 2SA841. This transistor is not on my list of infamous transistors prone to failure. However, testing each pair of transistors installed in the differential amplifier always makes sense to ensure they are still well-matched.

As can be seen from the test below, the mismatch of each pair of original transistors in the left and right channels is ~5% (left channel) and ~14% (right channel). The mismatch on the right channel is relatively high. I prefer to keep the mismatch between the two transistors in a differential amplifier as small as possible since it is critical to the correct operation of this circuit. So, I replaced each pair of the original 2SA841s with a closely matched pair (within 1%) of modern Fairchild KSA992 low-noise transistors. The new transistor has the same pinout as the original one.

Test results on original 2SA841 transistors

TR602 (left channel): hfe - 291, Vbe - 0.770V 

TR603 (left channel): hfe - 306, Vbe - 0.765V

TR602 (right channel): hfe - 346, Vbe - 0.763V 

TR603 (right channel): hfe - 304, Vbe - 0.760V

Test results on new KSA992FBU transistors

TR602 (right channel): hfe - 414, Vbe - 0.763V

TR603 (right channel): hfe - 413, Vbe - 0.762V

TR602 (right channel): hfe - 415, Vbe - 0.762V 

TR603 (right channel): hfe - 418, Vbe - 0.762V

All the original power transistors were removed, degreased, and tested with an Atlas DCA55 semiconductor analyzer. They all passed the test and I installed them back with new Mica pads and fresh thermal paste.

Power amplifier board from the left channel - before and after servicing

Power amplifier board from the right channel - before and after servicing

Tone Amplifier Board (NA-06586)

The serial number of the tone amplifier board (NA-06586) installed in this amplifier differs from the serial number in the service manual (NA-06337).

I tried my best to identify each electronic component correctly, but some errors may still exist due to the lack of the correct circuit diagram.

The tone amplifier board has 6 solid tantalum capacitors (C301, C302, C317, C318, C319, C320) installed in the signal path, and 5 aluminum electrolytic capacitors (C311, C312, C313, C314, C339).

The original tantalum capacitors (C301 and C302) with a nominal capacitance of 1uF were replaced with WIMA film polyester caps. The other 4 tantalum capacitors were replaced with low-leakage Nichicon UKL caps. The remaining e-caps were replaced with Nichicon UPW low-impedance capacitors.

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

C301: rated capacitance – 1uF, measured – 1.1uF, ESR – 8.2Ω, deviation: +10%

C302: rated capacitance – 1uF, measured – 1.0uF, ESR – 6.2Ω, deviation: 0%

C311: rated capacitance – 10uF, measured – 12uF, ESR – 1.36Ω, deviation: +20%

C312: rated capacitance – 10uF, measured – 12uF, ESR – 1.54Ω, deviation: +20%

C313: rated capacitance – 100uF, measured – 116uF, ESR – 0.71Ω, deviation: +16%

C314: rated capacitance – 100uF, measured – 120uF, ESR – 0.64Ω, deviation: +20%

C317: rated capacitance – 3.3uF, measured – 3.5uF, ESR – 3.1Ω, deviation: +6%

C318: rated capacitance – 3.3uF, measured – 3.5uF, ESR – 4.2Ω, deviation: +6%

C319: rated capacitance – 33uF, measured – 36uF, ESR – 0.74Ω, deviation: +9%

C320: rated capacitance – 33uF, measured – 34uF, ESR – 0.75Ω, deviation: +3%

C339: rated capacitance – 220uF, measured – 255uF, ESR – 0.42Ω, deviation: +16%

The six prone-to-failure Hitachi 2SC1345 transistors installed on this board were replaced with Fairchild KSC1845 transistors. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

The original fusible resistor FR301 was replaced with a new Vishay fusible metal film resistor.

Tone amplifier board - before and after

Filter Board (NA-06581)

The serial number of the tone amplifier board (NA-06581) installed in this amplifier differs from the serial number in the service manual (NA-06338).

I tried my best to identify each electronic component correctly, but some errors may still exist due to the lack of the correct circuit diagram.

The filter board has 6 solid tantalum capacitors (C511, C512, C531, C532, C540, C541) installed in the signal path, and 7 aluminum electrolytic capacitors (C509, C510, C521, C522, C529, C530, C539).

