[R-390] Servicing Advice on Signal Generator URM-25D

[Cecil Acuff]

There are brown colored capacitors that resemble the design of the black 
beauty cap.  I think it has been accepted by most that those caps are as 
prone to failure as the black beauty and they are being replaced for that 
reason.

Personally I haven't had a chance to check any of them to see if they have 
the same leakage characteristics as the infamous black beauty or not.  I 
can't say I have noticed any that are cracked, but I do believe I have heard 
stories about some that were found to be.

I'm not sure what the Pro's who are restoring these radio's for profit (I 
use that word loosely...I've been there) are doing...I expect a wholesale 
replacement of paper caps as evidenced by the cap kits that are offered by 
some.  Problem is if you leave them in you will always be nagged by that 
question floating around in your mind about whether the performance could be 
improved by replacing them.  You could lift one end and check them as long 
as you do it correctly by measuring them at their rated voltage...but by 
that time you are half way to replacing the darned thing so why not go ahead 
and finish the job...right!  I don't know...what's the right answer...is 
there really any harm in replacing them if you are up to doing the job and 
it's done properly.  Is there any documented evidence that replacement 
degrades performance...I would guess not.

Resistors...now they are a little easier.   Not hard to measure usually. 
Some spot checks will usually give you a pretty good idea if work is needed 
or not.

I certainly don't advise unnecessary replacement of parts....but for the 
most part paper caps have a very bad reputation.  I don't trust them!

Cecil Acuff
----- Original Message ----- 
From: "Dallas Lankford" <dallas at bayou.com>
To: <r-390 at mailman.qth.net>
Sent: Monday, January 10, 2005 8:47 PM
Subject: Re: [R-390] Servicing Advice on Signal Generator URM-25D


> The tube pin resistances seldom correspond to the values of resistors in 
> an
> R-390A (or other receiver).  And some of numbers in the tube resistance
> charts are not even correct.  I have yet to see a resistor in an R-390A,
> other than one that was burned brown or black (due to a short or 
> whatever),
> that needed replacing.  Like I said, if you want to know the value of a
> resistor in an R-390A, you will usually have to "lift" one end, and then 
> you
> might as well go ahead and replace it.  But then we are back to the
> "resistor replacing" game, aren't we?   (though perhaps not what the
> original rersistor replacers intended)
>
> Black beauties?  Where are the "black beauties" in an R-390A?  I have seen
> lots of R-390A's, from the first production Collins's and Motorola's to 
> the
> 1967-68 EAC's, and I have never seen a black capacitor in an R-390A. 
> Surely
> you are confused, and thinking of the SP-600.
>
> Have fun,
>
> Dallas
>
>
> ----- Original Message -----
> From: <DCrespy at aol.com>
> To: <r-390 at mailman.qth.net>
> Sent: Monday, January 10, 2005 8:45 PM
> Subject: Re: [R-390] Servicing Advice on Signal Generator URM-25D
>
>
>>
>> Dallas' note below.. He makes a great point, if it isn't broken, don't
> .....
>>
>> About resistors, just check the resistances from the tube pins to  ground
>> (most manuals have a chart).  I usually find at least one with  a problem
> to
>> correct. Otherwise, no wholesale replacements.  (I  confine recapping to
>> electrolytics and black beauties.)
>>
>> 73
>> Harry   KG5LO
>> Saline MI  :
>>
>> Sure.  Let me prerface my remarks by saying that much of what is  said
> about
>> preventive maintenance on the reflector is baloney.
>>
>> Take, for example, the "resistor replacers."  You can't measure  most
>> resistors accurately in circuit unless you "lift at least one end," and
> if you do
>> that, you might as well go ahead and replace  it.
>>
>>
>>
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>
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