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

[Ken Harpur]

Hi John,

Thanks for your comments...I had thought long and hard about this and most of the feedback I've had pretty much echos what had already crossed my mind, I guess I needed to hear it from someone else!
I'd got this idea of rebuilding my radio so it was as close to an 'as new' condition as possible therefore giving me years of trouble-free service...not unlike a person completely restoring an old hot-rod.

So, I am now going to leave them alone apart from the ones that are >20% out of spec, I'll do all the paper caps and any micas I can get out cleanly (these are the old 'domino' looking types that I have read somewhere in the archives can give trouble). I'll have a good look around for possible dry solder joints and redo any that may look bad also.

Interesting to note that some most of the resistors in this particular deck do indeed look like the poor quality ones that you describe although none are 50% off. I have seen the shiny ones that you refer to so I know the ones you mean...I have two more R-390As so I'll have a look at them and see if they are different.
One thing I have noticed about this particular deck (it is a Stewart Warner) is the component dress seems to be messy when compared to the Motorola and EAC decks that I have here. The Motorola and EAC decks are very tidy looking...the Stewart Warner looks a little like a rats nest...

Regards,
Ken


On 12/09/2011, at 1:12 AM, John Vendely wrote:

> 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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> 
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