[NJARC] Grounding question-Latest!
Scott Roberts
ng19delta at yahoo.com
Mon Feb 16 01:07:19 EST 2009
Ok- The hum is about 95% removed. The High Voltage negative to audio fixed most of it, the pi filter on the power supply fixed more.
I do have what sounds like brush noise form the dynamotor, so I will pull it for a look inside, as well as I am planning to re-repack the bearings: I had repacked them with synthetic grease, as recommended on one site, but I was not very happy with the stuff, so I am planning to repack with the original type grease, and go from there.
The radio is also doing something new- It is actually PICKING UP SIGNALS! However, only on one band so far. I am going to start checking through the alignment of the set. I imagine the ham who hacked it started fiddling with the adjustments trying to tune out the hum, which he put into the set!
Should it be a problem with the tuner sections, I do have a spare set for a BC-348-J which I will check to see if I can put into a 348-P.
I am quite happy with the fact it does seem to work.
A question for those who might know- The little neon tube(991?) used in the voltage regulator for the oscillator- What should it look like when operating? When the set is first turned on, it gets a large orange glow on one side of the little "hat" inside, which, as the set warms, shrinks and disappears altogether. Does this sound right, or should it have some other sort of glow?
Thanks!
Scott
--- On Sun, 2/15/09, Scott Roberts <ng19delta at yahoo.com> wrote:
> From: Scott Roberts <ng19delta at yahoo.com>
> Subject: Re: [NJARC] Grounding question- Good Lead!
> To: "New Jersey Antique Radio Club" <njarc at mailman.qth.net>
> Date: Sunday, February 15, 2009, 3:26 PM
> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Got into looking over the radio again this afternoon.
> Re-examined the big transformer at the back of the set,
> which houses 101A, 101B, 101C, and found something
> interesting...
>
> One of the regular mods done to the BC-348 model is that
> the output connection is switched from High Impedance
> output(early war) to Low Impedance output(later war). I had
> noticed a wire removed from what is usually the high, and
> one on the low. Only the low was marked, and I really
> didn't think about it, as it looked like others I've
> seen. Well, I took a closer look, and reread the schematic
> on the transformer itself, and realized a major difference
> from the schematic- instead of being paired in the normal
> way- three levels, from top to bottom, of two terminals
> each, and numbered 1 & 2, 3 & 4, 5 & 6, it is
> actually setup this way- 1 & 6, 2 & 3, 4 & 5.
>
> This caught my attention- ESPECIALLY when I noticed #6,
> which is hidden at the lowest right side connection, has
> "High" marked right above it- where you have to
> look hard to see it.
>
> So- Both High and Low impedance are connected- and No. 1 is
> not. And guess what is supposed to connect to No. 1- The
> High Voltage Negative output from the dynamotor.(and I
> retraced this circuit to confirm it is that) Go Figure-
> whoever hacked this radio never noticed the output
> transformer had a non-standard layout, at least not as
> regards many models, and did not bother to look at the
> label: he just followed the general direction to move the
> top left wire on it to the top right- which is correct on
> many of these radios, but apparently not all... The high
> impedance was cross-feeding with the high voltage negative,
> with nothing going through the 101B circuit... Wonder if
> that could be the overwhelming hum...
>
> I'm going to go swap the yellow Hi-V neg wire back to
> its proper position, and see what I get.
>
> Here's hoping- I'll post results.
>
> Scott
>
> --- On Sun, 2/15/09, Scott Roberts
> <ng19delta at yahoo.com> wrote:
>
> > From: Scott Roberts <ng19delta at yahoo.com>
> > Subject: Re: [NJARC] Grounding question- Result
> > To: "New Jersey Antique Radio Club"
> <njarc at mailman.qth.net>
> > Date: Sunday, February 15, 2009, 3:54 AM
> > Visit our web site - See http://www.njarc.org
> > _______________________________________________
> > No Change.
> >
> > Grumble.
> >
> > Scott
> >
> >
> > --- On Sun, 2/15/09, Scott Roberts
> > <ng19delta at yahoo.com> wrote:
> >
> > > From: Scott Roberts <ng19delta at yahoo.com>
> > > Subject: [NJARC] Grounding question
> > > To: "NJARC Radio Club"
> > <njarc at mailman.qth.net>
> > > Date: Sunday, February 15, 2009, 1:59 AM
> > > Visit our web site - See http://www.njarc.org
> > > _______________________________________________
> > > This may sound like a simple question, but I do
> need
> > an
> > > answer, although I am pretty sure I know it
> already...
> > >
> > > I am working on my BC-348-P, which I got at the
> > swapmeet
> > > back in August or September at Info Age.
> > >
> > > I had thought I had corrected all the
> modifications
> > made to
> > > the set, and returned it to original shape.
> However, I
> > still
> > > get a loud hum over any and all settings. I have
> been
> > > starting to check voltages and resistances at the
> > tubes with
> > > the I-56-A test set.
> > >
> > > Anyway, I just found a change I missed- The dial
> lamp
> > > circuit has been heavily modified- The power in
> lead
> > has
> > > been moved from the AVC-Off-MVC switch, front
> wafer,
> > > position 1, and moved to the VT-86(217, First
> I.F.)
> > tube
> > > socket terminal 7. Continuing through the
> circuit, the
> > > original series arrangement of the lamps has been
> > changed to
> > > parallel, and resistor 503(the flat 60 ohm dial
> lamp
> > > resistor) has been bridged across with a piece of
> bus
> > wire.
> > >
> > > Could these changes, having been left unnoticed
> &
> > > intact, be the cause of the overriding hum? I am
> about
> > to
> > > restore the circuit, as I would anyway, but
> wanted to
> > see
> > > what you guys think.
> > >
> > > I'll let you all know the results(hopefully
> > positive
> > > & hum free) after i complete it and test the
> unit
> > again.
> > >
> > > I am still not sure the neon tube in the
> oscillator
> > plate
> > > voltage regulator, (id 218, 2J991, or 2V991,
> JAN-991)
> > is
> > > operating properly, and wonder if it could be
> related
> > to
> > > either, or both issues- the convoluted lamp
> wiring, or
> > the
> > > hum...
> > >
> > > Thanks!
> > > Scott
> > >
> >
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