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
Overview
This advice only applies to sewing machines that have a separate motor box on the rear of the machine.
NOTE: the post is of an educational nature, work on live devices is intended exclusively for professionals, the risk of electric shock. !!
Replacement components
The type 707 motor Bernina used in these boxes is provided with a capacitor (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 poph 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 Class (earthed) or Class 2 (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.
The original capacitors (network suppressor) 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 nbr. 335.011.03) whilst this is internal In later versions and they have slightly different wire colours (part nbr. 335.200.03).
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 ........Early type........Later type......Replacement
(Part nbr) .......335.011.03......335.200.03....BD335201
Foot control....White...............White.............White
Motor case......Green/Yellow..Clear..............Green
Live ................Black...............Black.............Black
Neutral............Grey................Brown............Grey
Way
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 boh 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 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 frequentiy very brittle and care should be taken in case these parts are cracked.
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
The 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 capacitor-suppressor.
You may find it easier to remove the motor from the box in order to gain access to the terminal block on the side of the motor.
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 (arrow)
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...
What to say in the end, this is a bit of a controversial topic, network suppressors-capacitors today have no effect on radio and TV, if you rarely use the machine you can simply remove them, if you use your sewing machine very often, it is advisable replace the worn out capacitor.
Works of this type can be performed only by professionals. !!
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