Technics SA-5770 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Technics

Model: SA-5770

SN: AK7614B019

A rather rare Technics SA-5770 stereo receiver came in for restoration. This model was offered for only one year in 1977 with a list price of $800. This is one of the most powerful receivers ever made: 165 watts per channel into 8 ohms. The total harmonic distortion is less than 0.08% at rated power from 20 Hz to 20 kHz. This receiver is VERY heavy and weighs about 58 pounds without packaging.

This particular unit came in a non-working condition. Initial troubleshooting indicated that two fuses were blown: the 10A main fuse and the 2A fuse located on the dial light & power supply of the function indicator board (SUP9950B). The following in-depth troubleshooting revealed many faulty components in the main amplifier, power supply, speaker protection, and MPX circuits.

Main Amplifier Board (SUP9990)

The main amplifier board in this receiver looks rather suspicious. This PCB shows signs of significant thermal stress and several of the original transistors have already been replaced in the past. One resistor (R644) looks overheated. I decided to test every electronic component on this board to make sure there were no hidden surprises. It was worth doing, as many components turned out to be faulty:

Semiconductors:

TR610 (2SC1509) – open base-emitter

TR612 (2SC1573) – open base-emitter

D606 (MA150) – shorted

D605 (MA150) – shorted

D612 (MA150) – shorted

Passive components:

R670 – open

R644 – overheated

C601 – open

C629 – open

All failed components have been replaced with new parts in both channels. I always prefer to have the same parts installed in both amplifier channels, even if it's more costly.

Below is a list of original and replacement transistors that I have used. Note that I also replaced two pre-driver (TR607/TR608) and two current-limiting (TR613/TR614) transistors on this board. Those transistors were fine, but I replaced them with modern ones to make this board more reliable.

TR607: pre-driver, NPN, 2SC1628 (original), replaced with Fairchild KSC3503DSTU

TR608: pre-driver, NPN, 2SC1628 (original), replaced with Fairchild KSC3503DSTU

TR609: current limit, NPN, 2SC1509 (original), replaced with Fairchild KSC2690AYSTU

TR610: current limit, NPN, 2SC1509 (original), replaced with Fairchild KSC2690AYSTU

TR611: alarm indicator switching, NPN, 2SC1573 (original), replaced with Fairchild KSC3503DSTU

TR612: alarm indicator switching, NPN, 2SC1573 (original), replaced with Fairchild KSC3503DSTU

TR613: current limit, PNP, 2SA777 (original), replaced with Fairchild KSA1220AYS

TR614: current limit, PNP, 2SA777 (original), replaced with Fairchild KSA1220AYS

Four driver transistors (TR615/TR616, TR617/TR618) were removed from the small heat sink, degreased, and tested with Atlas DCA55 semiconductor analyzer. According to the datasheet, the measured DC current gain on all transistors was in spec. So, I mounted them back to the heat sink. The old thermal paste was refreshed with a new silicone thermal compound (Wakefield-Vette, 120 series).

Small signal diodes (D605, D606, D611, D612) were replaced with new Fairchild 1N4148.

Initially, I didn't change two dual PNP transistors 2SA798 installed on this board since they both are okay. However, after the restoration was completed I had trouble adjusting the DC offset on the right channel. The lowest DC offset was ~85mV which is rather high. On the left channel, I was able to adjust it to ~10mV. So, I unsoldered two dual transistors and tested each pair of p-n-p junctions with Atlas DCA55 semiconductor analyzer. The measured current gain of two p-n-p junctions of transistor TR601 (left channel) was 309 (hfe1) and 284 (hfe2), respectively. It gives an ~8% difference between them. Not too bad but according to the database, the hfe1/hfe1 ratio should be 0.98 (2% difference). This dual transistor works as a differential pair amplifier and is supposed to be gain matched as much as possible. Otherwise, the DC offset adjustment problem is inevitable. But again, even an 8% difference is usually acceptable and the DC offset will be very close to 0V.

