[R-390] Rising from the dead, again

[Drew P.]

Perry done wrote:

[snipped into bite-sized chunks]

"Both Chuck Ripple and Dallas Lankford have published audio circuit improvements for the R390A.  In his audio improvement circuit for the SP 600 chuck specifically advises to use metal film resistors for improved performance."

Metal film resistors are preferred over carbon compostition type in very low level circuits because of the lesser noise of the metal film types.  I don't think that the audio circuits of the R-390 series have low enough signal levels for carbon composition resistor noise to be a problem.  However, metal film types have a very high reliability, reason enough to recommend them.

"I tend to agree about doing a shotgun approach on some of the R390A modules as it is a real chore.  But if you have to go into a module to do caps I believe one should do resistors at the same time."

Defective or discolored resistors, yes.  But the failure rate of most of the carbon composition resistors in the R-390A is sufficiently low to, in most cases, leave them alone.  The exception would be, as Perry mentioned, in cases where access is poor and the module is already apart for some other invasive procedure.

"As for the SP 600, since the underside is relatively open, one should replace all the carbon film resistors as well as moving the power dropping resistors to the top side of the chassis.  Especially if one is replacing any BBODs."

Methinks you meant to say "carbon composition".  Carbon films can be a good replacement for carbon comps provided the carbon films have a sufficiently high voltage rating.  Carbon films are "quieter" than carbon comps but are still noisier than metal films.  

Again, is the failure rate of carbon comps in the SP-600 high enough to warrant wholesale replacement, especially in easy access areas?  

I have found carbon composition resistors to be failure prone in ciruits where the resistor has a high impressed voltage.  Screen dropping resistors can be failure prone, as can plate load resistors in resistance coupled stages.

"If you ever have to take out and repair the RF deck of a SP 600, by the time you get it removed you will want to put in the best, most reliable. resistors, capacitors, and new wires money can buy.  For  further proof Read The Fine Manual or ask someone who has none it; it will make you a believer."

I wholeheartedly agree.  Recapping the SP-600 RF section is severely invasive, risking damaging difficult to obtain parts. 'Tis best to insure that going in there is done only once.  Same for the Hallicrafters SX-28.

Drew