475/Repairs

Capacitors

The 475 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.

Problematic Transistors

151-0367-00

This part is known to fail. The 475 has eighteen listed in the service manual.

They're Q390, Q420* (or 151-0223-00), Q502, Q504, Q506, Q664, Q674, Q688* (or 151-0223-00), Q698* (or 151-0223-00), Q712, Q714, Q716, Q1004, Q1124, Q1134, Q1182, Q1184* (or 151-0127-00) and Q1138.

The Fairchild/OnSemi KSP10 functions in place of these, but note that the pinouts are different!

As Tek lists some of the parts with the alternative of 151-0223-00, I'd guess that some locations are more critical than others - hopefully we'll learn and share info here as we find it.

A tip for quickly finding these transistors in the 475 scope is that they will usually have the base bent back toward the round back of the transistor - while most other transistors in the unit have their base pin bent forward toward the flat part of the body. It is important that you double-check the part number on the transistor to make sure you didn't grab the wrong one. A few 2n3906s have their base pins bent forward so be careful.

Locations of this transistor on the trigger board for 475 oscilloscopes serial 250,000 and up are high lighted in this image.

Transistors with a reddish-pink top are the locations of the 151-0367-00 on the trigger board.

151-0434-00

This part is known to fail.

More info to follow.

High Voltage CRT Circuit

As with most 400 series scopes, the high voltage circuits decay over time. It will mainly be the non-ceramic axial capacitors that go bad.

This is indicated by a crackling constant arcing noise that can be triggered with the varying of intensity control knob.

Replacement capacitors are available on Digikey and Mouser. However, they will usually be available only in large bulk amounts so sourcing components may carry difficulties.

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.
5. Note that some of the manuals have many corrections at the back. The first Service Manual in the manuals list has over thirty pages of corrections. Some of the manuals have mistakes in the calibration sections.