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 contain 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

Marantz 2270 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Marantz

Model: 2270

SN: 52380J4

Today I'm showcasing a Marantz 2270 vintage receiver that came in for restoration. This is one of the most famous and desirable receivers of all time. Marantz 2270 was manufactured from 1972 to 1974. It produces 70 watts per channel into 8 ohms with no more than 0.3% total harmonic distortion. The damping factor at 8 ohms is 45. The optional walnut veneer cabinet for this model is WC-22. The retail price in 1972 was $549.95 (Ref. High Fidelity, April 1972, page 35).

According to the factory stamp on the chassis, this receiver was manufactured in April 1976.

Regulated Power Supply and Protection Relay Board (P800)

The power supply board has 8 aluminum electrolytic capacitors: C801, C802, C803, C804, C805, C806, C807, and C810.

All e-caps except the C801 on this board were replaced with Nichicon UPW/UPM low-impedance capacitors. Capacitor C801 was replaced with a new Cornell Dubilier type 381LX capacitor. The new capacitor has the same diameter and lead spacing as the original.

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:

C801: rated capacitance – 470uF, measured – 528uF, ESR – 0.26Ω, deviation: +12%

C802: rated capacitance – 330uF, measured – 310uF, ESR – 0.56Ω, deviation: -6%

C803: rated capacitance – 470uF, measured – 435uF, ESR – 0.52Ω, deviation: -7%

C804: rated capacitance – 330uF, measured – 303uF, ESR – 0.57Ω, deviation: -8%

C805: rated capacitance – 3.3uF, measured – 3.7uF, ESR – 1.62Ω, deviation: +12%

C806: rated capacitance – 10uF, measured – 13uF, ESR – 0.72Ω, deviation: +30%

C807: rated capacitance – 220uF, measured – 252uF, ESR – 0.63Ω, deviation: +15%

C810: rated capacitance – 47uF, measured – 58uF, ESR – 0.83Ω, deviation: +23%

The S1B-01-02 diodes (H801, H802, H811, H812, H813) were upgraded to Fairchild UF4005 fast recovery rectifiers.

All transistors except the H803 and H805 were also replaced to improve the reliability of the power supply. Below is a list of original and replacement parts that I have used.

Q806: NPN, 2SC734 (original), replaced with a new Fairchild KSC2383YTA

Q808: NPN, 2SC945 (original), replaced with a new Fairchild KSC2383YTA

Q809: NPN, 2SC1213A (original), replaced with a new Fairchild KSC2383YTA

Q8010: NPN, 2SC1213A (original), replaced with a new Fairchild KSC2383YTA

The large wire-wound resistor R801 was replaced with a new IRC / TT Electronics wire-wound resistor. The new resistor is smaller, does not have bulky corners, and has a rated power of 7W instead of 5W.

The original trimming resistor R809 was replaced with a new Bourns potentiometer.

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

After the servicing, the voltage between pin J802 and J803 (ground) was adjusted to +35.0V DC with trimming resistor R809.

Regulated power supply & protection relay circuit board - voltage regulator adjustment

Regulated power supply & protection relay circuit board - before and after

Tone Amplifier Board (P400)

The tone amplifier board has 8 solid tantalum capacitors (C403, C404, C407, C408, C410, C411, C414, C415) installed in the signal path and 3 aluminum electrolytic capacitors (C405, C406, C409) for local filtering.

All solid tantalum capacitors were replaced with high-quality WIMA film polyester caps. The remaining filtering capacitors were replaced with Nichicon UPW/UPM low-impedance capacitors. Note that the three original solid tantalum capacitors (C407, C410, and C411) have extremely high ESR resistance of over 20 ohms!

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

C403: rated capacitance – 1uF, measured – 1uF, ESR – 8.3Ω, deviation: 0%

C404: rated capacitance – 1uF, measured – 1uF, ESR – 5.4Ω, deviation: 0%

C405: rated capacitance – 10uF, measured – 11uF, ESR – 1.33Ω, deviation: +10%

C406: rated capacitance – 10uF, measured – 11uF, ESR – 1.56Ω, deviation: +10%

C407: rated capacitance – 3.3uF, measured – 3.8uF, ESR – 21.2Ω, deviation: +15%

C408: rated capacitance – 3.3uF, measured – 3.4uF, ESR – 1.68Ω, deviation: +3%

C409: rated capacitance – 100uF, measured – 106uF, ESR – 0.72Ω, deviation: +6%

C410: rated capacitance – 3.3uF, measured – 3.8uF, ESR – 24.4Ω, deviation: +15%

C411: rated capacitance – 3.3uF, measured – 4.2uF, ESR – 38.2Ω, deviation: +27%

C414: rated capacitance – 1uF, measured – 0.9uF, ESR – 2.4Ω, deviation: -10%

C415: rated capacitance – 1uF, measured – 0.9uF, ESR – 1.39Ω, deviation: -10%

Four NPN transistors (H401, H402, H405, H406) installed on this board are Toshiba 2SC1000. This transistor is on my list of transistors that are prone to failure. I replaced all the original 2SC1000s with new Fairchild KSC1845 transistors.

