Marantz 2250B Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Marantz

Model: 2250B

SN: U070883

Today I'm showcasing a Marantz 2250B vintage receiver that came in for restoration. Marantz 2250B was manufactured from 1976 to 1977. It produces 50 watts per channel into 8 ohms with no more than 0.25% total harmonic distortion. The damping factor at 8 ohms is 55. The optional walnut veneer cabinet for this model is WC-22. The retail price in 1977 was $549.95 (Ref. Stereo Review, Stereo Directory & Buying Guide 1977, page 50).

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

Power Supply Board (P800)

The power supply board has 8 aluminum electrolytic capacitors: C801, C802, C804, C805, C806, C807, C808, and C809.

I replaced all capacitors on this board with low-impedance Nichicon UPW/UPM caps.

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 – 220uF, measured – 221uF, ESR – 1.07Ω, deviation: +1%

C802: rated capacitance – 100uF, measured – 98uF, ESR – 1.59Ω, deviation: -2%

C804: rated capacitance – 22uF, measured – 25uF, ESR – 1.77Ω, deviation: +14%

C805: rated capacitance – 330uF, measured – 379uF, ESR – 0.84Ω, deviation: +15%

C806: rated capacitance – 470uF, measured – 445uF, ESR – 1.02Ω, deviation: -5%

C807: rated capacitance – 4.7uF, measured – 5.4uF, ESR – 2.4Ω, deviation: +15%

C808: rated capacitance – 47uF, measured – 52uF, ESR – 1.53Ω, deviation: +11%

C809: rated capacitance – 220uF, measured – 258uF, ESR – 1.35Ω, deviation: +17%

The four original capacitors installed on this board were secured at the factory with glue. Before installing the new capacitors, I removed the old glue, but noticed that two resistors (R805 and R808) were also covered in old glue. I decided to replace them with new KOA Speer metal film resistors to prevent possible degradation in the future.

All transistors and one Zener diode on this board were also replaced to enhance the reliability of the power supply circuit. Below is a list of original and replacement parts that I have used.

Q801: NPN, 2SD330 (original), replaced with a new Fairchild MJE15030G

Q802: NPN, 2SC1318 (original), replaced with a new Fairchild KSC2690AYSTU

Q803: NPN, 2SC945 (original), replaced with a new Fairchild KSC945CYTA

Q804: NPN, 2SD331 (original), replaced with a new Fairchild MJE15030G

Q805: NPN, 2SC945 (original), replaced with a new Fairchild KSC945CYTA

Q806: NPN, 2SC1318 (original), replaced with a new Fairchild KSC2383YTA

Q807: NPN, 2SC1318 (original), replaced with a new Fairchild KSC2383YTA

Q810: Zener, 14V/0.5W, WZ-140 (original), replaced with a new Fairchild 1N5244B

The large wire-wound resistor R807 was replaced with a new IRC / TT Electronics wire-wound resistor. The new resistor is smaller, has no bulky corners, and is rated at 7W instead of 5W.

The original trimming resistor R806 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 J804 and J805 (ground) was adjusted to +35.0V DC with trimming resistor R806.

Power supply board - before and after

Power supply board - voltage regulator adjustment

Phono Amplifier Board (P400)

The phono amplifier board has 2 solid tantalum capacitors (C401, C402) installed in the input signal path, 2 low-leakage e-caps (C403, C404), and an aluminum filtering e-cap C413. 

The original tantalum capacitors were replaced with high-quality film polyester WIMA caps. The original low-leakage e-caps were replaced with modern low-leakage Nichicon UKL caps. The design of the printed circuit board allows the installation of an axial or radial capacitor in position C413. I replaced the original axial e-cap C413 with a radial type low impedance Nichicon UPM cap.

The two original film capacitors (C414 and C415) installed in the output signal path were replaced with new Panasonic polyester film capacitors. I usually do not change film capacitors in vintage receivers. Film capacitors don't age badly like electrolytic capacitors. The only reason was to replace the old and bulky capacitors with new and more compact ones.

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

C401: rated capacitance – 1uF, measured – 1.1uF, ESR – 6.4Ω, deviation: +10%

C402: rated capacitance – 1uF, measured – 1.0uF, ESR – 6.1Ω, deviation: 0%

C403: rated capacitance – 47uF, measured – 47uF, ESR – 2.5Ω, deviation: 0%

C404: rated capacitance – 47uF, measured – 50uF, ESR – 1.9Ω, deviation: +6%

C413: rated capacitance – 100uF, measured – 112uF, ESR – 1.52Ω, deviation: +12%

C414: rated capacitance – 1uF, measured – 1.0uF, ESR – 0.86Ω, deviation: +10%

C415: rated capacitance – 1uF, measured – 1.0uF, ESR – 0.45Ω, deviation: +10%

The original 2SC1344 Hitachi transistor installed in positions H401 to H404 is on my list of transistors that are prone to failure. I replaced all 2SC1344s with low-noise Fairchild KSC1845 transistors. Watch the pinout on replacement transistors. The original transistor is BCE, and the new one is ECB. Also, make sure that the new transistors installed in the first stage of the phono stage have a high current gain of 400 or more.

