Technics SU-7300 Stereo Integrated Amplifier Restoration

Unit: Stereo Integrated Amplifier

Manufacturer: Technics

Model: SU-7300

SN: FD7818B030

Today I'm showcasing a Technics SU-7300 stereo integrated amplifier that came in for restoration. The SU-7300 was manufactured from 1976 to 1979. It produces 41 watts per channel into 8 ohms with no more than 0.08% total harmonic distortion. Low distortion and high signal-to-noise ratio in this model were achieved by implementing the matched transistor pair in a single package and the current-mirror loading in the differential amplifier. The unit has precise 41-step volume control and two attractive large-scale VU meters. It can easily drive a 4Ω impedance speakers with no overheating due to huge heat sinks. Overall, the Technics SU-7300 is a nice-looking integrated amplifier with great build quality and has plenty of power for home stereo needs.

Power Amplifier Board

The Technics SU-7300 is a service-friendly unit. One just needs to remove two side bracket panels from the chassis, remove four heat sink mounting screws, and then the front panel with the printed circuit board can be released. This design provides excellent access to all electronic components to be serviced.

Front panel with PCB released from the chassis - excellent access to all electronic components

The power supply in Technics SU-7300 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 D404 and D405, respectively. The balance power from the bridge rectifier and two filter capacitors C401/C402 is supplied to the main power stage. This circuit has five electrolytic capacitors C401, C402, C403, C404, and C2. I tested two large 8,200uF filter capacitors in-circuit with Atlas ESR70 capacitance meter and both have almost zero ESR. So, I didn't change them. Two aluminum e-caps C403 and C404 were replaced with low impedance and high-reliability Nichicon UPW caps. And the e-cap C2 was replaced with a high-quality film polyester WIMA MKS2 cap.

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

C403: rated capacitance – 220uF, measured – 174uF, ESR – 0.16Ω, deviation: -21%

C404: rated capacitance – 1000uF, measured – 1058uF, ESR – 0.07Ω, deviation: +6%

C2: rated capacitance – 1uF, measured – 1.3uF, ESR – 1.72Ω, deviation: +30%

The phono amplifier circuit has four low leakage e-caps C101, C102, C121, C122 installed in the signal path, and two aluminum e-caps C113, C114. The e-caps C101 and C102 were replaced with modern low leakage Nichicon UKL caps. The e-caps C121 and C122 were replaced with high-quality film polyester WIMA MKS2 caps. The remaining two aluminum capacitors C113 and C114 were replaced with low impedance Nichicon UPW caps.

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

C101: rated capacitance – 3.3uF, measured – 3.3uF, ESR – 2.2Ω, deviation: 0%

C102: rated capacitance – 3.3uF, measured – 3.3uF, ESR – 3.1Ω, deviation: 0%

C113: rated capacitance – 33uF, measured – 42uF, ESR – 0.54Ω, deviation: +27%

C114: rated capacitance – 33uF, measured – 43uF, ESR – 0.61Ω, deviation: +30%

C121: rated capacitance – 1uF, measured – 1uF, ESR – 2.3Ω, deviation: 0%

C122: rated capacitance – 1uF, measured – 1uF, ESR – 2.2Ω, deviation: 0%

The power amplifier circuit has two low leakage e-caps C201, C202 installed in the signal path and eight aluminum e-caps C209 thru C214 and C261, C262. The low leakage capacitors were replaced with film polyester WIMA MKS2 caps. And the remaining eight e-caps were replaced with low impedance Nichicon UPW caps.

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

C201: rated capacitance – 1uF, measured – 1uF, ESR – 2.6Ω, deviation: 0%

C202: rated capacitance – 1uF, measured – 1uF, ESR – 2.4Ω, deviation: 0%

C209: rated capacitance – 33uF, measured – 46uF, ESR – 0.62Ω, deviation: +39%

C210: rated capacitance – 33uF, measured – 42uF, ESR – 0.46Ω, deviation: +27%

C211: rated capacitance – 47uF, measured – 49uF, ESR – 0.37Ω, deviation: +4%

C212: rated capacitance – 47uF, measured – 56uF, ESR – 0.31Ω, deviation: +19%

C213: rated capacitance – 47uF, measured – 55uF, ESR – 0.26Ω, deviation: +17%

C214: rated capacitance – 47uF, measured – 56uF, ESR – 0.24Ω, deviation: +19%

C261: rated capacitance – 47uF, measured – 57uF, ESR – 0.56Ω, deviation: +21%

C262: rated capacitance – 47uF, measured – 64uF, ESR – 0.53Ω, deviation: +36%

The VU meters circuit has four aluminum e-caps C351 thru C354. I replaced them with low impedance Nichicon UPW caps.