However, the measured current gain of two p-n-p junctions of transistor TR602 (right channel) was 298 (hfe1) and only 157 (hfe2), respectively. So, it gives a 47% difference between them! It's way too much! And it was a reason why the lowest DC offset on the right channel was pretty high at ~85mV.

The PNP dual transistor 2SA798 can be replaced with two closely matched Fairchild transistors KSA992FBU thermally bonded to each other. But I still have new 2SA798's in stock and I replaced both original transistors with new "old stock" 2SA798's. Before installation, I tested new transistors and they were perfectly matched (see below). After replacement, the DC offset on both channels was easily adjusted close to 0V.

The measured current gain of two p-n-p junctions of new 2SA798's:

TR601: hfe1 = 399, hfe2 = 400

TR602: hfe1 = 373, hfe2 = 374

Original and new "old stock" PNP dual transistors 2SA798

The original carbon resistors (R643 & R644, 3.9kΩ, 1/4W) were replaced with modern KOA Speer metal film resistors (3.92kΩ, 1/2W). I also replaced four carbon resistors (R647 to R650, 100Ω, 1/4W) with KOA Speer metal film resistors (100Ω, 1/2W) as they were all subject to some thermal stress in the past.

The original wirewound emitter resistors (R669 to R672) were replaced with new Xicon wirewound resistors. I don't normally replace emitter resistors on vintage gears, but as I mentioned earlier, the R670 was electrically open in this unit. So, I decided to replace all of them.

Two low leakage e-caps (C601 & C602) installed in the signal path were replaced with high-quality film polyester WIMA caps. The remaining aluminum e-caps were replaced with low impedance Nichicon UPW/UPM caps.

The original e-caps were tested with an Atlas ESR70 capacitance meter and the results are below.

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

C601: rated capacitance – 1uF, measured – open circuit/low capacitance

C602: rated capacitance – 1uF, measured – 0.9uF, ESR – 5.8Ω, deviation: -10%

C629: rated capacitance – 33uF, measured – open circuit/low capacitance

C630: rated capacitance – 33uF, measured – 29uF, ESR – 0.47Ω, deviation: -12%

C631: rated capacitance – 100uF, measured – 132uF, ESR – 0.02Ω, deviation: +32%

C632: rated capacitance – 47uF, measured – 54uF, ESR – 0.16Ω, deviation: +15%

Main amplifier board - before and after

Power Transistors

Three out of four power transistors from the right channel (SUP9970C and SUP9970D) were shorted: TR624, TR626, and TR628. The fourth power transistor TR622 has a very low current gain of 6. I replaced all these transistors with new Fairchild MJ21193G/MJ21194G. The old thermal pads were also replaced with new Mica ones.

TR622 (2SD555) – very low current gain of 6

TR624 (2SB600) – shorted collector-emitter

TR626 (2SD555) – shorted base-emitter-collector

TR628 (2SB600) – shorted collector-base

TR622: NPN, 2SD555 (original), replaced with Fairchild MJ21194G

TR624: PNP, 2SB600 (original), replaced with Fairchild MJ21193G

TR626: NPN, 2SD555 (original), replaced with Fairchild MJ21194G

TR628: PNP, 2SB600 (original), replaced with Fairchild MJ21193G

The power transistors from the left channel (SUP9970A and SUP9970B) were tested with Atlas DCA55 semiconductor analyzer. The measured DC current gain on all transistors was in spec according to the datasheet. So, I replaced the old thermal pads, applied a fresh thermal compound, and mounted them back on the heat sink.

Speaker Protection Board (SUP9950D)

This is another PCB with many faulty components. Look at the picture below. Do you see anything weird? Yes, you are right. One transistor cracked! How is this even possible? I have no idea. I don't remember if I ever saw a cracked transistor in any of the vintage gears...