The NPN transistor 2SC458 installed in positions H405 and H406 has a bad reputation as a potential source of noise. I replaced both 2SC458 transistors with a low-noise Fairchild KSC1845 transistor. Watch the pinout on replacement transistors. The original transistor is BCE, and the new one is ECB.

The failure-prone VD1212 dual diode installed in position H409 was replaced with a pair of 1N4148 diodes in series.

Phono amplifier board - before and after

Pre & Tone Amplifier Board (PE01)

The pre- & tone amplifier board must be removed from the chassis for servicing. I recommend removing the phono amplifier board and one bracket underneath it before attempting to free the pre- & tone amplifier board. In addition, the metal shield under the pre- & tone amplifier board needs to be removed to properly clean all the switches underneath.

The pre- & tone amplifier board has 6 solid tantalum capacitors (CE13, CE14, CE33, CE34, CE35, CE36) installed in the signal path, 2 bipolar e-caps (CE37, CE38) installed in the output signal path, and 5 aluminum electrolytic capacitors (CE09, CE10, CE11, CE12, CE39).

Two original tantalum capacitors (CE33, CE34) with a rated capacitance of 1uF were replaced with film polyester WIMA caps. The other 4 tantalum e-caps were replaced with modern low-leakage Nichicon UKL caps. Two bipolar e-caps were replaced with new bipolar Nichicon UES caps. And the remaining aluminum e-caps were replaced with low-impedance Nichicon UPW caps.

The two original film capacitors (CE05 and CE06) installed in the input signal path were replaced with new Panasonic polyester film capacitors. The reason was the same as in the phono amplifier stage.

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

CE05: rated capacitance – 1uF, measured – 1.0uF, ESR – 0.88Ω, deviation: 0%

CE06: rated capacitance – 1uF, measured – 1.0uF, ESR – 0.84Ω, deviation: 0%

CE09: rated capacitance – 100uF, measured – 132uF, ESR – 1.71Ω, deviation: +32%

CE10: rated capacitance – 100uF, measured – 112uF, ESR – 2.1Ω, deviation: +12%

CE11: rated capacitance – 10uF, measured – 12uF, ESR – 2.2Ω, deviation: +20%

CE12: rated capacitance – 10uF, measured – 12uF, ESR – 2.5Ω, deviation: +20%

CE13: rated capacitance – 3.3uF, measured – 3.5uF, ESR – 3.0Ω, deviation: +6%

CE14: rated capacitance – 3.3uF, measured – 3.5uF, ESR – 3.4Ω, deviation: +6%

CE33: rated capacitance – 1uF, measured – 1.1uF, ESR – 6.4Ω, deviation: +10%

CE34: rated capacitance – 1uF, measured – 1.1uF, ESR – 6.2Ω, deviation: +10%

CE35: rated capacitance – 3.3uF, measured – 3.3uF, ESR – 3.6Ω, deviation: 0%

CE36: rated capacitance – 3.3uF, measured – 3.3uF, ESR – 2.4Ω, deviation: 0%

CE37: rated capacitance – 4.7uF, measured – 4.6uF, ESR – 2.9Ω, deviation: -2%

CE38: rated capacitance – 4.7uF, measured – 4.8uF, ESR – 1.9Ω, deviation: +2%

CE39: rated capacitance – 220uF, measured – 178uF, ESR – 1.1Ω, deviation: -19%

Two resistors (RE13 and RE14) were replaced with new KOA Speer metal film resistors, as the originals were covered with old glue.

The original 2SC1327 transistors installed in positions HE03 and HE04 were replaced with low-noise Fairchild KSC1845 transistors. And the original 2SA842 transistors installed in positions HE04 and HE06 were replaced with low-noise Fairchild KSA992 transistors.

Pre- & tone amplifier board - before and after

Power Amplifier Board (P700)

Marantz 2250B has two power amplifier boards. Each board has two power transistors and is mounted directly on the heatsink.

Each power amplifier board has two low-leakage capacitors (C701, C705) and 3 aluminum electrolytic capacitors (C702, C704, C706).

