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L-
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G
C 1
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F
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O e m i t t e r
T R l
a c d l e c t o r
EB 3
PZ6layout
ditions can be tested with an ohm meter, or by disconnecting one
wire from the transformer secondary: if the PZS functions in this
condition then the diode that is not disconnected is working
properly.
Service. About two thirds of PZSs returned for service do not have
a n y f a u l t a t a l l .
E B l 0
C o m m o n f a u l t s -
P Z O
The PZ6 is not protected, but includes a fuse to restrict damage. lf
the fuse blows, then it is possible that other damage has occured. lt
is not safe to replace the fuse and put the PZO back in circuit, since
other damage can cause the PZ6's voltage to increase to 55v (which
is the normal off-load voltage across C1). To test, disconnect the
P Z 6 f r o m c i r c u i t , r e p l a c e t h e f u s e ( 1 A a n t i s u r g e ) a n d s w i t c h o n .
Measure the output voltage, which should be adjustable between 20
and 50 approximately. lf this is correct, then it is safe to test the PZ6
i n c i r c u i t .
Fuse repeatedly fails. lf speakers are of low impedance (5
or less)
and the volume os turned too high, the fuse will fail with no other
d a m a g e o c c u r i n g . l f t h i s h a p p e n s t h e f u s e m a y b e u p r a t e d t o 1 . 2 5 A
antisu!'ge. lf the mains fuse repeatedly fails, then the extra equip-
ment driven off the fuse may be at fault, or the wrong type of fuse
mav be in use.
Overheating. The PZ6 in use does get quite warm, especially when
tuner and decoder are in use from it, or when high volume levels are
i n u s e . l t i s u n l i k e l y t h a t a f a u l t c a u s i n g o v e r h e a t i n g c o u l d o c c u r
within the PZ6 without causing severe audible symptons. The PZ6
transformer should not get much too hot to touch.
Service experience shows that PZ6 failure often accompanies failure
of one power amplifier. When an amplifier fails, the PZ6 should be
c h e c k e d .
EC PZB
E C l
C i r c u i t D e s c r i p t i o n
Tr4 and 5 form a long-tailed pair which compares the reference
v o l t a g e o n Z D 1 w i t h t h a t p r e s e n t o n t h e s l i d e r o f V R 1 . l t s o u t p u t
26
controls the Darlington
'triple'
Tr1 , 2 and 3 in such a way as to
minimize the difference between the two.
At the same time, if the output voltage is at its nominal value of
50v, T16 is turned hard on by the reference developed on its base by
R 1 0 a n d R 1 1 . T h e t a i l c u r r e n t t h r o u g h t h e d i f f e r e n t i a l p a i r i s t h e n
limited by the zener voltage, less Vbe in Tr4, across R8. As the
output current through Tr1 increases its base voltage increases by
v i r t u e o f t h e i n c r e a s i n g v o l t a g e d r o p i n R 1 . A s t h e b a s e v o l t a g e o n
the triple rises, R4's current increases, drawing more current through
T r 4 u n t i l , w h e n T r 4 i s c o n d u c t i n g t h e w h o l e o f t h e a v a i l a b l e t a i l
c u r r e n t t h e c i r c u i t s t o p s s t a b i l i z i n g a n d t h e o u t p u t v o l t a g e f a l l s a s
c u r r e n t l i m i t i n g o c c u r s . T h e c i r c u i t t h u s g i v e s a l i m i t e d c u r r e n t o f
between 4-6 amps at 50v, see the curve.
l f s e v e r e l i m i t i n g o c c u r s t h e o u t p u t v o l t a g e d r o p s . T h i s c o u l d
r e s u l t i n o v e r - d i s s i p a t i o n i n T r 1 , b u t t h e r e d u c e d o u t p u t v o l t a g e
c a u s e s T 1 6 t o t u r n o f f , t h r o u g h R 1 0 a n d R 1 1 , f u r t h e r r e d u c i n g t h e
limited current and dropping the output voltage. Positive feedback
o c c u r s a n d t h e c i r c u i t ' t r i p s ' i n t o a s e n s i n g s t a t e w h e n i t c a n n o t g i v e
more then 200-250mA. Tr1 still dissipates, about 10 watts (200 mA at
5 0 v a c r o s s i t ) b u t D 1 - D 3 a r e i n i n t i m a t e t h e r m a l c o n t a c t ( b e i n g
mounted on the heatsink below the transistor) and prevent thermal
r u n a w a y - i n f a c t b o t h t h e h i g h c u r r e n t a n d l o w c u r r e n t l i m i t s a r e
r e d u c e d s l i g h t l y w h e n t h e c i r c u i t i s h o t .
The circuit rests in its sensing state safely - until such time as the
o v e r l o a d v a n i s h e s , w h e n t h e o u t p u t v o l t a g e r i s e s a n d t h e c i r c u i t
t u r n s b a c k o n a g a i n .
An additional attractive feature is the circuitry of Tr7, D4 and C4.
Before switch on C4 is discharged. At switch on it charges up
through R5 and TrTs base. The value of C4 is then effectively
multiplied by the gain of Tr7 and thus charges very slowly, with it
rises the output voltage. Thus to start with, the output current is
heavily limited (200mA). lt is therefore incredibly gentle to the 260s,
w h i c h , d e p e n d i n g o n o u t p u t c a p a c i t o r s a n d s p e a k e r s , t a k e 2 - 1 5
s e c o n d s t o s w i t c h o n . D u r i n g t h i s t i m e m a n y l i m i t a t i o n s a n d i n t e r -
actions occur between the slowly rising potential, the slowly in-
c r e a s i n g c u r r e n t l i m i t , t h e c h a r g i n g o u t p u t c a p a c i t o r s , a n d t h e
switch on performance of the 260. The noise present in the 260 may
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