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465/Repairs: Difference between revisions
→CRT Circuit Troubles
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==CRT Circuit Troubles== | ==CRT Circuit Troubles== | ||
The 465 "Family" of scopes has been known to have issues with "no display" or a "shrunken" screen trace. This includes 465,465B,466,468 and 475 scopes. The schematic for each series is almost identical but each one uses different and component ID numbers in their schematic. | The 465 "Family" of scopes has been known to have issues with "no display" or a "shrunken" screen trace. This includes 465,465B,466(?),468 and 475 scopes. The schematic for each series is almost identical but each one uses different and component ID numbers in their schematic. | ||
1. This | '''CAUTION:''' There are VERY High Potentials (Lethal Voltages and Currents) at many points in this circuit. Make certain that you understand proper safety procedures and have the proper test equipment before measuring any voltages or attempting such a repair. | ||
1. This fault is commonly caused by a failure of the HV transformer and/or the HV Multiplier. | |||
a. These parts are generally shared across the entire family of instruments. | a. These parts are generally shared across the entire family of instruments. | ||
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6. Check and note the specific wave forms at each test point (indicated by a numbered Black Hexagon)on the schematic. These are illustrated on a page preceding the schematic. | 6. Check and note the specific wave forms at each test point (indicated by a numbered Black Hexagon)on the schematic. These are illustrated on a page preceding the schematic. | ||
7. Check the | 7. Check for a strong sine wave oscillation at the collector of Q14009 (Waveform 92 in the 465B early manual. A "weak" waveform of less than 2 or 3 V p/p is trouble. | ||
8. | 8. Next, check voltage at the base of Q4228 (on 465B-Early) schematic shows this should be about 0.7VDC. With a "shorted" HV Multiplier, you will likely see 4.5-4.8VDC on the base of Q4228. | ||
a. The regulation circuit is attempting to drive T4015 to the proper potential of about -2450VDC at TP4129 and 14-16kV at the CERT connector. | a. The regulation circuit is attempting to drive T4015 to the proper potential of about -2450VDC at TP4129 and 14-16kV at the CERT connector. | ||
b. My 468 was showing only about -330VDC and a blank CRT. | b. My 468 was showing only about -330VDC and a blank CRT. | ||
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d. You will find several other voltages are off as well throughout the circuit. | d. You will find several other voltages are off as well throughout the circuit. | ||
10. Locate Service Jumper W4032 and with the scope turned off, carefully de-solder and lift one end of this jumper from the circuit board. This isolates the HV Multiplier from the ground. | 9. Check the remaining voltages on your instrument and compare to those noted on the schematic. | ||
10. Locate Service Jumper W4032 and with the scope turned off, carefully de-solder and lift one end of this jumper from the circuit board. This isolates the HV Multiplier from the ground. | |||
a. Turn the scope power "on", if everything else is OK, you should see a trace which should be dim, but still visible. Dim room lighting helps spot the trace. | a. Turn the scope power "on", if everything else is OK, you should see a trace which should be dim, but still visible. Dim room lighting helps spot the trace. | ||
b. You should be able to fully control the trace position and other characteristics, however, it will be quite dim and somewhat unstable. | b. You should be able to fully control the trace position and other characteristics, however, it will be quite dim and somewhat unstable. | ||
c. The beam finder will cause the trace to contract about 10-15% below its previous size. | c. The beam finder will cause the trace to contract about 10-15% below its previous size. | ||
11. Re-Test all voltages in the CRT circuit and confirm that they generally agree with the schematic. | |||
12. Retest for the same wave forms (Step 6 and 8) as before, these should be more similar to the ones shown in the diagrams. | |||
13. Locate a known good HV Multiplier and replace as needed. | |||