[NJARC] BC-348 Hum

Tue Nov 11 15:37:54 EST 2008

Hi Al-
I suspect you are correct about the hum coming from the power supply: it is still present in the other BC-348, but not nearly as much. I do need to eliminate it, as well, but figure I have to get the other unit sound first as to reception, volume, etc. 

I did, however, swap in a different, larger choke from the junk pile- it is marked in marker 10H 240 ohm. And it seems to be doing something- but unfortunately it is overpowering the system! I am still showing 28VDC out, but the amps are not enough to spin the dynamotor... lol... Back to square one on that one I guess...

I did pick up a good signal with the BC-348Q using the dynamotor, and held it for a little bit. Now, what I might try is to use the solid state power supply with the BC-348P and see what result I get. Perhaps something else will occur.

As to the oscilloscope, I am still very much lost when it comes to using it. I would love to be able to understand how to hook it up to check the power supply, and signal trace with it, but it is still beyond me at the moment, and it frustrates me to no end. I have Riders book on interpreting traces, a few different manuals on oscilloscope use, several on how radios operate all sitting next to where I have been reading them, and still no clue as to what I am doing. I can make a pretty sine wave on the screen, but that's about it. And that is with no connections, either. My ocsilloscope makes me feel stupid.

Scott

--- On Tue, 11/11/08, Al Klase <al at ar88.net> wrote:

> From: Al Klase <al at ar88.net>
> Subject: Re: [NJARC] BC-348 Hum
> To: ng19delta at yahoo.com, "New Jersey Antique Radio Club" <njarc at mailman.qth.net>
> Date: Tuesday, November 11, 2008, 1:39 PM
> Scott,
> 
> I wouldn't worry about the filter downstream from the
> dynamotor at this point.  If you have hum, like 60 or 120
> cycle hum, it's not coming from the dynamotor.  The
> filter is designed to take care of the typical
> "whine" of the dyno.
> 
> Do you have an oscilloscope?  I sense you're just
> floundering around here.  As someone else suggested you need
> engage in signal tracing.  It's like plumbing:  The
> trouble lies between the last place you have water pressure
> and the first place you don't.
> 
> Start by grounding the center terminal (wiper) of the audio
> gain control.  (There are two pots in tandem, you have to
> figure out which is AF and which is RF.  Read the schematic
> etc.)  Does the hum go away?   If not, look at the plate and
> screen with the scope.
> 
> Al
> 
> Scott Roberts wrote:
> > Visit our web site - See http://www.njarc.org
> > _______________________________________________
> > Well, I had an idea tonight while working on solving
> the problem of the BC-348P hum. First, I checked to be sure
> I had a jumper in place on the 2 & 6 connections, in
> place of the tx relay, and I had none(a friend reminded me
> of this need). So I added one, but still no soap on any
> signal, and the hum is still persistent. So I tried a
> reverse substitution- As many may recall, I spent
> considerable time last year demodifying my late Uncle
> Bill's BC-348Q to its original wiring configuration,
> except that I had it set to accept a transformer setup in
> place of the dynamotor. Well, I pulled the transformer setup
> out, and changed the last two wires needed to convert it
> completely back to a dynamotor.(I checked band activity, etc
> first- set was working well, & receiving on several
> bands, no hum)
> > 
> > I installed the dynamotor I've been working with,
> and hooked it up. At first, there was no hum at all, but
> once things warmed up it was back, however- it is nowhere as
> loud as in the 348P. I am thinking it must be in the power
> supply I built, so I guess I need larger caps(I am using
> 47/160 electrolytics) and a larger choke(I have no idea its
> Henrys- it is a small one from an engine whine suppression
> kit) If I can get it cleared out entirely in the 348Q, then
> I can test it in the 348P. I am also thinking there may be
> other issues with the P- it seems the RCA 991 glow regulator
> is not glowing- so I need to check through all the tube
> wiring, and ensure I missed nothing.
> > 
> > I may attempt to wire my solid state power unit (from
> the 348Q) in, and see if there is any action there- another
> substitution to test things!
> > 
> > Any ideas/suggestions?
> > 
> > Thanks!
> > Scott
> > _______________________________________________
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> > NJARC at mailman.qth.net
> > http://mailman.qth.net/mailman/listinfo/njarc
> >
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> >   
> 
> -- Al Klase - N3FRQ
> Jersey City, NJ
> http://www.skywaves.ar88.net/