[R-390] wrong part in RF deck found

[Tom Norris]

If you look at the schematic of the RF deck, the rest of the bypass caps
are 5000 pf . Looking physically at the deck, the rest of the caps are .005
ceramic disks. In fact, if you look through the R-390A, .005 mf is the
value of the majority of the bypass caps used in the RF, Osc, mixer and
IF circuits. With only a couple exceptions, that is the value used through
about the 4th IF amp. The R390 non-A was apparently designed using more
common wisdom, as it is chock full of more normal values like .01mf.

The pictorial in all the manuals I have shows it as a ceramic disk instead
of a BBOD.  It has been a couple years since I have even had an RF deck
apart, so I don't remember on the if the older pre '54-55 decks had R-275
at this value or .033.

  In the original schematics, this cap is marked as a 5000 pf cap, but the
later manuals list the MOD note that it was changed to 3300. Online
MOD notes, such as Chuck Rippel's site has posted, list this value (3300)
as well. If it was a mistake, it has made it to every version of the schematic
I have from the original '51 to whatever the latest revision is. The latest
Army manual, change 4, 1988, does not have the value corrected to what
"common wisdom" says it should be.

All that being said, I seem to remember that there was a discussion on the
list years ago 'bout this very same issue. Couldn't find that thread in the
archive. Maybe is really is *supposed* to be what was in there. Regardless,
I replaced the thing with a .033 film.

73

Tom


>It is SUPPOSED to be .033 according to common wisdom, so it was assembled
>correctly.  Maybe the schematics are printed wrong in some cases, showing an
>incorrect value of .003.  When you thnk about it, a B+ bypass should be
>hefty in value, like .01 to .05.   The smaller value .003-.005 doesn't make
>sense.  Put a .033 Orange drop in there.
>
>Jim
>
>
>----- Original Message -----
>From: "Tom Norris" <r390a@bellsouth.net>
>To: <R-390@mailman.qth.net>
>Sent: Saturday, January 31, 2004 2:45 AM
>Subject: [R-390] wrong part in RF deck found
>
>
>>  This is an addendum to my last post about poking around the
>>  RF deck of the 63 Imperial --
>>
>>  I found a wrong part that looked as if it were there from production.
>>  C275, originally a .005 mf, changed to .003 mf somewhere along the lines
>>  according the mod notes in the TM. In mine it was a *.033 mf* big old
>paper
>>  cap. It's just a bypass on the B+ to the first mixer, but still...
>>  That big sheep lovin' BBOD appeared to have been there since the deck
>>  was put together and not just a maintenance mistake. In fact the whole
>radio
>>  doesn't look like it saw much use or maintenance at all. Other than a big
>Navy
>>  ID tag glued on the front, along with an engraved tag that says "RCVR 1"
>under
>>  the KC knob, the thing looked like it just came out of its crate.
>>  It's worked fine
>>  for me the past ten years or so, so it should keep on ticking for
>>  quite a few more
>>  years.
>>
>>  73
>>
>>  Tom KA4RKT
>>
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