Sherwood S-7100A Receiver Restoration

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 circuit noise. 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

Tone Amplifier Board

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

AM 1F Board

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

Sherwood S-7210 Receiver Restoration

Unit: AM/FM Stereo Receiver

Manufacturer: Sherwood

Model: S-7210

SN: 511645

Today I'm showcasing a Sherwood S-7210 AM/FM stereo receiver that came in for restoration. This model was introduced in 1974 and produces 26 watts per channel into 8 ohms with less than 0.6% total harmonic distortion. The damping factor is 30. The list price in 1974 was $299.95 (Ref. High Fidelity Magazine, September 1974, page 27).

The Sherwood S-7210 is a service-friendly receiver. Each board can be easily accessed for servicing.

Selector & Pre-Amp Board

The selector & pre-amp board has two solid tantalum capacitors (C501a, C501b) and twelve aluminum electrolytic capacitors (C503a, C503b, C504a, C504b, C505a, C505b, C506a, C506b, C512, C513, C514, C515). The solid tantalum capacitors are installed in the input signal path and I replaced them with modern low leakage Nichicon UKL e-caps. The ordinary aluminum e-caps C505a and C505b installed in the output signal path were replaced with high-quality film polyester WIMA caps. And, the remaining aluminum capacitors were replaced with low impedance Nichicon UPW caps.

The original e-caps were tested with an Atlas ESR70 capacitance meter and the results are below. Five of the fourteen capacitors are outside of factory spec.

Test results on original capacitors removed from the selector & pre-amp board:

C501a: rated capacitance – 4.7uF, measured – 5.3uF, ESR – 1.58Ω, deviation: +13%

C501b: rated capacitance – 4.7uF, measured – 4.9uF, ESR – 1.92Ω, deviation: +4%

C503a: rated capacitance – 22uF, measured – 28uF, ESR – 1.12Ω, deviation: +27%

C503b: rated capacitance – 22uF, measured – 28uF, ESR – 1.04Ω, deviation: +27%

C504a: rated capacitance – 33uF, measured – 36uF, ESR – 0.55Ω, deviation: +9%

C504b: rated capacitance – 33uF, measured – 34uF, ESR – 0.66Ω, deviation: +3%

C505a: rated capacitance – 1uF, measured – 1.3uF, ESR – 2.9Ω, deviation: +30%

C505b: rated capacitance – 1uF, measured – 1.2uF, ESR – 2.9Ω, deviation: +20%

C506a: rated capacitance – 10uF, measured – 11uF, ESR – 1.35Ω, deviation: +10%

C506b: rated capacitance – 10uF, measured – 11uF, ESR – 1.42Ω, deviation: +10%

C512: rated capacitance – 33uF, measured – 41uF, ESR – 0.72Ω, deviation: +24%

C513: rated capacitance – 33uF, measured – 41uF, ESR – 0.81Ω, deviation: +24%

C514: rated capacitance – 10uF, measured – 11uF, ESR – 1.39Ω, deviation: +10%

C515: rated capacitance – 10uF, measured – 11uF, ESR – 1.42Ω, deviation: +10%

Two NPN transistors (TR502a & TR502b) installed on this board are Toshiba 2SC1000. This transistor is known to get noisy over time. I replaced the original 2SC1000 transistors with modern low-noise Fairchild KSC1845s. The new transistor has the same pinout as the original one.

Selector & pre-amp board - before and after

Main Amp & Power Supply Board

The power supply, tone control amplifier, and main amplifier are all located on a single PCB.

The power supply circuit provides -11.3V DC to the tuner board, and balanced power to the main power amplifier from the bridge rectifier and two filter capacitors. The circuit has two aluminum electrolytic capacitors (C807, C808) and two axial filter capacitors (C805, C806). The two aluminum e-caps were replaced with low-impedance, high-reliability Nichicon UPW caps. Both axial 4700uF/35V filter capacitors were replaced with new 4700uF/40V Vishay 021ASM series caps. I usually don't replace large filter capacitors in vintage gears unless their ESR is high or the measured capacitance exceeds the nominal one by more than +/- 10%. But it makes sense in this particular case since the ESR of each original filter cap is relatively high and the measured capacitance is very low in comparison to the nominal value.

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

C805: rated capacitance – 4700uF, measured – 2362uF, ESR – 0.13Ω, deviation: -50%

C806: rated capacitance – 4700uF, measured – 2214uF, ESR – 0.14Ω, deviation: -53%

C807: rated capacitance – 33uF, measured – 32uF, ESR – 3.8Ω, deviation: -3%

C808: rated capacitance – 33uF, measured – 33uF, ESR – 3.8Ω, deviation: 0%

The tone control circuit has six aluminum electrolytic capacitors (C901a, C901b, C902a, C902b, C906a, C906b) installed in the signal path, and two filtering aluminum e-caps (C908 and C909). Four ordinary aluminum e-caps with a nominal capacitance of 1uF were replaced with film polyester WIMA caps to improve the signal-to-noise ratio. The other two e-caps with a nominal capacitance of 4.7uF were replaced with modern low leakage Nichicon UKL caps. And the remaining filtering aluminum capacitors were replaced with low impedance Nichicon UPM caps.

