[R-390] Capacitor Tester

[k2cby]

John,

You have asked a very simple question, and you are going to get a very
complicated answer – probably one that is going to provoke quite a bit of
comment from the rest of the guys on the list.

The very first point to consider is that most capacitors – especially the
failure-prone paper capacitors used in the R-390 R390-A known as “Brown
Beauties” fail short rather than open. This is true of all paper capacitors,
whether tubular or postage stamp format. Mica capacitors tend to fail open,
but failures of any kind are MUCH less common than with paper capacitors.

In my opinion, the most useful capacitance checker of all is a simple
voltmeter – a VTVM preferably or a 20,000 Ohms-per-volt VOM otherwise. Go
through the various stages and compare the tube pin voltages with the charts
shown in the service manual. (A tube socket extender makes the job much
easier.) 

1.	Too high a voltage on the control grid – suspect leakage on the
capacitor coupling the grid to the preceding plate.
2.	Too low a screen grid voltage – suspect leakage on the screen bypass
capacitor.
3.	Too low cathode voltage – suspect leakage (or more likely a dead
short) on the cathode bypass.
4.	No AGC, unreliable AGC, or bizarre AGC voltages (such as zero or
positive) – suspected one of the many AGC bus bypass capacitors is leaking.
5.	AGC hangs up – suspect an AGC bypass or time constant capacitor.
6.	Noise limiter doesn’t work except when “off” – time constant
capacitor in the noise limiter or AGC circuit.

That being said, the chief problem with any capacitance checker is that you
have to disconnect one side of the capacitor to test it. If you have gone to
all that trouble, you might as well disconnect the other side and replace
the capacitor (orange drops are a favorite among R-390/A enthusiasts) and be
done with it. (By the way, if you do this and keep a pile of the capacitors
you have pulled out and replaced, you will find that 99% (if not actually
100%) of them test “good” and the others are passable for non-critical
applications.)

If you are determined to use a capacitance checker, the Heathkit IT-11 is
about as good as it gets. Stay away from the CT-1 – it’s not very useful. My
next choice would be the EICO Model 950B R-C Bridge. It is about the same as
the Heathkit IT-11 but not quite so fancy and not as easy to use (but
usually  a lot cheaper). Next on my list would be the military ZM-11/U and
finally the ZM-3 capacitor analyzer. (This is the crème de la crème of
1950-1960 capacitance testers.)

Caveat: Both of the latter two are R-390 vintage tube-type equipment test
instruments – they are SUPER on tube-type equipment, but aren’t very helpful
for solid state electronics (especially electrolytic and tantalum capacitors
built to modern specs).

I’ll withhold comment on the Sencore instruments, since I’ve never used one.

73,

/Miles

Miles B. Anderson, K2CBY
16 Round Pond Lane
Sag Harbor, NY 11963
Tel.: (631) 725-4400

Fax.: (631) 725-2223
e-mail: k2cby at optonline.net