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