[Boatanchors] Backwards Microphonics?

Wed, 25 Dec 2002 19:50:08 -0500

Ed wrote:

On Wed, 25 Dec 2002 15:52:18 -0700 "ed sharpe" <[email protected]>
writes:
> Hmmm...... Demons!

More like the anonymous mad modifier ...

> the unit either needs a visit
> from a priest or be shredded!

Too nice for shredding, but may need an exorcism.

> be sure that no one added a speaker or headphone element inside the 
> case somewhere..  we have one at the museum that has a speaker inside
the 
> case!  the previous owner did this to reduce clutter on the desk... It
did 
> indeed scare me when I turened it on for the first time and  booming
sound 
> came out of it without the normal exterior speaker hooked up!

Like they say in those Hertz commercials -- not exactly.  There's no case
for a speaker to hide in and the audio emanations are too low for a
speaker but you got me thinking and guess what I found?

I removed the bottom cover of the chassis.  Now this rx looks stock from
the outside -- only thing is that someone removed the AC backwards
receptacle -- a recessed standard two-prong -- and direct-wired a cordset
and there's another open hole on the back of the chassis -- not sure what
might have been there.  Otherwise, no outward signs.

However, what I found was this:  Someone added a small audio transformer
underneath the original.  It looks like a PC mount type, with an
oversized piece of fishpaper and seven terminals -- 3 on the primary and
4 on the secondary.  One side of each is wired together and go to ground.
 The center terminal the 3 primary ones is connected to the 600 ohm
terminal of the original transformer which has one original lead still on
it.  An existing lead is connected to the 4th terminal of the secondary. 
It disappears into a wrapped wiring bundle and I can't trace it easily. 
I'm not sure which terminal on the stock transformer it originally went
to.  I suspect this is bringing the impedance down from 600 to 8 or
something. However, I don't know whether it's now in line to the 600 ohm
speaker terminals only or the headphone jack also.  I'll try some
ohmmeter sleuthing.  The headphone jack is a simple non-switching type --
no provision to cut the speaker feed.

Now get this:  The added transformer is suspended by its ground and 600
ohm tap connections -- on two stiff wires that look like 18 or 16 ga.
solid, harvested from a piece of romex.  If you tap on the thing, it
vibrates like a tuning fork.  I haven't tried this under power yet, but
it's highly suspect of being an unintentional transducer.  Also, it is an
open frame transformer and the top of the frame is about a half inch from
the chassis.  ..

Everything else looks fine.  This thing is really built.  How did they
manage to make a radio with hardly any caps.  I don't see any tubulars,
square ceramics or micas, round ceramics, etc.  Just about a half dozen
bathtub caps which are dry electrolytics, and four stud-mount
electrolytics on top.  OK, I haven't opened some covers yet, but
still.... ???

Amazing what you find.  I think I'll have to invest in a manual for this
so I can bone up on this late 30's technology.  Figures that the fault
isn't part of the original equipment.  Anybody know where I can find a
schematic on-line?

Barry

> 
> Please check our web site at
>  http://www.smecc.org
> to see other engineering fields, communications and computation 
> stuff we
> buy, and by all means  when in Arizona drop in and see us.
> 
> address:
> 
>  coury house / smecc
> 5802 w palmaire ave
> glendale az 85301
> 
> thanks Ed Sharpe archivist for SMECC
> 
> ----- Original Message -----
> From: "Barry Hauser" <[email protected]>
> To: <[email protected]>
> Sent: Wednesday, December 25, 2002 3:09 PM
> Subject: [Boatanchors] Backwards Microphonics?
> 
> 
> > Hi Gang
> >
> > I've come across microphonic tubes in my time -- like one in an 
> HQ180
> > where it was picking up my drumming fingers on the table and even 
> my
> > voice if I yelled at the front panel. That was a 6GH8, as I 
> recall.
> >
> > But the microphonic in this E.H. Scott SLR-F goes the other way. 
> It's
> > more of a "speakerphonic". I was checking it out using headphones. 
>  Took
> > them off and I could still hear something.  Unplugged the phones 
> and it
> > was still there.  (Yes, with no speaker connected and, yup -- the 
> sound
> > stops when I switch it off, just in case there are any jokers in 
> the
> > woodwork -- besides me, that is. ;-).
> >
> > Can a tube do that?  I would suspect the output transformer, but 
> the
> > sound is coming from the opposite side of the chassis.  Besides, 
> the
> > audio tranny is an oil-filled Thorardson so I doubt if the 
> windings could
> > be loose from the core or something.
> >
> > Aside from the rectifier, all the tubes are metal types.
> >
> > This is one of those pre-war/WWII receivers which was over-built 
> with
> > extra shielding for use by the Navy to minimize RF radiation and,
> > thereby, detection by the enemy.  I don't know about AF radiation 
> -- just
> > a sec ... Hey you!!!  Loose lips sink ships!!!.  Nah, I don't 
> think
> > that'll work.
> >
> > Are these like the AR88's audio setup?  The word is on AR88's that 
> the
> > output transformer can get wrecked if operated for more than a 
> short time
> > on headphones alone, without some load on the speaker terminals.
> >
> > Any clues will be appreciated.
> >
> > BTW -- Merry Xmas to All
> >
> > Barry
> >
> > ________________________________________________________________
> > Sign Up for Juno Platinum Internet Access Today
> > Only $9.95 per month!
> > Visit www.juno.com
> > _______________________________________________
> > Boatanchors mailing list
> > [email protected]
> > http://mailman.qth.net/mailman/listinfo/boatanchors
> >
> >
> 
> 
> _______________________________________________
> Boatanchors mailing list
> [email protected]
> http://mailman.qth.net/mailman/listinfo/boatanchors
> 
> 

________________________________________________________________
Sign Up for Juno Platinum Internet Access Today
Only $9.95 per month!
Visit www.juno.com