The Bernina Doctor Technical Advice Note Title: Bernina Electric Sewing Machine Suppressor Network Replacement 1.
Applicability
Bernina Record Models 730-to-732, 830-to-832 Bernina Sport Models 801-to-803 Bernina Minimatic Models 707-to-710, 717-to-719, 807-to-812, 817-to-819
2.
Overview
This advice only applies to sewing machines that have a separate motor box on the rear of the machine.
The type a707 motor used in these boxes is provided with a suppressor network to prevent interference to radio and television and to reduce arcing on the commutator contacts that leads to fouling of the contact blocks. Failure of these suppressors is now common due to age-related degradation of the product, this often being accompanied by a loud ‘pop’ and sometimes acrid-smelling smoke. In some cases, the machine may start running, uncontrolled, with potential safety implications so the machine should not be used until it is replaced. Some technicians simply remove failed suppressors without replacing them; this is very bad practice and is not recommended. Models of sewing machines covered include variants that are insulated to Class1 (earthed) or Class2 (double insulated) standards and it is important to take account of this when undertaking suppressor replacement to ensure that the relevant electrical safety is not compromised. All models and variants use a 4-pole connector on the motor box for the mains/foot-controller cable. Despite this there are variants that have different numbers of internal terminals, dependent upon whether a motor speed switch is incorporated into the box or the switch is external to it. Additionally, Class2 double-insulated variants have one pin/terminal missing to prevent connection of the mains earth to the machine’s chassis and later models using a type 232 or 239 electronic foot controller have a different shape connector. Iss3 May 2015
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The Bernina Doctor 3.
Replacement Component
The original suppressor network used on Bernina machines was available in a number of variants, but all had the same basic configuration with four leads terminated in push-on connectors. Early suppressors have a bleed resistor between the black & grey wires (Part Number 335.011.03) whilst this is internal in later versions and they have slightly different wire colours (Part Number 335.200.03). Original Types Early Later
Replacement Type
Bleed Resistor
Signs of failure of the original component are usually cracking of the housing and/or leakage of a sticky/waxy material. Depending upon the failure mode, the capacitance across terminals may appear low or a short-circuit may be found when using appropriate test equipment.
Wiring Colours: Function (Part Number) Foot Controller Motor Case Live Neutral
Iss3 May 2015
Early Type
Later Type
Replacement
335.011.03 White Green/Yellow Black Grey
335.200.03 White (ws) Clear (tr) Black (sw) Brown (bn)
BD335201 White (Wh) Green (Gn) Black (Bk) Grey (Gy)
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The Bernina Doctor 4.
Method
Remove the end panel of the sewing machine below the handwheel. Depending upon model, this is either by removing the three slotted screws in the end or by a single screw under the base. Slacken the hex socket bolt securing the drive-belt tensioner and remove the motor drive belt. Take care not to damage the belt.
Remove the two screws securing the motor box to the baseplate. On some models you will have to remove a plastic cover from the underside of the baseplate — this is secured by a single central screw.
Carefully tip the motor box over and recover the flexible spacer from between the box and baseplate. Note that the motor / spacer are frequently stuck to the base and may have oily deposits. Unscrew the two self-tapping screws holding the plastic backplate onto the motor box to gain access to internal cabling and terminations. Note that the backplate and mounting pillars are frequently very brittle and care should be taken in case these parts are cracked.
Iss3 May 2015
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The Bernina Doctor There are two types of motor box : Type 1 with an integral speed switch and 4-pole terminal and Type 2 with a separate speed switch and 5-pole terminal Type 1
Type 2
The above are shown without suppressor capacitor fitted Unplug and remove the old suppressor and, if necessary, clean the inside of the motor box to remove unwanted deposits before fitting the new suppressor. You may find it easier to remove the motor from the box in order to gain access to the terminal blade on the side of the motor (arrowed)
Replace the motor and route the cabling so it can’t be crushed when replacing the backplate. On some models the motor cables have filters that have to be accommodated in space adjacent to the terminal block (arrowed)
Finally, refit the motor box, taking great care to ensure the sewing light and external switch (if fitted) cables are correctly plugged onto the terminals. Conduct a functional test on the associated sewing machine and dispose of the old component responsibly. Iss3 May 2015
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The Bernina Doctor 5.
Appendix
5.1
Wiring Diagrams
5.1.1
Motor Box with Integral Speed Switch
Motor
Suppressor
Speed Switch
To Sewing Light
Iss3 May 2015
Earth Wire to bolt on chassis, if fitted (not on Class2 double-insulated machines)
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The Bernina Doctor 5.1.2
Motor Box with Separate Speed Switch
Motor
Suppressor
To Speed Switch To Sewing Light
Iss3 May 2015
Earth Wire to bolt on chassis, if fitted (not on Class2 double-insulated machines)
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The Bernina Doctor 5.2
Generic Circuit Diagram
Foot Controller
Sewing Machine Sewing Light
Wh Bk
Mains Plug
FC E L N
Gn Motor
Gy
Motor Suppressor
Speed Switch
Note that wiring shown in dashed lines is dependant upon model.
Iss3 May 2015
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The Bernina Doctor
Š The Bernina Doctor Midlands Farm Horndon Mary Tavy Devon PL19 9NQ 01822 810906 info@bernina.org.uk
Iss3 May 2015
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