[R-390] Is anyone selling R-390A Cap kits?

[2002tii]

Cecil wrote:

>I agree with your statement about mil qualified paper caps if you 
>are talking about the R-390/URR.  Not so with the cost reduced 
>R-390A.  The caps obviously were approved by the military for use in 
>the R-390A as offered in the cost reduced receiver.  The caps are 
>not any better than those offered in the SP-600 or any of my 
>Hallicrafters receivers with the Red Chiefs.  Failure rates are 
>pretty much the same...

>all fail the leakage testing on the Sencore cap analyzer which means 
>they have developed a resistive component.  One that can be present 
>at any voltage that cap may be subjected to in it's normal 
>use.  Once that process starts within the cap it only progresses.

That has not been my experience.  I've worked on several hundred 
R-390As, and have only had to replace a dozen or so bypass 
caps.  This is about the same rate of failure I've seen with caps in 
the R-390 (but I've only worked on about 50 of those).  I know a fair 
number of these owners, and ALL of their radios have been operating 
for years afterwards with no subsequent cap failures.

I've worked on several dozen SP-600s, and every single one had at 
least 5 or 6 dead paper bypass caps.  Often, they have already had 5 
to 10 caps replaced at what appear from the parts and workmanship to 
be different times (i.e., one at a time as they failed over the 
years).  So with SP-600s, I replace all of the paper caps the first 
time I work on the radio.

Note that paper caps had significant leakage resistance when they 
were new -- often in the several hundred kOhm range (i.e., leakage 
currents of as much as hundreds of uA with 500 V across them).  If 
the leakage resistance has not decreased significantly from that 
level, they are still in "as-installed" condition with respect to 
leakage.  One could argue that it is theoretically better to replace 
them with modern parts that have much less leakage -- but that is not 
necessary.  Leakage in the hundreds of uA per cap was normal from the 
day the radios were made, and leakage at that level today does NOT 
indicate the decline or immanent failure of the cap.  The vast 
majority have not changed since they were installed, and will not 
change over the next 60+ years.  A few will, and when they do they 
will produce operational symptoms.  When that happens, replace 
them.  If it happens twice or more to the same radio within several 
years, THEN consider replacing all the caps on the theory that that 
particular radio was built with a bad batch of caps or has been 
subjected to damaging environmental conditions.  By this standard, 
extrapolating from my observations, much fewer than 1% of 390As will 
be candidates for changing all of the caps over the lifetimes of 
their current owners.

Finally, one more reason to avoid mass replacement of 
capacitors:  Most owners who have not spent years using soldering 
irons in confined quarters and replacing components in point-to-point 
equipment on a daily basis, and who do not have proper vacuum 
desoldering equipment, will do MUCH more collateral damage to their 
radios by swapping out caps than bad caps ever would do.  This is 
particularly true with 390As, which have component leads wrapped 
mechanically very tightly (mil standard) and are full of fragile 
solder turrets.

Best regards,

Don


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