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Fixing a failing display by replacing bad capacitors

A hands-on teardown and repair of a B&K Precision 2540B oscilloscope with a dim, unstable LCD display.

Introduction

The B&K Precision 2540B is a rugged, professional oscilloscope that’s still perfectly useful decades after it was built. When mine developed a failing display—dim, unstable, and slow to warm up—I decided to repair it instead of retiring it.

This article documents the teardown, diagnosis, and repair process, with photos taken throughout.

Symptoms of Failure

Before opening the scope, several symptoms pointed toward failing electrolytic capacitors:

  • Dim or washed-out LCD display

  • Flickering or unstable brightness

  • Long warm-up time before the display stabilized

  • Front-panel controls behaving inconsistently

Safety Considerations

⚠️ **⚠️ General Electrical Safety
Although this oscilloscope uses an LCD and not a CRT, standard electrical safety precautions still apply when working on powered test equipment.

  • Disconnect mains power before opening the unit

  • Allow internal power supplies time to discharge

  • Avoid shorting components or probing energized boards unnecessarily

Basic bench safety and common sense go a long way.

Teardown Process

Disassembly of the B&K Precision 2540B is straightforward but requires patience.

Steps included:

  1. Removing the outer case panels

  2. Accessing the internal chassis

  3. Locating the display and power-related boards

  4. Identifying capacitor-dense sections near the LCD circuitry

B&K Precision’s internal layout is well designed, but careful documentation helps during reassembly.

Identifying the Problem

Once inside, the failure became obvious. Several electrolytic capacitors showed clear signs of aging:

  • Bulging tops

  • Discoloration

  • Possible leakage residue

These capacitors were located in the display-related circuitry, directly affecting brightness and stability.

Rather than replacing only the visibly failed components, I chose to replace all electrolytics in this section for long-term reliability.

Capacitor Replacement

Replacement parts were selected carefully:

  • Same capacitance values as original

  • Equal or higher voltage ratings

  • 105°C temperature-rated, name-brand capacitors

Each capacitor was removed with minimal heat to protect pads and traces. Polarity was verified before installation, and all solder joints were inspected.

Reassembly and Testing

After replacing the capacitors, the oscilloscope was reassembled in stages and powered on for testing.

Results were immediate:

  • Bright, stable LCD display

  • No flicker or warm-up issues

  • Normal control behavior restored

The repair successfully returned the scope to full working condition.

Lessons Learned

  • Electrolytic capacitors are a common failure point in vintage test equipment

  • Replacing an entire section improves long-term reliability

  • Good photos and documentation make reassembly much easier

Tools & Materials

  • Screwdrivers

  • Soldering iron and solder

  • Desoldering braid or pump

  • Replacement electrolytic capacitors

  • Multimeter

Conclusion

This repair is a good example of how older professional equipment can often be revived with basic tools and patience. If you own a B&K Precision 2540B—or similar vintage oscilloscope—a failing display doesn’t have to mean the end of its service life.

More hands-on repairs and technical projects like this are exactly what I enjoy documenting.