Transistor TR708 (speakers protection switching) is cracked

In addition to the cracked transistor, there were other faulty components on this board. Transistors TR709, TR710, and TR711 were found to be faulty:

TR709 (2SC828) – shorted base-collector

TR710 (2SC1509) – open collector-emitter

TR711 (2SC1509) – open base-emitter

I replaced all faulty transistors with new Fairchild transistors. I did not replace the overload alarm transistors (TR712 and TR713) on this board as they are both fine. Below is a list of original and replacement transistors that I have used.

TR708: speakers protection switching, NPN, 2SC828 (original), replaced with Fairchild KSC1845FTA

TR709: speakers protection switching, NPN, 2SC828 (original), replaced with Fairchild KSC1845FTA

TR710: relay driver, NPN, 2SC1509 (original), replaced with Fairchild KSC2383YTA

TR711: relay driver, NPN, 2SC1509 (original), replaced with Fairchild KSC2383YTA

The aluminum electrolytic capacitor C718 was found almost dead. The rated capacitance of this e-cap is 100uF, but the measured capacitance is only 17uF. So, the deviation is 83%! Another aluminum e-cap on this board was still in spec. I replaced both e-caps with low impedance Nichicon UPW caps.

Test results on original capacitors removed from the speaker protection board:

C718: rated capacitance – 100uF, measured – 17uF, ESR – 0.96Ω, deviation: -83%

C719: rated capacitance – 100uF, measured – 83uF, ESR – 0.44Ω, deviation: -17%

Speaker protection board - before and after

Power Amplifier Thermal Compensation Board (SUP9950F)

This small PCB is attached to the back side of the heat sinks. The original thermal compensation transistors TR619 and TR620 were replaced with new Fairchild KSC1815YTA.

Power amplifier thermal compensation board - before and after

Equalizer Amp, Tape Monitor, Tone Control, Tone Amp, Filter Amp, Muting Relay & Speaker Protection Relay (SUP9950A)

Yes, this PCB has such a long name, since all these circuits are assembled on this board.

Voltage Regulator Circuit

This circuit has seventeen aluminum electrolytic capacitors: C704 thru C709, C712, C713, C714, C720 thru C726, C733, and one bi-polar capacitor C717. All aluminum e-caps were replaced with low impedance Nichicon UPW/UPM caps. The original bi-polar capacitor was replaced with a modern bi-polar Nichicon UES cap. 

Note that the original bi-polar capacitor C171 has the largest deviation (more than 40%) from its nominal capacity among all other e-caps. This capacitor is marked on the schematic as one of the critical components.

Test results on original capacitors removed from the voltage regulator circuit:

