Tait T855 - Page 188

C5.12 T856/857 Fault Finding M850-00
31/09/98 Copyright TEL
5.7.3 Synthesiser
Refer to the synthesiser circuit diagram (sheet 7) in Section 6 and the VCO circuit dia-
gram in Part E.
N
A B
Check biasin
g
resistors, Q4 & Q5.
N
Y
N
8.3V present
at VCO?
DC V present
at Q4 & Q5?
9V present at
VCO Q4/Q5?
No VCO RF output.
Y
DC V present at
VCO, Q1, Q2 & Q3?
Check biasin
g
resistors, Q1,
Q2 & Q3.
Y
N
Capacitors OK?
Check for cracks,
shorts, etc.
N
Y
Replace VCO.
Replace
if faulty.
Y
N
Y
N
N
N
Y
Y
DC V present
on Q795 emitter?
Check 9V supply.
9V present on
Q795 divider buffer?
20V inverter
supply faulty.
20V present
on IC750 pin 8?
Check 5V re
g
. &
supply resistor
to each IC.
+5V present at
IC700, IC710,
IC740, IC720
& IC730?
VCO out of lock.
Y
Y
Y
Continued on the next pa
g
e
Y
VCO freq. correct?
Synthesiser OK.
VCO trimmer
freq. ran
g
e OK?
Check VCO tunin
g
caps & trimmer.
VCO OK.
N
N
NN
N
Y
Y
Recheck
pro
g
rammin
g
via PGM800Win.
Check Q5 C
multiplier
(synth).
Check biasin
g
resistors & Q795.
Check Tx re
g
.,
Q610 & 9V re
g
.
Does VCO trimmer
adjust for stable
control line
between 3 & 16V?
Does VCO trimmer
adjust for required
frequency ran
g
e
between 3 & 16V?
Check #C1, #C7, #C8,
C6 & D1-D2 in VCO.
Is 8MHz VCO ran
g
e
achieved between
3 & 16V?