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465/Repairs: Difference between revisions
CRT Circuit Troubles
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4. Service manual explains the schematic in detail, refer it for a detailed understanding of the section you are troubleshooting. | 4. Service manual explains the schematic in detail, refer it for a detailed understanding of the section you are troubleshooting. | ||
==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 scematic. | |||
1. This may be attributed to a failure of the HV transformer and/or the HV Multiplier. | |||
a. These parts are generally shared across the entire family of instruments. | |||
b. The HV Multiplier TEKTRONIX p/n 152-0552-00 or CMX234 boosts the Cathode voltage from T4015 to a much higher potential. | |||
c. The very high acceleration potential facilitates both full size and high brightness to the trace. | |||
2. Diagnosis is straight forward. Follow the steps in the troubleshooting block diagram and also study the Theory of Operation for the CRT Circuit. Use the Scope settings recommended in the manual. | |||
3. First check that all power rails are present at the appropriate Test points. The 55VDC must be correctly adjusted before any repairs are attempted. | |||
4. Repair any issues with your power supplies before moving to repair other issues. | |||
5. Study the CRT Supply Schematic (keep on mind that you need to use the schematic for your specific scope). | |||
6. Check and note the specific wave forms at each test point (indicated by a numbered Black Hexagon)on the schematic. | |||
7. Check the voltages on your instrument and compare to those noted on the schematic. | |||
8. If you have no display, 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. | |||
9. Next, check voltage at the base of Q4228 (on 465B-Early) schematic shows this should be about .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 the circuit 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. | |||
c. The HV Multiplier has likely shorted to ground and is demanding more current and voltage from T4015 than the transformer is able to deliver. | |||
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 un-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. | |||
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. | |||
11. Re-Test all voltages in the CRT circuit and confirm that they generally agree with the schematic. | |||
12. Retest for the same waveforms (Step 6 and 8) as before, these should be more close to the ones in the diagrams. | |||
13. Locate a known good HV Multiplier and replace as needed. | |||