This post documents my investigation of Sprague 672D Electrolytic capacitor failures in HP 70000 MMS test equipment.

Background

I have described the HP 70000 Modular Measurement System in depth previously. In summary, introduced in 1986, it is a highly configurable test equipment system comprising spectrum analyzers, DVM, power meter, oscilloscope, signal generators, phase analyzer and more. I have recently begun to restore several pieces of 70k equipment and in the process found that many of the original Sprague 672D electrolytic capacitors have failed. These capacitors are used for power supply filtering within each module.

The design of the 70k MMS comprises long, narrow modules with densely packed components and circuit cards. They are forced air cooled by necessity. Main power distribution is via a 40kHz sinewave from the mainframe power supply which is then used by each module to generate onboard voltages. This requires good filtering, where the Sprague 672D capacitors are used as switch mode power supply output filters.

Sprague 672D capacitors are a high quality, 105C, two or three wire aluminum electrolytic capacitor. They are a radial capacitor, with the third (axial) lead used only for mechanical stabilization and having no charge potential (not positive or negative). They have low ESR and high ripple current capability and could be ordered with an epoxy end seal:

Many 70k MMS systems were used in production testing and research from about 1986 onward. Therefore, they are likely to have thousands of hours of running time. This, together with the elevated internal temperature and limited cooling (even with forced air) environment has led to the failure of many 672D capacitors in the 70k MMS modules that I have been working on. I have found failed 672D capacitors in the following:

• 70900A Local Oscillator/Controller
• 70904A RF Section
• 70905A RF Section
• 70310A Precision Frequency Reference

Measurements and Results

I replaced thirty-four 672D capacitors from the modules noted above. For each capacitor removed, I measured capacitance, ESR, DF, and leakage current as follows:

• Capacitance, ESR, DF were measured at 120 Hz
• Leakage current was measured at the rated voltage of the capacitor
• Measurements were done with a BK Precision 879B which was checked against an HP 4263A
• All measurements (including leakage current) were repeated twice

From the above information and the manufacture’s data sheet I calculated the ESR limit, dC/C (change in capacitance), and allowable leakage current. The measured data was compared to the calculated data, and a pass/fail grade was given for each capacitor. The capacitors were divided into two groups: Those made up to and including 1989, and those made including and after 1990.

The following image is a capture of the spreadsheet data. The 1989 and older capacitors are grouped at the top of the image. Red and yellow mean failed or degraded, green means OK.

Of the thirty-four 672D capacitors measured:

• Nineteen were manufactured in 1989 or earlier
  • Of these, seven failed the tests outright
    • Of these, three failed open. All three showed visible signs of leakage.
    • And four failed due to a combination of high ESR, high DF, or excessive leakage current
  • Of those that did not fail outright, ten had leakage current that exceeded the manufacturer’s specification
  • Two passed all the tests
• Fifteen were manufactured in 1990 or later
  • Of these, none failed outright
  • Two failed due to excessive leakage current
  • Two failed due to having high capacitance
  • Eleven passed all the tests
  • All fifteen capacitors measured higher than expected capacitance* (See comment below)

Conclusions

• Any 672D capacitor showing signs of leakage must be replaced.
• It would be wise to replace any 672D capacitors older than 1990 manufacture as they are either failed or badly degraded and high prone to failure.
• *672D capacitors made from 1990-1999 are probably OK but seem to be drifting upward in capacitance value. This could be an indication of aging and possible failure. I cannot be sure of this because I did not measure the capacitors as new, but it is reasonable to surmise that Sprague sold the capacitors at or near the marked value.

Vishay-Sprague still make the 672D series and they are available at Mouser and Digi-Key. However not all voltage/capacitance combinations are available or in stock and some are expensive (up to US $14/each).

I chose to replace the Sprague 672D capacitors with Chemi-Con KYA and KZN series capacitors. These are both low impedance, high ripple current, and long-life rated capacitor series. As space allowed, I increased the voltage rating of the replacement capacitors. This has the added benefit of obtaining lower ESR and higher ripple current rating for a given capacitance value.

All 70k MMS modules are working fine after the repairs, and I expect no power supply related issues in the future.

Thanks for reading.

Shaun Merrigan November 2023

Electrolytic Capacitor Failure Electronics Restoration HP 70000 MMS Shaun Merrigan