Troubleshooting The High-Voltage Circuit; Troubleshooting The Sweep Circuits - Tektronix 475 Technical Manual

TM 11-6625-2735-14-1
2. Check that the current-limiting transistor is off
(Q1432 in the +50 volt supply) by measuring base-to-
emitter voltage. If the supply is current limiting, remove
the power from circuit boards, individually, until the
supply voltage returns to normal and the circuit board
drawing excess power is identified. To remove power to
circuit boards, disconnect plugs, or desolder and lift one
end of decoupling networks, or move circuit boards
enough to clear interconnecting pins.
3. Measure the reference voltage at the supply
operational amplifier and check for proper voltage (pin 3
of U1418 of +50 volt supply; negative input of the
operational amplifier of any supply).
Troubleshooting the High-Voltage Circuit. There are 3
major areas of concern when troubleshooting this cir-
cuitry; the high-voltage oscillator and regulator, the over-
voltage protection circuit, and the secondary load in-
cluding the crt and the high-voltage multiplier. By dis-
connecting the appropriate circuit, the high voltage
should come up. It may be helpful to use the following
information, in sequence, to troubleshoot an inoperative
high-voltage circuit. Remove the high voltage circuit cover.
Refer to figure 7-8 for component locations.
1. Check the collector of Q1316 for approximately
-0.7 volts to approximately +1.6 volts.
2. Check the positive end of C1318 for approximately
+21 volts.
3. Check the collector of Q1318 for approximately
+21 volts.
4. Disconnect and lift one end of CR1329.
5. Disconnect the crt base socket to eliminate crt
loading.
6. Check or replace C1316.
7. If the oscillator still does not come up, the high-
voltage multiplier could be defective. Remove the Vertical
Preamp board to expose the multiplier and disconnect the
center tap of the high-voltage transformer from the
multiplier tie point.
4-16
Troubleshooting the Sweep Circuits. The sweep cir-
cuits should be repaired in the following order:
A (Sweep)
A INTEN (Intensified Sweep)
B DLY'D (Sweep)
MIX (Sweep)
A Sweep and Logic
1. Remove all external signals to the instrument, set
TIME/DIV to 1 ms (lock knobs), set HORIZ DISPLAY to A
and set TRIG MODE to AUTO.
2. Remove Q572 from its socket. This will cause only
the Slow Non-Delaying Sweep Generator (A Sweep),
which normally should be in operation, to stay at a low
voltage level. Check the collector of (3984 for ap-
proximately +0.3 volt. Check the collector of Q1014 for
approxiately +16.4 volts. Check the collector of Q922
for approximately +146 volts.
Set TIME/DIV to 0.5
Slow-Non-Delaying Sweep Generator to run up and the
Fast Non-Delaying Sweep Generator to run down. Check
the collector of Q984 for approximately +16.4 volts and
check the collector of Q1014 for approximately +0.3 volt.
At this point, the A Sweep holdoff should be in a reset
condition. Check TP588 for approximately –4 volts.
Check the emitter of Q644 for approximately
When Q572 was removed from its socket, pin 16 of U600
went positive, which is the equivalent of a holdoff start
pulse. Check pin 16 of U600 for approximately +2.5 volts.
Check pin 17 of U600 for approximately +1.7 volts. Check
TP572 for approximately
0
approximately +0.2 volt.
3. Install Q572 and remove Q574 from its socket. This
wiII cause only the Fast Non-Delaying Sweep Generator,
which normally should be in operation, to stay at a start
(high) voltage level. Check the collector of Q1014 for
approximately +14 volts. Set TIME/DIV to 1 ms. Check
the collector of Q984 for approximately +14 volts.
At this point the A Sweep holdoff should be at a high
level. Check TP588 for approximately +2 volts. Check the
emitter of Q644 for approximately +4.3 volts.
or faster. This should cause the
volt.
0
volt. Check pin 3 of U600 for