[NJARC] BC-348 Hum

Tue Nov 11 21:52:02 EST 2008

Scott,

For the time being, you can keep it simple and just use a DVM. Measure the plate and screen voltages at the audio output stage, for starters. You *do* have a DVM don't you? 

You have an importnat clue -- you said the 991 regulator bulb is not firing. Ah hah! Assuming the bulb is good, that means you have no B+ (or low B+) at that point.

JR

--- 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, 7:30 PM
> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Hi Scott,
> 
> The Tektronix classic instructional text "The
> XYZ's of Oscilloscopes" is 
> available on the web:
> http://www.tek.com/Measurement/App_Notes/XYZs/03W_8605_2.pdf
> 
> Highly recommended reading.
> 
> Al
> 
> Scott Roberts wrote:
> > Visit our web site - See http://www.njarc.org
> > _______________________________________________
> > 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
> >>>
> _______________________________________________
> >>> NJARC mailing list
> >>> 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/
> >>     
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> 
> -- 
> Al Klase - N3FRQ
> Jersey City, NJ
> http://www.skywaves.ar88.net/
> 
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