Pioneer tuning fork Service Manual page 5

2 . TYPES OF SWITCHES
2 . 1 Classification by USES
They can be classified into three; power, selector and dig­
ital switches.
a) Power Switch
P o w e r switch connects or disconnects an A C power line
to the primary winding of a power transformer. This switch
has been so designed to stand high heat to deal with a large
current which generates arcs and heat when switched.
T h e contact pieces should, therefore, be m a d e of a mate­
rial which has a high melting point and should move quick­
ly t o minimize sparks, and the gap between the contact
points should be wide enough to prevent the pieces from
melting and sticking. If the gap is narrow or the opera­
tion is slow, the arc will continue for a long period. Fig.
3 shows a typical power switch. The contact points are
m a d e of silver cadmium oxide (AgCdO) and are operated
quickly with the help of a spring. All Pioneer products em­
ploy power switches which have preliminary been approved
by the authorities of safety standards in each country.
Major safety standards are U L (USA), CSA (Canada),
S E M K O (Sweden), D E M K O (Demmark), N E M K O (Nor­
way), BS (GB), VDE (West Germany), EI (Finland) and
SEV (Switzerland).
Terminals
Fig. 3 Contact Point of Power Switch
b) Selector Switch
Selector switch is a multiposition switch that permits one
or more conductors to be connected to one of other con­
ductors. This is used in the following circuits to the secon­
dary winding of a power transformer. Most switches are
for selecting a signal and some for switching a DC cur­
rent. Here we will discuss the switches for selecting signal
channels. As to the switches for opening and closing D C
circuits and for sensing various conditions, refer to the item
" 2 . 2 Switches Classified by F u n c t i o n s " .
Switches for selecting signals usually employ the sliding
mechanism which scrubs and clean the contact surface by
itself because the signals to be selected are very low in volt­
age, small in current and are highly affected by the resis­
tive dust film. Figure 4 shows the mechanism of a linear
sliding signal selector.
The clip or moving piece is m a d e of silver-plated phos­
phorous bronze and the fixed piece is silver-plated brass.
Low tenacity grease is applied o n the contact surfaces to
protect silver from being affected by the sulphur in the air.
The sulphur affects silver, forms a resistive film on the sil­
ver surface and causes poor contact.
There are shorting (make-before-break) and nonshorting
(break-before-make) types in this switch. Fig.5 shows the
difference. Type (a) once connects and type (b) once opens
all terminals of A, B and C when switching. Generally,
shorting type is used in muters, attenuators, mode-
selectors, etc. and nonshorting type is used in function
Bs (Ag-plated)
selectors, tape monitors, etc.
Bs or Cu
AgCdO
Bs or Cu
- Bs (Ag-plated)
Fig. 5 Shorting and Nonshorting Types of Contacts
c) Digital Switch
The switch exclusively used in a digital circuit is called
digital switch. This is also a kind of signal selectors.
Generally they are used in key input and data circuits.
Clip
Conductive piece
Terminals
Fig. 4 LINEAR Slider Type Signal Selector
3