C704: rated capacitance – 470uF, measured – 512uF, ESR – 0.02Ω, deviation: +9%

C705: rated capacitance – 470uF, measured – 595uF, ESR – 0.01Ω, deviation: +27%

C706: rated capacitance – 47uF, measured – 57uF, ESR – 0.21Ω, deviation: +21%

C707: rated capacitance – 47uF, measured – 56uF, ESR – 0.21Ω, deviation: +19%

C708: rated capacitance – 47uF, measured – 46uF, ESR – 0.27Ω, deviation: -2%

C709: rated capacitance – 47uF, measured – 52uF, ESR – 0.37Ω, deviation: +11%

C712: rated capacitance – 470uF, measured – 388uF, ESR – 0.31Ω, deviation: -17%

C713: rated capacitance – 220uF, measured – 250uF, ESR – 0.08Ω, deviation: +14%

C714: rated capacitance – 220uF, measured – 262uF, ESR – 0.06Ω, deviation: +19%

C717: rated capacitance – 220uF, measured – 310uF, ESR – 0.22Ω, deviation: +41%

C720: rated capacitance – 220uF, measured – 216uF, ESR – 0.17Ω, deviation: -2%

C721: rated capacitance – 220uF, measured – 132uF, ESR – 0.12Ω, deviation: -40%

C722: rated capacitance – 100uF, measured – 126uF, ESR – 0.14Ω, deviation: +26%

C723: rated capacitance – 220uF, measured – 191uF, ESR – 0.16Ω, deviation: -13%

C724: rated capacitance – 100uF, measured – 111uF, ESR – 0.22Ω, deviation: +11%

C725: rated capacitance – 100uF, measured – 116uF, ESR – 0.23Ω, deviation: +16%

C726: rated capacitance – 100uF, measured – 106uF, ESR – 0.14Ω, deviation: +6%

C733: rated capacitance – 220uF, measured – 229uF, ESR – 0.66Ω, deviation: +4%

The voltage regulator transistor TR707 is running rather hot under normal working conditions. I replaced it with a new Fairchild MJE15030G. I mounted a low-profile heat sink on this transistor to improve heat dissipation. The heat sink is small enough and doesn't touch any nearby components. The manufacturer's part number is 577404B00000G (Aavit).

A low-profile heat sink is mounted on a new TR707 (Fairchild MJE15030G)

I also replaced two NPN and two PNP transistors in this circuit: TR703/TR705 (2SC1509) and TR704/TR706 (2SA777). The transistors were still okay but I decided to replace them anyway to improve the overall reliability of the voltage regulator circuit. The package of original transistors (2SC1509 and 2SA777) is TO-92 whereas the package of replacement transistors (KSC2690 and KSA1220) is TO-126. The collector power dissipation is higher in package TO-126 in comparison to that in TO-92.

In this circuit, the Zener diode D708 and two ceramic capacitors (C715 and C716) turned out to be faulty. After restoration, the receiver sounded great. However, after about 2-3 minutes after turning it on, I heard a quiet whistling sound. The volume of this sound gradually increased over the course of a minute, until it became quite noticeable. I checked the other inputs (Phono, Aux, Tape) and this hissing sound was audible regardless of the input. I traced the 1kHz signal from the Aux input to the speaker terminals, looking for any faulty components, but found nothing. In the end, I checked every other component in the voltage regulator circuit and identified three suspects: the Zener diode D708 and two ceramic capacitors (C715 and C716). Each time one of them froze, the whistle would disappear until the component warmed up again. I unsoldered the Zener diode and tested it with Atlas DCA55 semiconductor analyzer. The Zener diode was normal with a forward voltage of about 0.76V. I decided to replace it anyway with a new Fairchild Zener diode 1N4744A. Then I unsoldered two ceramic capacitors and tested them. Capacitance measured was 0.0075 uF (C715) and 0.0095 uF (C716). The nominal capacitance of each cap is 0.01uF and the factory specification is +100%/-0%. So this gives a deviation of -25% for the C715 and -5% for the C716. I replaced both of them with new ceramic capacitors and the whistling sound is gone.

Equalizer Amplifier Circuit

This circuit has two low leakage capacitors (C201 & C202) installed in the input signal path, two bi-polar e-caps (C227 & C228) installed in the output signal path, and six aluminum e-caps (C213, C214, C223, C224, C231, and C232). The original low leakage capacitors were replaced with modern low leakage Nichicon UKL caps. Two bi-polar capacitors were replaced with high-quality film polyester WIMA caps. And, the remaining aluminum e-caps were replaced with low impedance Nichicon UPW caps.

