[R-390] R-390A IF Module Layout

[2002tii]

Richard wrote:

>Side note: Too bad the Audiophools have latched on to OD's.
>The price on e-Pay is absurd and assortments (at a reasonable
>price) are hard to come by.

Good grief.  Why would anybody look for commodity electronic 
components there?  Digi-Key, Mouser, Allied, to name just three 
proper parts vendors.  The list is long.

Jerry wrote (quoting Chuck Rippel):

>Been watching this thread.  First, do not perform a wholesale 
>replacement of all the capacitors in the radio.  The effort will not 
>yield any inprovement.  Beyond that,  there are some, especially in 
>extremely low value components in the RF deck, that are clearly 
>installed with lead dress considerations in mind.

Paper caps are bad news, wherever they are -- including in your parts 
bin.  The ones used in most 390s aren't the huge problem that the 
ones in Hammarlund SP-600s (and other boatanchors) are, but they were 
far from ideal even when they were new and are becoming failure-prone 
as time wears on.  Most (essentially all) paper caps in boatanchor 
receivers are RF/IF bypass caps.  Ceramic caps are MUCH better at 
this job than any film caps, including brand new Orange Drops (and I 
happen to think that the film-and-foil ODs are very fine capacitors, 
especially the polypropylene-dielectric ones).  Interesting that 
Hammarlund early on in SP-600 production adopted an engineering 
change that substituted disc ceramics for the tubular paper caps for 
this very reason, then apparently went right on making radios with 
the paper caps.  SP-600s with factory-installed ceramic bypass caps 
are few and far between -- I've only seen one, out of the hundreds 
I've examined.

I had direct experience with my SP-600, in which the previous owner 
had replaced "all" the paper caps with ODs.  When one of the paper 
caps in the RF amp failed (so much for "all"), I looked over the job 
and decided to replace not only the remaining paper caps, but also 
all of the new ODs with ceramics because the previous owner's 
workmanship was serviceable but not to a proper standard.  There had 
previously been substantial leakage of harmonics of the 455 kHz IF at 
910, 1365, 1820, and 2275 kHz, which totally disappeared when I 
replaced the ODs with ceramics.

Lesson:  even the best film caps are not optimal for bypass use at IF 
and RF frequencies.  Ceramics are much better.  If you're going to go 
to the trouble to replace them, why not use the parts that are best 
for the job?  Ceramics are generally cheaper, too.  Finally, lead 
dress (particularly, making leads as short as possible) is important 
for ALL bypass caps.

Buy a big bag of recently-manufactured 0.01 uF/1000 V or 0.02 uF/1000 
V disc ceramics, and use them everywhere.  You'll be glad you did.

Best regards,

Don