[Boatanchors] more sp-600 questions

[WA5CAB at cs.com]

Norm,

You should use the serial number that goes with the most recent modification. 
 I'm not by any means an SP-600 afficionado but unless someone just stuck the 
R-542/FRR nameplate on it because the receiver had none and they had the 
plate in the junkbox, it isn't "really" an R-274C/FRR any more.  Nor an 
SP-600-JX-14.  As a good example that I am familiar with, you wouldn't use an R-390A/URR 
manual to work on an R-725/URR except where the R-725 manual says to do so.

With regard to your second question about the 456 KC crystal, I'd guess that 
your second choice is more likely true.  The original crystal was replaced 
with one that (at least they thought) was series resonant at 456 KC and the note 
was added so that future alignments would be properly done.

Incidentally, and off your subject, I never could understand why people would 
use orange drop capacitors as replacements in vintage tube based gear.  
Although there is one plus if they did.  When modifications have been made that I 
want to remove, they make it much easier.  Step one is to remove all orange 
drops.  

There's nothing electrically wrong with orange drops, but they were developed 
for vacuum tube based TV's, radios and audio gear during the brief period 
when tube based equipment was still being built but on early printed circuit 
boards.  That's why they're radial lead.  When you bend the leads out to turn them 
into axial lead capacitors, the resultant workmanship always looks sloppy 
regardless of how competent you are.  There are perfectly good axial lead tubular 
and flat package capaciators available that are of recent manufacture and fit 
back into the original space that was designed to hold them.  If you use the 
current standard 630 volt rated units, the size won't be too different from 
the original 250-400 volt rated units and the resultant work looks like it was 
done by a competent (to be PC) repairperson.  Fair Radio currently offer a wide 
range of tubular 630 volt polyester capacitors that are good mechanical fits 
in a lot of our equipment.  And at good prices.  And they are good physical 
replacements for the Black Beauties (as well as those pale green abominations 
that are just as bad but no one ever seems to mention).

Incidentally, their .047's are perfect fits to replace the .05's in the six 
triple metal can capacitors in the BC-312/342.

In a message dated 12/4/2004 7:44:44 PM Central Standard Time, 
normn3ykf at stny.rr.com writes: 
> I've got a few more questions!
> 1: The various military manuals refer to a serial number for parts 
> identification as well as troubleshooting. The question is to which serial number does 
> it refer. My radio has two serial numbers. One is on the badge plate on the 
> front panel which says it is a r-542/frr sn 390.Doing a little detective work 
> revealed that it really is a r 274c/frr. 
> The plate on tuning unit assembly cover states that it is a jx-14 sn 7703. 
> Which one do I use?
> 2: T3 (if transformer, crystal filter) has a piece of masking tape on it 
> bearing the words 456 xtal (sic) installed. The rest of the sub assembly seems 
> to be original. Does anyone know why this xtal was changed from 455kc? I do 
> assume that this (value) was done with a purpose in mind, rather than that 
> value was on sale at the local electronics store that day. 
> Orange drops come Monday. I can go from the non-destructive phase to the 
> uh-oh I can't believe I just did that where in the hell am I gonna get one of 
> those phase.
> 

73
Robert Downs - Houston
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