Tektronix Webcam 070-8030-01 User Manual

Troubleshooting Procedures

1780R-Series Service Manual

13. Set the Variac to 40 V. Adjust the external DC supply for 18 V.

14. Use the test oscilloscope to check the waveform on the base of Q460 has

approximately 20% duty cycle and a period of about 4 

ms.

15. Connect the test oscilloscope probe to the anode of CR140 and check for an

approximate 150 V signal; voltage snubber working.

16. Move the probe to the collector of Q250 and check for a waveform.

17. Remove the short across VR570, the shorting strap from U360 pin 3, and the

external power supply leads.

At this point the power plug J600 can be reconnected or all supplies can be
externally loaded. However, if the power supply fails to operate the instrument, it
will be necessary to attempt to operate with an external load to determine if the
problem is with the power supply or in the load (instrument circuits).

18. Connect either the external load or reconnect J600. Table 6–4 has the

resistance values for the loads. Do not remove the isolation transformer.

Gnd.

31 – 39

+5 V

5 

W 5W

1.7 

W 15W

41 – 50

+15 V

100 

W 3W

11 

W 20W

17 – 22

–15 V

100 

W 3W

11 

W 20W

11 – 16

+60 V

1 k

W 4W

144 

W 25W

5 – 8

19. Set the variac for 110 V. Connect the test oscilloscope probe to W401 and

check for 

≈

+5 VDC, adjustable with R219.

20. Adjust the variac for mains potential of 90 V and then 130 V and check that

the test oscilloscope still reads the same as it did in step 19 (

≈

+5 VDC).

21. Adjust the voltage down with the variac, while watching the test oscillo-

scope. At 80 V the power supply should cease to operate (under voltage
lockout).

22. Set the variac to 110 V. Check the following waveforms (see Figure 6–8):

A.

Q250 collector requires X100 probe (

≈

750 V) 

B.

Q250 base 

C.

Q350 Q451 emitter