Unit: AM/FM Stereo Receiver
Manufacturer: Sherwood
Model: S-7100A
SN: 223009
Today I'm showcasing a Sherwood S-7100A stereo receiver that came in for restoration. This model was introduced in the early 70s and produces 22 watts per channel into 8 ohms with less than 0.9% total harmonic distortion. The damping factor is 30. The list price in 1974 was $219.95 (Ref. Stereo Reviews, Buyers Guide 1974, page 49).
The Sherwood S-7100A is a service-friendly receiver. Each board can be easily accessed for servicing.
Power Supply & Main Amplifier Board
The power supply and main amplifier are all located on a single PCB.
The power supply circuit has 5 aluminum electrolytic capacitors: C609, C610, C611, C612, and C614.
All of them 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 circuit:
C609: rated capacitance – 220uF, measured – 259uF, ESR – 2.6Ω, deviation: +18%
C610: rated capacitance – 220uF, measured – 292uF, ESR – 3.6Ω, deviation: +33%
C611: rated capacitance – 100uF, measured – 73uF, ESR – 4.5Ω, deviation: -27%
C612: rated capacitance – 220uF, measured – 298uF, ESR – 5.8Ω, deviation: +36%
C614: rated capacitance – 10uF, measured – 15uF, ESR – 5.1Ω, deviation: +50%
I also replaced the two original Panasonic 2SC1318 transistors, installed in positions TR606 and TR607, with new Onsemi KSC2690AYSTU transistors to enhance the reliability of the power supply circuit.
The main power amplifier circuit has 8 aluminum electrolytic capacitors (C601a, C601b, C602a, C602b, C604a, C604b, C605a, C605b), and two coupling capacitors (C651a and C651b).
The ordinary e-caps C601a and C601b, installed in the input signal path, were replaced with low-leakage Nichicon UKL capacitors to reduce noise in the circuit. The original coupling capacitors were replaced with new Nichicon LKG Gold Tune capacitors. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW/UPM caps.
Test results on original capacitors removed from the main amplifier circuit:
C601a: rated capacitance – 3.3uF, measured – 3.7uF, ESR – 5.1Ω, deviation: +12%
C601b: rated capacitance – 3.3uF, measured – 3.8uF, ESR – 6.2Ω, deviation: +15%
C602a: rated capacitance – 47uF, measured – 50uF, ESR – 4.2Ω, deviation: +6%
C602b: rated capacitance – 47uF, measured – 64uF, ESR – 3.2Ω, deviation: +36%
C604a: rated capacitance – 100uF, measured – 128uF, ESR – 3.6Ω, deviation: +28%
C604b: rated capacitance – 100uF, measured – 121uF, ESR – 2.8Ω, deviation: +21%
C605a: rated capacitance – 10uF, measured – 14uF, ESR – 2.6Ω, deviation: +40%
C605b: rated capacitance – 10uF, measured – 15uF, ESR – 2.2Ω, deviation: +50%
C651a: rated capacitance – 2200uF, measured – 3152uF, ESR – 1.1Ω, deviation: +43%
C651b: rated capacitance – 2200uF, measured – 3338uF, ESR – 1.2Ω, deviation: +52%
Coupling capacitors (C651a and C651b) - original and new Nichicon LKG Gold Tune
Power supply & main amplifier board - before and after
Pre-Amplifier Board
The pre-amplifier board has 4 solid tantalum capacitors (C501a, C501b, C507a, C507b) and 8 aluminum electrolytic capacitors (C502a, C502b, C505a, C505b C506a, C506b, C510a, C510b).
The original tantalum e-caps C501a and C501b, installed in the input signal path, were replaced with low-leakage Nichicon UKL capacitors to reduce noise in the circuit. The original tantalum e-caps C507a and C507b, installed in the output signal path, were also replaced with low-leakage Nichicon UKL capacitors for the same purpose. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW caps.
Note that the measured capacitance of the two original e-caps on this board exceeded their nominal value by more than 150%! This answers the question of whether electrolytic capacitors in vintage receivers need to be replaced.
Test results on original capacitors removed from the pre-amplifier board:
C501a: rated capacitance – 3.3uF, measured – 4.0uF, ESR – 3.4Ω, deviation: +21%
C501b: rated capacitance – 3.3uF, measured – 4.1uF, ESR – 3.2Ω, deviation: +24%
C502a: rated capacitance – 10uF, measured – 13uF, ESR – 2.3Ω, deviation: +30%
C502b: rated capacitance – 10uF, measured – 14uF, ESR – 2.2Ω, deviation: +40%
C505a: rated capacitance – 100uF, measured – 254uF, ESR – 1.9Ω, deviation: +154%
C505b: rated capacitance – 100uF, measured – 263uF, ESR – 1.4Ω, deviation: +163%
C506a: rated capacitance – 10uF, measured – 13uF, ESR – 3.1Ω, deviation: +30%
C506b: rated capacitance – 10uF, measured – 12uF, ESR – 3.2Ω, deviation: +20%
C507a: rated capacitance – 1.5uF, measured – 1.5uF, ESR – 4.7Ω, deviation: +50%
C507b: rated capacitance – 1.5uF, measured – 1.6uF, ESR – 5.2Ω, deviation: +60%
C510a: rated capacitance – 47uF, measured – 53uF, ESR – 1.5Ω, deviation: +13%
C510b: rated capacitance – 47uF, measured – 57uF, ESR – 1.3Ω, deviation: +21%
Original capacitor C505b - the measured capacitance exceeds its nominal value by 163%!
