[R-390] R-390 Meltdown - the Saga Continues

[2002tii]

Rick wrote:

>The results were mixed, now R220 is not burning up but there now seem to be
>a problem downstream so to speak. After letting her warm up & switching on
>the B+ I knew from the sound that something wasn't right. Shortly there
>after a little smoke started to come from somewhere else - this time it was
>on the topside of the RF deck

I believe I mentioned in my previous message that if one bypass cap 
has failed, all of the others in the radio are suspect.  You seem to 
indicate that the smoke is from somewhere else this time.  My guess 
it's another 2.2 kohm B+ feed resistor (the radio is chock full of 
them) feeding another bypass capacitor (ditto) that has gone 
leaky.  I hate to say it, but you will probably need to replace all 
of the paper caps in that radio before it is reliable.

On that subject, I'll repeat what I've said in the past ("OD" refers 
to Orange Drop film capacitors, originally made by Sprague, now by 
SBE, a division of Vishay):

>Disk ceramics are actually better than film caps in bypass 
>applications.  Film-and-foil ODs (or other film-and-foil caps of 
>equal quality) are fine -- they work at least as well as the 
>original paper caps, and will outlast you and your heirs.  If you 
>want the best film caps for bypass applications, those would be the 
>OD 779P series.  The PP film-and-foil types (715P/716P series) are 
>the next best, but even the PE film-and-foil types (225P) are much 
>better than the original paper caps ever were.  But for the BEST 
>performance, use disk ceramics in all bypass applications.
>
>Finally, avoid metallized film caps of any sort in tube equipment -- 
>use only film-and-foil types.  If a capacitor vendor won't show you 
>the manufacturer's data sheet stating that the caps are 
>film-and-foil, they are metallized film.  Any cap that is 
>significantly smaller than the equivalent film-and-foil OD is 
>metallized film.  NOTE that ODs are made in both metallized film and 
>film-and-foil versions, as well as a "hybrid" version for AC 
>applications, so you have to pay attention to the series numbers.
>
>Buy a big bag of recently-manufactured 0.01 uF/1000 V or 0.02 
>uF/1000 V disc ceramics, and use them everywhere.  You'll be glad you did.

Best regards,

Don