HP 54520A Oscilloscope – Fix

Introduction

The HP 54520A is a 500 MHz (1 GSa/s) 2-channel oscilloscope. I got this oscilloscope for free since it did not start properly and was going to be tossed. This article contains pictures of the interior of this oscilloscope, information about the problem, and a solution to getting it up and running.

Figure 1: Oscilloscope up and running after fix

Background

When powering on the oscilloscope the fan turns on and all buttons light up, however the screen remains off. Pressing the buttons do nothing and the display remains off, even after several minutes, see figure 2. Doing a bit of research I came across this link, indicating that the problem could be related to a lithium battery.

Figure 2: Oscilloscope booting up. Buttons light up but they are unresponsive. Fan turns on

Here is what one person( link) wrote:

“There may be a lithium cell which is used to maintain the static ram where calibration data is stored. In the 5454x series of ‘scopes the system locks up exactly as you describe when the lithium cell is discharged. This seems to happen about 15 years after manufacture.”

So apparently the issue is linked to a static RAM that could be a crucial part of the boot up sequence. Well, let’s see if we can find this lithium battery and verify that it is depleted. Opening the oscilloscope is done by doing the following:

  1. Unscrew plastic frame that extends around the front (5 Torx screws)
  2. Unscrew plastic feet at the rear (4 feet x 1 torx screw)
  3. Remove metal body chassi by sliding it in the direction: front to rear (Figure 3 and 4 shows the open chassi)
Figure 3: Removed the body cover
Figure 4: Rear panel

Locating the battery

The battery is placed on the motherboard at the rear left corner of the oscilloscope (viewed from the front), see figure 5. Unfortunately the placement of the battery requires that one disassemble the entire oscilloscope, just to be able to measure it. However, considering that this would be the probable cause, disassembleing the scope would be required to unsolder and replace the battery anyhow.

Figure 5: Battery location
Figure 6: Board for controlling crt monitor. Viewed unconnected and placed on top of the floppy drive and PSU

Removing the interior of the chassi involves the following:

  1. Remove floppy drive (2 torx srews + disconnect cable)
  2. The box situated under the floppy drive is the power supply. Removed by removing two plastic rods and disconnecting a couple of cables, see figure 7
  3. The PSU box can easily be slided to the right and lifted out (viewed from the front), see figure 8
  4. Remove back panel(5 torx screws) + three BNC connector rings
  5. Remove front panel BNC-connector rings and disconnect remaining cables on the motherboard
  6. Remove several torx screws that are securing the motherboard to the bottom plate
  7. Slide the motherboard in the direction front to rear. The removed motherboard and battery are shown in figure 9
Figure 7: Cables to the PSU
Figure 8 : PSU removed from the chassi. Motherboard seen in the bottom of the chassi
Figure 9: Motherboard with battery clearly visible
Figure 10 : Motherboard and probe block removed (top right corner)

Results

So, was the battery empty? Measuring the battery using a multimeter showed only 0.2V. The battery at full charge should read 3V!

Replacing the battery and assembling the oscilloscope did yield good results. The oscilloscope powered on and the display came to life! It indicated that calibration data was lost (as expected). By switching the READ RAM lock OFF, the oscilloscope will allow for self-calibration (see figure 4 for the switch; hole at the bottom left part of the rear plate, with white and blue sticker).

All in all this was a simple fix that added a vintage oscilloscope to my collection! Feel free to ask questions or comment.

Figure 11: Battery brand
Figure 12 : Battery voltage