[NJARC] Grounding question- Good Lead!

[Scott Roberts]

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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