The PNP transistor 2SA493 installed in positions H403, H404, H407, and H408 is not on my list of prone-to-failure transistors but is a complementary pair to the 2SC1000. So, I replaced all the original 2SC493s with new Fairchild KSC992 transistors.

Tone amplifier board - before and after

Phono Amplifier Board (P700)

The phono amplifier board has 4 solid tantalum capacitors (C701, C702, C703, C704) and one aluminum electrolytic capacitor C713.

The original solid tantalum capacitors (C701 and C702) installed in the input signal path were replaced with Panasonic film polyester caps. The other two solid tantalum capacitors (C703 and C704) were replaced with modern low-leakage Nichicon UKL caps. The local filtering capacitor C713 was replaced with a Nichicon UPM low-impedance capacitor.

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

C701: rated capacitance – 2.2uF, measured – 2.3uF, ESR – 5.6Ω, deviation: +5%

C702: rated capacitance – 2.2uF, measured – 2.3uF, ESR – 3.6Ω, deviation: +5%

C703: rated capacitance – 22uF, measured – 23uF, ESR – 0.74Ω, deviation: +5%

C704: rated capacitance – 22uF, measured – 24uF, ESR – 1.19Ω, deviation: +9%

C713: rated capacitance – 100uF, measured – 111uF, ESR – 0.56Ω, deviation: +11%

The prone-to-failure transistors 2SC1000 (H701, H702, H703, H704) and 2SC458LG (H705, H706) installed on this board were replaced with low-noise Fairchild KSC1845 transistors.

Both 1S1212 epoxy diodes installed in positions H707 and H708 were replaced with 1N4148 diodes.

The original VD1212 dual diode installed in position H709 was replaced with two 1N4148 diodes connected in series.

Phono amplifier board - before and after

Power Amplifier Board (P750)

Marantz 2270 has two power amplifier boars. Each board is mounted directly on the heatsink with two power transistors.

The right channel on this unit was previously toasted and many of the original components were replaced:

• three carbon resistors (R776, R782, R786)
• one wire-wound emitter resistor (R788)
• two driver transistors (H758, H759)
• one driver transistor (H766)
• one output transistor (H003)

Power amplifier board from the right channel - before servicing (heatsink is removed)

The original R776 resistor with a nominal resistance of 30kΩ was previously replaced with two 1.5kΩ resistors connected in series. As a result, the total resistance of R776 became 3kΩ instead of 30kΩ! I replaced it with a new KOA Speer metal film resistor (30.1kΩ, 1/2W).

The R782 carbon resistor (previously replaced with a single 1.5kΩ resistor) was replaced with a new KOA Speer metal film resistor (1.5kΩ, 1/2W). 

The R786 carbon resistor (previously replaced by two 47 ohm resistors in series) has been replaced with a new KOA Speer carbon film flameproof resistor (100Ω, 1W).

The original wire-wound emitter resistor R788 with a nominal resistance of 0.2Ω was previously replaced with a wire-wound resistor with a nominal resistance of 0.33Ω. I don't know why a 0.33Ω resistor was chosen instead of a 0.2Ω resistor. I replaced both emitter resistors (R788, R789) with new Vishay wire-wound resistors (0.2Ω, 5W).

The two original driver transistors (H758/H759) were previously replaced with Sony 2SC895/2SA762 transistors (TO-66 package). These transistors are now obsolete and were most likely replaced in the late 80s or early 90s. I decided to leave them.

The original H760 bias stabilization transistor in each channel is Toshiba 2SC496. One of them has a low current gain of 31 (according to the database this parameter should be >70 for rank O). I replaced each 2SC496 with a new Fairchild MJE243G transistor.

Each power amplifier board has one solid tantalum capacitor C761 and two aluminum electrolytic capacitors (C759, C760).

The original solid tantalum capacitor was replaced with a low-leakage Nichicon UKL cap. The remaining aluminum e-caps were replaced with Nichicon UPW low-impedance capacitors.

