Sherwood S 7110 B S 7125 A Service Manual

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8 I O m; _modulor III-Ii Pages 115-125 Courtesy of SHERWOOD ELECTRONICS LABS, INC.

PREFACE:

The 571108 and SIIZSA are identical electrically and mechanically, differing only in panel
model numbers and wood cases. The 571105 is used for illustration in this manual.

AMPLIFIER SERVICING AND ADJUSTMENT

NOTE:

For simplicity only the left channel and its related circuitry are described. The right
channel is identical except for reference symbol numbers (see Schematic Diagram). For
parts locations, refer to Figures 1A and 18 on the inside covers.

USE OF A VARIAC:

It is imperative that a variable voltage line source (Variac) equipped with a line watt-
meter to identify abnormal power consumption be used when servicing power amplifiers and
associated power supply circuitry. with the Volume (Loudness) control set at minimum,

the power consumption should not exceed 20 watts as the voltage is increased by the Variac
to the rated 120VAE. If the power consumption begins to exceed 20 watts. do NOT increase
the line voltage any further. Determine if the malfunction is in the power supply,
preamplifier, amplifiers, or tuner section of the receiver.

SPEAKER FUSE AND SPEAKER SYSTEM CHECK:

The 571103 and 57125A incorporate a speaker protection fuse in each channel. Verify that
both fuses are unopened and that both speaker systems and their associated wiring are
short-free.

LINE FUSE AND INITIAL CIRCUIT CHECKS:

Verify that the line fuse is unopened and check idling power consumption. The main causes
for abnormal power consumption, in order of decreasing occurance, are:

1. Open or shorted amplifier output, driver or ore-driver transistors TR602-609.
2. Open or shorted power supply diodes or filter transistor DBD1-504 and TRBD1.

3. Shorted power transformer.

NOTE:

For the following tests, an 8 ohm load resistor must be connected to each of the two
power amplifier (speaker) output terminals.

IIS

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SUGGESTED AMPLIFIER TEST BENCH SET-UP

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AMPLIFIER FAULT ANALYSIS:

If an amplifier channel is in question, check related circuit boards for burned parts and
replace. Check all the transistors with an ohmmeter for opens or shorts and replace if
defective.

'HARNING: Some ohmmeters may damage sensitive solid-state devices. Whenever possible,
use a high resistance range (at least RxID).

Use the centerpoint voltage (measured From the + speaker terminal to ground) as a guide.
The centerpoint voltage should always be Zero :SDmV. Any deviation suggests shorted or
open devices.

If channel operation is still faulty, verify that there are no shorted capacitors, open
resistors, etc., on the board.

Inspect the underside of the board for shorted pads, broken connections. etc.
When the board is restored. readjust Uutput Bias (see below).

DISTDRTION IN THE AMPLIFIER OUTPUT:

Distortion which exceeds the amplifier ratings (see Front Cover) may be due to the
following:

1. Miamatched output transistors.
2. Defective (low-beta) driver transistors.
3. Incorrectly adjusted output transistor bias.

DUTPUT TRANSISTDR BIAS ADJUSTMEN :

Proper output transistor operation and output bias adjustment ara most important to
assure cool, low-distortion operation of the amplifier. Bias adjustment is necessary if
the output transistors are replaced or if any of the transistors in the driver circuitry
of the amplifier exhibits one or more of the following symptoms:

1. Overheating of the output transistors under normal operating conditions.

2. Excessive low level Intermodulation Distortion (IMD) or Total Harmonic
Distortion (THD) - more than 0.3% at 3.0 volts across 8 ohms.

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