[R-390] Re: recent experiences (slightly OT)

[Roy Morgan]

At 11:37 AM 5/24/2007, W. Li wrote:
>...I thought I would share my recent experiences with ... a BC-348.
... a unique perculiarity in the power
>supply: that the B-minus was grounded through a choke
>incorporated in the audio output transformer. ... this error led to
>excessive current drain by the audio output tube, and
>no negative bias voltage to the mixer.

Wu,

Indeed. That "gotcha" has fried a lot of output tubes.  I understood that 
the choke is wired that way in only SOME models of BC-348, but I have not 
researched that.

>... Imagine my surprise to find a cosmetically perfect unit in our
>local thrift store ...interior was exceptionally clean, with that nice
>phenolic odor.

It sounds like you made a lucky find!

>... all the caps were metal-jacketed with styrene ends.
>There were no waxed paper caps or plastic caps
>anywhere.

You are lucky indeed.  In BC-348's with the rectangular black capacitors 
(they are really PAPER caps, not mica) there is at least one inside the BFO 
oscillator can requiring surgery to replace.  I overhauled one BC-348 that 
needed about 20 of them replaced.

I suspect the end seals on your caps are not styrene, but glass.  Try 
scratching the seal on one with a sharp thing.  Speaking of sharp things:

NOTE: In the past I've made a little tool to create a spiral on component 
leads for easy replacement of old parts without un-soldering the whole 
original connection.  I clip the old part out leaving a bit of lead, and 
slip the spiral of the new part over that for soldering.  I used phonograph 
needles for 78 RPM records and a quarter-inch threaded metal standoff 
epoxied into a small wooden tool handle.  I just discovered that normal 
push-pins available at office supply stores have a similar pin in each 
one.  Just break the plastic away and you are all set.  These same pins can 
be used in some VTVM probes having removable pins.

I have one BC-348 with it's original base and dynamotor.  The thing runs 
well and is a joy to operate.  No, I don't plan to do any of the 
modifications published years ago. (If I want better performance, I'll use 
an R-390A or 75A-4.)  Oh, I might snake a little wire out to hook up an 
external ARC-5 type radio as a "Q-Mulitplier".  Naturally, an LM frequency 
meter would be nearby for frequency calibration.

>Compared to our R390's, this piece of history won't
>compare in performance, but it makes a nice addition
>to one's vintage collection along my ARC-5's.

Yes, indeed.  Long live the old radios!

Roy



- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing
13033 Downey Mill Road, Lovettsville, VA 20180
Phone 540-822-5911   Cell 301-928-7794
Work: Voice: 301-975-3254,  Fax: 301-975-6097
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