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

left channel:

C759: rated capacitance – 10uF, measured – 12uF, ESR – 2.2Ω, deviation: +20%

C760: rated capacitance – 10uF, measured – 12uF, ESR – 2.4Ω, deviation: 20%

C761: rated capacitance – 22uF, measured – 24uF, ESR – 2.0Ω, deviation: +9%

right channel:

C759: rated capacitance – 10uF, measured – 12uF, ESR – 2.1Ω, deviation: +20%

C760: rated capacitance – 10uF, measured – 13uF, ESR – 2.4Ω, deviation: +30%

C761: rated capacitance – 22uF, measured – 25uF, ESR – 2.2Ω, deviation: +14%

The first stage of the power amplifier circuit is a differential amplifier consisting of two amplifying transistors with a common emitter: H751/H752. The original PNP transistor installed in these positions is 2SA640. 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 still very small: ~3% (left channel) and ~1% (right channel). However, I decided to replace each pair of the original 2SA640s with a closely matched pair (within 1%) of modern Fairchild KSA992 low-noise transistors to improve the reliability of the differential amplifier. I put a slight smear of new thermal paste on one face of KSA992 and pressed both transistors together. Heat shrink tubing was used to hold them together mechanically.

Test results on original 2SA640 transistors

H751 (left channel): hfe - 368, Vbe - 0.744V 

H752 (left channel): hfe - 356, Vbe - 0.748V

H751 (right channel): hfe - 364, Vbe - 0.757V 

H752 (right channel): hfe - 368, Vbe - 0.757V

Test results on new KSA992FBU transistors

H751 (left channel): hfe - 423, Vbe - 0.761V

H752 (left channel): hfe - 423, Vbe - 0.761V

H751 (right channel): hfe - 426, Vbe - 0.760V

H752 (right channel): hfe - 427, Vbe - 0.762V 

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

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

Power Transistors

All original power transistors were removed, degreased, and tested with Atlas DCA55 semiconductor analyzer. Each transistor passed the test, but one from the left channel has a relatively low current gain of ~27. Also, as I mentioned before, the original Fairchild transistor H003 from the right channel was previously replaced with a Motorola SJ2518 transistor.

Because of all these issues, I decided to replace the original power transistors in each channel with new ON Semiconductor MJ21194G / MJ21193G transistors. The new transistors are specifically designed for high-power audio output, have excellent gain linearity, and complement each other. The new Mica pads and fresh thermal compound were applied between each power transistor and heat sink.

Test results on original power transistors:

H001, S40442, PNP, hfe = 133, Vbe = 0.588V

H002, S40443, NPN, hfe = 27, Vbe = 0.545V

H003, SJ2518, PNP, hfe = 138, Vbe = 0.561V

H004, S40443, NPN, hfe = 41, Vbe = 0.552V

Test results on new ON Semiconductor power transistors:

H001, MJ21193G, PNP, hfe = 85, Vbe = 0.608V

H002, MJ21194G, NPN, hfe = 54, Vbe = 0.595V

H003, MJ21193G, PNP, hfe = 84, Vbe = 0.611V

H004, MJ21194G, NPN, hfe = 54, Vbe = 0.601V

Original power transistors - left and right channels

New ON Semiconductor power transistors - left and right channels

Dial, Meter, and Functional Lamps

The old incandescent bulbs were replaced with warm white LED lamps to maintain the original look and decrease heat. The original incandescent lamp behind the meter was replaced with a light blue LED lamp to match the illumination of the dial scale. The old yellowish vellum behind the dial scale was replaced with new heavyweight vellum paper.

The original functional lamps (Stereo, Phono 2, and Tape) in this unit have burned out. I replaced them with new bi-pin lamps (8V/40mA).

The original dial pointer lamp was also burned out. I replaced it with a new incandescent lamp (8V/60mA, 4.7mm).

Main Power Switch Replacement

The main power switch on this unit tends to get stuck in the ON position. I removed the front panel to see if the switch cover was rubbing against the plastic insert, but it was not. The problem was with the worn power switch. So, I decided to replace it with a new switch and snubber. 

Main power switch - original and new

DC offset and Bias Adjustments

The DC offset is measured across speaker terminals on each channel. It should be adjusted as close to zero volts as possible with the trimming resistor R762.

Be aware, that the service manual has an error for bias adjustment. The correct procedure is described below.

The bias on each channel is measured across resistors R788 and R789. The DC voltmeter should be connected between pins J754 (+) and J760 (-). Then, the voltage should be adjusted to ~10mV on each channel with the trimming resistor R763. It corresponds to the idling current of 25mA.

DC offset on the left and right channels after restoration

Bias on the left and right channels after restoration

Output Power Test

The final output power test was performed at the end of my restoration. The receiver was loaded with a low inductance 8Ω/100W dummy resistor for each channel. The oscilloscope was connected across the speaker terminals and a sine-wave signal of 1kHz was applied to the AUX jacks. The output sine-wave signal was perfectly symmetrical on both channels with no clipping up to 25.61 VRMS (left channel) and 25.66 VRMS (right channel). It corresponds to the output power of 82.0W on the left channel and 82.3W on the right channel.

Output power test

As usual, all the knobs and the face plate 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 awesome and sounds great! Please watch a short demo video at the end of this post. Thank you for reading.

Marantz 2270 - after restoration

Demo video after repair & restoration