Pre-amplifier board - before and after
The tone amplifier board has 4 aluminum electrolytic capacitors: C901a, C901b, C902a, and C902b.
All of them were low-leakage Nichicon UKL capacitors to reduce noise in the circuit.
Test results on original capacitors removed from the tone amplifier board:
C901a: rated capacitance – 4.7uF, measured – 5.6uF, ESR – 4.0Ω, deviation: +19%
C901b: rated capacitance – 4.7uF, measured – 5.1uF, ESR – 4.4Ω, deviation: +9%
C902a: rated capacitance – 10uF, measured – 14uF, ESR – 2.4Ω, deviation: +40%
C902b: rated capacitance – 10uF, measured – 14uF, ESR – 2.2Ω, deviation: +40%
Tone amplifier board - before and after
The AM 1F board has one solid tantalum capacitor (C418) and 4 aluminum electrolytic capacitors (C404, C409, C414, C420).
The original tantalum capacitor was replaced with a film polyester WIMA cap. The remaining aluminum e-caps were replaced with low-impedance Nichicon UPW caps.
Test results on original capacitors removed from the AM 1F board:
C404: rated capacitance – 10uF, measured – 15uF, ESR – 2.6Ω, deviation: +50%
C409: rated capacitance – 33uF, measured – 44uF, ESR – 2.4Ω, deviation: +33%
C414: rated capacitance – 100uF, measured – 204uF, ESR – 3.9Ω, deviation: +104%
C418: rated capacitance – 0.47uF, measured – 0.46uF, ESR – N/A, deviation: -2%
C420: rated capacitance – 100uF, measured – 151uF, ESR – 1.7Ω, deviation: +51%
AM 1F board - before and after
FM MPX Board
The FM MPX board has 10 aluminum electrolytic capacitors: C301, C302, C308, C309, C310, C311, C321, C322, C323, and C324.
The original capacitor C309 was replaced with a low-impedance Panasonic FCA series capacitor. Capacitor C323 (1uF/50V) was replaced with a new 33uF/50V capacitor, in accordance with Service Bulletin No. 119, issued on March 13, 1974. The original capacitors C321 and C322 were replaced with WIMA polyester film capacitors. The remaining aluminum electrolytic capacitors were replaced with low-impedance Nichicon UPW series capacitors.
Test results on original capacitors removed from the FM MPX board:
C301: rated capacitance – 10uF, measured – 13uF, ESR – 2.9Ω, deviation: +30%
C302: rated capacitance – 10uF, measured – 14uF, ESR – 3.2Ω, deviation: +40%
C308: rated capacitance – 10uF, measured – 15uF, ESR – 4.1Ω, deviation: +50%
C309: rated capacitance – 1uF, measured – 1.2uF, ESR – 6.4Ω, deviation: +20%
C310: rated capacitance – 10uF, measured – 15uF, ESR – 3.2Ω, deviation: +50%
C311: rated capacitance – 330uF, measured – 354uF, ESR – 3.2Ω, deviation: +7%
C321: rated capacitance – 1uF, measured – 1.3uF, ESR – 5.2Ω, deviation: +30%
C322: rated capacitance – 1uF, measured – 1.2uF, ESR – 5.1Ω, deviation: +20%
C323: rated capacitance – 1uF, measured – 1.3uF, ESR – 6.2Ω, deviation: +30%
C324: rated capacitance – 10uF, measured – 14uF, ESR – 3.8Ω, deviation: +40%
The original 1S1212 epoxy diode installed in position D301 was replaced with a 1N4148 diode.
The stereo lamp driver transistor TR305 was replaced with a new Fairchild KSC2383YTA. The original transistor installed in this position is 2SC1318, and it often fails due to thermal stress. The replacement transistor has the same pinout as the original.
Test results on the original stereo lamp driver transistor 2SC1318 (rank S) - the DC current gain is low
TR305: hfe - 122, Vbe - 0.732V
FM MPX board - before and after
Main Filter Capacitor
The original main filter capacitor C851 was replaced with a new Kemet screw-type capacitor. The new capacitor has the same diameter as the original.
Test results on original filter capacitors:
C851: rated capacitance – 2200uF, measured – 2434uF, ESR – 1.6Ω, deviation: +11%
Main filter capacitor - original and a new Kemet (ALS30 series)
Meter, Dial, and Stereo Indicator Lamps
The original meter lamp was replaced with a new incandescent bulb (bayonet base). The original dial illumination bulbs were previously replaced with cool blue LED lamps. The original stereo indicator lamp in this unit had burned out; I replaced it with a new incandescent lamp.
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 12.52 VRMS (left channel) and 12.56 VRMS (right channel). It corresponds to the output power of 19.6W on the left channel and 19.7W 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 receiver looks and sounds simply stunning! Please watch a short demo video at the end of this post. Thank you for reading.
Sherwood S-7100A - after restoration
Demo video after repair & restoration
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