[R-390] R-388 caps (OT)(not)

[Al Parker]

Hi folks,
    I haven't run into problems with the dogbones (yet), but you never know. 
The small micas are starting to go, in the R-388 particularly watch out for 
C-204, 100mmf coupling cap in the AVC section, just above V-110 in the skem. 
That one gets leaky, and really messes up the AVC, the rcvr will go deaf. 
It can be heat sensitive, best thing is to just replace it in every R-388, 
it's happened to me in 2 or 3 and I know of several others.
    The bathtubs can get leaky and make strange things happen, too, but not 
as drasticly as C-204.  You should look at the bias voltage generation 
setup, coming from the CT on the pwr Xfmr secondary, incl. C-205A, C-126, 
and the resistors iin that line that gooes twd C110 & back to ground.  I've 
got notes from someone on what should be there, maybe Lankford, I can't 
remember, but will hunt them down if you guys don't find them.  That, and 
teh PTO endpoints, are the only 2 other things that you really need to watch 
out for in the R-388.
    You can see some of my work on them on my website, in particular:
http://www.boatanchors.org/R388sn495.htm
and for a nice prod. det. installation see:
http://www.boatanchors.org/51J4sn3516.html
   the 51J-4 is essetntially an R-388 with mech filters.
    The R-388 is a terrific receiver, doesn't require a lot of work to make 
it very useful & enjoyable.
73,
Al, W8UT
New Bern, NC
www.boatanchors.org
www.hammarlund.info

"there is nothing -absolutely nothing- half as much worth doing as simply 
messing about in boats."
   Ratty, to Mole


----- Original Message ----- 
From: "John Flood" <kb1fqg at yahoo.com>
To: <wli98122 at yahoo.com>; <R-390 at mailman.qth.net>
Sent: Friday, July 25, 2008 11:12 AM
Subject: [R-390] R-388 caps (OT)


This interests me as well as I am doing a "quick" cleanup and checkout of a 
R388A for someone local who was "Given" one and had no clue what it was. He 
couldn't understand why I was drooling over the dust covered black thing and 
he couldn't get my attention to the complete shiny Hallicrafters station 
with matching speaker that was given to him at the same time. Then he said 
take it home and check it out and play if you want. Thought about saying no 
but not for long... where was I.... oh ya those dogbone caps. He doesn't 
want me to do a complete restore of it but I'd like to advise him about 
these and I have not run into these caps much before. This unit is running 
well at this point with little work, just a cleaning, a new tube and lamps. 
I know you'll all say check the caps but what I was mainly looking for here 
are these dogbones in the same class as the BBOD's and must be removed at 
all cost while the children are hidden to
 protect innocence or are they of higher quality breeding like single malt 
medicinal compound?

John Flood
KB1FQG
----- Original Message ----
From: wli <wli98122 at yahoo.com>
To: R-390 at mailman.qth.net
Sent: Thursday, July 24, 2008 10:52:31 PM
Subject: [R-390] re: R-388 caps (OT)

How durable the *dogbone* ceramic capacitors in the R-388/URR? They are 
widely employed as bypass caps... like .01µF everywhere it seems. How were 
they made?

Do they deteriorate like the paper caps of that era? My impression is no. 
But I do not really have any experience with this type of capacitor.

In contrast, I have seen many leaky *Black Beauties* in other receivers I 
have restored, and have replaced them with Orange Drops or other modern 
caps.

Thanks

W. Li




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