465/Repairs

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Capacitors

The 465 series scopes use tantalum capacitors. They are known to short out and bring the scope down.

Typical symptoms will be no trace, or powering on and going off in few seconds due to built in fold-back protection. The shorted capacitor in the power rails will bring down one (or more) of them causing entire scope to be non functional.

Quick way to troubleshoot is, after powering off - and ensuring all capacitors are discharged - check resistance to ground on each power supply rail and identify the shorted power rail. Then trace the offending tantalum capacitor in the shorted rail with the help of schematic. This could in any board/sub assembly, not necessary in the power supply or A9 board.

You can replace tantalum caps with general electrolytic. Remember to ensure you install them in the correct polarity, specially on the negative rails.

General Troubleshooting

Refer to service manual first for the overview of the equipment and general troubleshooting procedure with specific component level troubleshooting chart.

If you are new to the instrument, ensure that the scope is in the right settings. For example, trigger (in Auto), V and H positions, focus/intensity at the at correct levels and Time base selector switch is selected for right time base.

At a high level the process is to start with power supplies, and are ensure all power supply voltages are within the specified tolerance. There are test points on the main A9 board.

  1. Isolate the problem to a specific section first, for example Power supply, HV, Vertical, Horizontal, Trigger.
  2. There are DC voltages marked in every section of the schematic. Set the oscilloscope to the specific settings as mentioned in the service manual and verify all DC bias voltages at the schematic section in question are matching service manual. Once again remember to keep the scope in the specific settings as specified in the service manual. The failing section of the schematic can be easily spotted by a missing or incorrect DC bias value, mostly due to shorted or open components.
  3. There are AC waveforms shown in the schematic for every block of the schematic. Verify and confirm the same with another scope to isolate the failing component/section.
  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 CRT schematic for each series is almost identical but each one uses different component ID numbers in their schematic (most components are the same, but with different ID numbers).

EXERCISE EXTREME CAUTION: There are VERY high potentials (lethal voltages) at many points in this circuit. Make certain that you understand the circuit, use proper safety procedures and have the proper test equipment before measuring any voltages, wave forms or attempting such a repair. DO NOT work alone.

  1. This fault is commonly caused by a failure of the HV transformer and/or the HV Multiplier.
    1. These parts are generally shared across the entire family of instruments.
    2. The HV Multiplier TEKTRONIX p/n 152-0552-00 or CMX234 boosts the Cathode voltage from T4015 to a much higher potential.
    3. The very high acceleration potential facilitates both full size and high brightness to the trace.
    4. It is possible for either part to fail "open", "shorted" or a combination of both conditions. HV Multiplier "shorted to ground" is the condition that is primarily described here.
  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 55 VDC 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. These are illustrated on a page preceding the schematic.
  7. Check for a strong sine wave oscillation at the collector of Q14009 (Waveform 92 in the 465B early manual. A "weak" sine waveform of less than 2 or 3 V p/p indicates trouble.
  8. Next, check voltage at the base of Q4228 (on 465B-Early) schematic shows this should be about 0.7 VDC. With a "shorted" HV Multiplier, you will likely see 4.5-4.8 VDC on the base of Q4228.
    1. The regulation circuit is attempting to drive T4015 to the proper potential of about −2.45 kVDC at TP4129 and 14-16 kV at the CERT High Tension connector.
    2. My 468 was showing only about −330 V and a blank CRT.
    3. The HV Multiplier has likely shorted to ground and is demanding more current and voltage from T4015 than the transformer is able to deliver.
    4. You will find several other voltages are off as well throughout the circuit.
    5. This can cause fuse F4501 5 A 125 V on the 8 V power supply to open. (not sure if this is a "chicken or the egg scenario") My fuse was blown, along with the HV
  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 will isolate the HV Multiplier from the ground.
    1. 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.
    2. You should be able to fully control the trace position and other characteristics, however, it will be quite dim and somewhat unstable.
    3. The beam finder will cause the trace to contract about 10-15% below its previous size.
    4. Do not operate the scope for long periods in this condition. I do not know any detrimental effects, but don't take chances.
  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.