[R-390] The ultimate heresy?

[Jim M.]

I have found that replacing some caps has actually improved performance when 
it had degraded, even the disc .005 bypass caps.  In areas where I was 
having problems, especially along the AGC lines and screen bypass lines in 
the RF deck, replacement discs have helped.  They were more than likely 
marginally leaking.  I wouldn't do it wholesale, but in selected areas such 
as this.  Any resistor in a B+ or screen line to a tube should be suspect, 
even if it looks OK to the eye.  They will drift in value with age and 
heating.  Normally 2200 ohm mostly.  I replace them with 1 watt resistors. 
Beware, however, some modern resistors exhibit inductance, they are not pure 
carbon as the older ones are.   Replacing the straight wiring probably wont 
help, but I did have a case where the miniature coax cable carrying the 
diode load signal from the IF deck was breaking down and causing popping 
noises on strong signals... the dielectric in the cable started breaking 
down after many years, even with the low voltages there (less than 10 
volts).  Don; bother with ozone free oxygenated monster wire, it is a fraud.

----- Original Message ----- 
From: "Tim Shoppa" <shoppa_r390a at trailing-edge.com>
To: <r-390 at mailman.qth.net>; <aa9il at sbcglobal.net>
Sent: Friday, December 02, 2005 6:05 PM
Subject: Re: [R-390] The ultimate heresy?


> mike Kana <aa9il at sbcglobal.net> wrote:
>
>> Hi all
>>
>> Dangerous question....  Has anyone attempted to
>> completely
>> rebuild the modules or (entire?) R390 A radio with new
>> low noise
>> resistors, modern caps, wiring, etc - obviously all
>> the mechanics, tubes, slugs, major components, etc
>> stay.
>>
>> Regards
>> Mike
>> AA9IL
>>
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>
> Well, many of us have gone through and replaced any questionable caps
> and any questionable resistors in some/all modules.
>
> Doing everything everywhere could prove disastrous in the case of
> special tempco parts. There must be some of these in the PTO.
>
> And remember there's not just the caps in the chassis but also caps
> inside many of the coil cans.
>
> Replacing wiring sounds a more than just a little bit overboard.  You'd
> still have the same old connectors (which would be the weak point).  Some
> here have replaced individual bad cables etc. in the harness, and it's
> a fair amount of work to unlace the harness, remove the cable, put in the
> new cable, and lace it all back up.  While I've done some cable-lacing
> in the past I can guarantee you that I would never be able to put it all
> back together as well as it was done at the factory.
>
> The parts most likely to be in need of refurbishment are the mechanical
> controls and switch sections.  Many of these have custom wafers/contact
> arrangements that make them certainly NOT off-the-shelf items. Usually
> donor rigs are found and/or Deoxit + some very particular refurbishment
> takes care of the problem area.  I'm guessing the cost of fabricating
> new switch wafers/contacts throughout would run into thousands of
> dollars, very much discouraging en masse replacement from "new stock".
>
> Tim.
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