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

C351: rated capacitance – 10uF, measured – 11uF, ESR – 0.72Ω, deviation: +10%

C352: rated capacitance – 10uF, measured – 11uF, ESR – 0.61Ω, deviation: +10%

C353: rated capacitance – 10uF, measured – 11uF, ESR – 0.54Ω, deviation: +10%

C354: rated capacitance – 10uF, measured – 13uF, ESR – 0.68Ω, deviation: +30%

The last but not least circuit on the power amplifier board is a shock noise prevention circuit. This circuit restricts the current flowing through the pre-amplifier when the power supply is being turned ON or OFF. The time delay depends on the parameters of resistor R302 and capacitor C302 and set to be approximately 6 to 7 seconds. The circuit has four aluminum e-caps C301 thru C304, and I replaced them with low impedance Nichicon UPW caps.

Test results on original capacitors removed from the shock noise prevention circuit:

C301: rated capacitance – 33uF, measured – 35uF, ESR – 0.36Ω, deviation: +6%

C302: rated capacitance – 33uF, measured – 35uF, ESR – 0.38Ω, deviation: +6%

C303: rated capacitance – 47uF, measured – 53uF, ESR – 0.26Ω, deviation: +13%

C304: rated capacitance – 10uF, measured – 12uF, ESR – 1.18Ω, deviation: +20%

As one can see from the test results about 30% of all original electrolytic capacitors removed from the power amplifier board exceed a factory capacitance tolerance. Some e-caps have relatively high ESR values. I also noticed that many of the original capacitors are leaky and have corrosive leads due to electrolyte leakage. All of these observations show the importance of replacing the electrolytic capacitors in any vintage gears because they are most susceptible to aging and degradation.

Power amplifier board - before and after

Idle Current Adjustments

The idle current (or bias) should be confirmed whenever the power transistor is replaced. I didn't change any transistors in this unit but checked the bias after the restoration was finished. The bias in Technics SU-7300 is measured across the emitter resistor R237 on the left channel and across R238 on the right channel. According to the service manual, the voltage drop across each resistor should be under 25mV. However, if this voltage is over 25mV the corresponding lead wire should be cut off. I checked the voltage drop across each emitter resistor and it was ~9.5mV on the left channel and ~8.0mV on the right channel. So, no lead wires were cut off. Taking into account the resistance of the emitter resistor one can calculate the idle current based on Ohm's law. The resistance of the emitter resistor is 0.33Ω, and therefore the idle current is 29mA (9.5mV/0.33Ω) on the left channel and 24mA (8mV/0.33Ω) on the right channel.

Bias on the left and right channel after restoration

Output Power Test

At the end of my restoration, I loaded this receiver with a low inductance 8Ω/100W dummy resistor for each channel, connected my oscilloscope across the speaker terminals, and applied a sine-wave signal of 1kHz to the TAPE jacks. The output sine-wave signal was perfectly symmetrical on both channels with no clipping up to 18.63 VRMS (left channel) and 18.32 VRMS (right channel). So, the max output power can be calculated based on the following formula: P = (VRMS x VRMS)/8. And it gives us ~43.4W for the left channel and ~ 43.4W for the right channel, which meets and even exceeds a factory specification for this model.

Output power test - meets and even exceeds a factory specification!

The front panel and all the knobs were removed from the chassis and cleaned in warm water with dish soap. Also, 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 amplifier works great again, all inputs were tested and work properly as well. Please watch a short demo video at the end of this post. Thank you for reading.

Technics SU-7300 - after restoration

Demo video after repair & restoration