Tait T850 II Series, Service Manual

Page: 90 / 370

B5.16 T855 Fault Finding
M850-00
31/09/98 Copyright TEL
5.7.4 Noise Mute
Refer to audio
fault finding chart.
Ensure PL250 is set
for noise mute operation.
Check Q340
& Q350.
Check D310 is
rectifying noise.
Vary RF level &
observe voltage
on pad 236.
Pad 236 voltage
varies 0-1.8V?
Check 9V rail &
resistors R264,
R265 & R284.
Replace IC270.
Check/replace
IC270 & D240.
Replace Q250.
N Y
N
Y
N
Y
Y
N
Y
Y
Y
Y
N
Refer to
receiver fault
finding chart.
Check components
around active filter.
Mutable audio
on outputs?
Y
Voltage varies
0-2.5V?
N
OK
Does "Gate" LED
toggle on/off?
N
Check Q230, Q245 &
Q290 for operation.
Y
Adjust RV230
(gating sensitivity)
& observe voltage
on IC270 pin 3.
Set RF signal level to
-117dBm. Rotate
RV230 c/w & cc/w.
Monitor voltage on
IC270 pin 1. Switching
0-8V as RV230 rotated?
Monitor voltage on
IC270 pin 7. Switching
0-8V as RV230 rotated?
Monitor voltage on
pad 234. Switching
0-8V as RV230 rotated?
N
N
N
Does "Gate"
LED toggle? Check/replace
Q255 & D250.
Y
Y N
Check RV330
adjustment
(noise mute gain).
Approx. 0.6V DC rect-
ified noise on pad 236?
N
Switch SW201 (monitor
mute) to ensure
mute is enabled.
Rotate RV230
(gating sensitivity)
c/w & cc/w & view
"Gate" LED.
Note:
c/w = clockwise
cc/w = counterclockwise
or anticlockwise
Audio on
TP314?
Noise on
IC310 pin 11?
Amplified noise
on C380?
Apply on channel signal
@ -117dBm* to receiver.
*Modulated by 1kHz signal with
±3kHz (±2.4kHz) [±1.5kHz] deviation