[NJARC] Grounding question- Good Lead!

Sun Feb 15 16:39:50 EST 2009

Made the switch, and it removed most of the heavy hum- there is still a little bit of hum, but nowhere near as overwhelming as it had been- And I finally started hearing signals!

I think the remainder of the hum is from my 24 power supply- I have no pi filter on it at the moment, so I will put it back into the mix, and see if that fixes the rest of the hum.

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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