[R-390] Bypass capacitors, R390 and SP-600 [WAS: saga continues...]

[2002tii]

Tim wrote:

>I have to really doubt anything like a 99-point-something percent
>cap failure rate as applied to usability of the radio.
>
>That number was probably based on some original manufacturing
>spec for capacitor leakage.

The number is based on many, many reports of people who have had 
failures in their SP-600s from the '50s onward.  People who don't 
replace them all have failures over and over again, as the caps fail 
one by one.  The previous owner of my SP-600 had "replaced all the 
paper caps" with Orange Drops after having several failures.  He 
didn't replace the hard-to-find and hard-to-access ones, though, and 
when one failed on me it took out a power supply choke.  The radio 
had a kludged-in choke for over ten years until I finally found an 
original.  That radio has all 1 kV ceramic disks now, and performs 
significantly better than it did when it was new as a result.

>But in a filament or cathode bypass, who cares that leakage at
>300V rated voltage is in the microamps and above the original cap
>spec? Half the paper caps are being used at
>6.3VAC or 3-7VDC, and they could be leaking milliamps without affecting
>the circuit.

They will be causing ground loops and hum.

>But I'd rather be using the radio. Replace the ones that'll make it work
>or work better.

I prefer to work on radios once, then use them.  SP-600s today will 
not allow that.  If you don't replace all of the paper caps, you are 
going to be back inside it again and again to replace the caps as 
they fail (plus whatever resistors, rectifiers, chokes, etc. are 
taken out by the failing caps).

I do NOT see the need to shotgun 390s or 390As, except for the 
several critical and failure-prone caps -- they do not have the same 
record of unreliability as SP-600s.

Best regards,

Don