[R-390] Capacitor Tester

[rbethman]

John,

Miles makes excellent points. They are certainly not erroneous.

It becomes a matter of how one chooses to perform tests.

The Brown Beauties are definitely prone to failure and are essentially a 
known and automatically get replaced.

That being said, This also is why it is important to know precisely 
which radios were built by whom, and the specific date of contract.

'67 EACs do not have the aforementioned BBODs,, (Brown Beauties of 
Death) also some are Black BODs, although I have not located any of the 
Black ones in the R-390 series.

The '67 EACs have some good Sprague Vitamin Qs, and Aerovox caps.

So we have to look at different ways to test, AND determine which issues 
you are going to find and have to deal with.

Bob - N0DGN



On 3/6/2012 8:09 PM, k2cby wrote:
> 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
>