Most Accurate Clock Capacitors

[Larry McDavid]

First, let me thank all the list members who replied to my question
about my intermittent Heathkit Most Accurate Clock. I received some good
suggestions for troubleshooting and recommendations for modification.

I built this clock almost 18 years ago and it has been in continuous
operation since. Looking inside, I noted that the primary power supply
filter capacitor had extruded its bottom seal and leaked. This suggested
that all the wet electrolytic capacitors were likely defective; this
proved to be the case. I replaced ten of these with new similar
capacitors and the clock operation is vastly improved and now synchs to
WWV as it has for many years. "Calibration" of the PLLs was easy.

Buying fresh electrolytic capacitors demonstrated that component
technology has changed over the past 18 years! Current equivalent
capacitors are either much smaller or are rated at much higher
voltage--or both!

I have an LCR meter and I thought it would be interesting to measure the
capacitors I replaced. Here are the data (note the units!):

Display PWB
C102  1000 uF  measures  322 uF

Main PWB (Pwr Sply)
C203  2200 uF  measures     6 pF  OPEN
C205  1000 uF  measures   211 uF
C206  1000 uF  measures  0.68 uF  OPEN
C208   220 uF  measures   6.5 uF
C212    22 uF  measures   130 pF  OPEN

Tone Decoder PWB
C412     1 uF  measures  0.16 uF
C414    10 uF  measures   5.7 uF
C422  0.33 uF  measures  0.25 uF
C424   150 uF  measures    10 uF

Tests of the replacement capacitors showed that they were within their
rated tolerance.

The demonstrated loss of capacitance is not the whole story. As the
electrolyte is lost, the ESR (equivalent series resistance) of a
capacitor increases dramatically. This is especially important in the
filtering capacitors. In short, these capacitors were just plain BAD!

While the capacitors in the Tone Decoder 100 Hz PLL were degraded for
sure, I believe the main problem was the almost total loss of effective
power supply filtering, introducing excess ripple in all circuits.

I have yet to replace the few wet electrolytics on the (pre-built)
receiver PWB but I will do so shortly; I'm sure they are just as bad as
those on the boards I built.

I did not bother replacing the tantalum capacitors; those should have a
much longer lifetime since they don't have a wet electrolyte to leak or
evaporate.

As you know, the use of the LED-type displays for HH:MM:SS.S increases
the clock power dissipation and causes the clock to run rather warm.
This heat, over the period of 18 years, clearly took its toll!

This Most Accurate Clock sits prominently in my office/ham shack and I
count on it every day. I'm pleased to have it operating normally again!

Thanks again to the list members for their helpful comments and I hope
you find this report interesting. Now, go check your capacitors!
--
Best wishes,

Larry McDavid W6FUB
Anaheim, CA  (20 miles southeast of Los Angeles, near Disneyland)

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