[R-390] Rebuilding a R-390A IF Deck

[John Vendely]

Hello Ken,

Since you've asked for opinions, I'll offer mine.  In my opinion, folks 
are overly ambitious about component replacement in the R-390 series 
receivers.  Component quality was quite high in this equipment, and 
although certain well-known troublesome components are well worth 
replacing, I would not routinely replace carbon comp resistors which are 
at or near their tolerance limits.  It was common for resistor 
manufacturers to select from production lots those resistors which would 
be sold as 5%, 10%, and 20% tolerance.  This skewed the statistical 
distribution of values in each of these selected groups.  As a result, 
most 10% tolerance resistors ended up being 7 to 10% off the nominal 
value when new.  Your findings probably reflect this practice, and not 
component aging.  Equipment designers of that era were well aware of 
these practices, and very familiar with the characteristics and 
variability of the components worked with.  Their designs took all this 
into account, and will work usually well with tolerances significantly 
beyond nominal.

Routine mass component replacement is extremely time consuming, is 
generally unproductive, and risks collateral damage to the equipment.  
Frankly, it borders on the obsessive, particularly in the case of 
resistors.  In most cases, I would suggest replacing any 1/2 watt 10% 
resistor reading 20% or more off center--otherwise, leave well enough 
alone.  There will be plenty of other problems to attend to.  Of course, 
when making in-situ resistor measurements, you should be certain there 
are no parallel conductive paths to influence the reading.

I should point out that I'm referring to the good quality Ohmite or 
Allen Bradley carbon composition resistors commonly found in the R-390 
series receivers.  Certain earlier style carbon resistors are much more 
prone to aging and outright failure.  These can be identified by the 
dull, rough or "pebbly" surface texture of the resistor body, as opposed 
to the "shiny" appearance of the newer types.  Those older style 
resistors were truly crummy, and are often found to be 50% or more above 
tolerance.  Although they were still in use into the mid 1950s, I don't 
recall ever seeing these in the R-390/390A.  In the unlikely event you 
do find any, they would be worth replacing.

Best of luck in your restoration efforts.

73,

John K9WT

On 9/10/2011 11:57 PM, Ken Harpur wrote:
> Good Evening List,
>
> I have started restoring one of my R-390As....so far I've rebuilt the Audio Deck and have now turned my attention to the IF Deck and I was wondering what the general thoughts are on the list as to resistor and capacitor replacement. I have Cornell-Dublier silvered Micas and SBE 716 series going in and have a full set of resistors arriving from Mouser. Obviously all the paper caps are going to be replaced and the Micas are sometimes problematic so my idea was to go ahead and replace all the caps with the long term view of never having to worry about them again.
>
> My conundrum is with the resistors...I measured every one and about 40% of them are on, about, or just over 10% of their marked value so I figured I'd go ahead and replace those ones..apart from those that were at the edge of their tolerance when new my way of thinking is that they will just go even more out of spec over time. But what about the ones that are still well within their 10% tolerance? Is it reasonable to assume that if they haven't drifted out of spec by now they probably never will? Or do you guys think that in time they could be candidates for trouble/replacement if left alone?
>
> Any advice, thoughts or tips would be greatly appreciated as I want to make sure I do it right the first time!
>
> Best Regards,
> Ken
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