Tait T855 - Page 192

C5.16 T856/857 Fault Finding M850-00
31/09/98 Copyright TEL
5.7.5 T856 PA & Power Control
Check IC330,
Q310, Q315
& circuitry,
+13.8V supply.
Refer to Drive Amp.
Fault Findin
g
Chart.
Check supply
circuitry.
Tx deliverin
g
full power
(>30W)?
Set RV320
(power adjust)
fully clockwise;
synth. locked;
key Tx.
Y
NY
N
N
Y
N
Y
N
NY
Y
Y
Y
Power control
circuitry OK.
Note:
c/w = clockwise
cc/w = counterclockwise
or anticlockwise
Check Q410,
Q420 & circuitry,
LP filter.
10.8V
at L310?
Check drive at
SK310 >1.5W.
Check +13.8V at
Q410 & Q420.
<2V on
IC350 pin 6?
Turn RV310 ccw.
Check power
drops to zero
slowly in a
controlled manner.
Does variation
of RV310
cause either
full power
or no power?
Check for break
in power sense
circuitry; directional
coupler D420,
D440 & circuitry.
Set RV310 so that
output power = 25W;
check L310 approx. 7.5V.
Momentarily remove
Tx load. Volta
g
e at
L310 decreases to
approx. 5V?
Check lock det. circuit;
TP305 should be >8V.
Check temp. shutdown
circuit (IC350);
TP320 should be <0.6V.
Check Q320, RV330.
Normal operating conditions:
Forward power = 4.7V approx.
Reverse power = 0.7V approx.
IC350 pin 6 = 2.02V
IC350 pin 5 = 2V
IC350 pin 7 = 8.63V