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

C201: rated capacitance – 3.3uF, measured – 3.0uF, ESR – 2.1Ω, deviation: -9%

C202: rated capacitance – 3.3uF, measured – 3.1uF, ESR – 2.4Ω, deviation: -6%

C213: rated capacitance – 220uF, measured – 227uF, ESR – 0.11Ω, deviation: +3%

C214: rated capacitance – 220uF, measured – 233uF, ESR – 0.12Ω, deviation: +6%

C223: rated capacitance – 33uF, measured – 36uF, ESR – 0.65Ω, deviation: +9%

C224: rated capacitance – 33uF, measured – 39uF, ESR – 0.39Ω, deviation: +18%

C227: rated capacitance – 1uF, measured – 1.2uF, ESR – 2.4Ω, deviation: +20%

C228: rated capacitance – 1uF, measured – 1.3uF, ESR – 2.2Ω, deviation: +30%

C231: rated capacitance – 4.7uF, measured – 5.8uF, ESR – 1.32Ω, deviation: +23%

C232: rated capacitance – 4.7uF, measured – 5.8uF, ESR – 1.39Ω, deviation: +23%

Tone Amplifier Circuit

The tone amplifier circuit has two low leakage e-caps (C503 & C504) installed in the input signal path, four bi-polar e-caps (C515, C516, C537, C538) installed in the output signal path, and eight aluminum electrolytic capacitors (C507, C508, C509, C510, C511, C512, C513, C514). The original low leakage (C503 & C504) and two bi-polar (C537 & C538) e-caps were replaced with film polyester WIMA caps. The other two bi-polar e-caps (C515 & C516) were replaced with new bi-polar Nichicon UES caps. Two aluminum e-caps (C509 & C510) were replaced with film polyester WIMA caps. And, the remaining aluminum e-caps were replaced with low impedance Nichicon UPW caps.

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

C503: rated capacitance – 0.68uF, measured – 0.66uF, ESR – N/A, deviation: -3%

C504: rated capacitance – 0.68uF, measured – 0.70uF, ESR – N/A, deviation: +3%

C507: rated capacitance – 4.7uF, measured – 5.4uF, ESR – 1.81Ω, deviation: +15%

C508: rated capacitance – 4.7uF, measured – 5.6uF, ESR – 1.41Ω, deviation: +19%

C509: rated capacitance – 1uF, measured – 1.3uF, ESR – 2.2Ω, deviation: +30%

C510: rated capacitance – 1uF, measured – 1.3uF, ESR – 1.68Ω, deviation: +30%

C511: rated capacitance – 47uF, measured – 51uF, ESR – 0.44Ω, deviation: +9%

C512: rated capacitance – 47uF, measured – 55uF, ESR – 0.39Ω, deviation: +17%

C513: rated capacitance – 33uF, measured – 34uF, ESR – 0.58Ω, deviation: +3%

C514: rated capacitance – 33uF, measured – 33uF, ESR – 0.48Ω, deviation: 0%

C515: rated capacitance – 4.7uF, measured – 6.4uF, ESR – 2.1Ω, deviation: +36%

C516: rated capacitance – 4.7uF, measured – 6.1uF, ESR – 2.2Ω, deviation: +30%

C537: rated capacitance – 1uF, measured – 1.3uF, ESR – 2.4Ω, deviation: +30%

C538: rated capacitance – 1uF, measured – 1.3uF, ESR – 2.6Ω, deviation: +30%

This circuit also has four prone to failure transistors: 2SC1318 (TR507 & TR508) and 2SA720 (TR509 & TR510). I replaced the original 2SC1318 transistors with Fairchild KSC2383YTA. And, the original 2SA720 transistors were replaced with Fairchild KSA1013YBU.

Filter Amplifier Circuit

This circuit has two low leakage e-caps (C547 & C548) installed in the input signal path, and two bi-polar e-caps (C549 & C550) installed in the output signal path. All of them were replaced with film polyester WIMA caps.