The original low-leakage 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 power amplifier board:

left channel:

C701: rated capacitance – 3.3uF, measured – 3.0uF, ESR – 3.5Ω, deviation: -9%

C702: rated capacitance – 10uF, measured – 13uF, ESR – 2.6Ω, deviation: +30%

C704: rated capacitance – 47uF, measured – 58uF, ESR – 0.55Ω, deviation: +23%

C705: rated capacitance – 47uF, measured – 48uF, ESR – 1.18Ω, deviation: +2%

C706: rated capacitance – 220uF, measured – 221uF, ESR – 2.1Ω, deviation: +1%

right channel:

C701: rated capacitance – 3.3uF, measured – 3.1uF, ESR – 3.5Ω, deviation: -6%

C702: rated capacitance – 10uF, measured – 11uF, ESR – 2.5Ω, deviation: +10%

C704: rated capacitance – 47uF, measured – 62uF, ESR – 0.85Ω, deviation: +32%

C705: rated capacitance – 47uF, measured – 47uF, ESR – 1.22Ω, deviation: 0%

C706: rated capacitance – 220uF, measured – 234uF, ESR – 0.94Ω, deviation: +6%

The first stage of the main amplifier circuit is a differential amplifier consisting of two amplifying transistors with a common emitter: H702/H703. The original PNP transistor installed in these positions is 2SA722. This transistor is not on my list of infamous transistors that are 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 pair of transistors from the left channel was very well matched within ~5%. But the pair of original 2SA722s from the right channel had a mismatch of ~23%.

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 2SA722s with a closely matched pair (within 1%) of modern Fairchild KSA992 low-noise transistors.

Test results on original 2SA722 transistors

H702 (left channel): hfe - 379, Vbe - 0.786V 

H703 (left channel): hfe - 361, Vbe - 0.787V

H702 (right channel): hfe - 326, Vbe - 0.787V 

H703 (right channel): hfe - 400, Vbe - 0.788V

Test results on new KSA992FBU transistors

H702 (left channel): hfe - 523, Vbe - 0.759V

H703 (left channel): hfe - 523, Vbe - 0.759V

H702 (right channel): hfe - 524, Vbe - 0.758V

H703 (right channel): hfe - 525, Vbe - 0.759V

Each original MV-13 diode installed in positions H704 and H705 was replaced with two 1N4148 diodes connected in series.

Power amplifier board from the left channel - before and after

Power amplifier board from the right channel - before and after

AM Tuner Board (P150)

The AM tuner board has one low-leakage capacitor C171, and 6 aluminum electrolytic capacitors: C167 C168, C169, C170, C172, and C173.

The original low-leakage capacitor was replaced with a film polyester WIMA cap. The remaining aluminum e-caps were replaced with Nichicon UPW low-impedance capacitors.

Test results on original capacitors removed from the AM tuner board:

C167: rated capacitance – 22uF, measured – 27uF, ESR – 1.51Ω, deviation: +23%

C168: rated capacitance – 100uF, measured – 134uF, ESR – 1.1Ω, deviation: +34%

C169: rated capacitance – 4.7uF, measured – 5.7uF, ESR – 2.2Ω, deviation: +21%

C170: rated capacitance – 100uF, measured – 111uF, ESR – 2.1Ω, deviation: +11%

C171: rated capacitance – 1uF, measured – 1.0uF, ESR – 5.3Ω, deviation: 0%

C172: rated capacitance – 100uF, measured – 114uF, ESR – 2.2Ω, deviation: +14%

C173: rated capacitance – 4.7uF, measured – 5.6uF, ESR – 2.4Ω, deviation: +19%

AM tuner board - before and after

Dolby Level Board (PC01)

The Dolby level board has 4 solid tantalum capacitors: C01, C02, C03, and C04. 

All of them were replaced with film polyester WIMA caps.

Test results on original capacitors removed from the Dolby level board:

C01: rated capacitance – 1uF, measured – 1.0uF, ESR – 7.4Ω, deviation: 0%

C02: rated capacitance – 1uF, measured – 1.0uF, ESR – 4.5Ω, deviation: 0%

C03: rated capacitance – 1uF, measured – 1.0uF, ESR – 5.8Ω, deviation: 0%

C04: rated capacitance – 1uF, measured – 1.0uF, ESR – 5.5Ω, deviation: 0%

Dolby level board - before and after

FM MPX Board (P300)

The FM MPX board has 3 bipolar electrolytic capacitors (C307, C308, C309), 4 solid tantalum e-caps C323, C324, C325, C326), and 10 aluminum electrolytic capacitors (C302, C304, C306, C327, C328, C329, C331, C332, C337, C338).

The original bipolar and solid tantalum capacitors were replaced with film polyester WIMA and Kemet caps. Two ordinary aluminum e-caps with a rated capacitance of 1uF (C331, C338) were also replaced with film polyester WIMA caps. The remaining aluminum e-caps were replaced with low impedance Nichicon UPW/UPM caps.