Note that one of the original capacitors installed in this circuit completely failed the test (C906b, open circuit).

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

C901a: rated capacitance – 1uF, measured – 1.3uF, ESR – 1.66Ω, deviation: +30%

C901b: rated capacitance – 1uF, measured – 1.2uF, ESR – 2.4Ω, deviation: +20%

C902a: rated capacitance – 1uF, measured – 1.2uF, ESR – 2.2Ω, deviation: +20%

C902b: rated capacitance – 1uF, measured – 1.2uF, ESR – 2.1Ω, deviation: +20%

C906a: rated capacitance – 4.7uF, measured – 5.1uF, ESR – 1.8Ω, deviation: +9%

C906b: rated capacitance – 4.7uF, measured – open circuit

C908: rated capacitance – 220uF, measured – 298uF, ESR – 0.14Ω, deviation: +36%

C909: rated capacitance – 470uF, measured – 508uF, ESR – 0.08Ω, deviation: +8%

The main amplifier circuit has nine aluminum electrolytic capacitors: C601a, C601b, C604a, C604b, C605a, C605b, C606a, C606b, and C611. Two of them (C601a and C601b) are installed in the input signal path. I replaced them with film polyester WIMA caps to improve the signal-to-noise ratio. The other e-caps were replaced with low impedance Nichicon UPW/UHE caps.

Note, that two out of nine original capacitors installed in this circuit completely failed the test (C605b and C611, open circuit). Also, the measured capacitance of e-cap C604a is very close to the nominal but its ESR is very high.

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

C601a: rated capacitance – 1uF, measured – 1.2uF, ESR – 1.74Ω, deviation: %

C601b: rated capacitance – 1uF, measured – 1.2uF, ESR – 2.4Ω, deviation: %

C604a: rated capacitance – 33uF, measured – 34uF, ESR – 8.3Ω, deviation: %

C604b: rated capacitance – 33uF, measured – 33uF, ESR – 3.5Ω, deviation: %

C605a: rated capacitance – 33uF, measured – 33uF, ESR – 2.8Ω, deviation: %

C605b: rated capacitance – 33uF, measured – open circuit

C606a: rated capacitance – 100uF, measured – 87uF, ESR – 0.19Ω, deviation: %

C606b: rated capacitance – 100uF, measured – 86uF, ESR – 0.22Ω, deviation: %

C611: rated capacitance – 4.7uF, measured – open circuit

The first stage of the main amplifier circuit is a differential pair amplifier consisting of two amplifying transistors with a common emitter: TR601a/TR602a (left channel) and TR601b/TR602b (right channel). The original PNP transistors installed in these positions are 2SA640. These transistors are not on my list of notorious transistors prone to failure. However, before I started servicing this receiver I measured the DC offset across the speaker terminals and noticed a relatively high (but still safe) voltage in the right channel. The measured DC offset in the left channel was ~14mV (perfectly good) but in the right channel, the measured voltage was ~55mV. This voltage should be as close to zero volts as possible within +/- 50mV. Usually, if this voltage is relatively high (but still within a safe range) the problem might be with mismatched transistors installed in the differential amplifier. So, I unsoldered the original 2SA640 transistors and tested each pair with Atlas DCA55 semiconductor analyzer. The measured current gain of two transistors TR601a and TR602a (left channel) was 542 and 491, respectively. It gives an ~10% difference between them. Not too bad, but remember that transistors installed in the differential amplifier are supposed to be closely matched to operate properly in the circuit. Then, I measured the current gain of two transistors TR601b and TR602b installed in the right channel. The measured gain was 563 and 449, respectively. It gives ~25% difference between them which is rather substantial. 

I replaced all original 2SA640's with modern Fairchild KSA992 transistors. Before installation, each pair of KSA992 transistors was carefully matched by current gain and base-emitter voltage. After the new transistors were installed the DC offset on each channel became close to 0V within +/- 10mV.

I also noticed that the body of two resistors (R803 and R915) is damaged. The R803 is installed in the power supply circuit, and the R915 is in the tone control circuit. I didn't see any signs of overheating though. So, I am not sure what caused their damage. Despite the body being damaged the measured resistance of R803 and R915 is still very close to the nominal resistance. It makes sense to replace these two resistors as a part of preventive maintenance. The original R803 was replaced with a new Vishay power metal film resistor. And the R915 was replaced with a new KOA Speer metal film resistor.

The body of two original resistors R803 and R915 is damaged

New resistors R803 and R915 are installed

Main amp & power supply board - before and after

Dial and Meter Lamps

The dial and meter lamps in this unit have been already replaced with warm white LED lamps by the owner.

New dial and meter lamps - warm white LEDs

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 16.16 VRMS (left channel) and 15.88 VRMS (right channel). It corresponds to the output power of 32.6W on the left channel and 31.5W 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 final result can be seen in the photos below. Please watch a short demo video at the end of this post. Thank you for reading.

Sherwood S-7210 - after restoration

Demo video after repair & restoration