The original tantalum capacitors (C511 and C512) with a nominal capacitance of 1uF were replaced with WIMA film polyester caps. The other 4 tantalum capacitors were replaced with low-leakage Nichicon UKL caps. The remaining e-caps were replaced with Nichicon UPW low-impedance capacitors.

Test results on original capacitors removed from the filter board:

C509: rated capacitance – 10uF, measured – 12uF, ESR – 1.28Ω, deviation: +20%

C510: rated capacitance – 10uF, measured – 12uF, ESR – 1.76Ω, deviation: +20%

C511: rated capacitance – 1uF, measured – 1uF, ESR – 4.2Ω, deviation: 0%

C512: rated capacitance – 1uF, measured – 1uF, ESR – 4.1Ω, deviation: 0%

C521: rated capacitance – 10uF, measured – 13uF, ESR – 1.22Ω, deviation: +30%

C522: rated capacitance – 10uF, measured – 12uF, ESR – 1.24Ω, deviation: +20%

C529: rated capacitance – 100uF, measured – 128uF, ESR – 0.72Ω, deviation: +28%

C530: rated capacitance – 100uF, measured – 127uF, ESR – 0.82Ω, deviation: +27%

C531: rated capacitance – 4.7uF, measured – 5.1uF, ESR – 0.93Ω, deviation: +9%

C532: rated capacitance – 4.7uF, measured – 5.1uF, ESR – 1.04Ω, deviation: +9%

C539: rated capacitance – 220uF, measured – 246uF, ESR – 0.54Ω, deviation: +12%

C540: rated capacitance – 2.2uF, measured – 2.3uF, ESR – 3.6Ω, deviation: +5%

C541: rated capacitance – 2.2uF, measured – 2.2uF, ESR – 4.8Ω, deviation: 0%

The four prone-to-failure Hitachi 2SC1345 transistors installed on this board were replaced with Fairchild KSC1845 transistors. Watch the pinout on replacement transistors. The original transistor is BCE and the new one is ECB.

The original fusible resistor FR501 was replaced with a new Vishay fusible metal film resistor.

Filter board - before and after

Operation Board (NA-06590)

The two original snap-action microswitches have been replaced with new Omron ones. These switches change the voltage on the secondary winding of the transformer between class A and class B. These two snap-action contacts can be damaged/welded together when switching from Class A to Normal mode with power on. So this is more of a preventative maintenance.

Snap-action Omron microswitches - original and new

Operational board - after servicing

Primary Stage, DC offset and Bias Adjustments

Four adjustments should be done to each main amplifier board: (i) primary stage differential amplification circuit current, (ii) mid-point voltage (or DC offset), (iii) idling current (or Bias, Class B), and (iv) idling current (or Bias, Class A). All steps are clearly described in the service manual.

(i) Connect the DC voltmeter between terminals TP1 (-) and TP2 (+). Adjust the voltage to 15V +/- 1V with the trimming resistor VR602.

(ii) Connect the DC voltmeter between terminals SP Out and E. Adjust the voltage to 0V +/- 10mV with the trimming resistor VR601.

(iii) Set the Operation switch to Normal. Connect the DC voltmeter between terminals TP3 (+) and TP4 (-). Adjust the voltage to 47mV +/- 10mV with the trimming resistor VR603.

(iv) Set the Operation switch to Class A. Connect the DC voltmeter between terminals TP3 (+) and TP4 (-). Adjust the voltage to 940mV +/- 50mV with the trimming resistor VR604.

Primary stage differential amplification circuit current on the left and right channels after restoration

Mid-point voltage (or DC offset) on the left and right channels after restoration

Idling current (or Bias, Class B) on the left and right channels after restoration

Idling current (or Bias, Class A) 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 26.41 VRMS (left channel) and 26.97 VRMS (right channel) for Class B mode. It corresponds to the output power of 87.2W on the left channel and 90.9W on the right channel. For Class A mode, the maximum output sine-wave signal before clipping was 11.80 VRMS (left channel) and 11.92 VRMS (right channel). It corresponds to the output power of 17.4W on the left channel and 17.8W on the right channel.

Output power test (Class B)

Output power test (Class A)

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 truly amazing. Please watch a short demo video at the end of this post. Thank you for reading.

Yamaha CA-1000 - after restoration

Demo video after repair & restoration