Test results on original capacitors removed from the FM MPX board:

C302: rated capacitance – 33uF, measured – 42uF, ESR – 2.2Ω, deviation: +27%

C304: rated capacitance – 22uF, measured – 25uF, ESR – 2.4Ω, deviation: +14%

C306: rated capacitance – 22uF, measured – 25uF, ESR – 2.1Ω, deviation: +14%

C307: rated capacitance – 0.47uF, measured – 0.47uF, ESR – N/A, deviation: 0%

C308: rated capacitance – 0.22uF, measured – 0.25uF, ESR – N/A deviation: +14%

C309: rated capacitance – 0.22uF, measured – 0.26uF, ESR – N/A, deviation: +18%

C323: rated capacitance – 0.22uF, measured – 0.23uF, ESR – N/A, deviation: +5%

C324: rated capacitance – 0.22uF, measured – 0.23uF, ESR – N/A, deviation: +5%

C325: rated capacitance – 1uF, measured – 1.0uF, ESR – 5.4Ω, deviation: 0%

C326: rated capacitance – 1uF, measured – 1.1uF, ESR – 4.8Ω, deviation: +10%

C327: rated capacitance – 220uF, measured – 244uF, ESR – 0.94Ω, deviation: +11%

C328: rated capacitance – 220uF, measured – 215uF, ESR – 1.6Ω, deviation: -2%

C329: rated capacitance – 10uF, measured – 13uF, ESR – 2.8Ω, deviation: +30%

C331: rated capacitance – 1uF, measured – 1.1uF, ESR – 3.4Ω, deviation: +10%

C332: rated capacitance – 10uF, measured – 12uF, ESR – 2.6Ω, deviation: +20%

C337: rated capacitance – 4.7uF, measured – 5.8uF, ESR – 1.84Ω, deviation: +23%

C338: rated capacitance – 1uF, measured – 1.0uF, ESR – 2.8Ω, deviation: 0%

MPX stereo decoding amplifier board - before and after

FM IF Board (P200)

The FM IF board has 6 aluminum electrolytic capacitors (C237, C240, C242, C244, C245, C246). 

All original e-caps were replaced with low impedance Nichicon UPW caps.

Test results on original capacitors removed from the FM IF board:

C237: rated capacitance – 10uF, measured – 13uF, ESR – 3.4Ω, deviation: +30%

C240: rated capacitance – 100uF, measured – 127uF, ESR – 1.12Ω, deviation: +27%

C242: rated capacitance – 100uF, measured – 108uF, ESR – 1.16Ω, deviation: +8%

C244: rated capacitance – 10uF, measured – 12uF, ESR – 4.0Ω, deviation: +20%

C245: rated capacitance – 22uF, measured – 25uF, ESR – 2.1Ω, deviation: +14%

C246: rated capacitance – 10uF, measured – 12uF, ESR – 3.2Ω, deviation: +20%

FM IF board - before and after

Filter Capacitors

I don't usually replace filter capacitors in vintage gears unless their ESR is high or the capacitance tolerance is more than +/-10%. In this unit, the two main filter capacitor still have a low ESR but the measured capacitance is more than 20% below the nominal one. So, I replaced them with new Nichicon LKG e-caps. The LKG series was designed for high-grade audio equipment and, in my opinion, is the best replacement for the original filter and/or coupling capacitors on the market. The new Nichicon LKG e-caps are the same diameter as the original ones. Thus, the same clamps can be used to attach them to the chassis.

Test results on the original filter capacitors:

C004: rated capacitance – 10000uF, measured – 7453uF, ESR – 1.69Ω, deviation: -26%

C005: rated capacitance – 10000uF, measured – 7622uF, ESR – 1.07Ω, deviation: -24%

New Nichicon LKG filter capacitors

Dial and Meter Lamps

The original dial and meter lights on this receiver were previously replaced with blue LED lamps. I replaced them with warm white diffused LED lights to keep the original look. The old yellowish paper behind the dial scale was replaced with new vellum paper.

Dial scale removed - previously replaced dial lamps

Dial scale removed - warm white diffused LED lamps

Original vellum paper

New vellum paper

Previously replaced meter lamps

Warm white diffused LED lamps

Main Power Switch

The main power switch on this receiver was previously replaced. The technician who replaced it did it carelessly and damaged the threads. As a result, this switch became slightly loose. Also, the diameter of the switch knob is about 8.8mm, which is smaller than all the original knobs, which are about 10.9mm in diameter. I replaced it with a new switch and knob to match with original knobs.

Previously replaced main power switch

New main power switch

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 R707.

The bias on each channel is measured across emitter resistors R732 and R733. So, the DC voltmeter should be connected between pins J708 (+) and J710 (-). Then, the voltage should be adjusted to ~10mV on each channel with the trimming resistor R719. 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 22.31 VRMS (left channel) and 22.14 VRMS (right channel). It corresponds to the output power of 62.2W on the left channel and 61.3W 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 the sound is clear and warm. Classic Marantz sound from the late 70's! Please watch a short demo video at the end of this post. Thank you for reading.

Marantz 2250B - 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 boards. 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 a 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