Chapter 3. Service Precaution - Sharp ER-A750 Service Manual

CHAPTER 3. SERVICE PRECAUTION

1. Adjustment for SRN (IN-LINE) interface
circuit
If transistor Q10 in the transmitter/receiver section has been re-
placed or if the SRN level requires readjustment, the following align-
ment is required:
1) Tools and Instruments Required
1 Oscilloscope (50MHz or better) . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 ER-A750 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2) Dummy Network Specifications
R1 100
R2 150
C1 0.01µ F (mylar firm)
Fig. 1 Dummy network
The oscillator should be connected to the points indicated by
.
$
: Connect the positive side of the oscillator.
: Connect the negative side of the oscillator.
$
3) Connections
Main PWB
Attach the BNC connector to the SRN connector (CON 15) on the
main PWB.
4) Alignment Procedure
1 When Using an Oscillator
a) Checking the 1MHz oscillator output
Using an oscilloscope check the 1MHz oscillator's output waveform.
5V
0V
0.5µ S 0.5µ S
Fig. 3 1MHz oscillator output waveform
NOTE: The oscillator used should have an output impedance of
50 .
R2
R1
C1
J (1/4W carbon)
J (1/4W carbon)
BNC connector
Fig. 2
b) Connecting the oscillator and its adjustment
Connect a dummy network or branch-trunk network to the output of
the SRN connector (CON 15), and connect the oscillator to the
dummy or branch-trunk network.
* Waveform adjustment
Adjust VR4 until the signal waveform as shown in Fig. 5 is obtained
across IC34 (pin 1 of the 75115) and GND pin.
Turning VR4 clockwise extends the interval of T1.
T1 T2
T1 = 580 to 620ms
T2 = 380 to 420ms
Fig. 5 Receiver regeneration waveform (with dummy network)
Main PWB
and
2 When the Branch Trunk Network and Two POS'S are Available.
a) Connecting terminals
Both ends of the network must be terminated with a 50
If only two active terminals are tested and left on the network, dis-
connect all other terminals from the network. (In this case as well,
both ends of the trunk network must be terminated with 50 ).
R50
ER-A750
3 – 1
Fig. 6 Board location
Fig. 7 Terminal connection
VOH
VOL
terminator.
R50
ER-A750