Singer 621B Sewing Machine Service Manual

The Singer 621B is a classic straight-stitch sewing machine known for its durability and reliability. This service manual provides comprehensive instructions for maintenance, troubleshooting, adjustment, and repair procedures. It covers key sections on safety, machine features, disassembly, lubrication, timing adjustment, tension settings, common issues, and parts identification to assist technicians and experienced users in proper servicing.

Features at a Glance

Key components: Cast-iron body, oscillating hook system, reverse stitch lever, built-in light, adjustable stitch length, drop feed for darning, and a standard domestic motor.

Safety Precautions

Always unplug the machine before performing any service or maintenance.

1. Electrical Safety: Disconnect power cord from outlet before opening covers or accessing internal components.
2. Sharp Parts: Handle needles, hooks, and other sharp components with care to avoid injury.
3. Lubricants: Use only recommended sewing machine oil. Keep away from heat sources and open flames.
4. Stability: Ensure machine is placed on a stable, level surface during service to prevent tipping.
5. Moving Parts: Keep fingers and tools clear of the needle bar, hook, and gears when testing.

WARNING! Never operate the machine with protective covers removed. Serious injury or electrical shock may occur.

Tools & Lubricants Required

Gather the following before starting service procedures.

1. Screwdrivers: Small flat-head and Phillips-head.
2. Needle Nose Pliers & Tweezers.
3. Lint Brush and Soft Cloth.
4. Sewing Machine Oil (high-grade).
5. Timing Gauge or Feeler Gauge Set.
6. Multimeter (for electrical checks).
7. Hex Wrenches (if applicable for motor mounting).

CAUTION! Using incorrect tools or lubricants can cause permanent damage to the machine.

Machine Specifications

• Model: Singer 621B.
• Class: 15 Oscillating Hook.
• Stitch: Straight stitch only.
• Stitch Length: 0 to 4 mm, adjustable.
• Maximum Sewing Speed: Approx. 1100 stitches per minute.
• Needle System: 15x1 (HAx1) standard domestic.
• Bobbin: Class 15 metal bobbin.
• Thread: All-purpose sewing thread (size 40-50 recommended).
• Power: 110-120V AC, 60Hz (US/Canada standard).
• Dimensions: Approx. 40 cm (L) x 18 cm (W) x 30 cm (H).

External Controls & Components

Familiarize yourself with the main user-accessible parts.

Front Panel: Stitch length dial, reverse stitch lever, tension dial, presser foot lifter, handwheel.

Needle Area: Needle clamp screw, presser foot, feed dogs, throat plate, bobbin cover.

Side/Bottom: Motor belt access panel, light switch, power socket, foot controller port.

Operation: Turn handwheel TOWARDS you only. Use reverse lever for short back-tacks. Adjust stitch length for fabric thickness.

Internal Mechanism & Disassembly

Step-by-step guide to accessing the main internal components for service.

1. Unplug machine and remove needle, presser foot, and throat plate.
2. Remove the top cover (usually held by screws at the rear).
3. Remove the side and front face plates to access the hook drive and gear train.
4. To access the lower shaft and hook, remove the bobbin case and shuttle race.
5. For motor service, remove the belt guard and loosen the motor mounting screws.

Tip: Lay out screws in order of removal and take photos for reassembly reference.

Timing & Hook Adjustment

Correct timing is critical for proper stitch formation and preventing skipped stitches.

1. Set the needle at its highest position by turning the handwheel.
2. Loosen the needle bar clamp screw and adjust the needle bar so the eye of the needle is level with the bottom of the presser foot.
3. For hook timing: With needle descending, the point of the hook should meet the needle just above the eye (approx. 1.5-2.0 mm) as it begins its upward stroke.
4. Adjust by loosening the hook drive gear set screw and rotating the hook assembly slightly.
5. Tighten all screws and test with scrap fabric. Adjust as needed.

WARNING! Incorrect timing can cause needle breakage, thread jams, and permanent damage to the hook.

Thread Tension & Stitch Formation

Balanced tension creates even stitches on both sides of the fabric.

Upper Tension: Controlled by the numbered dial. Higher number = tighter tension.

Bobbin Tension: Adjusted via the small screw on the bobbin case spring. Turn clockwise to tighten.

Test & Adjust: Sew on two layers of medium-weight fabric. If top thread loops on bottom, tighten upper tension or loosen bobbin tension. If bobbin thread loops on top, do the opposite.

Presser Foot Pressure: Adjust the spring screw on top of the machine for thick or thin fabrics.

Motor & Electrical System

Basic service for the motor, light, and foot controller.

1. Motor Runs but Machine is Slow/Weak: Check belt tension. It should have about 1/2 inch of play. Adjust motor position.
2. Motor Doesn't Run: Check power cord, wall outlet, and foot controller with a multimeter. Inspect internal wiring for breaks.
3. Light Doesn't Work: Replace bulb (standard screw-base). Check socket for corrosion.
4. Foot Controller Issues: Ensure cord is not frayed. Internal carbon brushes may need replacement if motor speed is erratic.

CAUTION! Always disconnect power before checking electrical components.

Parts List & Diagrams

Reference for common replacement parts (partial list).

For complete diagrams and part numbers, refer to the official Singer parts catalog for model 621B.

Troubleshooting Guide

Final Check: After any service, run the machine slowly without thread to ensure smooth operation before sewing.

Service Support: For genuine Singer parts and advanced technical support, contact Singer Customer Service or visit www.singer.com.