Power Supply And Regulators - Tait T880 II Series Adjustment Manual

B2.8 T885 Circuit Operation
The audio can also be disabled using the "RX-DISABLE" inputs, pads 225 or 228, having
connected the "RX-DISABLE" link between pins 1 & 2 of PL260. An adjustable time
delay (RV220) is provided on these lines. In order to disable the audio, either pad must
be pulled to 0V.
An undedicated relay is provided (RL210) for transmitter keying or other functions and
this can be operated from the mute line by linking PL270.
2.8

Power Supply And Regulators

(Refer to the regulators circuit diagram (sheet 6) in Section 6.2.)
13.8V Nom.
From Rear
D-Range 1
Crowbar
Diode
13.8V
Nom.
Figure 2.4 T885 Power Supply And Regulators Block Diagram
The T885 is designed to operate off a 10.8-16V DC supply (13.8V nominal). A 5.3V regu-
lator (IC630) runs directly from the 13.8V rail, driving much of the synthesiser circuitry.
It is also used as the reference for a DC amplifier (IC640, Q630 & Q620) which provides
a medium current capability 9V supply.
A switching power supply, based on Q670 and Q660, runs off the 9V supply and pro-
vides a low current capability +20V supply. This is used to drive the synthesiser loop
filter (IC740), giving a VCO control voltage of up to 20V.
The 13.8V supply drives both output audio amplifiers without additional regulation. A
separate 5V regulator (IC610) drives the microprocessor and associated digital circuitry.
The output of this regulator is monitored by the Low Voltage Interrupt (LVI) circuit
(IC650). An additional 5V regulator (located in the IF cavity) supplies the first IF ampli-
fier (Q301, Q302) and the demodulator IC (IC300).
A crowbar diode is fitted for protection against connection to a power supply of incor-
rect polarity. It also provides transient overvoltage protection.
Note: A fuse must be fitted in the power supply line for the diode to provide effec-
tive protection.
10/07/00
DC
Amp
5V
Switching
Reg
PS
5V 9V
5V Dig
LVI LVI
Reg
20V 5V Dig
M880-00
5V IF
Reg
µP 5V IF
Reset
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