[R-390] Rising from the dead, again

[2002tii]

Drew wrote:

>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.

Don't forget that carbon comp ("CC") resistors have a higher 
tolerance for abuse (current surges, overcurrent due to bad tubes or 
leaky caps, etc.) than either CF or MF.  In some cases, one wants the 
"fuse-like" protection of a film resistor, but in the context of a 
radio that will likely suffer some overcurrent conditions as a matter 
of course, CCs can save you having to go back in because they 
tolerate a lot.  Metal oxide resistors can be a good choice to 
replace CCs in such locations.

>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.

CF resistors are being rapidly discontinued by almost everyone, for 
good reason (not least of which are the awful crimped-on end caps and 
the spiral elements of most values).  MFs are the way to 
go.  Regarding noise, I doubt there is a single resistor in a 390A 
where you'd be able to hear or measure the reduction in circuit noise 
going from CC to MF.

>"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.  *   *   *   "
>
>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.

I actually don't find re-capping SP-600s that much of a chore.  Not 
nearly as bad as a 390A (IMO), where you have to remove the modules 
from the chassis before you can do anything.  Working steadily, I can 
do an SP-600 in 4-5 hours, including a few non-cap changes, cleaning 
and lubricating contacts and controls, etc.  I replace a bunch of the 
hardware with socket-head cap screws when I put 'em back together, to 
make future disassembly easier. Note that I use a vacuum desoldering 
tool (indispensable for repairing almost everything electronic, IMO) 
and several different soldering irons -- the right tools make things 
much easier.  I almost never desolder until I've clipped the old 
component out -- that also makes things much easier.  Finally, it 
speeds things up if one is familiar enough with the circuit to remove 
several caps at once, then desolder/remove the old leads, and finally 
reinstall new caps, without losing one's place.

Best regards,

Don