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

C547: rated capacitance – 0.68uF, measured – 0.68uF, ESR – N/A, deviation: 0%

C548: rated capacitance – 0.68uF, measured – 0.67uF, ESR – N/A, deviation: -2%

C549: rated capacitance – 1uF, measured – 1.1uF, ESR – 2.6Ω, deviation: +10%

C550: rated capacitance – 1uF, measured – 1.1uF, ESR – 3.2Ω, deviation: +10%

FM Output Amplifier

This circuit has two aluminum electrolytic capacitors (C165 & C166) installed in the output path of the MPX circuit, two low leakage e-caps (C167 & C168) installed in the input signal path, and two low leakage e-caps(C169 & C170) installed in the output signal path. I replaced the e-caps installed in the input path with modern low leakage Nichicon UKL caps. The e-caps installed in the output signal path were replaced with film polyester WIMA caps. The remaining aluminum e-caps were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from the FM output amplifier circuit:

C165: rated capacitance – 3.3uF, measured – 3.3uF, ESR – 1.9Ω, deviation: 0%

C166: rated capacitance – 3.3uF, measured – 3.3uF, ESR – 2.2Ω, deviation: 0%

C167: rated capacitance – 3.3uF, measured – 3.2uF, ESR – 2.1Ω, deviation: -3%

C168: rated capacitance – 3.3uF, measured – 3.0uF, ESR – 2.4Ω, deviation: -9%

C169: rated capacitance – 1uF, measured – 1.1uF, ESR – 4.4Ω, deviation: +10%

C170: rated capacitance – 1uF, measured – 1.1uF, ESR – 5.7Ω, deviation: +10%

Speaker Protection Relay

The original speaker protection relay RLY601 in this unit was faulty and I replaced it with a new Omron LY2-0-DC24 relay.

New Omron LY2-0-DC24 relay is installed

Equalizer amp, tape monitor, tone control, tone amp, filter amp, muting relay & speaker protection relay - before servicing

Equalizer amp, tape monitor, tone control, tone amp, filter amp, muting relay & speaker protection relay - after servicing

FM Stereo Demodulator

The original stereo indicator lamp was burned out in this unit which is a fairly common problem in almost any vintage equipment. I replaced it with a new incandescent bulb, but a new problem arose. Now the new lamp was always on, whether the receiver was tuned to a station or not. There was also no sound when switching from Stereo to Mono. And the indicator of the signal-strength meter moved to the lower position when switching from Stereo to Mono.

First of all, I unsoldered the TR110 transistor and tested it. This transistor drives the stereo indicator lamp and often fails. However, the TR110 transistor was still good. The next logical step was to test the IC104 FM multiplexer IC. The diagram shows the reference voltage on each pin of IC104 for both Mono and Stereo reception. So, I measured the voltage on each pin and compared it against the schematic. Based on the testing results, the measured voltage on pins 1, 4, 5, 6, and 8 didn't match the reference voltage on the schematic. Something is definitely wrong with the IC104.

Test results on IC104 in Mono and Stereo modes:

pin 1: ref: 13.8V (Mono), measured: 9.8V (Mono), ref: 13.6V (Stereo), measured: 13.5V (Stereo)

pin 2: ref: 3V (Mono), measured: 2.9V (Mono), ref: 3V (Stereo), measured: 3.1V (Stereo)

pin 3: ref: 5.1V (Mono), measured: 5.4V (Mono), ref: 5.1V (Stereo), measured: 5.7V (Stereo)

pin 4: ref: 10.4V (Mono), measured: 6.9V (Mono), ref: 10.3V (Stereo), measured: 10.2V (Stereo)

pin 5: ref: 10.4V (Mono), measured: 7.0V (Mono), ref: 10.3V (Stereo), measured: 10.2V (Stereo)

pin 6: ref: 15.1V (Mono), measured: 10.8V (Mono), ref: 0.6V (Stereo), measured: 0.5V (Stereo)

pin 7: ref: 0V (Mono), measured: 0V (Mono), ref: 0V (Stereo), measured: 0V (Stereo)

pin 8: ref: 0.8V (Mono), measured: 0V (Mono), ref: 2.4V (Stereo), measured: 4V (Stereo)

pin 9: ref: 2.3V (Mono), measured: 2.3V (Mono), ref: 2.3V (Stereo), measured: 2.5V (Stereo)

