[R-390] To swap or not to swap those aging "reddish" rectangular mica caps

[jkharvie]

 Jacques,I apologize if in any way I implied that you suggested that all the "red" capacitors HAVE TO  BE CHANGED.  I did not mean that and certainly you did not provide this input.
I looked at the new old stock of parts I have, and see all the red ones have packaging/ production codes dating back to the 1950's and early 1960's, and that all the green ones I have date appear to be from early 70's.I also have a bunch of of the trim caps to service those that may need it.  I have not (yet) located any cracked ceramic wafers or failed neoprene elastomeric washers associated with these variable capacitors.Your quite correct.  One option is to get the receiver going, await for the next failure, fix it, and move forward.  
I do like the idea of leveraging group experience.  If the members have experienced a statistically higher failure rate on the red caps, maybe go ahead and replace these.  
>From my listening to the broadcast bands, times have changed and I am not hearing much worth listening to.  So part of this is getting it going for the sake of getting it going.
If this is not a sound strategy (no pun intended), and a better strategy is ONLY fix that which has failed, (expect for those high risk part failures the group has already clearly documented where a failure results in additional component failures), that also works for me.


thanks
John N3JKE




    On Thursday, July 4, 2024, 3:51:29 PM EDT, Jacques Fortin <jacques.f at videotron.ca> wrote:   

 John, I never suggested that all the "red" ones HAVE TO be changed.
The warning was just that due to their age, they CAN fail, but not all...
If you want to completely rebuild a R-390A, go for it, but...!
IMHO, the only capacitors that NEED to be changed are the Sprague
"bumblebees" paper ones  if you find any inside your modules.
Otherwise, wait for the failures to happen, if any !
All depend also of the "running time" you expect for your R-390A.
My own are OFF most of the time, so I do not expect a high number of mica
cap failures... 

73, Jacques, VE2JFE in Montreal

Guided discovery continues as I make progress on the RF sub chassis.I
received C327 replacement for "high Q" 100pf, 1,000 volt mica capacitor to
install on the primary output of T207.I received group input that the green
mica mold caps provides acceptable operational service with very low failure
rates, and the "reddish" color caps are older generation and appear to have
a statistically higher failure rate, and should be replaced with new high q
mica caps.Two examples in the receiver, (1) with all reddish mica
capacitors, and (2) 1 out of 2 reddish capacitors.My course of action is to
keep the green, replace the reddish capacitors.
Or does the group make the case that while the patient out and ready for
surgery to replace both capacitors while opportunity is good?
Thanks in advance to your vast repository of deep informed knowledge as I
work my first end to end refurbishment on this magnificent receiver.Happy
4th of July, celebrate independence day.
John N3JKE