pin 10: ref: 1.5V (Mono), measured: 1.4V (Mono), ref: 1.5V (Stereo), measured: 1.6V (Stereo)

pin 11: ref: 2.2V (Mono), measured: 2.2V (Mono), ref: 2.4V (Stereo), measured: 2.6V (Stereo)

pin 12: ref: 2.4V (Mono), measured: 2.3V (Mono), ref: 2.4V (Stereo), measured: 2.5V (Stereo)

pin 13: ref: 2.4V (Mono), measured: 2.3V (Mono), ref: 2.4V (Stereo), measured: 2.5V (Stereo)

pin 14: ref: 3.2V (Mono), measured: 3.3V (Mono), ref: 3.2V (Stereo), measured: 3.6V (Stereo)

I unsoldered the IC104 from the tuner board and installed a new HA1156 chip on the 16-pin DIP socket. The new chip solved the problem. I measured the voltage on each pin of the new chip and it perfectly matches the reference voltage on the schematic. The stereo indicator lamp is now properly working and the sound is clear in both Mono and Stereo modes. The signal-strength meter also works as it should.

The original (faulty) IC104 (HA1156W) 

New HA1156-6M1 was installed on the 16-pin DIP socket

Volume Control Potentiometer VR502

The volume control potentiometer VR502 in this receiver is a real PITA. It is a stack of two pots bolted together with no access for cleaning! This pot was making a very squeaky sound and needed to be cleaned. But how to do it?! I decided to use a heated tip of the soldering iron to melt the hard plastic. I made three small holes in each section and injected DeoxIT FaderLube 5% spray. The result is excellent - no creaking! Three holes were covered with aluminum foil to prevent dust from entering.

Volume control potentiometer - with holes for cleaning

Volume control potentiometer - "sealed" after cleaning and lubricating

Dial Light & Power Supply of Function Indicator Board (SUP9950B)

This PCB really surprised me. Someone installed 25A/32V automotive fuses instead of 380mA/12.6V incandescent bulbs to illuminate the dial. As a result, the 2A fuse has blown due to a short circuit as it was connected in series with three 25A fuses. I replaced the 2A fuse and installed new warm white LED festoon lamps. Two aluminum electrolytic capacitors C710 and C711 were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from the dial light & power supply of the function indicator board:

C710: rated capacitance – 220uF, measured – 214uF, ESR – 0.19Ω, deviation: -3%

C711: rated capacitance – 33uF, measured – 29uF, ESR – 0.66Ω, deviation: -12%

25A/32V automotive fuses were installed instead of dial lamps - not a smart decision

Dial light & power supply of function indicator board - after servicing (sorry, forgot to take a picture before servicing)

DC offset and Bias Adjustments

The DC offset on the left channel is measured between pins W and V (ground). On the right channel, it is measured between pins Z and V (ground). The DC offset should be adjusted as close to zero volts as possible with the trimming resistors VR601 and VR602.

DC offset on the left and right channels after restoration

The Bias on the left channel is measured across emitter resistor R669 (0.33Ω/5W). So, the voltmeter should be connected between pins X (+) and W (-). On the right channel, the bias is measured across emitter resistor R672 (0.33Ω/5W). So, the voltmeter should be connected between pins Y (+) and Z (-). The Bias should be adjusted to ~7mV on each channel with the trimming resistors VR603 and VR604. It corresponds to the idle current of ~21mA.

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 28.44 VRMS (left channel) and 27.93 VRMS (right channel). It corresponds to the output power of 101.1W on the left channel and 97.5W on the right channel. I was not able to test this receiver up to its 165W rating simply because my dummy resistor can only dissipate 100W max. But I have no doubts that this beast can easily handle it.

Output power test

As usual, all the knobs and the front panel were gently cleaned in warm water with dish soap. All pots and switches 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 brand new again and sounds fantastic! Please watch a short demo video at the end of this post. Thank you for reading.

Technics SA-5770 - after restoration

Demo video after repair & restoration